Qin Y. Micromanufacturing Engineering and Technology

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thin films [49], which may offer anti-adhesive

properties in relation to cells.

Structured by multiple layers, texturing or by

the addition of nano-fillers, these new-generation

polymer thin films are currently being develo ped

and will give plastic, paper and steel substrates

high performance properties.

Summary on Vacuum-deposited

Coatings

Polymer thin films obtained by PECVD have a

great deal of relevance for various industrial appli-

cations today, mainly due to their lower cost.

Moreover, their structure is now very well con-

trolled. This has given rise to systems with excellent

new chemical and physical characteristics, such as

barrier, friction and hydrophobic properties, etc.

The chemical nature of the precursors and their

chemical reaction mechanisms are two significant

factors in the vapor-phase deposition process

which in turn affect reactor architecture.

Process param eters strongly influence the

PECVD process. These parameters, including

power, temperature and pressure, allow modifica-

tions to film structure that can be controlled. The

resulting properties can be very powerful. Numer-

ous studies have been conducted on how to apply

the final materials.

The application areas include packaging, with

a focus on high barrier films, as well as steel,

where the accent is on corrosion protection or

adding properties such as high surface energy. A

large number of emerging applications are also

incorporating such films. Uses include biological,

physical and chemical sensors along with elec-

tronic, molecular and non-linear optical systems.

Despite multiple studies in this area, these materi-

als have only been successfully used for a small

number of electr onics and optical applications.

This is primarily due to the low thermal an d

chemical stability of thin films as well as their

weak mec hanical solidity. Hence the relevance

of creating high quality polymer thin films for a

broad range of technolo gical applications. The

next generation of organic flat screens requires

drastic encapsulation of their systems, which in

turn necessitates jumping at least three to four

orders of magnitude in barrier efficiency com-

pared to the requirements of traditional industries

such as packaging.

Today these thin films and the associated pro-

cesses are used for the electronics market. Certain

micro-electronics manufacturers, such as Applied

Materials, are trying to push these proces ses to

their limits, but when production of 65 nm tran-

sistors starts, they may no longer be satisfactory.

A competing technology know n as ALD (Atomic

Layer chemical vapor Deposition) seems very

promising. For example, it should be possible to

deposit 2 nm barrier films using ALD. Anot her

emerging technology, electrografting, will be a

competitor as well. Electrografting is an electro-

chemical technique. The chemical species present

in a liquid bath migrate between an electrode and

the part to be coated. Deposition is controlled

through the electrical conditions.

CONCLUSIONS

There are various processes for producing poly-

mer thin films, but whatever the deposition pro-

cess (extrusion coating, wet or dry methods), the

properties of the resulting film will depend

strongly on the chemical nature of the polymer

used. However, the numerous interactions

between the process and material parameters

significantly influence the final properties. Also

it is very important to control the process

parameters for maximum optimization of the

target properties.

With control of these processes becoming more

and more developed, films with an increasingly

fine structure can be obtained (e.g. nanometric

scale for PECVD). This in turn produces proper-

ties with very high performance.

The resulting thin films are used by traditional

industries such as packaging and steel, but

because there is enormous progress in the area

of structure and properties, new emerging mar-

kets are focusing on the latest generation of poly-

mer films. The re are new products in optics, elec-

tronics, batteries and micro-sources for which

precision, multiple layers and much localized

CHAPTER 15 Polymer Thin Films – Processes, Parameters and Property Control 261

deposition with micro-structures and texturing

are required to attain the desired properties.

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CHAPTER 15 Polymer Thin Films – Processes, Parameters and Property Control 263

Micro-/Nano-Fibers

by Electrospinning

Technology: Processing,

Properties and Applications

Ioannis S. Chronakis

INTRODUCTION

Human beings have used fibers for centuries. In

5000 BC, our ancestors used natural fibers such as

wool, cotton silk and animal fur for clothing. Mass

production of fibers dates back to the early stages of

the industrial revolution. The first man-made fiber –

viscose – was presented in 1889 at the World Exhi-

bition in Paris. Developments in the polymer and

chemical industries – as well as in electronics and

mechanics – have led to the introduction of new

types of man-made fibers, especially the first syn-

thetic fibers, such as nylon, polypropylene and poly-

ester. The needs and further progress allowed the

production of high functionality fibers (antistatic,

flame resistant, etc.) and high performance fibers

(carbon fibers in 1960 from viscose and aramid

fibers in 1965) that showed high strength, a high

modulus and great heat resistance. These fibers are

used not only in clothing but also in hygienic pro-

ducts, in medical and automotive applications, in

geo-textiles and in other applications.

Traditional methods for polymer fiber produc-

tion include melt spinning, dry sp inning, wet spin-

ning and gel-state spinning. These methods rely

on mechanical forces to produce fibe rs by extrud-

ing a polymer melt or solution through a spinneret

and subsequently drawing the resulting filaments

as they solidify or coagulate. These methods allow

the production of fiber diameters typically in the

range of 5 to 500 microns. At variance, electro-

spinning technology allows the production of

fibers of much smaller dimensions. The fibers

are produced by using an electrostatic field [1].

Electrospinning is a fiber-spinning technology

used to produce long, three-dimensional, ultra-

fine fibers with diameters in the range of a few

nanometers to a few microns (more typically

100 nm to 1 micron) and lengths up to kilo-

meters (Fig. 16-1). When used in products, the

unique properties of nano-fibers are utiliz ed,

such as e xtraordi naril y high surface area per unit

mass, very high porosity, tunable pore size, tun-

able surface properties, layer thinness, high per-

meability, low basic weight, ability to r etain

electrostatic charges and cost effectiveness,

among others [2].

While electrospinning technology was developed

and patented by Formhals [3] in the 1930s, it was

only about fifteen years ago that actual developments

were triggered by Reneker and co-workers [4].

CHAPTER

264

Interest today is greater than ever and this cost-effec-

tive technique has made its way into several scientific

areas, such as biomedicine, filtration, electronics, sen-

sors, catalysis and composites [5,6]. Electrospinning

is a continuous technique and is hence suitable for

high volume production of nano-fibers. The ability to

customize micro-/nano-fibers to meet the require-

ments of specific applications gives electrospinning

an advantage over other, larger-scale, micro-/nano-

production methods. Carbon and ceramic nano-

fibers made of polymeric precursors further expand

the list of possible uses of electrospun nano-fibers [7].

WORKING PRINCIPLE

AND CONFIGURATION OF

ELECTROSPINNING PROCESSING

Electrospinning is increasingly being used to pro-

duce ultra-thin fibers from a wide range of poly-

mer materials. This non-mechanical, electrostatic

technique involves the use of a high voltage elec-

trostatic field to charge the surface of a polymer-

solution droplet, thereby inducing the ejection of

a liquid jet through a spinneret (Fig. 16-2). In a

typical process, an electrical potential is applied

between a droplet of a polymer solution held at

the end of a capillary tube and a grounded target.

When the electric field that is applied overcomes

the surface tension of the droplet, a charged jet of

polymer solut ion is ejected. On the way to the

collector, the jet will be subjected to forces that

allow it to stretch immensely . Simultaneously, the

jet will partial ly or fully solidify through solvent

evaporation or cooling, and an electrically

charged fiber will remain, which can be directed

or accelerated by electrical forces and then col-

lected in sheets or other useful shapes.

A characteristic feature of the electrospinning

process is the extremely rapid formation of the

nano-fiber structure, which occurs on a millisec-

ond scale. Other notable features of electrospin-

ning are a huge material elongation rate of the

order of 1000 s

1

and a reduction of the cross-

sectional area of the order of 10

to 10

, which

have been shown to affect the orientation of the

structural elements in the fiber.

The Electrospinning Mechanism

In spite of the simple set-up for electrospinning,

the actual sp inning mechanism is quite complex.

Although extensive studies have been conducted

to explore the mechanism, some aspects and phe-

nomena are not yet fully understood.

Formation of the Taylor Cone and Subsequent

Fluid Jet. When the high voltage field is applied,

the droplet of polymer solution at the tip of the

needle will become highly electrified and the

charges induced will be evenly distributed over

the polymer solution surface. The droplet will

FIGURE 16-1 (a) SEM image of poly(ethylene terephthal-

ate) (PET) nano-fiber web. The nano-fibers were electro-

spun from a PET solution in THF:DMF. The diameter of the

fibers is about 200 nm. (b) PET nano-fiber web – compari-

son with human hair [1].

CHAPTER 16 Micro-/Nano-Fibers by Electrospinning Technology 265

experience two types of electrostatic forces: elec-

trostatic repulsion between the charges on the

surface and Coulombic forces in the external

field. Under the influence of these two forces, the

droplet will be elongated and finally distorted into

a so-called Taylor cone. As the voltage increases,

the electrostatic forces will become stronger and

eventually overcome the surface tension, and a

charged jet of fluid will be ejected.

Both electrostatic and fluid dynamic instabil-

ities can contribute to the basic operation of the

process.

Reznik et al. [8] experimentally and numeri-

cally studied the shape evolution of small droplets

attached to a conducting surface that was sub-

jected to relatively strong electric fields. Three

different scenarios of droplet shape evolution

are distinguished, based on numerical solution

of the Stokes equations for perfectly conducting

droplets:

1. In sufficiently weak (subcritical) electric fields,

the droplets are stretched by the electric Max-

well stresses and acquire steady-state shapes

where equilibrium is achieved by means of sur-

face tension.

2. In stronger (supercritical) electric fields the

Maxwell stresses overcome t he surface ten-

sion, and jetting is initiated f rom the droplet

tip if the static (initial) contact angle of the

droplet with the c onducting electrode is

< 0.8p; in this case, the jet base acquires

a quasi-steady, nearly conical, shape with a

vertical semi-angle of b  30



,whichissig-

nificantly smaller than that of the Taylor cone

= 49.3



3. In supercritical electric fields acting on droplets

with a contact angle in the range 0.8p < a

/<p,

there is no jetting and almost the whol e droplet

jumps off: this is similar to gravity or drop-on-

demand dripping.

FIGURE 16-2 Schematic illustration of the conventional set-up for electrospinning. The insets show a drawing of the

electrified Taylor cone, bending instability and a typical SEM image of the non-woven mat of PET nano-fibers deposited on

the collector. The bending instability is a transversal vibration of the electrospinning jet. It is enhanced by electrostatic

repulsion and suppressed by surface tension.

266 CHAPTER 16 Micro-/Nano-Fibers by Electrospinning Technology

The drople t-jet transitional region and the jet

region proper are studied in detail for the second

case using quasi-one-dimensional equations,

taking into account the inertial effects an d addi-

tional features such as the dielectric properties

of the liquid (leaky dielectrics). The flow in the

transitional and jet region is matched to that in the

droplet. This is used to predict the currentvol-

tage characteristic, I = I(U), and the volumetric

flow rate, Q, in electrospun viscous jets, given

the potential difference applied. The predicted

dependence, I = I(U), is nonlinear due to the con-

vective mechanism of the charge redistribution

superimposed on the conductive (ohmic) mecha-

nism. Realistic current values I = O(10

nA) have

been predicted for U = O(10 kV) and fluid con-

ductivity s =10

4

1

Thinning of the Fluid Jet. Beyond the conical

base, immediately at the end of the capillary

tip, the jet continues to become thinner. This

jetting mode is known a s the electrohydrody-

namic cone jet. T he jet will initially travel in a

straight line towards the collector but will

eventually become unstable. To the naked eye,

it looks like the jet splits into multiple jets and

it was thought before 1999 that this was the

main reason for the small diameter of the elec-

trospun fibers. However, when the jet is exam-

ined with a high speed camera, it can clearly be

seen that the splaying is actually one single

fiber rapidly bending or whipping, causing the

fiber to make lateral excursions that grow into

spiraling loops.

Jet splitting does occur, but it is not as com-

mon as previously thought and it is not the

dominant process that occurs during spinning.

Bending or whipping is caused by a phenome-

non called bending instability and can occur in

electrified fluid jets. Every loop then grows

larger in diameter and the jet becomes thinner.

New bending instabilities arise when the jet is

thin enough and enough s tress relaxation of the

viscoelastic stress has taken place. This is

called the second instability region and is very

similar to the first instability region but acts on

a much smaller scale. A tertiary-bending insta-

bility has also been documented. Each cycle of

bending instability can be described in three

steps:

1. A smooth, straight or slightly curved segment

starts to bend.

2. The segment of the jet in each bend elongates

and a spiral of growing loops develops.

3. As the perimeter of the loops increases, the

diameter of the jet decreases. When the perim-

eter of the loop is large enough and the diam-

eter of the jet is small enough, the conditions of

the first step of the cycle are fulfilled. The next

cycle of bending instability then begins.

Several research groups have attempted to

explain the bending instability by mathematical

models.

ELECTROSPINNING PROCESSING

PARAMETERS – CONTROL

OF THE MICRO-NANO-FIBER

MORPHOLOGY

The fiber morphology has been shown to be

dependent on process parameters, namely solu-

tion properties (system parameters), process con-

ditions (operational parameters) and ambient

conditions [1,2].

Solution Properties

Solution properties are those such as molecular

weight, molecular weight distribution and archi-

tecture of the polymer, and properties such as

viscosity, conductivity, dielectric constant and

surface tension. The polymer solution must have

a concentration high enough to cause polymer

entanglements, yet not so high that the viscosity

prevents polymer motion induced by the electric

field. The resulting fibers’ diameters usually

increase with the concentration of the solut ion

according to a power law relationship. Decreas-

ing the polymer concentration in the solution

produces thinner fibers. Decreasing the concen-

tration below a threshold value causes the uni-

form fiber morphology to change into beads [9].

The main factors affecting the formation of beads

(Fig. 16-3) during electrospinning have been

shown to be solution viscosity, surface tension

CHAPTER 16 Micro-/Nano-Fibers by Electrospinning Technology 267

and the net charge density carried by the electro-

spinning jet. Higher surface tension results in a

greater number of bead structures, in contrast to

the parameters of viscosity and net charge density,

for which higher values favor fibers with fewer

beads. This reduction in thickness is due to the

solution conductivity, which reflects the charge

density of the jet and thus the elongation level.

The surface tension also controls the distribution

and the width of the fibers, which can be

decreased by adding a surfactant to the solution.

Adding a surfactant or a salt to the solution is a

way of increasing the net charge density and thus

reducing the formation of beads. Finally, the

choice of solvent(s) directly affects all of the prop-

erties mentioned and is of major importance to the

fiber morphology.

Process Conditions

The parameters in the process are spinning voltage,

distance between the tip of the capillary and the

collector, solution flow rate (feed rate), needle

diameter and, finally, the motion of the target

screen. Voltage and feed rate show different ten-

dencies and are less effective in controlling fiber

morphology as compared to the solution proper-

ties. Too high a voltage might result in splaying and

irregularities in the fibers. A bead structure is evi-

dent when the voltage is either too low or too high.

However, a higher voltage also leads to a higher

evaporation rate of the solvent, which in turn

might lead to solidification at the tip and instability

in the jet. Morphological changes in the nano-

fibers can also occur upon changing the distance

between the syringe needle and the substrate.

Increasing the distance or decreasing the electrical

field decreases the bead density, regardless of the

concentration of the polymer in the solution.

Ambient Conditions

Ambient conditions include factors such as

humidity and temperature, air velocity in the spin-

ning chamber and atmospheric pressure. Humid-

ity primarily controls the formation of pores on

the surface of the fibers. Above a certain threshold

level of humidity, pores begin to appear and, as

the level increases, so does the number and size of

the pores.

The precise mechanism behind the formation

of pores and texturing on the surface is complex

and is thought t o be dependent on a combination

of breath figure format ion and phase separation.

Breath figures are imprints formed due to the

evaporative cooling during evaporation of the

FIGURE 16-3 Example of bead formation during electro-

spinning: SEM micrographs of poly(propyl carbonate) (PPC)

beads prepared by electrospinning a PPC solution in dichlor-

omethane [9].

268 CHAPTER 16 Micro-/Nano-Fibers by Electrospinning Technology

solvent, which results in condense solvent drops

on the surface and, later, pores. Surface porosity

(Fig. 16-4) can also be achieved by selective

removal of one of the components in the poly-

mer blend after spinning. The pores formed

on the fiber surface can be used, for example,

to capture nano-particles, act as a cradle for

enzymes or increase the surface area for filtra-

tion applications.

Baumgarten studied the spinning velocities in

addition to the effect of the flow rate, voltage, gap

and the surrounding atmosphere [10]. He was

able to determine the spinning velocity using the

power balance:

IV ¼

ð1Þ

where V is the potential, I is the current and

and

are the mass flow rate and the spinning veloci-

ties, respectively. The calculation showed velocities

close to the velocity of sound in air. Other research-

ers calculated velocities of the fibers reaching the

collector to be 140 to 160 m/s. Obviously, these

speeds must depend on the process parameters and

solution used.

Increasing the solution temperature is also a

method for speeding up the process, but it might

cause morphological imperfections, such as the

formation of beads. Furthermore, the regulation

of scale and bifurcation-like instability in electro-

spinning are intriguing problems that remain to be

solved. Regulatory mechanisms for controlling the

radius of electrospun fibers at the different states

are clearly illustrated in the work by He et al. [11].

ELECTROSPINNING SET-UPS

AND TOOLS

Novel Set-ups

The traditional set-up for electrospinning has

been modified in a number of ways during the last

few years in order to be able to control the elec-

trospinning process and tailor the structure of

micro-nano-fibers.

Yarin and Zussman achieved upward electro-

spinning of fibers from multiple jets without the

use of nozzles; instead using the spiking effect of a

magnetic liquid [12]. The concept (Fig. 16-5) con-

sists of a bath filled with a laye r of magnetic liquid

(a). This liquid is covered by the solution to be

spun (b). An electrode is submerged into the

FIGURE 16-4 SEM images of (a) porous poly(L-lactide)

(PLA) nano-fibers prepared by electrospinning a solution

of PLA in dichloromethane [5]. (b) Poly(propyl carbonate)

(PPC) nano-fibers with a porous surface electrospun from a

PPC solution in dichloromethane [9].

CHAPTER 16 Micro-/Nano-Fibers by Electrospinning Technology 269

magnetic fluid (d). A counter-electrode (c) as a

collector is placed a certain distance above this

bath. A strong permanent magnet or electromag-

net (f) is placed under the bath around the elec-

trode. When a magnetic field is applied, the spik-

ing effect causes some of the polymer solution to

protrude into the electrical field applied between

the electrodes (c and d). The protrusion is suffi-

cient to initiate multiple jets of polymeric fibers

traveling towards the collector. The production

rate was reported to be about 12 times that of a

conventional set-up. This approach also avoids

clogging problems.

Using supercritical CO

-assisted electrospin-

ning, polymer fibers of high molecular weight

polydimethylsiloxane (PDMS) and poly(D,L-lac-

tic acid) (PLA) were produced by means of only

electrostatic forces an d without the use of a liquid

solvent. The fibers were formed between two elec-

trodes in a high pressure view cell. This supported

the idea that the supercritical CO

reduces the

polymer visco sity sufficiently to allow fibers to

be pulled electrostatically from an undissolved

bulk polymer sampl e.

Electrospinning in a vacuum is also a novel set-

up. Compared to electrospinning in air, a vacuum

allows higher electric field strength over large dis-

tances and higher temperatures compared to what

can be achieved in air, which influences both the

spinning process and the morphology of the fibers

that are produced. Other attempts have been

made to incorporate vibration technology in poly-

mer electrospinning. The idea is to produce finer

nano-fibers under lower applied voltage by vibra-

tion technology. Other electrospinning set-ups are

discussed in a recent review by Teo and Ramak-

rishna [6].

Set-ups Involving Dual Syringes

A set-up was developed for electrospinning

involving a dual syringe spinneret (Fig. 16-6).

The development enables spinning highly func-

tional nano-fibers such as hollow nano-fibers,

nano-tubes and fibers with a core-shell structure

[13]. A recent study describes the formation of

hollow nano-tubular fibers in a single step using

electrospinning and sol-gel chemistry. The

method exploits electrohydrodynamic forces that

form coaxial jets of liquids with microscopic

dimensions. A high voltage is applied to a pair

of concentric needles used to inject two immisci-

ble liquids that lead to the formation of a two-

component liquid cone that elongates into coaxial

liquid jets and forms hollow nano-fibers.

Set-ups Controlling the Orientation

and Alignment of Micro-nano-fibers

A number of set-ups that allow control over the

orientation of fibers have been developed. The

orientation is crucial for different applications of

nano-fibers and opens new opportunities for

manufacturing yarn, micro-nano-wire devices,

etc. Most of the set-ups are based on rotating

collection devices.

FIGURE 16-5 Schematic representation of the upward electrospinning set-up [12].

270 CHAPTER 16 Micro-/Nano-Fibers by Electrospinning Technology