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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
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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
4.8 Summary and conclusions............................................................................... 64
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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
6.8 Summary and conclusions............................................................................... 126
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