Electrospinning Polymer Nanofibers - Electrical and Optical

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ELECTROSPINNING POLYMER NANOFIBERS - ELECTRICAL AND OPTICAL

CHARACTERIZATION

A dissertation presented to

the faculty of

the College of Arts and Science of Ohio University

In partial fulfillment

of the requirements for the degree

Doctor of Philosophy

Saima N. Khan

November 2007

This dissertation titled

ELECTROSPINNING POLYMER NANOFIBERS ─ ELECTRICAL AND OPTICAL

CHARACTERIZATION

SAIMA N. KHAN

has been approved for

the Department of Physics and Astronomy

and the College of Arts and Science by

Martin E. Kordesch

Professor of Physics and Astronomy

Benjamin M. Ogles

Dean, College of Arts and Science

Abstract

KHAN, SAIMA, Ph.D., November 2007, Physics and Astronomy

ELECTROSPINNING POLYMER NANOFIBERS - ELECTRICAL AND OPTICAL

CHARACTERIZATION (107 pp.)

Director of Thesis: Martin E. Kordesch

Electro spinning is a technique used for the production of thin continuous fibers

from a variety of materials including polymers, composites and ceramics [1-3]. The

extremely small diameters (~ nm) and high surface to volume and aspect ratios found in

electrospun fibers can not be achieved through conventional spinning.

Electrically conducting polymers are materials which simultaneously possess the

physical and chemical properties of organic polymers and the electronic characteristics of

metals. In this work fibers were electrospun from polymer blends of polyaniline doped

with Camphorsulfonic acid (PAn.HCSA) and polyethylene oxide (PEO) in chloroform.

Electrical conductivities of the fibers were measured using the four-point-probe method.

The conductivities of the cast films were measured for comparison purposes. It was

noticed that the conductivity of both the fibers and films increase exponentially with the

concentration of (PAn.HCSA), the conductivity of the film however is higher than that of

the mat for any given concentration of PAn.HCSA in PEO. Electrical conductivities of

single fibers containing different PAn: HCSA concentrations were measured for the first

time and were found to be the highest (3.2S/cm) among the mats and films. The effect of

the non-conductive PEO on the conductivity of the polyaniline fibers was studied.

Keeping the PAn.HCSA concentration constant films and fibers were obtained from blends containing PEO (300,000 g/mol) and PEO (900,000 g/mol). Higher electrical conductivities were recorded in fibers and mats containing PEO (900,000 g/mol) than those containing PEO (300,000 g/mol).

Silicon Carbide (SiC) fibers were obtained by electrospinning a blend of SiC and

PEO in chloroform and sintering the as spun fibers at temperatures of 800ºC and 1000ºC.

The compositional analysis of the annealed samples confirmed the presence of (30-40)

µm long SiC fibers with diameters in the range (1-3) µm. Optical spectra of the fibers show red emission extending to the infrared.

For the first time complexes of ruthenium with picolinate and polypyridine ligands were introduced into nanofibers. Fibers containing ruthenium picolinate ([Ru

(pic)2 (dmso)2]) turn orange from pale yellow on exposure to ultraviolet radiation

(350nm). Fibers containing ruthenium bi-pyridine ([Ru(bpy)2](PF6)2) exhibit photoluminescence with steady state red emission upon 450 nm excitation.

Approved:

Martin E. Kordesch

Professor of Physics and Astronomy

Dedication

To my father Mumtaz Khan,

Who provided me a strong academic foundation and whose determination to provide me education serves as an inspirational example to a society and culture where women have

always been underestimated and under-treated. He is not in this world any more but I hope he knows that I can’t be proud enough of being his daughter and that I love him as

much as he loved me.

Acknowledgements

First and foremost I would like to express my heartfelt gratitude towards my supervisor Professor Martin E. Kordesch for his academic guidance, continuous support and encouragement throughout this project. He is a pleasant person, a remarkable teacher and a great human being. I appreciate his generosity in providing me the opportunity to attend various conferences that gave me good academic exposure. He has been very co- operative all along and gave me his precious time and advice whenever I needed it.

I am very thankful to Dr. Saw Wai Hla, Dr. Alexander Neiman and Dr. P. G. Van

Patten for being members of my thesis committee. I am sincerely thankful to Dr. Jeffery

Rack for his help and co-operation in this work.

My heartfelt gratitude extends to Dr. Aurangzeb Khan who makes me proud for being my loving husband, wonderful friend and a nice colleague. I can’t thank him enough for believing in me and making me believe in myself.

I want to acknowledge the little sacrifices our lovely daughter Gulsanga Khan has been making every day, for three years now. I love her and I owe her every bit of the pride I take in my degree today.

I want to offer a very special thanks to my mother and my brothers who always took pride in my academic achievements and facilitated me in all the ways possible.

I offer my special thanks to my mother-in-law for her love and support. She is an extraordinary woman and truly believes in the value of education. I also want to thank my friends and relatives for their support and co-operation. 7

Table of Contents

ABSTRACT...... 3

DEDICATION...... 5

ACKNOWLEDGEMENTS ...... 6

LIST OF FIGURES ...... 11

CHAPTER 1...... 15

BACKGROUND AND EXPERIMENTAL TOOLS ...... 15

1.1 Introduction...... 15

1.2 Electrospinning ...... 16

1.3 Working principle...... 21

1.4 Scanning electron microscope (SEM) ...... 25

1.5 Transmission electron microscope (TEM)...... 27

1.6 Energy dispersive x-ray spectrometry (EDX)...... 29

1.7 Four-Point-Probe method of conductivity measurement...... 31

1.8 Cathodoluminescence ...... 34 8

1.9 Conclusion ...... 34

CHAPTER2...... 36

ELECTROSPINNING POLYETHYLENE OXIDE NANOFIBERS...... 36

2.1 Introduction...... 36

2.2 Sample preparation and obtaining fibers ...... 37

2.3 Important process parameters ...... 39

2.3.1 Applied voltage...... 39

2.3.2 Distance between the nozzle and the collector ...... 42

2.4 Conclusion ...... 44

CHAPTER 3...... 45

CONDUCTING POLYMER NANOFIBERS ...... 45

3.1 Introduction...... 45

3.2 Different types of doping...... 48

3.2.1 Redox doping...... 48

3.2.2 Photo doping...... 48

3.2.3 Charge-injection doping...... 48

3.2.4 Non-Redox doping or Protonic-acid doping...... 49 9

3.3 Polyaniline ...... 50

3.3.1 Oxidation states of polyaniline ...... 51

3.3.2 Doping polyaniline...... 52

3.4 Sample preparation...... 55

3.5 Electrospinning fibers...... 56

3.6 Characterization of the nanofibers...... 57

3.7 Electrical Conductivity measurement...... 60

3.7.1 Using PEO (300,000 g/mol) as the matrix...... 61

3.7.2 Using PEO (900,000 g/mol) as the matrix...... 63

3.7.3 Effect of PEO on the conductivity of PAn...... 67

3.8 Conclusion ...... 70

CHAPTER 4...... 71

FABRICATION AND OPTICAL CHARACTERIZATION OF SILICON

CARBIDE NANOFIBERS...... 71

4.1 Introduction...... 71

4.2 Silicon Carbide Nanostructures ...... 73

4.3 Electrospinning silicon carbide nanofibers...... 75 10

4.4 Structural and Compositional Analysis ...... 81

4.5 Cathodoluminescence ...... 83

4.6 Conclusion ...... 84

CHAPTER 5...... 85

POLYMER NANOFIBERS CONTAINING RUTHENIUM COMPOUNDS...... 85

5.1 Introduction...... 85

5.2 Experimental...... 87

5.3 Results and Discussion...... 87

5.4 Fibers Containing [Ru (BPY) 3] (PF6)2 ...... 92

5.4 Conclusion ...... 94

CHAPTER 6...... 96

CONCLUSION AND FUTURE PROSPECTS...... 96

BIBLIOGRAPHY...... 99

List of Publications ...... 106

Conferences Attended...... 107 11

List of Figures

FIGURE 1.1: SCHEMATIC DIAGRAM OF THE ELECTROSPINNING PROCESS...... 20

FIGURE 1.2: SEM IMAGES OF THE PATTERNS FORMED BY FIBERS DURING ELECTROSPINNING (A) V

=5 KV, D = 10 CM (V = 7 KV, D= 10 CM)...... 22

FIGURE 1.3: DIGITAL PHOTOGRAPH OF THE EXPERIMENTAL SET UP USED FOR ELECTROSPINNING.

...... 24

FIGURE 1.4: SCHEMATIC DIAGRAM OF SCANNING ELECTRON MICROSCOPE...... 26

FIGURE 1.5: SCHEMATIC DIAGRAM OF TRANSMISSION ELECTRON MICROSCOPE...... 28

FIGURE 1.6: X-RAYS SCATTERING FROM THE CRYSTAL PLANES OF THE SAMPLE...... 30

FIGURE 1.7: SCHEMATIC DIAGRAM OF THE OF EDX SPECTROMETER...... 30

FIGURE 1.8: SCHEMATIC OF THE FOUR-POINT-PROBE SET UP...... 33

FIGURE 1.9: OPTICAL IMAGE OF THE COMMERCIAL FOUR PROBE CHIP...... 33

FIGURE 1.10: SCHEMATIC OF THE CL SET UP...... 35

FIGURE 2.1: SEM IMAGES OF ELECTROSPUN FIBERS WITH BEAD DEFECTS (A) 1500X (B) 750X (C)

200X (D) 750X. IMAGES WERE TAKEN USING 30 KV AND 80 ΜA...... 38

FIGURE 2.2: EFFECT OF THE APPLIED VOLTAGE ON THE DIAMETER OF FIBERS ELECTROSPUN FROM

PEO/CHLOROFORM AT A CONSTANT NOZZLE TO COLLECTOR DISTANCE OF 10 CM...... 40

FIGURE 2.3: ELECTROSPINNING CURRENT MEASURED AS A FUNCTION OF APPLIED VOLTAGE. ....41

FIGURE 2.4: EFFECT OF NOZZLE TO COLLECTOR DISTANCE ON THE DIAMETER OF FIBERS

ELECTROSPUN FROM PEO AT A CONSTANT VOLTAGE OF 7 KV...... 43

FIGURE 2.5: SEM IMAGES OF ELECTROSPUN FIBERS WITH MINIMUM BEAD DEFECTS (A) 500X (B)

1200X...... 44 12

FIGURE 3.1: BAND STRUCTURE IN ELECTRICALLY CONDUCTING POLYMERS. EACH BAND

CONSISTS OF CLOSELY PLACED DISCRETE STATES IN POLYMERS UNLIKE THE CONTINUOUS

ONES AS IN SEMI-CONDUCTORS...... 46

FIGURE 3.2: EXAMPLES OF CONDUCTING POLYMERS...... 47

FIGURE 3.3: NON-REDOX DOPING PROCESS OF POLYANILINE [45]...... 49

FIGURE 3.4: MAIN POLYANILINE STRUCTURES N + M = 1, X = DEGREE OF POLYMERIZATION...... 51

FIGURE 3.5: MOLECULAR STRUCTURE OF CAMPHORSULFONIC ACID...... 53

FIGURE 3.6: (A) INSULATING EMERALDINE BASE FORM OF POLYANILINE (B) CONDUCTING

EMERALDINE SALT FORM OF POLYANILINE (POLARON STRUCTURE) (C) BIPOLARON

STRUCTURE IN DOPED POLYANILINE...... 54

FIGURE 3.7: (A) PAN AND CAMPHORSULFONIC ACID RIGHT AFTER MIXING IN CHLOROFORM. (B).

AFTER 6 HOURS OF STIRRING...... 55

FIGURE 3.8: DIGITAL PHOTOGRAPH OF THE FIBROUS WEB ELECTROSPUN FROM PAN/PEO BLEND.

(D = 10 CM AND V = 7 KV WERE USED DURING ELECTROSPINNING)...... 57

FIGURE 3.9: SEM IMAGES OF THE FIBERS ELECTROSPUN FROM PAN.HCSA/PEO BLEND (A) 200X

(B) 350X (C) 750X (D) 1000X...... 58

FIGURE 3.10: TEM IMAGES OF BEADED FIBERS. IMAGES WERE TAKEN USING 80 KV POTENTIAL.59

FIGURE 3.11: TEM IMAGES (80 KV) OF BEADLESS FIBERS ELECTROSPUN FROM PAN: HCSA/PEO

BLEND SCALE BAR IS (A) 1 UM (B) 500 NM...... 60

FIGURE 3.12: FOUR PROBE IV’S OF FIBROUS MATS ELECTROSPUN FROM (A) 50 WT. % (B) 58 WT.

% PAN.HCSA SOLUTION. THE CURRENT VALUES WERE AVERAGED 10 MEASUREMENTS...61

FIGURE 3.13: ELECTRICAL CONDUCTIVITY OF THE FIBROUS MATS ELECTROSPUN FROM

PAN.HCSA/PEO (300,000 G/MOL) BLEND. CONDUCTIVITY VALUES WERE AVERAGED OVER

10 TRIALS, EACH TRIAL VALUE BEING THE AVERAGE OF 20 ITERATIONS...... 62 13

FIGURE 3.14: ELECTRICAL CONDUCTIVITY OF THE FIBROUS MATS ELECTROSPUN FROM

PAN.HCSA/PEO (900,000 G/MOL) BLEND...... 64

FIGURE 3.15: FOUR-PROBE I-V CHARACTERISTICS OF A SINGLE NANOFIBER ELECTROSPUN FROM

60 WT. % PAN.HCSA IN PEO (MW ~ 900,000 G/MOL). THE CURRENT VALUES SHOWN ARE

THE AVERAGE OF TEN MEASUREMENTS...... 65

FIGURE 3.16: (A) OPTICAL IMAGE OF THE 4-PROBE CHIP. (B) SEM IMAGE OF THE FIBER PLACED

ON THE CHIP FOR CONDUCTIVITY MEASUREMENT...... 66

FIGURE 3.17: ELECTRICAL CONDUCTIVITIES OF SINGLE FIBERS, CAST FILMS AND FIBROUS MATS

MEASURED AS FUNCTION OF PAN.HCSA CONCENTRATION IN THE POLYMERS SOLUTION..67

FIGURE3.18: ELECTRICAL CONDUCTIVITIES OF THE FIBERS AND FILMS ELECTROSPUN FROM

PAN.HCSA BLENDED WITH PEO (900,000 G/MOL) (UPPER THREE LAYERS) AND PEO

(300,000 G/MOL) (LOWER TWO LAYERS)...... 68

FIGURE 4.1: TETRAHEDRAL ARRANGEMENT OF SI AND C ATOMS IN (A) 4H AND (B) 6H SIC

STRUCTURES [60]...... 72

FIGURE 4.2: SCHEMATIC OF THE FACE CENTERED CUBIC (3C) CRYSTAL OF SIC (Β PHASE)...... 73

FIGURE 4.3: SEM IMAGE OF SILICON CARBIDE PARTICLES. AVERAGE PARTICLE SIZE IS 800 NM.76

FIGURE 4.4: SEM IMAGES OF FIBERS ELECTROSPUN FROM A DISPERSION SOLUTION OF SIC IN PEO

(A) LOW RESOLUTION (1000 X) AND (B) HIGH RESOLUTION (5000 X)...... 77

FIGURE 4.5: SEM IMAGE OF THE SIC/PEO FIBERS AFTER ANNEALING AT 800˚C FOR 7 HOURS...78

FIGURE 4.6: SEM IMAGES OF SIC FIBERS OBTAINED AFTER ANNEALING THE SAMPLE AT 800˚C

FOR 7 HOURS...... 79

FIGURE 4.7: SEM IMAGES OF SIC FIBERS FORMED AFTER ANNEALING THE SIC/PEO FIBERS AT

1000˚C FOR 10 HOURS (A) 2000X (B) 2000X (C) 1500X (D) 1500X. IMAGES WERE TAKEN

USING 30 KV, 80 UA...... 80

FIGURE 4.8: EDX PATTERN OF SIC/PEO FIBERS AFTER ANNEALING...... 81 14

FIGURE 4.9:XRD PATTERNS OF SIC/PEO FIBERS AFTER ANNEALING...... 82

FIGURE 4.10: CL SPECTRUM COLLECTED FROM SIC FIBERS AT 5 KEV...... 83

FIGURE 4.11: CL IMAGES OF SILICON CARBIDE FIBERS. SCALE BAR IS THE SAME ON THE

PICTURES. (5 KEV BEAM WAS USED)...... 84

FIGURE 5.1: MOLECULAR STRUCTURE OF [RU (PIC)2 (DMSO)2] [84]...... 86

FIGURE 5.2: OPTICAL IMAGES OF THE ELECTROSPUN MAT (A) BEFORE AND (B) AFTER EXPOSURE

TO UV (Λ~350 NM) WITH A MASK ON IT. SCALE BAR IS THE SAME ON (A) AND (B)...... 88

FIGURE 5.3: SEM IMAGE OF THE FIBROUS MAT ELECTROSPUN FROM [RU (PIC)2 (DMSO)2]

BLENDED IN PEO (30 KV, 80 UA)...... 89

FIGURE 5.4: UV-VISIBLE ABSORPTION SPECTRUM OF [RU (PIC) 2 (DMSO) 2] / ETHANOL CHANGES

UPON IRRADIATION AT 344 NM.(SPECTRUM TAKEN BY RACHFORD ET AL. [84])...... 91

FIGURE 5.5: SEM IMAGE OF FIBERS ELECTROSPUN FROM ([RU (BPY) 3] (PF6)2)/PEO IN

METHYLENE CHLORIDE...... 93

FIGURE 5.6: FLUORESCENCE SPECTRA CORRESPONDING TO LOCATIONS A-G IN THE

FLUORESCENCE IMAGE OF A SINGLE FIBER AFTER EXCITATION WITH 532 NM...... 94

FIGURE 5.7: NSOM IMAGES OF (A) FIBER CROSS-SECTION WITH THE BRIGHT SPOTS SHOWING [RU

(BPY) 3] (PF6)2 DISTRIBUTED IN THE FIBER AND (B) SINGLE LUMINESCENT FIBER,

ELECTROSPUN FROM [RU (BPY) 3] (PF6)2 /PEO IN CHLOROFORM ON EXCITATION WITH 532

NM...... 95

Chapter 1

Background and experimental tools

1.1 Introduction

According to the National Science Foundation (NSF) and in fiber science related literature, nanofibers are defined as structures having at least one dimension of 100 nanometer (nm) or less, but generally nanofibers are considered as having a diameter of less than one micron. The name derives from the nanometer, a scientific measurement unit representing a billionth of a meter, or three to four atoms wide.

Nanofibers due to their extremely high surface to volume ratio compared to conventional fibers exhibit special properties such as low density, low specific mass and high pore volume which make them appropriate for a wide range of applications such as filtration and energy storage[4]. Nanofibrous mats with specific pore size can be used as chemical and mechanical filters. These are ideally suited for filtering submicron particles from air or water. Properly designed fibrous mat can actually trap and dissolve certain chemical and biological components through chemical reactions. These fibers combined with other nonwoven products have potential uses in a wide range of filtration applications such as aerosol filters, facemasks, and protective clothing. Other uses are made in personal care products, wipes, garments and insulation. Fabrics made with micro fibers claim stain resistance and extremely soft hand. At present, military fabrics under 16 development designed for chemical and biological protection have been enhanced by laminating a layer of nanofibers between the body side layer and the carbon fibers [5].

Nanofibers are also used in medical applications, which include, drug and gene delivery, artificial blood vessels, artificial organs, tissue engineering and medical facemasks [6].

For example, carbon fiber hollow nano tubes, smaller than blood cells, have potential to carry drugs in to blood cells [7]. Other applications of nanofibers are made in aerospace capacitors, transistors, battery separators, energy storage, fuel cells, and information technology.

Nanofibers ob conducting polymers are predicted to possess unique electronic and optical properties that can be tuned through doping. Theses fibers carry a whole package of applications in chemical and biological sensors, light emitting diodes, rechargeable batteries nanoelectronic devices, electromagnetic shielding and wearable electronics.

Similarly nanofibers derived from ceramic materials such as zinc oxide and silicon carbide possess optical characteristics (luminescence) that can be made use of in light and field emitters. The fibers are also used extensively as reinforcements in the development of nano composites.

1.2 Electrospinning

Electro spinning is a simple technology known since 1930s’ for the production of continuous fibers (as thin as 5 nm) from a variety of materials including polymers, composites and ceramics. The electrospun fibers possess properties not found in conventional fibers e.g. they have a high surface to volume ratio, high aspect ratio, 17

controlled pore size and superior mechanical performance. The superior mechanical

properties associated with the e-spun fibers arise from the decrease in diameter that can not be achieved through conventional spinning processes.

The process of electro spinning was first patented by Farmhals in 1934 [8].

Although the process of spinning artificial threads was experimented with even before

Farmhals, it did not get much attention because of technical difficulties such as fiber drying and collection. Farmhals’ process consisted of a movable thread-collecting device to collect the threads in a stretched condition. However drying the fiber was still a problem. In order to overcome this disadvantage, Farmhals repatented his work in 1940.

In the refined process, the distance between the feeding nozzle and the screen was altered

to give more drying time for the electrospun fibers.

In the 1960’s, Taylor initiated studies on the jet forming process [9]. Taylor studied the

shape of the polymer droplet produced at the tip of the needle in detail when an electric

field is applied and showed that it is a cone and the jets are ejected from the vertices of

the cone [9]. This cone shape is known as “Taylor cone” in the literature. The conical

shape of the jet defines the onset of the extension in the fiber forming process. In

subsequent years, much attention was given to the structural morphology of the fibers.

Researchers took great interest in the structural characterization of the fibers and

understanding the relationship between the structural features and the process parameters.

Wide angle x-ray diffraction (WAXD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) were used by researchers to characterize the electrospun nanofibers. 18

In 1971, Baumgarten reported the electrospinning of acrylic micro fibers whose diameters ranged from 500 to 1100 nm [10] . He focused on how the diameter of the fibers changed with the changing electric field. Larrondo and Mandley produced polyethylene and polypropylene fibers from the melt, which were larger in diameter than the solvent spun fibers [11, 12].

In 1987, work by Hayati et al. focused on the effects of electric field, experimental conditions, and the factors affecting the fiber characteristics. They showed that highly conducting fluids with increasing applied voltage produced highly unstable streams that whipped around in different directions [13].

Doshi and Reneker [14] used electrospinning to make fibers from water soluble

poly(ethylene oxide) with diameters of 0.05 to 5 μm. They described the electro spinning

process, the processing conditions, fiber morphology and possible uses of electrospun

fibers. Jaeger (1998) et al. [15] observed chain packing in electrospun polyethylene oxide

(PEO) via the atomic force microscopy (AFM). They concluded that at the molecular

level the electrospun PEO fibers possess a highly ordered surface layer. That ordered

surface layer could be the result of the electro spinning process, or it could be the residue

of the droplet formation.

Gibson and Rivin [16] studied the transport properties of electrospun fiber mats.

They showed that the electro spun layers present minimal impedance to moisture vapor diffusional transport required for evaporative cooling.

Nanofibers of atomic heterocyclic polybenzimidazole PBI were electrospun by

Kim [17] with diameter of the fibers around 300 nm. The mechanical strength of these 19

fibers was increased by treatment with sulfuric acid and heat. The effect of the spinning

voltage and solution concentration, on the morphology of the electrospun fibers was

systematically studied by Deitzel [18] et al. in 2001.

Demir et al. [19] successfully produced ultra fine elastic fibers with submicron

diameters by electro spinning of polyurethane urea solutions. Fiber diameters in the range

7 nm to 1500 nm were obtained by varying the solution concentration. It was shown that fiber diameters increase as the third power of solution concentration. Besides all these efforts to understand the process of electro spinning and make fiber growth adaptable in various technologies, new materials were adapted and their properties were studied at

micro/nano scale.

The electronic properties of various types of nano fibers were studied by Wang et

al. [20]. Low temperature electronic transport properties of PAN-derived Carbon nano

fibers were studied very recently [20]. The semi-conducting nature of the fiber was revealed by an increase in conductivity with temperature. A similar temperature dependence of conductivity was found in Carbon micro fibers and was explained using a simple two-band model with temperature dependent mobility.

Figure 1.1: Schematic diagram of the electrospinning process. 21

1.3 Working principle

In the electrostatic technique, a high electric field is generated between the polymer solution contained in a syringe and a grounded metallic collection plate by connecting the

needle of the syringe to a high voltage power supply as shown in the Fig.1.1. At a certain threshold voltage, the polymer droplet is charged enough to overcome the surface tension of the solution. A fluid jet is thus ejected out of the droplet which is drawn down by acceleration towards the grounded collector. The jet undergoes a series of electrically

induced bending instabilities and displays a whipping motion [21] that result in the

stretching of the relatively thick jet in to fiber accompanied by the evaporation of the

solvent. The diameter of electrospun fibers is at least one order of magnitude smaller

than those made by conventional spinning techniques, and the fibers are typically

deposited in the form of a nonwoven mats.

The concentration of the polymer solution is critical to the spinning process.

Electrospinning of very concentrated polymer solution results in fibers with

discontinuities. On the other hand solutions with insufficient viscosity lead to

electrospray the solution instead of electrospinning.

The fiber jet travels down in the electric field eventually hitting the collection

plate making patterns shown in Fig. 1.2. These patterns can help visualize the whipping

motion of the jet in the electric field. The dry fibers are collected on the grounded

collector in the form of a non-woven mat. The distance between the nozzle and the

collector is generally varied between 15 and 30 cm. The process is carried out at room

temperature unless heat is required to keep the polymer in liquid state. 22

Figure 1.2: SEM images of the patterns formed by fibers during electrospinning (a)

V =5 kV, D = 10 cm (V = 7 kV, D= 10 cm).

The morphology of the fibers depends on the type of polymer used and the spinning conditions. Fiber fineness can be regulated from ten to a thousand nanometers in diameter. The actual set up devised for electrospinning is shown in Fig. 1.3

It consists of a variable high voltage low current Bertan power supply model

205B-20R. The polymer solution is contained in a BD 3ml syringe with a BD 25G1 needle. A positive potential is applied to the polymer blend by attaching the power lead to the tip of the vertically placed needle. Fibers are collected on 5x5 cm2 grounded

aluminum plate 23

The morphology, structure and chemical composition of the fibers was studied using various characterization tools present in our lab. These include the scanning electron microscope (SEM) with an inbuilt energy dispersive x-ray spectrometer (EDX), transmission electron microscope (TEM) and x-ray diffraction (XRD). The optical characteristics of the (SiC) were studied using the Cathodoluminescence

(CL).spectroscopy. The electrical conductivity of the electrospun fibers was measured using the four point probe method. A short theoretical description of the Characterization tools is given in the following section. 24

Figure 1.3: Digital photograph of the experimental set up used for electrospinning. 25

1.4 Scanning electron microscope (SEM)

The scanning electron microscope (SEM) is an important tool capable of producing high-

resolution images of a sample surface. A schematic diagram of a typical SEM is shown in

Fig. 1.4. Electrons are thermionically emitted from the tungsten cathode and are

accelerated towards an anode. Tungsten has the highest melting point and lowest vapor pressure of all metals and therefore can be heated enough for electron emission. The electron beam carries energy typically ranging from a few hundred to 100 keV. The beam is focused by one or two condenser lenses into al spot sized of 1 to 5 nm. The beam passes through pairs of scanning coils in the objective lens, which deflect the beam horizontally and vertically so that it scans in a raster fashion over a rectangular area of the sample surface. When the primary electron beam interacts with the sample, the electrons lose energy by repeated scattering and absorption within a specimen volume known as the interaction volume that extends from less than 100 nm to around 5 µm into the surface. The energy exchange between the electron beam and the sample results in the emission of secondary electrons and electromagnetic radiation from the sample which can be detected and amplified by a photomultiplier tube and used to produce an image[22]. The black and white image of the sample is obtained by correlating the sample scan position with the resulting signal. Scanning electron microscope Joel JSM

5300 was used in this work. The SEM images in this document were taken using a voltage and current of 30 kV and 80 μA respectively. Details about the construction and working principle of the SEM are available elsewhere [23]. 26

Figure 1.4: Schematic diagram of scanning electron microscope.

1.5 Transmission electron microscope (TEM)

The working principle of a TEM is depicted in Fig. 1.5. The transmission electron microscope (TEM) uses a highly energetic beam of electrons that passes through the specimen. These electrons are scattered from the sample at different angles depending on the density of atoms encountered by the electron beam. The transmitted electron signal is magnified by a series of electromagnetic lenses and is observed through electron diffraction or direct electron imaging. The enlarged version of the sample thus appears on a fluorescent screen or a layer of photographic film. The obtained electron diffraction patterns are used to determine the crystallographic nature of the sample [24]. The microstructures e.g. the grain size and lattice defects are investigated using TEM in the imaging mode. TEM in the diffraction mode is used to investigate the crystalline structure of the specimen. A 100 keV TEM [Joel 1010] in was used in this work to evaluate the dimensions of the electrospun fibers Other details about TEM can be found elsewhere [25]. 28

Figure 1.5: Schematic diagram of transmission electron microscope. 29

1.6 Energy dispersive x-ray spectrometry (EDX)

Energy dispersive X-ray spectroscopy (EDS or EDX) is an important non-destructive analytical tool predominantly used for the chemical characterization of all kinds of matter. Each crystalline solid has a characteristic X-ray pattern associated with it that is used for its identification. In the EDX the sample is investigated through interactions with x-rays.

Its characterization capabilities are due to the fundamental principle that each element of the periodic table has a unique electronic structure and, thus, a unique response to electromagnetic waves.There are four primary components of the EDS setup: the beam source; the X-ray detector; the pulse processor and the analyzer as shown in the schematic diagram [26] in Fig. 1.7. The EDX set up used in this work is installed within the scanning electron microscopes. Electron microscopes with in-built EDS set ups are equipped with a cathode and magnetic lenses to create and focus the electron beam and thus carry the elemental analysis capabilities. X-rays are emitted from the sample atoms after being hit by the energetic electrons. The x-ray energy is converted into a voltage signal by the detector. This information is sent in to a pulse processor, which measures the signals (count rate) and pass them onto an analyzer for data display and analysis. The

X-rays are thus used to identify the elements present, quantify their relative or absolute concentration, and map their distribution [27]. An X-ray set up Rigaka Geigerflex

DMAX-B with an X-ray source of wavelength Kα = 1.5405 A° was used in this work. 30

Figure 1.6: X-rays scattering from the crystal planes of the sample.

Figure 1.7: Schematic diagram of the of EDX spectrometer. 31

1.7 Four-Point-Probe method of conductivity measurement

The most common way of measuring the resistivity of a semiconductor material is

by using a four-point collinear probe. This technique involves bringing four equally

spaced probes in contact with the material of unknown resistance

The two outer probes are used for sourcing current and inner two probes are used

for measuring the resulting voltage drop across the sample. Each probe has a resistance

Rp, a probe contact resistance Rcp and a spreading resistance Rsp associated with it.

However, these resistances can be neglected for the two voltage probes because the

voltage is measured with a high impedance voltmeter, which draws very little current.

Thus the voltage drops across these resistances are insignificantly small and the voltage

reading from the voltmeter is approximately equal to the voltage drop across the

semiconductor sheet resistance. A schematic of the process is shown in Fig. 1.8.

By using the four-point probe method, the semiconductor sheet resistance can be

calculated as;

V R = K s I

Where V is the voltage reading from the voltmeter, I is the current carried by the two

current carrying probes, and K is a correction factor. For collinear or in-line probes with

equal probe spacing, the correction factor K can be written as a product of three separate correction factors, K= k1k2k3

Where k1 corrects for finite sample thickness, k2 corrects for finite lateral sample

dimensions, and k3 corrects for placement of the probes with finite distances from the

sample edges. For very thin samples with the probes being far from the sample edge, k2 32 and k3 are approximately equal to one [28], and the expression of the semiconductor sheet resistance becomes:

π ⎛V ⎞ Rs = ⎜ ⎟ ln 2 ⎝ I ⎠ The volume resistivity is calculated as follows:

π V ρ = × × t × K ln 2 I Where: ρ is the volume resistivity in (Ω-cm) V is the measured voltage in volts, I is the source current in amperes, t is the sample thickness in cm and k is a correction factor based on the ratio of the probe to the sample diameter and on the ratio of the sample thickness to probe separation.[28]. The electrical conductivity of the sample (fibrous mats and films with rectangular geometry) was calculated according to the equation [29];

(ln 2)L ⎛ I ⎞ σ = ⎜ ⎟ kπwt ⎝V ⎠

Where L and w are respectively the length and width of the mat (~mm).

The thickness t of the mat was measured carefully with a digital micrometer with a 1µm resolution. The actual set used in our lab consists of a Keithley 220 programmable current source and a Keithley 2000-20 Multimeter. A commercially available four probe chip was used to make contact with the fibers/mats. An optical image of the chip is shown in Fig 1.9.

Fibrous mat I

Keithley 220 Programmable Current source

Keithley 2000-20 Multimeter

Figure 1.8: Schematic of the four-point-probe set up.

Figure 1.9: Optical image of the commercial four probe chip. 34

1.8 Cathodoluminescence

Cathodoluminescence (CL) is a method used to analyze the crystal structure of a specimen including lattice defects , lattice distortion and carrier dynamics [24]. CL is an optical and electrical phenomenon used for the characterization of luminescent materials especially semiconductors. The working principle of CL is depicted in Fig. 1.9. The electron beam generated by the electron gun hits the target material placed in the gyrostat) at 45º angle. These electrons carry energy up to 5 keV and cause the target to emit visible light. Cathodoluminescence occurs because the impingement of a high energy electron beam onto a semiconductor will result in the promotion of electrons from the valence band into the conduction band, leaving behind a hole. When an electron and a hole recombine, it is possible for a photon to be emitted. The energy (color) of the photon, and the probability that a photon and not a phonon will be emitted, depends on the material, its purity, and its defect state [30]. The emitted light passes through a converging lens and a monochromator and is detected by a camera.

1.9 Conclusion

A set up was devised for electrospinning polymer nanofibers. Background knowledge of different tools was obtained that is required for characterization of the fibers. The four probe conductivity measurement of polymeric fibers and cast films was practiced with convenience.

Figure 1.10: Schematic of the CL set up. 36

Chapter2 Electrospinning polyethylene oxide nanofibers

2.1 Introduction

Various applications of nanofibers in medicine and surgery are known so far and have been mentioned elsewhere [31]. These include scaffolds used in tissue engineering, wound dressing, drug and medicine delivery, artificial organs and vascular grafts. The kind of fibers required for such purposes are obtained from polymers that are biocompatible and biodegradable. One such polymer is polyethylene oxide (PEO) which carries the additional advantages of processibility and low cost. PEO is also used in combination with other polymers in order to enhance their processibility and biocompatibility.

Polyethylene oxide (PEO) is a commercially available polymer represented by the chemical formula C2nH4n+2On+1. PEO is non-toxic and is used in a variety of products including clinical products such as laxatives and skin creams. The environmental stability of PEO and its ability to easily dissolve in a number of solvents such as water and chloroform were the reasons that encouraged our choice of the polymer for electrospinning fibers.

2.2 Sample preparation and obtaining fibers

250 mg of PEO (Mw~ 300,000 g/mol) was added to 10 mL Chloroform at room temperature. The mixture was magnetic stirred for 2 hours to make a homogeneous solution. The obtained solution was viscous enough for electrospinning fibers. Using the custom made set up fibers were electrospun from the solution at a voltage of 5kV. A threshold voltage of 4kV was established for electrospinning PEO fibers on trial basis.

The nozzle to collector distance was varied between 8 and 15 cm to focus the spinning jet on the collector. Fibers were electrospun for an hour and were collected on Aluminum plate in the form of a non-woven mat.

The fibers shown in Fig. 2.1 were electrospun at the threshold voltage and at a nozzle to collector distance of 15 cm. The fibers exhibit high density of defects in the form of bead like structures as has been described in literature [32] that introduce non- uniformities into the fiber thickness. These defects arise from various factors including the viscosity, conductivity and flow rate of the polymer solution and the experimental parameters such as the applied voltage and nozzle to collector distance. Some of these parameter were studied in detail to better understand the electrospinning process and to be able to obtain fibers free of defects. 38

Figure 2.1: SEM images of electrospun fibers with bead defects (a) 1500X (b) 750X (c) 200X (d) 750X. Images were taken using 30 kV and 80 μA. 39

2.3 Important process parameters

2.3.1 Applied voltage

The voltage applied during the electrospinning process plays important role in determining the shape and diameter of the fibers. The formation of beaded fibers during the electrospinning depends on the applied voltage together with other parameters such as viscosity of the solution and its flow rate. An increase in the applied voltage reportedly

[33] favors the formation of fibers with high bead density. In this study however we investigate the effect of the spinning voltage on the fiber diameter keeping other parameters constant.

A 5 wt. % solution of polyethylene oxide in chloroform was used for this purpose.

Non woven mats of fibers were electrospun at spinning voltages of 7, 9, 10 and 12 kV.

The collector was maintained at a distance of 10 cm from the nozzle throughout the experiment. Three samples were prepared for each particular voltage. These samples were observed in the scanning electron microscope (SEM). The average fiber diameter was evaluated by randomly selecting up to 5 different portions of the fibrous mat and averaging the diameter over the number of portion selected in each portion.

The data collected is shown in Fig. 2.2. The average diameter drops down from

950 to 500 nm as the voltage is increased from 7 to 9kV and gradually attains a 300 nm value at 12kV. Keeping the nozzle to collector constant, an increase in the applied voltage increases the intensity of the electric field across the solution and the collector that further accelerates the whipping motion of the jet resulting in the thinning of the fibers. These observations reported here are in qualitative agreement with literature [34]. 40

Figure 2.2: Effect of the applied voltage on the diameter of fibers electrospun from PEO/Chloroform at a constant nozzle to collector distance of 10 cm.

The effect of the spinning voltage on the current was also studied. As the charged fiber jet initiating from the nozzle hits the collector, the circuit is closed and current flow can be monitored. In this part of the experiment the fibers were electrospun from a 5 % PEO solution in Chloroform at a constant nozzle to collector distance of 8 cm. The voltage was varied from 4 to 9kV in steps of one and the current was monitored through an inbuilt ammeter in the power supply. The obtained I-V relation is shown in Fig. 2.3 indicating a linear behavior of the spinning current with respect to the applied voltage.

Figure 2.3: Electrospinning current measured as a function of applied voltage.

This result is not in complete quantitative agreement with the previous studies [33] but pretty much explains various related effects such as the increase in bead formation with the voltage. In fact the high spinning current (resulting from a high voltage) increases the extraction rate of the solution from the nozzle that result in discontinuities (beads) in the fiber morphology. 42

2.3.2 Distance between the nozzle and the collector

The distance between the nozzle and the collector (D) plays an equally important role in determining the ultimate diameter of the fibers during the electrospinning process. As part of understanding the electrospinning process the effect of nozzle to collector distance was carefully investigated in this work.

Fibers were electrospun from a 2 wt. % PEO solution in chloroform. The nozzle to collector distance (D) was adjusted between 8 and 24cm keeping a constant voltage of

7 kV. Four samples were prepared keeping D = 8, 14, 20 and 24 cm. The samples were dried overnight. The average fiber diameter was evaluated in the SEM using a voltage of

30 kV and 80 μA current. The variation in diameter of the fibers with the distance D is shown in Fig. 2.4. The diameter decreases exponentially from 700 nm to 360 nm as D is increased from 8 cm to 20 cm and slowly goes down to 350 nm as D is increased to 23 cm. We understand that the fibers with smaller diameters are obtained by allowing the jet to cover more distance in the electric field. This also allows more time for the evaporation of solvent from the fibers that can result in a decrease in the diameter.

Other parameters affecting the diameter of the electrospun fibers include the concentration, charge conduction and flow rate of the solution, nature of the solvent used and the surrounding temperature etc. Some of these have been studied to some extent and are found in literature [33, 35, 36].

Figure 2.4: Effect of nozzle to collector distance on the diameter of fibers electrospun from PEO at a constant voltage of 7 kV.

The study of the process parameters provided a useful insight into the electrospinning process. The density of beads was successfully minimized by adjusting process parameters and fibers with uniform thickness were electrospun in a repetitive manner.

SEM images of the beadless electrospun fibrous mats are shown in Fig. 2.5. 44

Figure 2.5: SEM images of electrospun fibers with minimum bead defects (a) 500X

(b) 1200X.

2.4 Conclusion

Electrospinning of polymer fibers was demonstrated and fibers of PEO were obtained. These fibers find important applications in medicine and surgery. The effect of some process parameters was studied that helped understand the electrospinning process and obtain fibers with optimal dimensions. 45

Chapter 3

Conducting polymer nanofibers

3.1 Introduction

Electrically conducting polymers are organic semi-conductors comprising conjugated molecules. Most commercially produced organic polymers are electrical insulators. Conductive organic polymers have extended delocalized bonds composed of aromatic units. These bonds create a band structure similar to that of a semi-conductor, but with localized states as has been depicted in Fig. 3.1. When charge carriers are introduced into the conduction or valence bands the electrical conductivity increases dramatically. Technically almost all known conductive polymers are semiconductors due to the band structure and low electronic mobility. The most notable difference between conductive polymers and inorganic semiconductors is the carrier mobility, which is dramatically lower in conductive polymers.

Generally the conductivity mechanism in conducting polymers is based on the motion of charged defects within the conjugated framework [37]. These defects can be positive (p-type) or negative (n-type) and result from the oxidation or reduction of the polymer respectively. In contrast, typical "doping" in the polyacetylene-derived conductive polymers involves actually oxidizing the compound. Oxidation of the 46

Figure 3.1: Band structure in electrically conducting polymers. Each band consists of closely placed discrete states in polymers unlike the continuous ones as in semiconductors. polymer initially generates a radical cation with both spin and charge. This species is referred to as a polaron and comprises both the hole site and the structural distortion which accompanies it. The cation and radical form a bound species, since any increase in the distance between them would necessitate the creation of additional higher energy quinoid units.

Polyacetylene

Polyaniline

Polyphenylene Vinylene

Polypyrrole

Polythiophene

Figure 3.2: Examples of conducting polymers.

The genesis of the conducting polymers can be traced back to the mid 1970s when polyacetylene, the first polymer capable of conducting electricity was reportedly prepared by accident by Shirakawa [38]. The subsequent discovery by Heeger and MacDiarmid

[39] that the polymer would undergo an increase in conductivity of 12 orders of magnitude by oxidative doping lead to initiating an intensive search for other conducting polymers. 48

The simplest possible form of conjugated polymers is the archetype polyacetylene (CH)X shown in Fig. 3.2. Polyacetylene itself is too unstable to be of any value but its structure constitutes the core of all conjugated polymers and is therefore well suited to ab-initio and empirical calculations required to elucidate other conducting polymers. Some examples of conjugated polymer are given in Fig. 3.2.

3.2 Different types of doping

3.2.1 Redox doping

All conducting polymers and most of their derivatives e.g. polypyrrole, polythiophene, polyfuran and polyaniline undergo either p- (partial oxidation) and/or n-

(partial reduction) redox doping by chemical and/or electrochemical processes during which the number of electrons associated with the polymer backbone changes [40].

3.2.2 Photo doping

This type of doping does not involve dopants. The polymer is exposed to radiation of energy greater than its band-gap. The electrons are thus promoted across the gap and the polymer undergoes ‘photo-doping [41].

3.2.3 Charge-injection doping

This type of doping is most conveniently carried out using a metal/insulator/semiconductor (MIS) configuration involving a metal and a conducting polymer separated by a thin layer of a high dielectric strength insulator [42, 43]. 49

3.2.4 Non-Redox doping or Protonic-acid doping

This type of doping differs from redox doping in that the number of electrons associated with the polymer backbone does not change during the doping process. The energy levels are rearranged during doping. The emeraldine base form of polyaniline was the first example of this process [44].

An example of non-redox doping process is depicted in Fig. 3.3. Conductive polyaniline can be obtained by chemical oxidation (p-doping) of Leucoemeraldine base.

It’s the reaction of a solution of chlorine in carbon tetrachloride proceeds to give emeraldine hydrochloride.

Figure 3.3: Non-redox doping process of Polyaniline [45].

3.3 Polyaniline

Polyaniline among the family of conductive polymers is unique in that it is a type of semiconductor and can be configured to conduct across a wide range, from being utterly non-conductive for insulation to highly conductive for other electrical purposes. It was discovered in 1934 as “aniline black”. In the late 1990s it became evident that polyaniline was a flexible and highly useful polymer, and could be used in applications ranging from intelligent windows to computer chips.

Polyaniline is used in a variety of applications, because it can be easily combined with other polymers to form desired shapes. It is frequently utilized in the computer industry where it is incorporated into static free packaging, flexible electronic components, and in testing to shield against electromagnetic radiation. New uses of these fibers are constantly being discovered, with more manufacturers adopting the versatile material for a wide range of applications every day.

Depending on the desired conductivity of the polyaniline, the resulting polymer will be exposed to other chemicals in a process called doping. Doping polyaniline leads to a more stable polymer, and will also allow it to conduct current evenly.

Among conducting plastics polyaniline stands out due to its outstanding properties. It is one of the so-called doped polymers, in which conductivity results from a process of partial oxidation or reduction. Polyaniline compounds can be designed to achieve the required conductivity for a given application. The resultant blends can be as conductive as silicon and germanium or as insulating as glass. Another advantage is that it is both melt and solution processable. This means that the compound can be easily mixed with 51 conventional polymers and that it is easy to fabricate polyaniline products into required shapes. Moreover products consisting of polyaniline compounds can be easily disposed of without environmental risks.

The electrical conductivity of polyaniline based compositions can be closely controlled over a wide range. For neat polyaniline compositions, conductivity levels as high as 100 S/cm can be achieved. The full range of conductivity levels from less than

10-10 to 10-1 S/cm (melt processing) and 10 S/cm (solution processing), can be achieved for polymer blends containing polyaniline compositions [46].

3.3.1 Oxidation states of polyaniline

The structural unit of Polyaniline is shown in Fig. 3.4. Polyaniline can be found in one of five distinct oxidation states [47]:

1. Leucoemeraldine, with n = 1, m = 0 is the fully reduced state.

2. Protoemeralsine (n = 0.7, m = 0.3)

3. Emeraldine (n = m = 0.5), is either neutral or only partially reduced or oxidized.

4. Nigraniline (n = 0.3, m = 0.7)

5. Pernigraniline is the fully oxidized state (n = 0, m = 1) with imine links only.

Figure 3.4: Main polyaniline structures n + m = 1, x = degree of polymerization. 52

3.3.2 Doping polyaniline

The emeraldine (n = m = 0.5) form of polyaniline, often referred to as emeraldine base (EB), is either neutral or only partially reduced or oxidized. Emeraldine base is regarded as the most useful form of polyaniline due to its high stability at room temperature, compared to the easily oxidized Leucoemeraldine and the easily degraded

Pernigraniline.

The emeraldine base is different from other conducting polymers in several respects e.g. (1) Its energy gap arises from the electronic structure of the constituent C6 units instead of changes in bond lengths as in polyacetylene, polythiophene [48] etc. (2)

EB can be converted from an insulating to metallic state (ES) if protons are added to the

─N═ sites and the number of electrons in the chain is held constant [49]. Additionally, the emeraldine base polyaniline can function as a semiconductor when doped by a protonic acid [46] such as HCl, HNO3 etc.

More recently, it was found that the emeraldine base form of polyaniline could be made soluble in a number of organic solvents by choosing functionalized protonic acids as dopants [50]. One such dopant is an organic acid called the Camphorsulfonic acid,

HCSA. The molecular structure of HCSA (chemical formula: C10H16O4S) is shown in

Fig. 3.5 and the doping process of PAn with HCSA is shown in Fig.3.5. 2 moles of

HCSA is required to dope 1 mole of polyaniline. The process is carried out in a compatible solvent such as chloroform. The imine sites in the PAn molecule in Fig.

3.6(a) are oxidized by the H+ ions of the HCSA in chloroform. A hole is thus created on the site that restricts the CSA- ion to itself. due to coulombic forces. The emeraldine salt 53

Figure 3.5: Molecular structure of Camphorsulfonic acid.

PAn is shown in Fig. 3.6 (b). The hole sites polarize the surrounding medium and travel across the molecular chain carrying along the polarization field associated with them when electrons from the adjacent amine sites jump to fill these sites. These holes together with the phonons resulting from the medium distortion are called polarons and render the polymer its conductive nature.

Theoretical treatments [51, 52] have demonstrated that two nearby polarons combine to form the lower energy bipolaron as shown in Fig. 3.5 (c). One bipolaron is more stable than two polarons despite the coulombic repulsion of the two ions. This bipolaron with an infinite conjugation chain on either side can migrate in either direction without affecting the energy of the backbone. It is this charge carrier mobility that leads to the high conductivity of these polymers. Experimental data [53] however supports the polaronic band structure in polyamine.

Figure 3.6: (a) Insulating emeraldine base form of Polyaniline (b) Conducting emeraldine salt form of polyaniline (Polaron structure) (c) Bipolaron structure in doped polyaniline. 55

Figure 3.7: (a) PAn and Camphorsulfonic acid right after mixing in Chloroform. (b). after 6 hours of stirring.

3.4 Sample preparation

Polyethylene oxide (PEO) 10-Camphorsulfonic acid (HCSA) and Polyaniline (Mw~

300,000g/mol) in the emeraldine base (EB) form were purchased from Aldrich and were used as received to prepare solutions. Chloroform was used as solvent. A 100 mg of

Polyaniline (EB) was doped with a129 mg of Camphorsulfonic acid (HCSA) in 10mL

Chloroform. The solution, originally in color blue was magnetic stirred for 4 h and filtered using a No. 40 Whatman filter paper in order to remove any undissolved matter.

The solution gets a dark green appearance after stirring indicating the conducting emeraldine salt form (PAn: HCSA) of polyaniline as shown in Fig. 3.7. PEO (Mw~ 56

300,000g/mol) was added to the filtrate and stirred for another 2 h to make a homogeneous solution. The PEO enhances the viscosity of the emeraldine salt solution so that it can be used for electrospinning fibers. A number of polymer blends were thus prepared at room temperature with the concentration of doped Polyaniline i.e.

PAn.HCSA varying from 30 to 70 wt. %.

3.5 Electrospinning fibers

Nanofibers were electrospun from each of the polymer blends using the electrospinning technique. Fibers were collected for an hour at room temperature using a high voltage of 7 kV. A 10 cm nozzle to collector distance was maintained during the spinning process. Due to the conductive nature of doped polyaniline it is hard to confine the fiber jet to a particular position on the collector. Therefore instead of making a fibrous mat on the collection plate, the electrospun fibers form a web extending beyond the collector as shown in the digital photograph in Fig. 3.8. The fibers were dried in air at room temperature overnight in order to minimize the solvent (Chloroform) content and used for characterization and electrical conductivity measurements. To avoid any damage due to handling, fibers were directly electrospun onto 1 × 1 cm2 pieces of Silicon and

TEM grids in order to be investigated in the SEM and TEM.

Figure 3.8: Digital photograph of the fibrous web electrospun from PAn/PEO blend. (D = 10 cm and V = 7 kV were used during electrospinning).

3.6 Characterization of the nanofibers

Fig. 3.9 represents the SEM images of the fibers electrospun from PAn:

HCSA/PEO blend. The fibers are randomly oriented in the non woven mat and the bead density in the fibers is almost negligible. The thickness of the fibers is uniform along the length the distribution of diameter however varies across the mat. Overall fibers seem to have circular cross-sections. It is worth mentioning that these fibers are very fragile and are deformed when exposed to intense electron beam (I> 82 µA) or regular electron beam for longer intervals. 58

Figure 3.9: SEM images of the fibers electrospun from PAn.HCSA/PEO blend (a) 200X (b) 350X (c) 750X (d) 1000X.

The irregular surfaces of the fibers shown in Fig. 3.8 (d) resulted from a 3 minute exposure to the electron beam in the SEM using a high current of 82 µA. Transmission electron microscope (TEM) was used to accurately evaluate the dimensions of the electrospun fibers. Few fibers with beads that were not visible in the SEM were seen in

Figure 3.10: TEM images of beaded fibers. Images were taken using 80 kV potential.

the TEM as shown in Fig. 3.11. Some of these fibers seem to wind around each other which indicate that they are thinner than they seem to be in the TEM images. Others just cross over each other giving a bulky expression at the junctions. The average fiber diameter was evaluated using the TEM. Fiber diameters were recorded in different potions of the sample and averaged over the number of fibers in that portion. The diameter was further averaged over the number of portions selected on a given sample.

This procedure was repeated for three different samples prepared under identical spinning conditions. An average fiber diameter of 300 nm was estimated in this manner although the minimum fiber diameter observed was only 50 nm. 60

Figure 3.11: TEM images (80 kV) of beadless fibers electrospun from PAn: HCSA/PEO blend Scale bar is (a) 1 um (b) 500 nm.

3.7 Electrical Conductivity measurement

The electrical conductivity of the electrospun fibers was measured at room temperature using the four-point-probe method. In four-point-probe method the resistance introduced into the circuit by the contacts between the mat and the four probe chip is effectively omitted. The current voltage behavior (IV) of the electrospun fibrous mats obtained from polymer blends with different PAn: HCSA compositions are shown in

Fig.3.12. These mats could conduct current on the micro meter scale with resistance ~

(2.8-2) MΩ. The ohmic nature of the IVs’ confirms that the four-probe resistance measured is intrinsic to the fibers. 61

Figure 3.12: Four probe IV’s of Fibrous mats electrospun from (a) 50 wt. % (b) 58 wt. % PAn.HCSA solution. The current values were averaged 10 measurements.

3.7.1 Using PEO (300,000 g/mol) as the matrix

Fig. 3.13 represents the electrical conductivity values of the electrospun fibrous mats and cast films for different weight % PAn: HCSA in the PEO. The conductivity of the mat increases as the amount of doped PAn in the blend is increased from 30 to 50 wt.

% and tends to saturate beyond. The highest electrical conductivity value recorded for 70 wt. % PAn.HCSA/PEO fibers is 0.001 S/cm. The conductivity for fibers with PAn.HCSA concentration higher than 70 wt. % however was not measured since fibers can not be electrospun from the blends due to their inadequate viscosity. For comparison purposes the electrical conductivities of the cast films were also measured using the four probes set

Figure 3.13: Electrical conductivity of the fibrous mats electrospun from PAn.HCSA/PEO (300,000 g/mol) blend. Conductivity values were averaged over 10 trials, each trial value being the average of 20 iterations.

up. The films were directly cast on to the four-probe-chip using a syringe and were kept in air at room temperature for 24 hours to dry out.

The electrical conductivity of the cast films is increased from 8.4 x 10-6 S/cm to

0.019 S/cm as PAn: HCSA concentration is varied from 30 to 70 wt. %, a behavior identical to the fibrous mats as shown in Fig. 3.13. The conductivity of the cast films however is higher than that of the fibrous mat for any given concentration of the doped 63

PAn in the PEO matrix. This has been previously attributed to the presence of voids in the electrospun mats by Norris et al [54] who measured the conductivity of polyaniline fibers in which the concentration of PAn: HCSA was varied only up to 50 wt.% in PEO

(Mw~ 900,000 g/mol).

3.7.2 Using PEO (900,000 g/mol) as the matrix

Another set of experiments was performed using a higher molecular weight PEO

(molecular weight ~900,000 g/mol) as the matrix polymer. The concentration of the

PAn.HCSA was varied between 30 and 70 weight % in the polymer blends and the blends were used to prepare cast films and fibers as described earlier.

The four-probe conductivity values of the fibrous mats and cast films were measured for different wt % concentration of PAn: HCSA in PEO (900,000 g/mol). The conductivity values shown in Fig. 3.14 represent the average over 10 trials, each trial value being the average of 20 iterations. The electrical conductivity of the fibrous mats was found to increase sharply up to a 50 wt. % concentration of the PAn: HCSA in the blend and started to slow down afterwards assuming a value of 0.192 S/cm for 70 %

PAn: HCSA in PEO [55]. The electrical conductivity of the cast films increased in an identical manner with the concentration of doped PAn i.e. PAn.HCSA in the polymer blend.

The conductivity values of the fibrous mats are consistently lower than their cast film counter parts for any given concentration of the PAn.HCSA in the blend. Given that the four probe method measures the volume conductivity of a system, the anticipated increase in the conductivity of the fibrous mats compared to the films was not observed 64

Figure 3.14: Electrical conductivity of the fibrous mats electrospun from PAn.HCSA/PEO (900,000 g/mol) blend.

probably because of the presence of voids in these mats. It is therefore important to measure the electrical conductivities in single fibers.

The four-probe IV’s of single fibers electrospun from PAn.HCSA/PEO (Mw

~900,000 Da) blends were measured using the same set up. These fibers were (2-3) cm long. SEM was used to measure the thickness of the fibers. Fig. 3.15 shows the IV characteristics of a 760 nm thick single nanofiber electrospun from 60 wt. % PAn.HCSA

Figure 3.15: Four-probe I-V characteristics of a single nanofiber electrospun from 60 wt. % PAn.HCSA in PEO (Mw ~ 900,000 g/mol). The current values shown are the average of ten measurements.

in PEO (900,000 g/mol). The resistance of the fiber is 10 MΩ that corresponds to a conductivity value of 3.29 S/cm [55]. Electrical conductivity values of (3.29 ±0.19) S/cm were recorded in fibers in the diameter range (750 ± 30) nm. The conductivity value reported here is higher than that reported earlier by Zhou et al. [56] for a single fiber of

70 nm diameter. The SEM image of a single fiber placed on a four probe chip is shown in

Fig. 3.16.

Figure 3.16: (a) Optical image of the 4-probe chip. (b) SEM image of the fiber placed on the chip for conductivity measurement.

The conductivity of the single fiber increases with respect to the amount of PAn:

HCSA in the blend in a fashion similar to mats and films as shown in Fig. 3.16 but is higher than that of both the cast film and the fibrous mat for a given concentration of

PAn.HCSA in PEO. A highest conductivity value of 3.29 S/cm was measured in fibers with 70 wt. % PAn: HCSA in PEO (900,000 g/mol).The higher electrical conductivity of single fibers compared to fibrous mats is obvious from the fact that unlike the mats they do not contain any voids. The poor contact among the fibers in the electrospun mats can also account for the lower electrical conduction in these mats. This is not the case in the fibers. The higher conductivity in single fibers as compared to the cast films is not unexpected in that the 67

Figure 3.17: Electrical conductivities of single fibers, cast films and fibrous mats measured as function of PAn.HCSA concentration in the polymers solution.

polymer chains in the fibers are more aligned as a result of the electrospinning process.

The carrier mobility and hence the conductivity is higher in single fibers.

3.7.3 Effect of PEO on the conductivity of PAn

The electrical conductivities of fibers and cast films obtained from polymeric blends for doped PAn using (1) PEO (Mw ~ 300,000 g/mol) and (2) PEO (Mw ~ 900,000 g/mol) as the matrix were shown in the previous section. 68

Figure3.18: Electrical conductivities of the fibers and films electrospun from PAn.HCSA blended with PEO (900,000 g/mol) (upper three layers) and PEO (300,000 g/mol) (lower two layers).

Qualitatively the variation in conductivity with respect to the amount of doped

PAn is identical in both cases. The conductivity values however are far higher in the case of fibers and films obtained with PEO (900,000 g/mol).A comparison of the two cases is made in Fig. 3.18. The conductivity measurements were performed over 10 times and the conductivity value was average over 20 iterations each time in order to establish a high degree of experimental accuracy. 69

The different electrical conductivities associated with films and fibers derived from blends using different molecular weight PEO matrices can be explained in terms of the relative miscibility of the polymers in the blends and the molecular chain conformations associated with them. It has been reported earlier that longer molecular chain polymers have poor dissolution in a miscible polymer as compared to a shorter chain polymer [23]. The PEO (Mw~900,000 g/mol) in this case have longer molecular chains and hence does not diffuse appreciably in Polyaniline as compared to the PEO

(Mw~300,000 g/mol). This results in a lower density of twisting defects among the polymer molecules in the blends allowing the PAn chains to maintain a stable polaronic structure, and hence an enhanced electrical conductivity.

Secondly the higher molecular weight PEO is more crystalline with straight chains as compared to the branched chains present in the comparatively less crystalline

PEO (Mw~300,000 g/mol). The fibers and films prepared with PEO (Mw~900,000 g/mol) there possess higher electrical conductivity as compare to those obtained from solutions using PEO (Mw~300,000 g/mol) as the matrix polymer in which the twisting defects shield the adjacent isolated polarons resulting in the localization of the polaron band. The carrier mobility and hence the intrachain conductivity decreases.

This leaves us with the conclusion that the molecular weight of the polymer matrix used does have an effect on the conductivity of the doped polymer in the blend and that a higher molecular weight matrix i.e. PEO in this case enhances the electrical conductivity of the fibers. The two cases have been summarized in the Fig. 3.16.

3.8 Conclusion

Electrically conducting fibers of PAn.HCSA/PEO were prepared using the electrospinning technique. The fiber diameter varied between 0.3-2µm in the fibrous mat.

Electrical conductivities (0.192 S/cm) comparable to the bulk value (2.04 S/cm) were obtained in the fibrous mats by varying the amount of doped Polyaniline in the polymer blend. Electrical conductivities greater than the bulk value were recorded in single fibers

(3.2 S/cm). A linear current voltage behavior was observed in the fibrous mats as well as in a single fiber. The convenience of producing electrically conducting polymer fibers was demonstrated that promise new functional opportunities in the field of nanoelectronic devices together with the advantage of low cost. 71

Chapter 4

Fabrication and optical characterization of silicon carbide nanofibers

4.1 Introduction

Silicon carbide (SiC) holds an important position among the semi conducting ceramic materials because of its high (higher than silicon) breakdown electric field (3-

5MV/cm), high thermal conductivity (350-490 W m-1 K-1) and low mass density [57-59].

Silicon carbide exists in at least 70 crystalline forms. The most commonly encountered polymorph has a hexagonal crystal structure similar to Wurtzite and is called the alpha silicon carbide (α-SiC) [60]. Alpha SiC is formed at temperatures greater than 2000 °C.

Pure α-SiC is an intrinsic semiconductor with band gaps of 3.28 eV (4H) and 3.03 eV

(6H) respectively [61]. The 4H and 6H SiC crystals are shown in Fig. 4.1. The face- centered cubic crystal structure similar to diamond and zincblend is formed at temperatures below 2000°C [61] and is called β-SiC shown in Fig. 4.2. 72

Figure 4.1: Tetrahedral arrangement of Si and C atoms in (a) 4H and (b) 6H SiC structures [60].

Figure 4.2: Schematic of the face centered cubic (3C) crystal of SiC (β phase).

4.2 Silicon Carbide Nanostructures

Nanostructures of SiC are attracting a great deal of attention for their tremendous potential in applications such as reinforcements in the development of nanocomposites

[62, 63]. Silicon carbide (SiC) nanofibers possess high mechanical strength and oxidation resistance at elevated temperature [62] and provide a perfect alternative for carbon 74 nanotubes in the development of metal matrix composites. Carbon nanotubes carry the disadvantage of degradation and can not be used in such composites [63]. These composite materials are used to prepare equipment parts capable of operating in high temperature environments. In addition, electronic devices based on SiC nanostructures are capable of operating at high powers, high temperatures and high frequencies [64, 65].

Nanostructures of SiC are of great interest because of their broad range of potential applications such as field electron emitters and light emitters[66]. In particular, the nanofibers of SiC possess high aspect ratios, which make them more suitable for applications as field emitters [67-70].

Attempts have been made to synthesize SiC fibers using various methods such as chemical vapor deposition (CVD) [69, 71], high power microwave plasma chemical vapor deposition [72], heat treating carbon nanotubes covered with Si [73] and from a precursor of two types of polycarbosilanes (type-A and type-L) [74]. The procedures mentioned above however, either require sophisticated equipment or expensive carbon nanotubes or involve several preparation steps. Moreover, the SiC fibers thus obtained are only less than a 100 nm long. The synthesis of these fibers therefore remains a challenge and needs a great deal of attention.

In this chapter we report for the first time (to the best of our knowledge) the synthesis of SiC nanofibers from a dispersion of SiC powder in polymer solution using the electrospinning technique. The as spun SiC/polymer fibers were sintered at a temperature of 1000˚C at a low pressure of 10mTorr for 8 hours. The fibers obtained are

(40-50) μm long and have uniform thickness. The optical properties of the SiC fibers 75 were studied using PL and CL systems. The Cathodoluminescence (CL) spectrum collected from the fibers lie in the red region of the spectrum.

4.3 Electrospinning silicon carbide nanofibers

SiC powder in the alpha-phase was purchased from Advanced Materials

Technology Company and polyethylene oxide was purchased from Aldrich. An 800 nm grain size of SiC powder was estimated from SEM images shown in Fig. 4.3.

The dispersion solution was prepared by mixing 500 mg of SiC powder in 5mL

Chloroform (99.8 %). The solution was stirred for 3 hours to ensure a uniform distribution of the powder in chloroform. 250mg of Polyethylene oxide was added to the solution. The mixture was stirred for another 3 hours in order to ensure a uniform dispersion of SiC particles in the polymer. The solution was used to obtain fibers using the electrospinning technique.

The spinning process was carried out at a high voltage of 10kV and a nozzle to collector distance of 12cm. Fibers were electrospun for 1h at room temperature. The fibrous mats of the SiC/PEO fibers were collected on Silicon and quartz substrates and kept in air for 15 h to dry out. Several samples were obtained in this manner and annealed at 800˚C and 1000˚C temperature at a lower air pressure of 10mTorr for 8 hours. The samples were cooled down in steps at the rate of 200˚C/30 minutes. The morphology and dimensions of the electrospun fibers as well as the quantitative elemental analysis of the fibers were investigated by scanning electron microscope (SEM) model Joel JSM 5300 76

Figure 4.3: SEM image of silicon carbide particles. Average particle size is 800 nm.

with an energy dispersion X-ray (EDX) spectrometer. The XRD was carried in order to investigate any crystalline phases formed during the annealing process of the fibers.

Cathodoluminescence (CL) was done with 20kV. accelerating electron and ocean optics setup, described in detail elsewhere [75].

Fig. 4.4 represents the SEM images of the as-spun fibrous mats electrospun from

SiC/PEO dispersion solution. The fibers appear to have non uniform surfaces and exhibit a dual complexion. The comparatively brighter regions of the fibers are believed to arise from the clustering of the SiC particle. 77

Figure 4.4: SEM images of fibers electrospun from a dispersion solution of SiC in PEO (a) Low resolution (1000 x) and (b) high resolution (5000 x).

These clusters render the fibers a non-uniform thickness as can be seen in the

SEM image shown in Fig. 4.4(b). The distribution of fiber diameter highly varied across the mats. Fiber diameters in the range (200-800) nm were observed in the SEM. The density of beads in the fibrous mats was almost negligible due to the apparently high viscosity of the solution and an appropriate combination of the spinning parameters such as voltage (10 kV) and nozzle to collector distance D (12 cm).

Fig. 4.5 shows the SEM images of the electrospun fibers after annealing at 800˚C. Large portions of the sample appear like “piles of dust” in the SEM and mainly contain the PEO residual. Within these piles are present (20-40) μm long aggregates of particles which unlike the polymer fibers remain structurally stable under the electron beam for any length of exposure time. 78

Figure 4.5: SEM image of the SiC/PEO fibers after annealing at 800˚C for 7 hours.

As shown in the high resolution SEM images in Fig. 4.6 the particles in the fibers are not closely packed rendering the fibers their rough appearance. Fig.4.7 represents SEM images of the electrospun fibers after annealing at 1000˚C. These fibers possess relatively smaller diameters and uniform surfaces. 79

Figure 4.6: SEM images of SiC fibers obtained after annealing the sample at 800˚C for 7 hours.

Figure 4.7: SEM images of SiC fibers formed after annealing the SiC/PEO fibers at 1000˚C for 10 hours (a) 2000X (b) 2000X (c) 1500X (d) 1500X. Images were taken using 30 kV, 80 uA.

Figure 4.8: EDX pattern of SiC/PEO fibers after annealing.

4.4 Structural and Compositional Analysis

The SiC nature of the obtained fibers was established by carrying out the compositional analysis of the fibers using the energy dispersive X-ray (EDX) spectrometer attached to the SEM. The peaks at 280 and 1740 (keV) indicate respectively the presence of Carbon and Silicon in the sample. The intensity peak at 535 keV indicates the presence of oxygen in the fibers. It is anticipated that teat treatment of the sample at higher temperature will completely eliminate the oxygen and enhance the crystallinity of the fibers. The intensity 82

Figure 4.9:XRD patterns of SiC/PEO fibers after annealing.

peak at 1515 keV represents aluminum that is coming from the SEM grid. No other significant peaks were seen in the EDX pattern as shown in Fig. 4.8.

The XRD study of the sample was also carried out in order to see any possible phase changes in the SiC. Fig. 4.9 represents the x-ray diffraction pattern of the sample annealed at 1000 deg ºC. All the intensity peaks were indexed to α-SiC [ICDD, card# 00-

002-1042], hence no phase change occurred during the heating process. 83

Figure 4.10: CL spectrum collected from SiC fibers at 5 keV.

4.5 Cathodoluminescence (CL)

The CL set up used in this work uses an electron beam with up to 5keV energy. Fig. 4.9 shows the Cathoduluminescence spectrum collected from the silicon carbide fibers.

These fibers emit two peaks in the red regions upon excitation with 20kV electron gun in the CL setup. The main peak is centered at 774 nm while the shoulder on the left is at

740nm. The optical microscope image taken during the CL is shown as inset in Fig. 4.10. 84

Figure 4.11: CL images of silicon carbide fibers. Scale bar is the same on the pictures. (5 keV beam was used).

4.6 Conclusion

Silicon Carbide nanofibers were synthesized by sintering the as-spun fibers obtained from SiC powder dispersed in polyethylene oxide (PEO) solution in Chloroform using the electrospinning technique. Fibers as long as 40 μm were obtained.

Cathodoluminescence spectrum of the annealed fibers shows emission in the red region of the spectrum. Apart from their importance as reinforcements in the development of nanocomposites, SiC fibers also show potential for applications in light emitting diodes and in field emitters. 85

Chapter 5

Polymer Nanofibers containing Ruthenium Compounds

5.1 Introduction

One of the many important applications of electrospun fibers is made in nano- sensors [63], [76] that utilize the electronic properties associated with these fibers. The sensor applications of nanofibers known so far however involve measurement of some physical property associated with them, electrical conductivity for instance, before and after the fibers are exposed to the particular agents/environments [77].

Complexes of ruthenium with polypyridine ligands have been deeply studied from an experimental point of view [78, 79] because of their tremendous potential industrial applications such as photovoltaics (as components in solar batteries) [80], light switches

[81]and biochemistry , DNA binding for instance[82]. The peculiar photochemical properties of these complexes were discovered 30 years ago [83] but effort is still going on to enhance these properties.

In this chapter we report for the first time, the electrospinning of nanofibers from polymer solutions containing the ruthenium compounds [Ru (pic)2 (dmso)2]and [Ru (bpy)

3] (PF6)2 . 86

Figure 5.1: Molecular structure of [Ru (pic)2 (dmso)2] [84].

The fibers containing [Ru (pic)2 (dmso)2] are excellent sensors for ultraviolet light

(wavelength ~ 350 nm). These fibers originally colorless or pale yellow turn orange when exposed to the UV radiation. The UV sensing is thus apparent from the change of color of these fibers and does not involve any complicated measurements. The molecular structure of [Ru (pic)2 (dmso)2 ] is shown in Fig. 5.1.The picolinate ligands are in a cis- arrangement so that the carboxylate oxygen of one pic ligand (O1) is trans-to the pyridine 87 of the second picolinate 20 (N2). One dmso ligand (S1) is trans to a pyridine nitrogen (N1) while the second dmso (S2) is trans to a carboxylate oxygen (O3) [84].

5.2 Experimental

The ruthenium complexes [Ru (pic)2 (dmso)2], (pic is 2-pyridinecarboxylate; dmso is dimethylsulfoxide) (chemical formula; C16H24N2O8RuS2) and [Ru (bpy) 3] (PF6)2 used in this work were obtained from Department of Chemistry and Biochemistry Ohio

University Athens Ohio. PEO (Mw~ 300,000 g/mol) was purchased from Aldrich and was used as received to prepare solutions. Chloroform was used as a solvent.

165 mg PEO was mixed with 5 g Chloroform and stirred for 2 hours to prepare a

3 wt. % polymer solution. 0.5 g of [Ru (pic)2 (dmso)2] was added to the solution and stirred for another 2 hours in order to ensure a homogeneous mixture. Another solution containing 0.5 g of [Ru (bpy) 3] (PF6)2 was prepared in a similar manner. Fibers were electrospun from both the solutions using the electrospinning technique.

5.3 Results and Discussion

Laser Flash Photolysis Spectroscopy (LFPS) was performed on the as-spun nanofibrous mat using light wavelength of 350 nm. The mat changed color from pale yellow to orange as soon as it was exposed to UV. This change in color is not permanent and the fibers return to their original color in 2-3 days. The nanofibrous mat prepared from a solution of PEO in Chloroform does not change color when exposed to UV. Thus it is fair to say that the color changing ability of the fibers comes from the introduction of 88 the photo chromic compound [Ru (pic)2 (dmso)2]. Fig. 5.2(a) shows the digital photograph of the non-woven nanofibrous mat before exposure to UV. It appears pale

Figure 5.2: Optical images of the electrospun mat (a) before and (b) after exposure to UV (λ~350 nm) with a mask on it. Scale bar is the same on (a) and (b). 89

yellow in the beginning and turns orange when exposed to UV [85] as shown in Fig. 5.2

(b). The intensity of the color increases with the exposure time and the mat turns dark orange in 2-3 minutes.Fig. 5.3 shows the SEM image of the electrospun fibers. The diameter of the fibers is pretty much uniform along their length. Overall the diameter ranges from 0.1-1 μm. The SEM studies of the electrospun fibrous mat show that the morphology of the fibers remains unchanged after they have been exposed to ultra violet radiation.

The fibrous mat electrospun from [Ru (pic)2 (dmso)2]/PEO blend responds to UV in a fashion similar to [Ru (pic) 2(dmso) 2] solution in ethanol. The color of the fibers after exposure to UV is apparently the same as that of [Ru (pic) 2(dmso) 2]/ethanol

Figure 5.3: SEM image of the fibrous mat electrospun from [Ru (pic)2 (dmso)2] blended in PEO (30 kV, 80 uA).

solution. The absorption spectrum of the electrospun mat was not taken for technical reasons. The UV sensing behavior of the fibers however was explained using the UV- visible absorption spectrum of [Ru (pic)2(dmso)2]/ethanol solution reported by Rack et. al., [84] as shown in Fig. 5.4. The spectrum features an intense π*←π absorption at 250

-1 -1 -1 nm (18940 M cm ) and a broad (∆νfwhm = 11000 cm ) less intense absorption centered at 327 nm (6400 M-1cm-1), with a low energy tail of significant intensity extending past

450 nm. Upon excitation in ethanolic solution, the 327 nm absorption shifts 3700 cm-1 lower in energy (372 nm), consistent with formation of O-bonded dmso. Similar observations following UV irradiation or exposure to sunlight are made in benzyl alcohol and propylene carbonate as well as in the microcrystalline solid and polymer film. The high energy MLCT (metal-to-ligand charge transfer) is a consequence of the relatively 91

Figure 5.4: UV-visible absorption spectrum of [Ru (pic) 2 (dmso) 2] / ethanol changes upon irradiation at 344 nm.(spectrum taken by Rachford et al. [84]).

high energy π* orbital of pic compared to that of bpy. The one-electron reduction potential of picolinate occurs at -1.75 V in this complex. In contrast, the one-electron reduction potential for tpy occurs at ~ -1.1 V and ~ -1.5 V for bpy in related complexes

[84]. The energy difference between the Ru3+/2+ E°′ (1.28 V) and pic-/pic2- E°′ (-1.75 V) suggests that the lowest energy Ru dπ→ pic π* metal-to-ligand charge-transfer (MLCT) is estimated to be ~25000 cm-1. Thus, the absorption at 327 nm (30580 cm-1) is assigned 92 as an MLCT transition. Evidence for isomerization is best observed in the UV-visible absorption spectrum. In alcoholic solutions, irradiation of [Ru (pic)2 (dmso) 2] at 344nm results in a new absorption at 372 nm and extends past 500nm. This new band is attributable to O, O-[Ru (pic) 2 (dmso) 2].

5.4 Fibers Containing [Ru (BPY) 3] (PF6)2

Following the experimental procedures described in section 5.2, polymeric fibers containing ruthenium bi-pyridine i.e. ([Ru (bpy) 3] (PF6)2) were prepared using the electrospinning process. Methylene Chloride was usedas solvent since the complex is not appreciably soluble in Chloroform. The electrospun fibers represent an identical morphology to those containing [Ru (pic)2 (dmso)2] as shown in Fig.5.5. The optical characteristics of these fibers however are different.

Figure 5.5: SEM image of fibers electrospun from ([Ru (bpy) 3] (PF6)2)/PEO in methylene chloride.

Optical spectra were collected from the fibers using the near field scanning optical microscopy (NSOM) for improved resolution. One such spectrum is shown in Fig. 5.6 that shows fluorescence recorded from different locations (A-G) in the electrospun fiber.

The intensity of the light collected from different positions varies depending on the concentration of [Ru (bpy) 3] (PF6)2) present at the particular position. An NSOM image of the fiber representing the specified locations with letters A-G is shown in Fig. 5.7(a).

Figure 5.6: Fluorescence Spectra corresponding to locations A-G in the fluorescence image of a single fiber after excitation with 532 nm.

5.4 Conclusion Photo chromic compounds of ruthenium with picolinate and bi-pyridine ligands were introduced into polymeric solutions and successfully electrospun into nanofibers. The nanofibers possess nice luminescent properties and are suitable for applications as photosensors. 95

Figure 5.7: NSOM images of (a) fiber cross-section with the bright spots showing [Ru (bpy) 3] (PF6)2 distributed in the fiber and (b) single luminescent fiber, electrospun from [Ru (bpy) 3] (PF6)2 /PEO in Chloroform on excitation with 532 nm. 96

Chapter 6

Conclusion and future prospects

Nanotechnology has got tremendous attention over the last two decades. One important field of this nanotechnology is producing fibers with the nanoscale dimensions.

Electrostatic generation of ultra fine fibers “electrospinning” has been known since

1930s. Fiber mechanical properties are improved with the decrease in the dimensions and electrospinning is the best technique known so far to achieve minimum diameter in the fibers. The high surface to volume ratio of the electrospun fibers make them attractive for various applications such as high performance filters and scaffolds in tissue engineering.

Other uses are made as reinforcements in the development of nanocomposites, in stain resistant textiles, sensors, electromagnetic shielding, biomedical grafts and drug delivery.

In this dissertation, we started by devising a custom made set up for electrospinning. Electrospinning of fibers was successfully practiced using various polymer solutions. The effect of some important process parameters on the size and shape of the fibers was investigated. The problem of bead defects in the electrospun fibers was successfully eliminated by adjusting the experimental parameters such as the applied voltage and the distance between the nozzle and the collector. SEM and TEM were used to characterize the fibers in detail. 97

Our work also focused on minimizing the thickness of the electrospun fibers and fibers with diameters as small as 50 nm were successfully achieved. Next we focused on making our fibers functional. This was done in three steps.

(1) Electrospinning of conducting polyaniline fibers was demonstrated and the

conductivity values were optimized by changing the amount of doped polymer in

the polymer blends used for this purpose. The electrical conductivities of these

fibers were measured using the four probe method. Compared to the cast films

and fibrous mats, the electrical conductivity was improved by an order of

magnitude in single fibers.

(2) SiC was introduced into the polymer solutions and used for spinning fibers. The

as spun fibers were heat treated at 1000˚C. Crystallization of SiC particles was

achieved in this manner. XRD and EDX were used for the structural and

compositional analysis of these fibers. The obtained fibers were optically

characterized using CL and PL. Optical spectra of the fibers were recorded

showing emission in the red spectral region.

SiC is an important structural material and is speculated as an important candidate to replace the Si technology. Nanofibers of SiC are oxidation resistant and can stand temperatures as high as 2700 oC which makes them superior to Carbon nanotubes in applications such as metal matrix composites. The optical properties of the fibers such as those reported in this work further add to their value. Our work on conveniently synthesizing these fibers will therefore continue in future.

(3) Metal to ligand charge transfer (MLCT) complex of ruthenium

[Ru(pic)2(dmso)2] was utilized for the first time in order to obtain fibers with the

ability to sense ultra-violet radiation. These complexes were used in solution

form together with the polymer. The polymer fibers alone do not show any UV

sensing characteristics. It was therefore concluded that the UV sensing ability

comes from the ruthenium complex alone.

(4) Another ruthenium complex [Ru (bpy)2](PF6)2 was introduced into the

nanofibers and the photoluminescence properties were studied.

The introduction of these ruthenium complexes into nano fibers is an important

addition to optically active nano materials. These materials possess potential for

various applications such as sensors and light emitting diodes. The work presented

here however is only the beginning and a lot more remains to be done in order to

exploit these fibers in a useful manner. 99

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APPENDIX A

List of Publications

Saima N. Khan, Aurangzeb Khan and M. E. Kordesch, “Electrical Conductivity

Measurements of Nanofibers Electrospun from Polyaniline/Polyethylene

Oxide Blend and Its Dependence on the Matrix Material” Material Research

Society Symposium Proceedings 948 (2007).

Saima N.Khan, Jeffrey J. Rack, Aaron A. Rachford, M. E. Kordesch,

“Electrospinning Polyethylene Oxide nanofibers and their application as

ultraviolet sensors” Material Research Society Symposium Proceedings 948

(2007).

Aurangzeb Khan, Wojciech M. Jadwisienczak, Saima N. Khan, and Martin E.

Kordesch, “ZnO Nanofibers Doped with Ga, In and Er Fabricated with

Electrospinning Technique”, Material Research Society Symposium

Proceedings 957 (2007).

Saima N. Khan, Aurangzeb Khan and M. E. Kordesch, "Fabrication and Optical

Characterization of SiC Nanofibers"- (Accepted) Material Research Society

Symposium Proceedings (2007).). 107

APPENDIX B

Conferences Attended

APS March Meeting 2006, Baltimore, MD, March 13-17 2006, Poster

contributed.

2006 International Institute for Nanotechnology Symposium, Northwestern

University, Evanston, IL.

MRS 2006 Fall Meeting, Boston, MA, Nov-Dec 2006, Poster and Oral

Presentation contributed.

APS March Meeting 2007, Denver, Colorado, March 5-9 2006, Oral

Presentation contributed.

MRS 2007 Spring Meeting, San Francisco, CA, April (9-13) 2007, Oral Presentation contributed.