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DESIGN AND ENGINEERING OF

SUBMICRON STRUCTURES BY ELECTROSPINNING PROCESS

A Dissertation

Presented to

The Graduate Faculty of the University o f Akron

In Partial Fulfillment

of the Requirement for the Degree

Doctor o f Philosophy

Zhaohui Sun

August, 2005

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UMI Number: 3184574

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DESIGN AND ENGINEERING OF

SUBMICRON STRUCTURES BY ELECTROSPINNING PROCESS

Zhaohui Sun

Dissertation

Approved:

Advisor

Darrell H. Reneker

Committee Memb

William J. Brittain

Committea^Clember

Gary R. Harrl

Comtnittee Member

Stephen Z. D. Cheng

Committee Member

Rex D. Ramsier

Dep ^tme nt Chair

Stephen Z. D. Cheng

tonDean o f the College

Frank N. Kelly

)ean o f tnef Graduate School

George RVNewkome

Date v

ii

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ABSTRACT

Electrospinning is an effective method to produce polymer nanofibers by creating an

electrically charged jet, o f a polymer solution or a polymer melt. In the electrospinning

process, a je t travels straight for a certain distance, and then develops a series of loops

moving downw ard and outward. During the elongation of a liquid jet, solvent evaporates

and fibers accumulate on a grounded collector. To reconstruct three dimensional

structures, a two-camera system was used to obtain stereo images of the instantaneous

traj ectory of the j et.

The objective of this work is to design and engineer sub-micron structures using

electrospinning process.

A novel silver dressing was developed by incorporating a silver complex in

electrospun nanofibers. A homogeneous solution of silver complex and polyurethane

(Tecophilic) was obtained by mixing the two components in ethanol. As-spun fibers

from the above solution were homogeneous without observable aggregates.

Nanoparticles were observed after exposing as-spun fibers to water. A sustained release

of silver ions was triggered by introducing water to the fibers. The silver dressing from

electrospun fibers showed a greater killing effect on bacteria and fungi than silver nitrate

and silver sulfadiazine presently used clinically.

Clay sheets were incorporated in electrospun polyimide fibers. Plasma etching was

used to reveal clay sheets by controllable gasification of polyimide. The shape, size

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distribution, flexibility and arrangement of clay sheets were observed by electron

microscopy. Gas barrier films were developed by filtering a suspension of clay sheets

water through electrospun fibers. When clay sheets are larger than the interstices

between fibers, they tend to lie flat on the fiber mat and cover the interstices. The

resulting film was about 10 pm thick and self-supporting over tens of centimeters.

A carbon material with super high surface areas was produced by growing carbon

nanotubes on carbonized nanofibers. It was realized by carbonization of electrospun

polyacrylonitrile fibers with metal catalysts, reduction of metal catalyst into metal

nanoparticles, and the following growth of carbon nanotubes from metal particles on

fiber surface. The highly porous hierarchical structure promises a greatly improved

electrode material for fuel cells and photovoltaic cells.



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ACKNOWLEDGEMENTS

I would like to express my deepest gratitude to my advisor, Dr. Darrell H. Reneker,

for his continuous support, guidance, and encouragement throughout the course of my

study and research. He has been a great mentor with his enthusiasm, understanding and

willingness to help students both professionally and personally. I would also like to

thank Dr. William J. Brittain, Dr. Gary R. Hamed, Dr. Stephen Z. D. Cheng, and Dr. Rex

D. Ramsier for serving on my committee.

I would like to thank all my former and current group members for their help and

friendships. I also thank my collaborators from other departments and universities.

The Financial support for this research was provided by NA SA Glenn Research

Center and CFNC, and is greatly acknowledged.

Most of all, I would like to thank my parents, my husband, Jie, and my sister, for all

those times they stood by me and all the joy they brought to my life. They have done

everything they possibly could to make my dreams come true. I am everything I am

because of their undying love and support throughout my life.

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TABLE OF CONTENTS

Page

LIST OF TABLES.............................................................................................................. xi

LIST OF FIGURES............................................................................................................ xii

CHAPTER

I. INTRODUCTION...................................................................................................... 1

II. DEVELOPMENTS IN ELECTROSPINNING .................................................... 8

2.1 Understanding electrospinning behav ior..................................................... 9

2.2 Diversity o f materials used in electrospinning.......................................... 11

2.3 Modification o f electrospinning set-up...................................................... 12

2.3.1 Power supply...................................................................................... 12

2.3.2 Spinnerets........................................................................................... 13

2.3.3 Environment....................................................................................... 14

2.3.4 Collector............................................................................................. 14

2.4 Control of electro spinning fibers................................................................. 16

2.5 Applications.................................................................................................... 17

III. STEREO IMAGING OF ELECTROSPINNING PRO CESS .......................... 19

3.1 Introduction.................................................................................................... 19

3.2 Experimental.................................................................................................. 21

3.2.1 Electro spinning process for observation....................................... 21

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3.2.2 Illumination of the electrospinning je t............................................. 21

3.2.3 Single camera system with a pr ism .................................................. 22

3.2.4 Two-camera system.............................................................................23

3.3 Observation o f electrospinning process with stereo sys tems ..................... 24

3.3.1 Single camera with a prism ................................................................. 24

3.3.2 Two-camera system with NTSC signals........................................... 25

3.3.2.1 NTSC analog signals............................................................... 25

3.3.2.2 Stereo image o f a still object captured by a

two-camera system................................................................................ 27

3.3.2.3 Stereo image of electrospinning captured by a

Two-camera system............................................................................... 29

3.3.2.4 Influence o f exposure time ...................................................... 31

3.3.2.5 Improvement on illumination................................................. 33

3.3.2.6 Monitoring electrospinning process....................................... 34

3.4 Summary and conclusions............................................................................... 37

IV. ELECTRO SPUN FIBERS ENCAPSULATING SILVER COM PLEX .......... 38

4.1 Introduction........................................................................................................ 38

4.2 Experiemtal......................................................................................................... 41

4.2.1 Electrospun fibers from silver complex and Tecophilic ................ 41

4.2.2 Silver ions released in de-ionized water.............................................. 42

4.2.3 Antimicrobial tests of the fiber mats containing silver complex......42

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4.2.4 Kinetic test of bactericidal activity................................................... 42

4.3 Controlled release o f silver ions from the fiber ma t.................................... 43

4.3.1 Formation o f silver particles in moisturized environment .............. 43

4.3.2 Silver ion concentration....................................................................... 45

4.4 Antimicrobial activity test on fiber mats ........................................................ 47

4.4.1 Bactericidal tests................................................................................... 47

4.4.2 Antifugal tests....................................................................................... 48

4.4.3 Bactericidal tests on fiber mats and silver compounds ...................49

4.5 Microscopy o f fiber mats after bactericidal tests......................................... 50

4.5.1 Structure o f fiber revealed by stereo m icroscopy ............................51

4.5.2 Aggregates formed after the bactericidal tests................................ 53

4.6 Mechanical strength o f the fibers and fiber mats........................................ 54

4.6.1 Tensile strength by In stron ................................................................ 54

4.6.2 Other methods for measuring stress of electrospun fibers........... 56

4.7 Tri-silver complex (Ag3T) with antimicrobial activities......................... 59

4.7.1 Electro spun fibers from Ag3T and Tecophilic............................. 59

4.7.2 Formation of silver particles triggered by water.............................. 59

4.7.3 Determination o f chemical composition of silver particles............ 61

4.7.4 Release of silver ions in water............................................................ 63


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V. ELECTROSPUN FIBERS FROM CLAY AND POLY MER........................... 65

5.1 Introduction.................................................................................................... ....65

5.2 Experimental........................................................................................................ 67

5.2.1 Materials................................................................................................ 67

5.2.2 Electrospun polymer fibers containing clay sheets.......................... 67

5.2.3 P lasma etching technique...................................................................... 68

5.2.4 Gas barrier film from electrospun fibers and clay sheets..................68

5.3 Electrospun fibers from polymer and clay sheets........................................... 68

5.3.1 Ribbon shaped fiber of polyimide with clay....................................... 69

5.3.2 Plasma etching effect.............................................................................. 70

5.3.2.1 Plasma etching set-up................................................................. 70

5.3.2.2 Plasma etching applied to various systems .............................. 72

5.3.3 Clay sheets revealed by plasma etching............................................... 76

5.3.4 Arrangement o f clay sheets inside fibers ............................................. 81

5.3.5 O bservation of single clay sheets ......................................................... 85

5.3.5.1 Single clay sheets attached to surfaceof electrospun fibers..85

5.3.5.2 Single clay sheets revealed by plasma etching ..................... 88

5.3.5.3 Single clay sheets imbedded in a film ...................................... 90

5.4 Gas barrier film from clay and polymer nanofibers ....................................... 91

5.4.1 Laponite supported on top of electrospun polymer fibers........... 93

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5.4.2 Montmorillonite supported on electrospun polymer fibers .............. 94

5.4.3 Li+-fluorohectorite supported on electrospun polymer fibers .......... 95

5.4.4 Polymer film reinforced by electrospun fibe rs................................. 101

5.4.4.1 Spincoated Polym er film reinforced by electrospun fibers. 101

5.4.4.2 Polymer cast film reinforced by electrospun fibers ..............105

5.4.5 Gas permeability measurement.......................................................... 106

5.4.5.1 Frazier differential pressure air permeability test................. 106

5.4.5.2 Volumetric gas transmission measurem ent.......................... 108

5.5 Summary andconclusions ............................................................................. 109

VI. HIERARCHICALSTRUCTURE FOR FUEL CELL APPLICATION S 111

6.1 Introduction........................................................................................................ 112

6.2 Experimental...................................................................................................... 112

6.3 Growth of carbon nanotubes............................................................................ 113

6.4 Unique properties.............................................................................................. 115

6.5 Fuel cell and fuel cell electrodes .................................................................... 119

6.6 Preparation o f platinum catalyzed hierarchical s tructure ............................ 120

6.7 Design of fuel cell............................................................................................ 125


VII. SUMMARY............................................................................................................ 127

REFERENCES........................................................................................................ 130

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LIST OF TABLES

TABLE Page

2.1 Achievements and challenges in electro spinning....................................................... 9

4.1 History of silver and silver compounds in woundcare............................................ 39

4.2 Tensile strength and strain at break of thefiber mats................................................ 55

4.3 Forces along single fibers............................................................................................. 58

5.1 Intrinsic permeability of composite films................................................................. 107

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LIST OF FIGURES

FIGURE Page

1.1 Drawing o f an electrospinning setup; the inset shows an instantaneous path

of a jet ....................................................................................................................... 1

1.2 Scanning electron micrograph of a human hair, nylon textile fibers and

electrospun polyethylene oxide fibers................................................................. 3

1.3 A cut glass stone supported on a thin layer of electrospun Tecophilic

fibers across a rin g.................................................................................................. 4

1.4 Electrospun nanofibers on a substrate were used to catch clay particles

suspended in water in a filtration process ............................................................ 5

1.5 Electrospun fibers with encapsulated medicine can be used in wound

dressing: (a) a bandage and (b) electrospinning on wound surface ................. 5

1.6 Structure of an artery: (a) a drawing of the layered structure of an artery;

(b) scanning electron micrograph of a segment of artery; (c) electrospun

collagen fibers were used to prepare artificial artery ........................................ 6

2.1 Images of electrospinning jet with different exposure times by video

camera: (a) 16.7 ms, (b) 1 ms, and by high speed camera (c) 0.25 ms 10

2.2 Viscoelastic model: (a) a system of beads connected by viscoelastic

elements; (b) temporal growth of the bending instability; (c) three-

dimensional reconstruction of the bending je t .................................................... 11

2.3 Designs of collectors: (a) a typical flat plate; (b) a mesh; (c) a frame on-a

plate; (d) a wheel with sharp edge; (e) a rotating drum; (f) two bars; (g) two

rings and (h) biased rings along je ts .................................................................... 15

3.1 The stereo system of a single camera and a prism: (a) top view; (b) side

view; (c) the equivalent stereo system with two virtual cameras .................... 22

3.2 Setup for two-camera stereo system ..................................................................... 23

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3.3 Stereo images of electrospinning Tecophilic captured by a single camerawith a prism: a) single jet with branching; (b) two jets from the same

droplet with branching; pa rt o f jet was missing because o f the limited field

of view ...................................................................................................................... 24

3.4 NSTC signal has 525 horizontal lines; a full frame is made up of two

interlaced fields: an odd field (solid lines) and an even field (dot lines) 25

3.5 Working principle of two-camera system by splitting the two fields in

NTSC; one cam era was designed to catch the odd field (solid lines) and the

other camera was used to catch only the even field (dot lines)........................ 26

3.6 Setup for two-camera system to capture a still object; the inset shows the

top view o f the set-up............................................................................................. 27

3.7 Stereo image captured by two-camera system: (a) image out of camera; (b)

odd field image; (c) even field image; (d) reconstructed odd field image;

and (e) reconstructed even field im ag e ............................................................... 28

3.8 A pair of stereo images of electrospinning from polyethylene oxide in

water: (a) reconstructed odd field image; (b) reconstructed even field

image........................................................................................................................ 29

3.9 A stereo image of electro spinning from PEO in water: (a) odd field image;

(b) even field image; (c) hand tracing of the trajectory from odd field

image; (d) hand tracing of the trajectory from even field image ...................... 31

3.10 Electrospinning o f Tecophilic in ethanol at 8 KV captured by one camera

at different shutter speeds: (a) 1/10000 s, (b) 1/4000 s and (c) 1/2000 s 32

3.11 Introducing a reflecting mirror to the setup to improve illumination............... 33

3.12 Electrospinning of Tecophilic in ethanol at 8 KV: (a) without reflecting

mirror (setup as in Figure 3.2); (b) with reflecting mirror (setup as in Figure3.11).......................................................................................................................... 34

3.13 Electrospinning of Tecophilic in ethanol with a gap distance of 20 cm;

high voltages used in the experiments are (a) 7 KV, (b) 8 KV, (c) 9 KV, (d)

10 KV, (e) 11 KV and (f) 12 KV; the exposure time was 1/4000 s ................ 35

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3.14 Further improvement on illumination by introducing a spherical reflector sothat the light will be reflected inside randomly .................................................. 36

4.1 Thermal ellipsoid plot of silver complex with the thermal ellipsoid drawn at

50% probability level. The counter anions are omitted for clarity .................. 40

4.2 Electrospun fibers with a composition of 25 wt% of silver complex and 75

wt% Tecophilic: (a) as-spun fiber; (b) fiber exposed to water...................... 44

4.3 Formation of silver particles as a function of time in electrospun fibers

from 50 wt% of silver complex and 50 wt% Tecophilic: (a) set-up for

detecting the formation of silver particles in a humid environment; (b) as-spun fiber mat; (c) fiber mat exposed to moisture for 0.5h; (d) fiber mat

exposed to moisture for 65h .................................................................................. 45

4.4 A fiber mat containing 50 wt% of silver complex was soaked in water

(right); the concentration of silver, detected by atomic absorption

spectrophotometer, was plotted as a function o f soaking time (1 mg o f fiber

mat in 1 mL de-ionized water)............................................................................. 46

4.5 Fiber mats placed on lawns of Staphylococcus aureus and incubated

overnight at 35 C: (a,d) pure Tecophilic fiber mat; (b,e) fiber mat from

25 wt% silver complex and 75 wt% Tecophilic; (c,f) fiber mat from 75wt% silver complex and 25 wt% Tecophilic; (d,e,f) scanning electron

micrographs o f the fiber mats ............................................................................... 48

4.6 Plot of colony forming unit (CFU) as a function of time for different

samples on Staphylococcus aureus; inset shows the colonies grown on an

agar p la te ................................................................................................................ 50

4.7 A stereo pair of micrographs (5 tilt) on a segment of fiber after

antibacterial test; the electrospun fiber had a composition of 75 wt% silver

complex and 25wt% Tecophilic ........................................................................ 51

4.8 3D reconstruction process shows the relative position of particles within a

fiber: (a) referencing the particles and fibers between two images; (b)

structure viewed in 3D viewer; (c) structure viewed along the fiber a x is 52

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4.9 Electron micrograph of the fiber mat with 75% of silver complex afterbactericidal tests: (a) SEM image shows the topology of the fiber mat; (b)

TEM image of a single fiber shows nanoparticles as well as a big

aggregate................................................................................................................... 53

4.10 A plot of stress as a function of strain for three samples from pure

Tecophilic fiber m a ts ......................................................................................... 54

4.11 Scanning electron micrographs of a fiber mat containing 75% of silver

complex and 25% o f Tecophilic: (a) as-spun fiber mat and (b) fiber mat

after tensile stress measurement........................................................................... 55

4.12 A thin layer of electrospun Tecophilic fibers: (a) across a gap between

glass slides; (b) a rod with certain weight was placed on the fibers ................ 57

4.13 Force analysis: (a) top view of two fibers across the gap with extreme

orientations; (b) force analysis o f a deformed fiber.......................................... 57

4.14 Chemical structure of Ag3T ................................................................................... 59

4.15 As-spun fibers from a solution of Ag3T and Tecophilic with composition

of (a) 25% Ag3T, (c) 67% Ag3T and (e) 80% Ag3T; fibers after exposing

to water: (b) 25% Ag3T, (d) 67% Ag3T and (f) 80% Ag3T ............................ 60

4.16 Elemental analysis on the particles: (a) transmission electron micrograph of

silver particles on fiber surface; (b) X-ray energy dispersive spectroscopy

obtained from the particles; (c) field emission scanning electron

micrographs (backscattered FE-SEM) o f fibers from Ag3T (33%) and

Tecophilic (67%) after exposing to water; (d) X-ray energy dispersive

spectroscopy corresponding to (c )........................................................................ 61

4.17 Bright field TEM micrograph and electron diffraction pattern obtained

from Tecophilic fibers with silver particles.................................................... 62

4.18 Silver concentration as a function of soaking time in water detected by

atomic absorption spectrophotometer.................................................................. 64

5.1 Scanning electron micrographs of electrospun fibers from a solution of

polyimide (BPADA -BAPP) in tetrahydrofuran at (a) low and (b) high

magnification........................................................................................................... 69

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5.2 Plasma etching apparatus used to remove polymer; the inset shows thevacuum chamber where the etching process took place .................................... 71

5.3 Optical micrographs of electrospun polyimide fibers collected on a glass

slide after plasma etching for (a) 0 min, (b) 75 min, and (c) 135 min 71

5.4 Electrospun polyacrylonitrile fibers containing carbon nanotubes: (a, b, c)

before etching; (d) etching effect on polymer was more obvious than on

carbon nanotubes due to the difference in reactivity to ion species................ 73

5.5 Transmission electron micrograph of a sample of polystyrene and

montmorillonite nanocomposite prepared by plasma etching.......................... 74

5.6 Electron micrograph of a sample from carbon black filled natural rubber:

scanning electron micrographs of rubber sample surface (a) after and (b)

before plasma etching; (c) transmission electron micrograph of a sample

thinned by plasma etching..................................................................................... 75

5.7 SEM micrographs of polyimide fiber (a) before and (b) after plasma

etching; electrospun fibers from polyimide and clay (c) before and (d) after

etching ................................................................................................................ 76

5.8 Arrangement of clay sheets in a relatively large fiber revealed by removinga thin layer o f polymer from the surface of the fiber....................................... 78

5.9 Size distribution of the parts of clay sheets revealed by plasma etching: (a)

original scanning electron micrograph of a segment o f fiber surface after

plasma etching; (b) image after threshold and watershed; (c) image of eight

largest clay sheets exposed; (d) size distribution of all exposed clay sheets... 79

5.10 Exfoliation degree analysis: (a) a segment of electrospun fiber of polyimide

with clay after plasma etching; (b) an area containing two stacks of clay

sheets were enlarged for analysis; (c) plot profile showing the average

intensity o f a region inside the dotted rectangle in (b) ...................................... 80

5.11 Arrangement of clay sheets inside a fiber: (a) a 2 pm electrospun polyimide

fiber with clay sheets after plasma etching at 8 Torr, 8 KV and 0.5 cm for

lh; (b) electrospun fibers (1 pm) o f polyimide (BPADA-BAPP) and 4%

bentonite H after plasma etching at 6 Torr, 8 KV and 0.5 cm for 0 .5h........... 81

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5.12 Transmission electron microscopy of clay sheets crumpled in theelectrospun polystyrene fibers with a diameter of 800 nm: top left shows an

as-spun fiber; bottom left shows a fiber after plasma etching at 3 Torr, 6

KV and 0.5 cm for lh; the inset at right shows a model of clay sheets

crumpled inside a fiber.......................................................................................... 83

5.13 Electrospun fibers with smaller size: (a) electrospun fibers of polyimide

(BPADA-BAPP) and 4% bentonite H after plasma etching at 6 Torr, 8 KV

and 0.5 cm for 0.5h; (b) electrospun fibers of polystyrene and

montmorillonite after plasma etching at 3 Torr, 6 KV and 0.5 cm for lh;

inset shows electron diffraction pattern from a selected area shown in the

brighter c ircle........................................................................................................... 84

5.14 Observation of single clay sheets: (a) model of single clay sheets with

layered structure and irregular shape; (b) model of a stack of clay sheets;

(c) stacks of clay sheets attached to surface of a fiber by filtr atio n .............. 85

5.15 A stack of clay sheets (PGV-C12) attached to the surface of a fiber by

filtering a suspension o f clay in water through a fiber mat of

polyacrylonitrile; inset shows the electron diffraction p att ern ........................ 86

5.16 A stack of clay sheets (bentonite H) attached to the surface of polyimide

fibers (not shown in the TEM image) and inset shows electron diffractionpat tern ....................................................................................................................... 87

5.17 Clay sheets attached to the surface of polycaprolactone fibers: (a) low

magnification micrograph with an inset showing electron diffraction

pattern; (b) single clay sheets observed at higher magnification .................... 88

5.18 Transmission electron micrograph of a ribbon shaped fiber of polyimide

(BPADA-BAPP) containing clay sheets (PGV-C12); the fiber was thinned

by plasma etching at 8 Torr and 8 KV for lh ...................................................... 88

5.19 Transmission electron micrographs o f polyimide (BPADA-BAPP) fiberscontaining clay sheets (PGV-C12); the fibers were thinned by plasma

etching at 3 Torr and 6 KV for 0.5h: (a) arrangement of clay sheets inside

fibers; (b) single clay sheets observed at the surface of a fiber at high

magnification and inset shows the diffraction pattern obtained from a

selected area indicated in the circle ...................................................................... 89

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5.20 An ultra thin film prepared from a dilute solution of polyimide and claysheets in tetrahydrofuran; a drop of solution was floated on water and the

resulted film was transferred to a TEM grid: (a) morphology and electron

diffraction (inset) of the film; (b) selected area diffraction pattern ................. 90

5.21 TEM images of spincoated films from (a) polyethylene oxide and clay

sheets with electron diffraction pattern (inset); (b) polyimide and clay

sheets; both films were thinned by plasma etching for TEM observation 91

5.22 Schematic drawing of gas path through (a) a pure polymer film and (b) a

polymer and clay composite fi lm ......................................................................... 92

5.23 A continuous film of Laponite supported on a nylon-6 fiber mat: (a) top

view; (b) reverse side of the composite film; (b) top view of the composite

film at high magnification.................... 93

5.24 Montmorillonite supported on electrospun fiber mats by filtration: (a) a few

stacks of clay sheets collected on electrospun polyimide fibers; (b) higher

loading of clay sheets on fibers; (c) top view o f a continuous film of clay

supported on polyimide fibers; (d) cutting edge of the composite film

revealed the structure: clay film (region 1), polyimide fibers (region 2), and

filter paper substrate (region 3)............................................................................ 94

5.25 Li+-fluorohectorite deposited flatly on carbon film: (a) TEM image shows

morphology and (b) electron diffraction pattern obtained from a selected

area in (a)................................................................................................................. 96

5.26 Li+-fluorohectorite supported on electrospun polyacrylonitrile fibers: (a) a

few layers of Li+-fluorohectorite supported on the fibers; (b) many layers of

Li+-fluorohectorite tended to fill the interstices between the fibers and inset

shows the electron diffraction pattern ................................................................. 97

5.27 Electrospun fiber mats from polyacrylonitrile (PAN) in DMAc at (a) low

and (b) h igh magnification; top view o f a continuous film o f Li+-fluorohectorite supported on PAN fiber mat at (c) low and (d) high

magnification........................................................................................................... 98

5.28 Tearing edge of the composite film from Li+-fluorohectorite supported on

polyacrylonitrile fibers: (a) a side view; (b) top view of a tearingedge 99

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5.29 A piece of gas barrier film prepared from Li+-fluorohectorite andelectrospun nylon 6 fibers; inset shows a scanning electron micrograph on a

cutting edge o f the composite film ....................................................................... 99

5.30 Thin films o f clay on nano fibers: (a) thin film of Li+-fluorohectorite on

PAN fibers prepared by electrospraying a suspension of clay in water; (b) a

layer of smaller fibers on one layer of larger fibers as a substrate .................. 100

5.31 Spincoated films of polyimide (6FDA PMFB) with clay sheets (bentonite

H) reinforced by polyimide (BPADA BAPP) fibers: (a) a continuous film;

(b) porous film obtained by reducing the amount o f solution used in

spincoating............................................................................................................... 102

5.32 TEM image of a spincoated film of polyimide (6FDA PMFB) with clay

sheets (bentonite H) reinforced by polyimide (BP AD A BAPP) fibers: (a)

before etching; (b) after etching at 3 Torr and 5 KY for 1.5h.......................... 103

5.33 Arrangement of clay sheets in the vicinity of a fiber revealed by plasma

etching on a composite film: (a) bright field TEM image; (b) negative TEM

image........................................................................................................................ 104

5.34 The composite film after etching can be used to characterize the clay sheets

by electron diffraction patterns as shown in (a), (b), (c) and (d )...................... 105

5.35 A film of polyimide (6FDA PMFB) and clay cast on polyimide (BP AD A

BAPP) fiber mat: (a) top view; (b) reverse side of the film ............................. 105

5.36 Frazier differential pressure air permeability measuring mach ine................... 106

5.37 Set up according to ASTM D 1431-82: (a) instrument assembly; (b)

diagram shows the gas flow of a volumetric gas transmission test cell 108

5.38 Volumetric gas transmission test: rise of the indicator fluid at a function of

time for a control sample o f filter paper.............................................................. 108

6.1 Chemical vapor deposition setup for the growth of carbon nanotubes on

carbon n anofibers................................................................................................... 112

6.2 Temperature profile inside furnace measured by using thermo couple 113

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6.3 Preparation process of carbon nanotubes on carbon nanofibers: (a) as-spunfiber o f iron acetylacetonate and polyacrylonitrile (weight ratio=T :5); (b)

nanoparticles of iron formed by hydrogen reduction; (c) introducing hexane

for 6 min; (d) Introducing hexane for 12 min; 3-D cartoons show the

structures ................................................................................................................. 114

6.4 Carbon nanotubes grown from (a, b) iron and (c, d) nickel nanoparticles on

the surface of electrospun fibers........................................................................... 115

6.5 Pieces of hierarchical structure with controllable thickness: (a) a relatively-j

thick piece with a mass per unit area of 4 g/m ; (b) a thinner piece was

semi-transparent with a mass per unit area o f 0.5 g/m2; (c) the thin piecewas curved to show flexibility and strength........................................................ 116

6.6 The hierarchical structure viewed at different scales.......................................... 117

6.7 Setup to measure the current at different voltages: (a) schematic drawing of

the setup; (b) samples were attached to a metal wire and insulated ................ 118

6.8 Plot of current at different voltages (0 KV to 2 KV); inset shows the

readings for three cycles........................................................................................ 118

6.9 Electrospun fibers of platinum acetylacetonate and polyacrylonitrile (1:10);the fibers were heated in hydrogen for 4h at 550 C; the temperature was

increased to 700 C and the flowing argon was bubbled through hexane for

6 min......................................................................................................................... 120

6.10 Electrospun fibers of polyacrylonitrile containing platinum acetylacetonate

(10:1) were carbonized and hexane was introduced for 10 min at 850 C;

micrographs (a, b, c, and d) were obtained by moving the sample in TEM.... 121

6.11 Electrospun fibers of polyacrylonitrile containing platinum acetylacetonate

(10:1) were treated with a reducing agent, hydrazine, carbonized and

exposed to hexane for 10 min at 850 C ............................................................. 122

6.12 Platinum was sputtered on electrospun fibers o f pure polyacrylonitrile for

30s; the resulting fibers were carbonized and then hexane was introduced in

the carrier gas o f argon for 10 min at 850 C....................................................... 123

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6.13 Platinum was sputtered on electrospun fibers of pure polyacrylonitrile for 1min; the coated fibers were carbonized in argon and hexane was introduced

in the carrier gas o f argon for 10 min at 850 C................................................. 124

6.14 Platinum was sputtered on electrospun fibers of polyacrylonitrile

containing platinum acetylacetonate (weight ratio = 20/1) for 2 min; the

resulting fibers were carbonized and hexane was introduced in the carrier

gas of argon for 10 min at 850 C; (a, b, c) were obtained at different

magnification in TEM ........................................................................................... 124

6.15 Design o f a stack of fuel cell...................... , ......................................................... 126

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CHAPTER I

INTRODUCTION

Electrospinning is a straightforward process to produce polymer fibers from

electrically charged polymer solutions or polymer melts1. A typical setup of

electrospinning from polymer solutions is shown in Figure 1.1.

Polymer Solution

Electrospun

Fibers

Grounded

Collector

Figure 1.1 Drawing of an electrospinning setup; the inset shows an instantaneous path of

a jet.

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A polymer solution was held in a container with a hole at the bottom, such as a

pipette or a metal cone. High voltage (up to 60 KV) was applied to the solution and

introduced charges to the solution. Since these charges have the same polarity, Coulomb

forces between them are repulsive and tend to produce liquid jets from the surface of a

pendent droplet of the polymer solution. A liquid je t is initiated when the Coulomb force

is greater than surface tension of the polymer solution.

After initiation, the jet traveled straight toward grounded collector for a certain

distance, defined as je t length. With a small perturbation, the je t became unstable and

developed into a series of loops moving downward and outward. Secondary and higher

order of bending instabilities may happen in a self similar way1. During the bending

instability, the jet was elongated and stretched thousands o f times. If no evaporation

occurred, the cross sectional area was reduced by a similar amount to conserve volume.

The diameter of a je t was decreased by the square root o f the elongation ratio. However,

in an electrospinning process from polymer solutions, solvent evaporation occurred and

volume was not conserved. After evaporation o f solvent, dry fibers were accumulated on

a grounded collector. The diameter of dry fibers can be estimated from elongation ratio

and concentration o f the solution.

A jet, from a hanging droplet, could be a few micrometers or even larger in diameter

traveling at a speed of a few meters per second2. After enormous elongation, the length

of electrospun fibers produced per second could be a few hundred meters or even longer.

Polymer fibers, prepared by electrospinning, range from a few nanometers to a few

micrometers. However, the typical diameter of electrospun fibers is a few hundred

nanometers. Figure 1.2 shows a size comparison o f electrospun polyethylene oxide fibers

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(about 200 nm), textile fibers (about 10 micrometers) and a human hair (about 100

micrometers). With the same amount o f polymer, a 200 nm electrospun fiber is 2500

times longer and 50 times larger in surface area than a 10 micron textile fiber.

Electrospinning is an effective method to cover large areas with a thin layer of nanofibers.

Figure 1.2 Scanning electron micrograph of a human hair, nylon textile fibers and

electrospun polyethylene oxide fibers.

In practical applications, mechanical properties o f electrospun fibers become a

concern. A relatively small force is needed to break electrospun nanofibers only because

of their small diameters. Tensile strength test, by nano tensile tester, showed a stress at

break from 20 MPa to 60 MPa for a single electrospun polycaprolactone (PCL) fiber4. In

Figure 1.3, a stone was supported on a thin layer of electrospun fibers from elastic

polyurethaneTecophilic. Only a small number of fibers were collected and the

supporting ring was observable through the fibers. Deformation was caused by the

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weight of stone, which demonstrated the strength and elasticity o f fibers. The stress of a

single fiber can be estimated by knowing the size of the gap, the mass of the stone,

number o f nanofibers, and diameter of nanofibers.

Figure 1.3 A cut glass stone supported on a thin layer of electrospun Tecophilic fibers

across a ring.

With small diameter, ultra high surface area per unit mass, electrospun fibers are

g 6 7being used or can be potentially used in f iltration , wound dressing , tissue engineering ,

sensors8, space applications9,10, nano devices11, and composite materials12,13,14.

An effective filter can be made by applying an ultra thin layer of electrospun fibers

on a substrate with very little increase in pressure drop across the filter. As shown in

Figure 1.4, particles were caught on the thin layer of electrospun fibers, while the

substrate (filter paper) supported the nanofibers. A good example o f scale up products is

PowerCore air filters from Donaldson Company, Inc. The PowerCore air filters are 10

times as efficient and are more compact at a given performance level than standard

cellulose filters by using nanofiber filtration15. Besides solid particles, tiny oil droplets,

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were caught and drained o ff with the aid of electrospun nanofibers with similar diameter

to oil droplets16.

Figure 1.4 Electrospun nanofibers on a substrate were used to catch clay particles

suspended in water in a filtration process.

Figure 1.5 Electrospun fibers with encapsulated medicine can be used in wound dressing:

(a) a bandage and (b) electro spinning on wound surface.

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Medicines can be encapsulated in electrospun nanofibers by mixing medicines with

polymer solutions. Electrospun fiber mats with antimicrobial activities can be made into

bandages for further application (Figure 1.5a). An alternative way is to electrospin

directly on a wound surface (Figure 1,5b). Fibrous structures are widely observed in

IT ISmuscle, skin, and blood vessels . Electrospun fiber mats, with controllable and

similar texture, can be used to mimic the tissues and support the growth of cells. Figure

1.6 shows a drawing of the layered structure of an artery (Figure 1.6a) and a micrograph

of a segment of an artery (Figure 1.6b). Electrospun collagen fibers were wound into

tubular s tructure to make an artificial artery (Figure 1.6c)19.

Norma! Layers of Artery

AdventitiaMedia

Intima

?/ jy

e n r t o t f w i h a i l i n i n g t m o o t h m i n d *

I C p r t s

Figure 1.6 Structure o f an artery: (a) a drawing o f the layered structure of an artery; (b)

scanning electron micrograph of a segment of artery; (c) electrospun collagen fibers were

used to prepare artificial artery19.

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A|

Sensors, developed from electrospun fibers, included gas sensor , chemical sensor ,

"J1 'I'X thermal sensor, fluorescence sensor , piezoelectric sensor and so on . Future space

applications of nanofibers, such as a solar sail, are based on low mass of electrospun

fibers. Due to their nano-scale size, electrospun fibers were used as templates to produce

24 25 26nanotubes or nanofibers from metal and ceramics . A layer of target material was

chemically or physically deposited on electrospun fibers. Nanotubes were produced by

the removal o f polymer fiber templates. Ceramic nanofibers (such as SiC>2, TiC^, AI2O3

and ZrC>2) for high temperature applications were prepared by sol gel process from

electrospun fibers of ceramic precursors. An alternative way to produce ceramic fibers

was to blend a ceramic precursor with a sacrificing polymer matrix in a solution.

Electrospun fibers from the above solution were heated to elevated temperature to

remove the polymer matrix and convert the precursor to ceramic. Electrospinning has

27 been applied to a wide variety of polymers . Electronic and photonic devices were

designed based on electrospun fiber of conducting polymers and polymers with photonic

effects. There are many more applications needed to be explored.

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CHAPTER II

DEVELOPMENTS IN ELECTROSPINNING

28 29Electrical spinning was first disclosed in patents by Formhals in the 1930s .

Artificial threads or filaments were produced from cellulose acetate and rayon solutions

-JA "11by electrical field . Little interest and few publications on electrical spinning were

known thereafter. The technique o f producing fine fibers by electrical field was then

named as electrostatic spinning by Childs in 1941 . From 1970s to early 1990s, fibersA i p 1 / i w

(


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challenges in various directions o f electrospinning process. Detailed examples in each

direction will be discussed.

Table 2.1 Achievem ents and challenges in electro spinning

Directions Achievements Challenges

Spinning behavior Visualization

Modeling

Modeling on controlling

size and morphology

Diversity of

materials

> 60 natural and

synthetic polymers

High performance and

functional materials with

low solubility

Modification of Micro-tips Controllable size and

spinning process Co-spinning

Multiple jets

Environment

Collectors

mass production

Electrospinning Control Size

Shape

Features

Alignment

Pattern

Writing a letter with

electrospinning

Applications Filtration

Biomedical

Sensors

Nanodevice

Space applications

TemplatesComposites

Interdisciplinary, more

novel applications need to

be explored

2.1 Understanding electrospinning behavior

Although electrospinning has been widely used to prepare fine fibers for a few

decades, a splitting mechan ism dominated the formation of fibers44. A major

breakthrough in electrospinning involved visualization and modeling of instantaneous jet

trajectory. Figure 2 .11shows snapshots of electrospinning from a polyethylene oxide

solution at different exposure times. At longer exposure time (16.7 ms), an envelope

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cone was observed by video camera (Figure 2.1a). Part of straight jet and loops were

visible with an exposure time o f 1 ms (Figure 2.1b). With the aid of high speed camera,

higher frame rate (2000 frames/s) and shorter exposure time (0.25 ms) were achieved.

Tapering of the jet and smaller loops on a segment of jet were clearly visualized in Figure

2.1c. A je t was straight for certain distance and then developed a series of spiraling loops

that moved outward and downward. Smaller loops were observed from a segment of jet

downstream. This behavior was described as an electrically driven bending instability1.

2 m m

Figure 2.1 Images of electrospinning je t with different exposure times by video camera:

(a) 16.7 ms, (b) 1 ms, and by high speed camera (c) 0.25 ms1.

Based on viscoelastic model (Figure 2.2a) of rectilinear electrified liquid jet, growth

of bending instability was modeled (Figure 2.2b) and a three-dimensional reconstruction

of the je t was realized (Figure 2.2c)1. The moving speed of loops was so fast that the

downward motion o f bright spots caused by specular reflections created the misleading

impression of jet splitting. More recently, Rutledge group presented a whipping theory

based on observation and modeling45. Diameters of fibers were predic ted according to

the whipping model46. Other works on stretching o f viscoelastic je t47,48, allometric

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scaling o f current and voltage49, and Taylor cone50 also contributed to the understanding

of electrospinning process. More comprehensive models will help to improve the control

on size and morphology of electrospun fibers.

pen den t d rop

20s

100

1 5 -

Y (mm)

Figure 2.2 Viscoelastic model: (a) a system of beads connected by viscoelastic elements;

(b) temporal growth o f the bending instability; (c) three-dimensional reconstruction o f the

bending je t1.

2.2 Diversity of materials used in electrospinning

Methods to prepare nanofibers or nanowires included crystal growth, template

synthesis, physical deposition, chemical vapor reaction, and self-assembly. These

methods usually involve long reaction time, complex synthesis, low length to diameter

ratio, and poor manipulation51. Electrospinning supplies a controllable and efficient way

to produce nanofibers from polymers, ceramics and other materials. More than 60

natural and synthetic polymers were made into fibers by electrospinning from solutions

or melts. The polymers included conventional polymers (such as polyolefine, polyamide,

and polyester), biopolymers (protein, DNA, polypeptides) and other functional materials

(conducting and photonic polymers). Comprehensive lists of polymers are available in

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11

several publications . More polymers will be added to the list if the advantages of

electrospinning are realized in various fields. Besides, blends and composite materials

further expand the versatility o f electrospinning. Efforts in electrospinning from new

materials not only enrich the diversity of polymers but also lead to more applications.

Since electrospinning is still a relatively new field, more explorations are needed to have

a comprehensive understanding of the process. The trend is to utilize electrospun fibers

in special applications instead of commodity applications. Preparing fibers from

conducting and photonic materials is a challenge. Some of these polymers have limited

molecular weight, poor chemical and physical stability, and low solubility.

2.3 Modification of electro spinning set-up

A typical apparatus of electro spinning included kilovolt power supply, spinneret and

collector. Modification o f apparatus for better control can be done in at least four ways:

power supply, spinneret, electro spinning environment and collector.

2.3.1 Power supply

Direct current (DC) power supplies, with both positive and negative polarity, are

widely used in electrospinning. Voltages up to 60 KV with maximum currents lower

than 200 micro-amperes are useful in ordinary experiments. Application of alternate

current (AC) power supply in electrospinning demonstrated a reducing bending

instability and a lower charge build-up54.

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2.3.2 Spinnerets

Containers for solutions vary in size and shape. Novel spinnerets were designed to

make smaller fibers, manipulate single fibers, and achieve mass production. The

diameter of the opening on a spinneret is not critical if it is larger than the diameter of a

jet. Capillaries and microchannels55 were used for electrospinning but the flow of

viscous fluid through channels was a challenge. Craighead and co-workers used

microfluidic channel with a triangular tip in electrospray56. A silicon scanning tip was

then used in electrospinning to produce aligned nanofibers5758, which were then made

into nanofluidic channels59,60. Kessick and Tepper produced single fibers from

microscale droplets of a concentrated polymer solution on patterned electrodes61.

Supercritical carbon dioxide was used to reduce viscosity of polymers to assist

electrospinning process by Levit and Tepper. Fibers of polydimethylsiloxane (PDMS)

and poly(D,L-lactic acid) (PLA) were formed from between two electrodes in a high

62pressure carbon dioxide cell without liquid solvent .

Coaxial capillaries with separate feedings were introduced to electrospinning from

two components63. Core-shell64, hollow tubes65 or porous structures66 were resulted from

two components such as oil67, ceramic and polymers. Besides, gas flow in a coaxial

design was also used to realize the control over transition from electrospray to

electro spinning68.

Although electrospinning is very efficient in producing huge surface area per unit

mass, the mass produced per unit time is low, which may limit potential applications.

Several approaches were taken to improve the mass production rate by increasing the

number o f jets. Multiple jets were observed on a pendent droplet of polymer solution69 in

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a typical electrospinning process. Stabilizing these jets may lead to a higher mass

production rate. Multiple spinnerets70, patterned capillaries or channels, could be another

solution only if there is no interference between different spinnerets. Yarin proposed a

needleless electrospinning method by introducing a ferromagnetic suspension below a

polymer solution. Multiple jets were initiated on the surface o f polymer solution by the

perturbations from magnetic suspension71.

2.3.3 Environment

Environmental parameters for electrospinning include temperature, pressure and

humidity. Temperature influences both rheological behavior o f a polymer solution and

vapor pressure of the solvent. In polycaprolactone system, fused fibers with beads were

observed at low temperature (15 C) while dry fibers free of beads were obtained at

higher temperature (23 C) with other parameter kept constant27. Similar behavior of

72electrospinning was observed under vacuum and at ambient pressure .

2.3.4 Collector

Electrospun fibers can be collected on metal, semiconductor and insulator.

Collectors o f electrospinning have been modified in many ways to achieve better

alignment of electrospun fibers. Instead of using typical flat plate (Figure 2.3a) as a

collector, mesh (Figure 2.3b), paper, and frames (Figure 2.3c) were used. A wheel

with a tapered edge, rotating at a high speed, was used to collect aligned nanofibers

(Figure 2.3d). A straight je t was followed by a conical envelop cone, which then became

an inverted cone close to the sharp edge73. Uniform fiber mats were collected on a drum

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(Figure 2.3e) rotating at a controllable speed by Reneker and Kim 74. Xia and co-workers

75developed a method to produce uniaxially aligned arrays over large areas . Two strips

of conductive materials were grounded and separated by void gaps or gaps between

stripes of insulating materials (Figure 2.3f). Two grounded rings were placed

symmetrically below a spinneret (Figure 2.3g) and an array o f fibers was formed between

the rings. Further rotation o f one ring resulted in a fiber yarn with a diameter smaller

than 5 micrometers76. Bending instability is a characteristic feature o f electrospinning.

Electrostatic lenses (biased rings) were introduced to a spinning process. The

introduction of electrostatic lenses demonstrated feasibility to delay the onset o f bending

77instability and control the deposition of nanofibers (Figure 2.3h) .

Power

supply (+)Jet

Power

supply (+)

Biased rings

Power

y * supply (-)

Figure 2.3 Designs o f collectors: (a) a typical flat plate; (b) a mesh; (c) a frame on a plate;

(d) a wheel with sharp edge; (e) a rotating drum; (f) two bars; (g) two rings and (h) biased

rings along jets.

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2.4 Control o f electrospinning fibers

Morphological control of electrospun fibers was focused on size, shape and surface

features. The influences o f processing parameters on fiber morphology (size and

presence of beads) were systematically investigated by Reneker and Fong78. The

param eters studied included viscosity, surface tension, high voltage, gap distance, and

feeding rate. A smooth electrospun fiber normally has a round cross section. Other

shapes, such as flat ribbon, were observed when volatile solvents were used. With the

solvent evaporation, a polymer skin was formed on the surface of a jet. The skin

collapsed and formed a ribbon shaped fiber after the stretching and drainage o f the

solution79. Helical structures (coils) were produced from a mixture o f conductive and

nonconductive polymers. A mechanism to explain the presence of coils was proposed as

on

partia l charge neutralization and a following viscoelastic contraction . Secondary

structures, such as pores and pits, were observed on the surface of nanofibers. The fibers

with enlarged surface area could be used in applications such as sensors and catalyst

carriers81.

Certain alignment of electrospun fibers in some system were ach ieved by various

approaches shown in spinneret and collector modification. Rotating wheels and gaps

between conductive stripes w orked well in the alignment o f electrospun fibers. Step by

O'*

step rotation of the collecto r in certain degrees led to a cross-bar or patterned fiber mats

83,84. Another patterning method was realized by electrospinning directly on a grounded

mesh. More fibers were co llected on metal wires before the fibers were lying down

across the wires. Fiber mats, with visible pattern replica, were tested for blowing failure

and showed an improved blow resistant property85.

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Although massive p rogress has been made on understanding electrospinning process

and controlling electrospun fibers, there is always need for further exploration. Pioneer

works on ideas, such as stretching a single molecule into a fiber by electrospinning and

writing a letter with electrospun fibers, may lead electrospinning to a novel stage.

2.5 Applications

One o f the most important reasons for electrospinning to become a fast moving front

o /

in material science is the tremendous applications associated with these fine fibers.

8 7 O D

Applications, such as filtration, wound dressing, tissue engineering , sensing, space

applications, were described in Chapter one. The above applications were usually

realized by incorporating functional materials or fillers to electrospun polymer fibers.

OQ

Fillers, in the shapes o f particles, rods and sheets , can be incorporate in electrospun

fibers by mixing the fillers with polymer solutions. Even i f the filler has a relatively

large size, it can still be carried along the jet and encapsulated or fixed by fibers.

Particulate fillers, such as calcium carbonate (CaCCE)14, titanium oxide (Ti02), carbon

black, and silica dioxide (SiCh), were dispersed in polym er solutions for electrospinning.

Metal salts (eg. PdCl2) were dissolved in polymer solution, made into fibers and then

reduced by reducing agents (hydrogen gas, hydrazine) to form nanoparticles inside and

on the surface o f fibers90. High concentration of carbon nanotubes was also incorporated

in electrospun fibers to improve the conductivity and mechanical strength of fiber mats91.

Carbon nanotubes tended to align along electrospun fibers due to the flow during

electrospinning process. However, some rigid nanotubes were also observed to stick out

on the surface of fibers. Clay sheets with layered structure were also incorporated into

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electrospun fibers, which supplied a simple way to study the morphology of clay sheets

and arrangement o f clay sheets in a confined environment.

Electrospinning supplied a unique path to design, fabricate and engineer sub-micron

structures. This work focused on the fabrication of sub-micron structures for various

applications. The goal is to demonstrate the simplicity and versatility of electrospinning

technique.

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CHAPTER III

STEREO IMAGING OF ELECTROSPINNING PROCESS

3.1 Introduction

Stereo vision is a technique used to visualize or reconstruct three dimensional

structures by using a pair of stereo images obtained from two distinct viewpoints92,93,94.

One example in nature is stereo vision by human eyes. Two slightly different images are

taken by human eyes and depth is perceived by combining the two images in human

brain. Stereo images are usually obtained by a single camera with mirrors or prisms93,95,

96, 97,98 ancj two cameras separated by a certain distance. Matching the features between

stereo images was followed by depth perception and three dimensional recoveries.

Both one camera and two-camera systems were used to obtain the stereo vision of

electrospinning from polymer solutions. In one camera system, a prism was aligned in

front of a charge coupled device (CCD) camera to produce an equivalent stereo camera

system with two virtual cameras. The overlapped region between the two fields of view

(FOV) of virtual cameras defines the FOV of the prism system. The advantages

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associated with single camera system include simple set-up, identical optical properties,

easy calibration and no synchronization needed. However, a relatively small field of

view is associated with the system o f a single camera and a prism. Besides, interference

area limits the usage o f a single camera system in applications where a large field o f view

is required.

Two-camera system was used as an alternative method to monitor electrospinning

process. To mimic human eyes, two cameras with identical optical parameters were

separated at a tunable distance and aimed at the object of interest. To observe the

instantaneous path of an electrically charged jet, strobe flashes were used to illuminate

the jet and stop its motion. A co-axial metal ring at high potential was positioned around

the charged droplet of polym er solution to stabilize the electrospinning process. To

achieve a uniform illumination o f the jet trajectory, flash positions were tuned and

reflecting materials were used to scatter light in random directions.

A great challenge for two-camera system is synchronization, which is especially

critical for objects in motion. National Television System Committee (NTSC) standard

refers to the analog signal for television broadcasting system. A full field in NTSC

standard is displaying every 1/30 of a second and is made up of two interlacing fields. A

field is defined as a set of even lines, or odd lines99,10. The two cameras were designed

to capture only even field or odd field independently. The whole frame was then split

into two distinct images, which made up a pair of synchronized stereo images. The

resulted pair o f stereo images was used for further analysis and reconstruction o f three

dimensional structures.

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3.2 Experimental

3.2.1 Electrospinning process for observation

In electrospinning process, a polymer solution was held in a metal cone with an

opening of about 300 micrometers at the tip. The solution was delivered to the opening

by its own w eight and no pressure was applied to control the flow rate. The metal cone

holding the solution was connected to a high voltage power supply with a capacity up to

30 KV. To stabilize the electrospinning process, a metal ring with a diameter of 5 cm

was connected to equal high potential and positioned at the same horizontal level with the

tip. A grounded metal plate was placed 20 cm below the tip to collect the electrospun

fibers. Polymer solutions used in this work were polyethylene oxide in water and

Tecophilic in ethanol. Polyethylene oxide (PEO), chemical formula [CH2CH20 ]n, with

a molecular weight o f 400,000 g/mol, was dissolved in water at a weight concentration of

6%. The PEO was purchased from Scientific Polymer Products, Inc. A solution of 7%

Tecophilic (SP-80A-150) in ethanol was prepared at 60 C without stirring.

Tecophilic was purchased from Noveon Thermedics Polymer Products, Co.

3.2.2 Illumination of the electrospinning jet

Xenon strobes purchased from RadioShack were used to illuminate and stop the

motion o f the electrospinning jet. Strobes were installed with light sensors, which

enabled the triggering of strobes by a burst of light, such as a flash controlled by

computer. The instantaneous trajectory of the electrically charged je t was captured by

two-camera stereo imaging system. The synchronization between two cameras and

strobes was realized by triggering the strobes and capturing the images at the same time.

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3.2.3 Single camera system with a prism

(c)I?

%tkL

Black background

Prism

a

A

pT

(

... i t

Xenon .strobe

Virtual Electrospinning

Cameras

Figure 3.1 The stereo system of a single camera and a prism: (a) top view; (b) side view;

(c) the equivalent stereo system with two virtual cameras.

A single camera with a prism, shown in Figure 3.1a, was used to obtain stereo views

of an electrospinning jet. The distance between the camera and prism, which influences

the interference area, is adjustable from 0.5 cm to tens of centimeters. A prism was

placed at a distance that was far enough not to disturb the electrospinning process and

close enough to get a reasonable field of view. A xenon strobe was tilted and aligned

with the metal cone at an angle of 20 (Figure 3.1b). The equivalent stereo system is

shown in Figure 3.1c. An object on the optical axis of the real camera was transformed

into two objects by the two inclined planes of the prism. The deviation (3, the angle

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between a real point and one of the two virtual points, is a function of refractive index n

and the angle a o f the prism. Sony camcorder was used as the real camera. With the

optical zoom lens, stereo images with different field view were obtained easily. A

solution o f Tecophilic and super absorbent particles (Waterlock) was electrospun

from a metal cone so that the solution can be continuously supplied. A high voltage of 30

KV and a gap distance o f 30 cm were used in the electrospinning process.

3.2.4 Two-cam era system

Xenon strobe

CameraTubing

Electrospinning Black hole

background

Figure 3.2 Setup for two-camera stereo system.

The whole setup of two-camera system (Figure 3.2) was placed on a frame to

facilitate the alignment. Two identical CCD cameras were mounted on a beam and

separated by a distance o f 12 cm. Zoom lenses were used and extension tubes were

mounted on the lenses to protect the cameras from redundant light. The distance between

the two cameras and electrospinning set-up was adjustable as well. Black tubing was

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used as a black hole background so that the reflected light that could enter the camera

was minimized. Two xenon strobes for each camera were shaded so that no direct

illumination was captured by the cameras. A third xenon strobe, covered with a slit

opening, was introduced to the system to illuminate the straight part of the jet.

3.3 Observation of electrospinning process with stereo systems

3.3.1 Single camera with a prism

Figure 3.3 Stereo images of electrospinning Tecophilic captured by a single camera

with a prism: a) single jet with branching; (b) two jets from the same droplet with

branching; part of je t was missing because of the limited field of view.

By using the stereo system with a single camera and a prism, two images appeared

on the same frame side by side. Illumination was improved by putting reflecting

materials around the electrospinning set up so that the light from strobe flash was

reflected in multiple directions. Figure 3.3 showed the stereo images obtained by a single

camera system. The strobe flash was used to stop the motion to obtain the instantaneous

trajectory o f the jet. The behavior o f electrospinning and 3-D structure of the jet can be

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visualized on the above images by human eyes. In Figure 3.3a, the je t traveled in a

complex path and small branches were observed along the jet. There were some brighter

spots along the je t that were resulted from the particles in the solution. Figure 3.3b shows

another behavior in the electrospinning processmultiple jets from the same drop of

solution. At high voltages, multiple jets were initiated from the droplet o f polymer

solution hanging at the tip of the metal cone. The jets were not stable, and single-jet

spinning was resumed after the other jets died. The disadvantages with single camera

system were found to be a limited field of view and interference areas. A part of one jet

was missing from the right image in Figure 3.3b because of the limited field of view.

The interference area increases as the decrease in the distance from camera to prism, and

the increase in thickness o f the prism. The larger the interference area is, the smaller the

field of view becomes.

3.3.2 Two-camera system with NTSC signals

3.3.2.1 NTSC analog signals

ZEZEHHEHEIE* Even

.......................... Field@*

9 9

9 9

9

Figure 3.4 NSTC signal has 525 horizontal lines; a full frame is made up of two

interlaced fields: an odd field (solid lines) and an even field (dot lines).

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O d d

Fie ld



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Evenfun-ra Camera 2F i e l d

Field

Split imageO dd

E v e n

FieldField

Right eyeLeft eye

Figure 3.5 Working principle o f two-camera system by splitting the two fields in NTSC;

one camera was designed to catch the odd field (solid lines) and the other camera was

used to catch only the even field (dot lines).

Synchronization is a key issue when a two-camera system is used for capturing the

stereo images o f a moving object. Two images have to be obtained at exactly the same

time to reconstruct the instantaneous structure. National Television System Committee

(NTSC) was used in this work to realize the synchronization. NTSC analog signal is

made up of 525 horizontal lines (Figure 3.4), which define the vertical resolution of a full

frame. Two interlaced fields, an odd field (solid lines) and an even field (dot lines), are

displaying subsequently at a speed of 60 fields per second to make up a full frame. Two

cameras were set up to view the object of interest from different viewing angles. The

working principle is shown in Figure 3.5. Two cameras captured images at the same time

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from two different viewing angles. Odd field was extracted from the left camera and

even field was from the right camera. The full frame was then split into two fields so that

a pair of synchronized stereo images was resulted. Because each field only contained

half of the 525 lines, lines were added to each field to make an image with a resolution of

525 lines. Scion image was used to split the stereo image and restore the resolution

with a macro w ritten in our laboratory.

3.3.2.2 Stereo image o f a still object captured by a two-camera system

Two-camera system was tested on a still object with a setup shown in Figure 3.6.

Two cameras were symmetrically separated to view the object from different directions.

The inset shows the top view o f the setup. The two cameras were aligned and calibrated

with a scale chart before the still object was captured.

Black holeTop view

Camera 1 (left eye

Object

Camera 2 (right eye)

Figure 3.6 Setup for two-camera system to capture a still object; the inset shows the top

view o f the setup.

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Figure 3.7 Stereo image captured by two-camera system: (a) image out of camera; (b)

odd field image; (c) even field image; (d) reconstructed odd field image; and (e)

reconstructed even field image.

Figure 3.7a shows the stereo image out of the cameras. Two images were imbedded

in the stereo image because odd field and even field signals were obtained from different

viewing angles. Scion image was used for image processing. By separating the two

fields from Figure 3.7a, two images were obtained as Figure 3.7b and 3.7c. Since one

field has only half of the lines out of a full frame, the vertical resolution by splitting the

two fields was only half of the full frame. The images were distorted by losing half of

the lines. To restore the image resolution, another set of lines were added to the image by

using the macro written in our laboratory. The macro interlaced one line between every

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two adjacent lines. The information o f the line added was predicted by the mean value of

the two adjacent lines. Figure 3.7d and 3.7e were the images restored from Figure 3.7b

and 3.7c respectively. The two restored images made up a stereo pair and 3-D structure

was visualized by human eyes. Visualization aids and software are also available to

facilitate the 3-D reconstruction.

3.3.2.3 Stereo image o f electrospinning captured by a two-camera system

Figure 3.8 A pair of stereo images of electrospinning from polyethylene oxide in water:

(a) reconstructed odd field image; (b) reconstructed even field image.

The possibility to capture stereo images by a two-camera system was demonstrated

above. Xenon strobes were used to stop the motion of the electrospinning process. The

flash duration was 200 ps and the strobe was controlled by a computer software

(Flashpoint 3D). The software also contr

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Diseño de fibras submicronicas por medio de la tecnica de electrohilado