Mechanical Properties of Individual Polymeric Micro and Nano Fibers using Atomic Force Microscopy (AFM)

ABSTRACT

HATIBOGLU, BILGE. Mechanical Properties of Individual Polymeric Micro and Nano Fibers using Atomic force Microscopy (AFM). (Under the direction of Dr. Behnam Pourdeyhimi and Dr. Juan P. Hinestroza.)

Being the raw materials, fibers have the biggest importance in textiles industry. With the invention of the first man-made fiber, nylon, in 1930s, fiber industry gained a new aspect. During the recent years, fibers gained another aspect with the term “nanotechnology”. Now, today’s science is focused on the nano materials. Analyzing and finding new applications for them are some of the concerns. Textile industry is affected from the fiber part of the nanomaterials. The introduction of the nanofibers added a lot of interesting applications to textiles. Drug delivery, tissue engineering, reinforcement for composite materials, filtration are some of the interesting applications of nanofibers. Having a lot of exciting applications, nanofibers require to be examined well. However, since they are fairly small materials, it is not very easy to analyze them.

MECHANICAL PROPERTIES OF

INDIVIDUAL POLYMERIC MICRO AND

NANO FIBERS USING ATOMIC FORCE

MICROSCOPY (AFM)

by

Bilge Hatiboglu

A thesis submitted to the Graduate Faculty of North Carolina State University

in partial fulfillment of the requirements for the Degree of Master of Science

Textile Engineering

Raleigh, North Carolina 2006

Approved By:

Dr. Behnam Pourdeyhimi Dr. Juan P. Hinestroza

Chair of Advisory Committee Co-chair of Advisory Committee

BIOGRAPHY

ACKNOWLEDGEMENTS

I would like to express my gratitude to my advisors Dr. Behnam Pourdeyhimi and Dr. Juan P. Hinestroza for giving me the opportunity to work with them and for their continuous patience and guidance. This work would not have been possible without their knowledge and support. I also appreciate the support given by the other members of my advisory committee, Dr. Phillip Russell and Dr. Orlando Rojas.

I also appreciate the financial support provided by Nonwovens Cooperative Research Center. I would like to thank Chuck Mooney, Mike Salmon, David Nackashi, Roberto Garcia and Dale Batchelor for their help and support with the experimental work. I extented my great appreciation to Jeffrey Krauss.

My special thanks go to Umut Kivanc Sahin and Erkmen Ercan for their support and encouragement throughout this work.

My last but not least gratitude goes to my parents and my sister for being with me, supporting me and trusting me at every moment of my life…

TABLE OF CONTENTS

LIST OF TABLES………..……...vii

LIST OF FIGURES………viii

1. INTRODUCTION………..……….1

2. LITERATURE REVIEW………...3

2.1. Fibers……….3

2.1.1. Natural Fibers……….4

2.1.2. Man-Made Fibers………...4

2.2. General Fiber Properties………...4

2.2.1. Geometric Characteristics………..5

2.2.2. Physical Properties……….5

2.2.3. Chemical Properties………...6

2.2.4. Mechanical Properties………6

2.3. Properties of Polyamide-6 (Nylon-6) and Polyester (PET)………..7

2.3.1. Nylon-6 Fibers………...7

2.3.2. PET Fibers………...10

2.4. Microfibers………..12

2.5. Nanofibers………...13

2.5.1. Fabrication of Nanofibers………16

2.5.1.1. Drawing……….14

2.5.1.2. Template Synthesis………...14

2.5.1.3. Phase Separation………...15

2.5.1.5. Self-Assembly………...18

2.5.1.6. Electrospinning……….18

2.5.1.7. Other Techniques………..20

2.5.2. Applications of Nanofibers………..20

2.5.2.1. Filters………21

2.5.2.2. Biomedical Applications………...22

2.5.2.3. Protective Clothing………...24

2.5.2.4. Reinforcement for Composite Materials………..24

2.5.2.5. Sensors………..25

2.5.3. Analytical Techniques………...…..25

2.5.3.1. Scanning Electron Microscopy (SEM).………....25

2.5.3.2. Transmission Electron Microscopy (TEM)…..…………31

2.5.3.3. Atomic Force Microscopy (AFM)………....35

3. EXPERIMENTAL APPROACH………...47

3.1. Materials……….47

3.2. Instruments………..49

3.2.1. Focused Ion Beam (FIB)………..49

3.2.2. Scanning Electron Microscopy (SEM)………50

3.2.3. Dynamic Mechanic Analyzer (DMA)……….52

3.2.4. Atomic Force Microscopy (AFM)………...54

3.3. Experimental Procedures………59

3.3.3. Third Approach………61

3.3.4. Final Approach……….64

4. RESULTS AND DISCUSSION………..66

4.1. Cross-sectioning and imaging islands-in-the-sea form fibers……….66

4.2. Method Validation………..71

4.3. AFM Imaging and Indentations………..72

4.4. Determination of the tip radius………...78

4.5. Determination of the cantilevers’ size……….…...79

4.6. Determination of the cantilevers’ spring constant………..80

4.7. Data processing and obtaining Force vs. Displacement curves………..81

4.8. Calculation of elastic modulus values……….99

4.9. Results for PET micro and nanofibers………..103

4.10. Results for Nylon-6 micro and nanofibers………..105

4.11. Results for Nylon-6 hollow micro and nanofibers………..107

5. SUMMARY AND CONCLUSIONS………108

6. REFERENCES………...110

LIST OF TABLES

Page Table 2.1 Effect of drawing on Elastic modulus of Nylon-6 fibers [3]………...8 Table 2.2 Typical Properties of nylon fibers [3]………10 Table 2.3 Typical properties of Polyester fibers [3]………..11 Table 4.1 Elastic modulus values [GPa] of the PET film, obtained

under different testing conditions………..71 Table 4.2 An example of the variation on the same cantilevers’ spring

LIST OF FIGURES

Page

Figure 2.1 Cross-sections of bicomponent fibers………..16

Figure 2.2 Classical bilateral bicomponent spinning (A, B: Polymers) [12]……….17

Figure 2.3 Pipe-in-pipe mixers [12]………...17

Figure 2.4 Schematic figure of Elecrtospinnig process [13]………..19

Figure 2.5 Fractional efficiency (Filtration Efficiency vs. particle size) for a standard cellulose media and nanofiber filter media [22]………..22

Figure 2.6 SEM images of nickel titanate fibers: a)as-prepared composite fibers, b) fibers calcinated at 1273 K [28]……….27

Figure 2.7 SEM images of the V2O5 fibers [29]………27

Figure 2.8 SEM images of elastomeric nanofiber membranes under two different levels of biaxial strain a) 100%, b) 0 % [24]………...28

Figure 2.9 SEM photograph of PVA/lithium chloride/manganese acetate composite fiber samples [30]……….29

Figure 2.10 SEM images of a) polyaniline nanofibers b) polyaniline nanofibers and polyaniline/CeO2 composite microspheres [32]……….30

Figure 2.11 TEM image of polyaniline nanofibers [32]………31

Figure 2.12 TEM images of a) Twisted nantubes; b) and c) Aligned, nanotubes in PEO nanofibers [35]………..32

Figure 2.13 TEM image of an individual Collagen-r-PCL composite nanofiber a) collagen as the shell material, and PCL the support b) is the TEM image of a pure PCL nanofiber [34]………..33

Figure 2.14 Transmission electron micrographs of a) PA6 fiber, a segment almost constant in diameter b) PLA nanofiber fibers with modulations [37]…………...34

Figure 2.16 a) NCAFM image (25 ím _ 25 ím) of dendrimer 1 nanofibers prepared by drop-casting a 2.0 _ 10-6 M dendrimer 1 solution in THF on a silicon surface in a saturated environment of THF, b) NCAFM image

(25 ím _ 25 ím) of dendrimer 1 nanofibers repared by drop-casting a 2.0 _ 10-6 M dendrimer 1 solution in THF in a saturated environment of THF:H2O ) 90:10 (v/v) on a silicon surface, c) NCAFM image (50 ím _ 50 ím) of dendrimer 1 nanofibers prepared by drop-casting a 2.0 _ 10-6 M dendrimer 1 solution in THF in a saturated

environment of THF:H2O ) 80:20 (v/v) on a silicon

surface [31]………37 Figure 2.17 a) Schematic diagram of three-point bending test and b) actual

AFM scanning data on fiber (i) and pore (ii) [41]……….38 Figure 2.18 Young’s modulus (E) vs. diameter of TiO2–PVP and

TiO2 nanofibers [41]………..39 Figure 2.19 (a and b) AFM images of indents on a silver nanowire c) height

profile of an indent on the wire, and d) a representative

nanoindentation load-displacement curve for a silver nanowire [42]………40 Figure 2.20 (a) Indentation load-displacement curves made on a solid

Cu2O nanocube and (b) a hollow Cu2O nanocube [43]……….41 Figure 2.21 Schematic of AFM tip depressing suspended nanofiber [44]………42 Figure 2.22 a) Average Young’s modulus versus diameter for several PEO

nanofibres produced by electrospinning b)Average Young’s modulus versus diameter for several polysiloxane and glass nanofibres

produced by electrospinning [44]………..…42 Figure 2.23 Nanofibers suspended over etched grooves of silicon wafer:

a)Electron micrograph of PLLA nanofibers deposited onto the silicon wafer; b) AFM contact mode image of a single nanofiber (300 nm diameter)

suspended over an etched groove; c) schematic diagram of a nanofiber withmid-span deflected by an AFM tip [45]……….43

Figure 2.24 Variation of elastic modulus with fiber diameter for nanoindentation

of PLLA nanofiber [46]……….44

Figure 2.25 (a) AFM image of a SWNT rope adhered to the polished alumina ultrafiltration membrane, with a portion bridging a pore

Figure 2.26 Measured reduced modulus, Er , for ten different SWNT ropes with

diameters between 3 and 20 nm (circles) [49]………...46

Figure 3.1 Custom-made TEM grids……….48

Figure 3.2 SEM image of the calibration grating used for tip radius determination………...49

Figure 3.3 Schematic figure of FIB [60]………49

Figure 3.4 Schematic Diagram of an SEM [49]………52

Figure 3.5 Schematic of how DMA works [49]………53

Figure 3.6 Essential elements of AFM [59]………..56

Figure 3.7 a) Force calibration Z waveform, b) a typical force-distance curve for a tip in contact with a sample [49]……….………58

Figure 3.8 A microscope image of an epoxy coated PET/PE islands in the se form fiber……….60

Figure 3.9 AFM images of PET nanofibers coated with adhesive………62

Figure 3.10 SEM images of the PET nanofibers on perforated aluminum plates……….63

Figure 3.11. Schematic of the sample preparation method; a) threading the I/S fiber through the windows of the grids, b) Submerging and keeping the grid in the appropriate solution, c) Obtaining the PET and Nylon6 nanofibers and Nylon6 hollow fibers on the grid………65

Figure 4.1 SEM images of a) PET/PE islands-in-the-sea fibers and b) Nylon6/Evoh islands-in-the-sea hollow fiber………67

Figure 4.2 FIB images of a) PET/PE and b) Nylon6/Evoh islands-in-the-sea fibers……68

Figure 4.3 AFM images of a) PET/PE b) Nylon-6/Evoh islands-in-the-sea fibers……...70

Figure 4.4 AFM images of quartz sample a) before and b) after indentation………72

Figure 4.5 A typical Piezo Movement vs. Tip Deflection Curve for Quartz………….…73

Figure 4.6 Schematic of AFM tip imaging the nanofiber………..74

Figure 4.8 Examples of Tip Displacement vs. Piezo Movement curves of a

a)PET and a b)Nylon6 nanofiber………...77

Figure 4.9 Size relations between some of the AFM tips and the fibers………...77

Figure 4.10 a) An AFM image of the tip obtained imaging calibration gratings and b) Cross-section of the AFM tip……….78

Figure 4.11 SEM images of some of the cantilevers……….79

Figure 4.12 Some examples of F vs. d curves’ retracting parts of PET micro and nanofibers………87

Figure 4.13 Some examples of F vs. d curves’ retracting parts of Nylon6 micro and nanofibers………..93

Figure 4.14 Some examples of F vs. d curves’ retracting parts of Nylon6 hollow micro and nanofibers………..98

Figure 4.15 Cross-section of two solids a) before and b) after deformation…………...100

Figure 4.16 Elastic modulus vs. Fiber diameter relation for PET micro and Nanofibers……….………..….103

Figure 4.17 Elastic modulus vs. Fiber diameter relation for Nylon-6 micro and nanotubes………105

Figure 4.18 Elastic modulus vs. Fiber diameter relation for Nylon-6 hollow micro and nanofibers……….………...107

Figure 7.1 3D image of PET nanofiber………116

Figure 7.2 3D image of Nylon-6 nanofiber……….116

Figure 7.3 Raw indentation Curve of PET microfiber (φ =2.5±0.18μm)…………....117

Figure 7.4 Raw indentation Curve of PET microfiber (φ =1.8±0.11μm)……….117

Figure 7.5 Raw indentation Curve of PET microfiber (φ =700±50nm)………...118

Figure 7.6 Raw indentation Curve of PET microfiber (φ =400±30nm)………...118

Figure 7.9 Raw indentation curve of Nylon 6 Nanofiber (φ =1.3±0.09μm)………...120

Figure 7.10 Raw indentation curve of Nylon 6 Nanofiber (φ =1.2±0.08μm)………..120

Figure 7.11 Raw indentation curve of Nylon 6 Nanofiber (φ =1±0.07μm)………….121

Figure 7.12 Raw indentation curve of Nylon 6 Nanofiber (φ =900±60nm)…………121

Figure 7.13 Raw indentation curve of Nylon 6 Nanofiber (φ =800±55nm)………….122

Figure 7.14 Raw indentation curve of Nylon 6 Nanofiber (φ =700±50nm)…………122

Figure 7.15 Raw indentation curve of Nylon 6 Nanofiber (φ =600±40nm)…………123

Figure 7.16 Raw indentation curve of Nylon 6 Nanofiber (φ =500±35nm)………….123

Figure 7.17 Raw indentation curve of Nylon 6 Nanofiber (φ =300±20nm)………....124

Figure 7.18 Raw indentation curve of Nylon 6 Nanofiber (φ =200±15nm)………….124

Figure 7.19 Raw indentation curve of Nylon 6 hollow

Microfiber (φ =1.3±0.09μm)……….125

Figure 7.20 Raw indentation curve of Nylon 6 hollow

Microfiber (φ =1.1±0.07μm)……….125

Figure 7.21 Raw indentation curve of Nylon 6 hollow

Microfiber (φ =1±0.07μm)………126

Figure 7.22 Raw indentation curve of Nylon 6 hollow

Microfiber (φ =500±35nm)………...…126

Figure 7.23 Raw indentation curve of Nylon 6 hollow

Microfiber (φ =400±30nm)………..127

Figure 7.24 Raw indentation curve of Nylon 6 hollow

Microfiber (φ =300±20nm)………..127

Figure 7.25 Raw indentation curve of Nylon 6 hollow

1. INTRODUCTION

The objective of this work was to develop a method capable of analyzing the mechanical

properties of individual micro and nanofibers using Atomic Force Microscopy. An

optimized protocol for sample preparation was developed and the properties of polyester

and nylon-6 islands-in-the-sea fibers were probed by applying Hertzian Contact Theory

to the experimental data obtained via AFM.

The introduction of polymeric nanofibers has spurred a great number of new and

interesting applications to the field of textiles. Some of these applications include drug

delivery, tissue engineering, reinforcement for composite materials, and filtration.

During the last ten years, due to advances in instrumentation and the nanotechnology

revolution, it has been established that some material properties may be size dependent.

However, most of current manufacturing and testing techniques for micro/nano scale

devices are still based on bulk material properties that do not consider size dependent

phenomenon. Some improved microscopy techniques, including Atomic Force

Microscopy (AFM), Transmission Electron Microscopy (TEM) and Scanning Electron

Microscopy (SEM) are currently used to analyze micro and nano-sized materials.

Atomic Force Microscopy (AFM) has been chosen for this study because this technique

obtained by cross sectioning samples and analyzing the sections via TEM. AFM can be

used not only as an imaging technique, but it shows an increasing potential for direct

2. LITERATURE REVIEW

2.1. Fibers

In early history only natural fibers such as cotton, linen and jute from plants as well as

wool and mohair from the fleece of sheep and goats, and silk from the cocoon of the

silkworm, were available. During the twentieth century some derivative fibers were

created from existing natural fibers and more importantly new synthetic fibers were

developed using by using by-products of the coal and petroleum industries. Today , it is

possible to synthesize polymeric fibers with almost any desired property by manipulating

their chemistry[1].

Fibers can be seen everywhere. Even the fundamental blocks of living systems are

formed by fibrous materials in nanometer scale such as DNA molecules, cytoskeleton

filaments, rod cells of the eyes… etc. [2]. Fibers are also raw material for all kinds of

textiles. Having a combination of high specific surface area, flexibility and superior

directional strength, fibers are preferred materials for applications ranging from clothing

to reinforcements in aerospace applications [3].

2.1.1. Natural Fibers

Natural fibers are mostly derived directly from animals, vegetables or minerals. With the

exception of silk, which is extruded by silkworms as a continuous filament, natural fibers

fibers for textile purposes is also affected by their fineness, presence of impurities, color,

absorption of water and dyestuffs, thermal and environmental stability, resistance to

chemical degradation [3].

2.1.2. Man-Made Fibers

As it is apparent from their name, all fibers manufactured by man are called man-made

fibers; distinct from those which occur naturally. Man-made fibers are grouped into two

major categories: natural organic polymer fibers and synthetic organic polymer fibers [4].

Natural organic polymer fibers can be either regenerated or derivative fibers. Regenerated

fibers are formed by dissolving and extruding a natural polymer or a derivative thereof

retaining the chemical nature of the originating natural polymer

Since their commercialization in 1940, synthetic organic polymers have revolutionized

the textile industry. There are many synthetic polymers that have fiber forming ability.

However the most widely used synthetic polymers are based on polyamides, polyesters,

polyolefins... etc. [3]

2.2. General Fiber Properties

After years of research and experience, the relationship between fiber properties and

end-use performance has been roughly established for several families of synthetic fibers

facilitating the selection of the best fiber for a particular application. Fiber properties can

2.2.1. Geometric characteristics

These properties include fiber properties such as length, cross-sectional area, shape and

crimp. Fiber length uniformity, cross-sectional area of the fiber (fiber diameter) and fiber

fineness affects processing efficiency and the quality of the final product. As the fibers

become finer, the number of fibers in the cross section of a yarn will increase creating

more regularity.

Crimp describes the waviness or longitudinal shape of the fiber. Conventional textile

manufacturing equipments require some degree of fiber crimp. All natural fibers are

crimped; however synthetic fibers must be crimped artificially to be processed into spun

yarns.

It is very difficult to control the length; fineness and crimp of natural fibers and the

economic value of these fibers are mostly dependent on the uniformity of these

properties. In terms of synthetic fibers, the length can be set to almost any desired value

and their uniformity can be easily controlled. [3].

2.2.2. Physical properties

Subjected to elevated temperatures, textile fibers must have high melting or degradation

points while other fibers properties must be relatively constant over a useful temperature

In terms of electric properties, textile fibers are classified as insulators which cause static

electrification. This problem is common in fibers with low water absorption preventing

them from forming an electrically conducting system able to dissipate the static charges.

Furthermore, inter-fiber friction and geometric roughness also affect the process ability

and final product performance of synthetic fibers [3].

2.2.3. Chemical Properties

Textile fibers are required to be resistant to the effects of acids, alkalies, reducing agents,

oxidizing agents, besides electromagnetic and particulate irradiation. Due to their

chemical structure, some textile fibers are generally capable of absorbing large amounts

of moisture from atmosphere. The amount of moisture uptake has a great effect on their

electrical and mechanical properties [3].

2.2.4. Mechanical Properties

The mechanical properties of fibers can be described as the responses of a fiber to

deforming loads under conditions that induce tension, compression, torsion or bending.

The mechanical properties of fibers are usually evaluated under standard conditions of

temperature and humidity (65% rh, 21ºC). Mechanical properties can be described in

terms of strength, extensibility, stiffness, elasticity and toughness.

Fiber strength is the stress required to produce rupture in units of mass per unit cross

section. In common fiber terminology, strength is expressed as the tenacity at break or

deformation of the fiber that is produced by a given stress. Quantitatively it is defined by

the ultimate strain. The units for extensibility or strain are length-per-unit-length

expressed as percentage. Stiffness describes the resistance of the fiber to deformation.

The elastic stiffness is equivalent to the elastic modulus or Young’s modulus of elasticity

and has the units of stress-per unit-strain [3].

2.3. Properties of Polyamide-6 (Nylon-6) and Polyester (PET)

Being the main materials used this research work, Poly(ethyleneterephthalate) (PET,

Polyester) and Polyamide-6 (Nylon-6) fibers are explained in detail.

2.3.1. Nylon 6 Fibers

Nylon was the first of the synthetic fibers. Nylon’s story begins in 1928 at DuPont

Company with the hiring of Dr. Wallace H. Carothers. By 1935 the first nylon 6,6

polymers had been prepared and pilot plant production started in 1938. In 1939, the first

nylon fiber plant went into production and the first stockings went on sale in the same

year. Most production during World War II was focused on military uses, especially on

parachute fabrics. Nylon was available for domestic uses in 1946. During that time, in

1931, in Germany, a parallel development was occurred leading nylon 6. Some coarse

monofilaments were produced in 1939 with small scale production of continuous

Nylon fibers have monomer units joined by amide groups

[

]

usually prepared from diamines and dicarboxylic acids, or, in the case of

[

]

from lactams. If R and R’ are aliphatic, alicylic, or mixtures containing less than 85 wt %

aromatic moieties, the polyamides usually are referred to as Nylon. If more than 85 wt %

of the repeating units is aromatic structure, the fibers are called Aramids [6].

Nylon filaments are usually manufactured via melt spinning. The melting point of

nylon-6 is about 215 ºC [3]. Basically the molten polymer is extruded through a spinneret into a

chamber where the melt solidifies in filament form. However, in order to achieve

desirable properties in terms of molecular orientation and crystallinity, the newly formed

filaments must be drawn. Since the glass transition temperature, Tg, of nylon is below the

room temperature, nylon can be cold drawn. Nylon filaments can be drawn up to several

times their initial length. With the drawing promoting molecular orientation and hence

increase in the elastic modulus of the Nylon fibers [3].

Table 2.1 Effect of drawing on Elastic modulus of Nylon-6 fibers [3] nHN

(CH2)5

C O H NH (CH2)5 C

O

OH n Nylon-6

Draw Ratio Elastic Modulus [GPa]

1 1.97 2 2.74 3 3.70 4 4.59 5 5.77 6 6.74

Draw Ratio Elastic Modulus [GPa]

Instead of having extrusion and drawing as two separate processes, a one step high-speed

spinning can also being used. In this process, the filament windup speed is significantly

higher than the extrusion speed that orientation and crystallinity in the fibers develop in

the spin line [3].

Nylon-6 is a semi-crystalline polymer with several possible crystalline polymorphs.

Nylon-6, being a linear aliphatic polyamide, is able to crystallize because of strong

intermolecular hydrogen bonds through the amide groups and van der Waals forces

between the methylene chains [3].

The mechanical properties of nylon-6 depend on processing, drawing and the nature of

heat setting. Typical drawn nylon filaments are strong, highly resilient and sensitive to

moisture. Even though it is often thought to be a hydrophobic fiber, in practice it is

significantly hydrophilic and can absorb water within the structure. Water is able to

penetrate the amorphous regions and hydrogen bond to the amide groups. Being a good

plasticizer for nylon-6, water increases the mobility of the molecular chains and reduces

the tenacity, modulus and the Tg. [3, 5] Polyamide fibers are resistant to chemical and

microbial degradation. They are also electrical insulators. The fibers can be heat and

moisture set and they return to their set shape after deformation if the setting conditions

Table 2.2 Typical Properties of nylon fibers [3]

2.3.2. PET Fibers

Polyester fibers (poly(ethylene terephthalate) (PET)) fibers dominate the world synthetic

fibers industry. A polyester fiber is composed of any long-chain synthetic polymer

including at least 85 wt % of an ester of a dihydric alcohol (HOROH) and terephthalic

acid (p-HOOCC6H4COOH) [5, 6].

Property Continuous Staple

Filament

Tenacity at break, N/Tex

65% rh, 21ºC 0.40-0.71 0.35-0.44

Wet 0.35-0.62 0.31-0.40

Extension at break, %

65% rh, 21ºC 15-30 30-45

Wet 20-40 30-50

Elastic Modulus, N/Tex

65% rh, 21ºC 3.5 3.5

Moisture regain at 65% rh, % 4.0-4.5

Specific Gravity 1.14

Approx. volumetric swelling in water, % 2-10

Property Continuous Staple

Filament

Tenacity at break, N/Tex

65% rh, 21ºC 0.40-0.71 0.35-0.44

Wet 0.35-0.62 0.31-0.40

Extension at break, %

65% rh, 21ºC 15-30 30-45

Wet 20-40 30-50

Elastic Modulus, N/Tex

65% rh, 21ºC 3.5 3.5

Moisture regain at 65% rh, % 4.0-4.5

Specific Gravity 1.14

Approx. volumetric swelling in water, % 2-10

O CH2 CH2 O C

O

C O

n

PET monomer

O CH2 CH2 O C

O

C O

n

The free terephthalic acid, or its methyl ester, is polymerized with ethylene glycol in

vacuum by a condensation mechanism at elevated temperatures. The polymer may be

isolated and formed into chips for subsequent handling, but the current trend is toward

continuous processes where fiber formation immediately follows polymerization.

Polyester fibers are usually produced via melt-spinning. The molted polymer jets solidify

almost immediately after extrusion. Then, the filaments are drawn in order to improve

their molecular orientation and crystallinity.

As in the case of polyamide fibers, high speed spinning is beginning to replace the

traditional two-step spinning and drawing process. Similar but fully equivalent,

crystalline structures are developed in polyester by high speed spinning as well as by the

two-step process. The properties of typical polyester fibers are summarized in Table 2.3.

Table 2.3 Typical properties of Polyester fibers [3]

Property Continuous Staple

Filament

Tenacity at break, N/Tex

65% rh, 21ºC 0.35-0.53 0.31-0.44

Wet 0.35-0.53 0.31-0.44

Extension at break, %

65% rh, 21ºC 15-30 25-45

Wet 15-30 25-45

Elastic Modulus, N/Tex

65% rh, 21ºC 7.9 7.9

Moisture regain at 65% rh, % 0.4 0.4

Specific gravity 1.38 1.38

Property Continuous Staple

Filament

Tenacity at break, N/Tex

65% rh, 21ºC 0.35-0.53 0.31-0.44

Wet 0.35-0.53 0.31-0.44

Extension at break, %

65% rh, 21ºC 15-30 25-45

Wet 15-30 25-45

Elastic Modulus, N/Tex

65% rh, 21ºC 7.9 7.9

Moisture regain at 65% rh, % 0.4 0.4

The tensile stiffness or elastic modulus at low strains is much higher for drawn polyester

than for corresponding polyamides. Polyester exhibits high elastic recovery, especially

for small deformations. A very important property of polyester is that its mechanical

properties in the wet state and under standard conditions are practically the same. PET

fibers have excellent resistance to acids, alkalies and microbial attack. They also have

good resistance to light and actinic degradation. Polyester fibers have moisture regains

about 0.4% under standard conditions which results with the fibers’ high electrical

receptivity and creation of static electrification. The interactions between polyester and

interactive chemical systems can lead to depression of Tg, secondary crystallization and

loss of orientation, which have an important affect on physical and mechanical properties

[3].

2.4. Microfibers

For comparison, microfibers are half the diameter of a silk fiber, one-third the diameter of

cotton fiber, one-quarter the diameter of fine wool fiber and one hundred times finer

human hair. In order to be called a microfiber, a fiber must be less than one denier which

is the weight in grams of a 9000m length of fiber or yarn. Many microfibers are 0.5 to 0.6

denier. Besides having a luxurious body and drape, microfiber fabrics are also

lightweight resilient. They can retain their shape and resist pilling. Compared to other

fabrics of similar weight, they are relatively strong and durable. Since fine yarns can be

packed tightly together, microfiber fabrics have good wind resistance and water

repellency. As the number of filaments in a yarn of given linear density increases, the

Liquid water is prevented by surface tension from penetrating the fabric, which will have

a degree of water repellency. On the other hand, the spaces between the yarns are porous

enough to breathe and wick body moisture way from the body [6, 7].

The production of microfibers depends on the fiber fineness. For the fibers up to 7µm in

diameter, conventional melt extrusion can be used. For finer microfibers, the

islands-in-the-sea (I/S) method can be used. In the I/S method, a number of bicomponent

sheath-core polymer flows are combined into a single flow in the spinneret and extruded as a

single fiber. A similar method involves the use of two polymers with poor adhesion to

each other. After extrusion the polymer are separated and microfibers are obtained [6].

2.5. Nanofibers

In general, the definition of nano is one millionth (1/106) of a millimeter or 10-9 meter.

When the term is applied to technology, nanotechnology, the common definition is the

precise manipulation of individual atoms and molecules. For the polymeric nanofibers the

smallest practical size is approximately 50 nm as a polyester crystallite has dimensions in

the order of 40 nm so structures approaching this size would begin to become an ordered

array of atoms and would not have typical fiber morphology [8]. Similar to the nature’s

design, polymeric nanofibers and their composites can provide fundamental building

blocks for the construction of devices and structures. Drug delivery systems, scaffolds

for tissue engineering; wires, capacitors, transistors and diodes for information

fuel cells, and structural composites for aerospace applications are expected to be

impacted by the development of nanofibers [2].

2.5.1.1.Drawing

The drawing process can be considered as dry spinning at a molecular level. The process

can only be applied to viscoelastic materials that can undergo strong deformations while

remaining cohesive enough to support the stresses developed during pulling. A typical

drawing process requires a SiO2 surface; a micropipette and a micromanipulator to

produce nanofibers. However, this is a laboratory-scale process in which the nanofibers

have to be produced one by one preventing it from being scaled up to industrial level [9].

2.5.1.2.Template Synthesis

In template synthesis nanofibers are formed from specific materials within the pores of

nanoporous membranes. The membranes contain cylindrical pores with uniform

diameters that run through the complete thickness of the membranes, which is typically

on the order of 5-50mm. Each pore can be considered as a beaker in which a

nanostructure of desired material is electrochemically or chemically synthesized by a

myriad of methods and oxidative polymerization. Because these pores are cylindrical, a

nano cylinder of the desired material is obtained in each pore. Depending on the material

and the chemistry of the pore wall, the nanocylinders may be fibrils or tubules. The

template synthesis method has been used to prepare nanotubules and nanofibrils of

standard laboratory equipment. Nanofibers of different diameters can be produced with

different templates. However, it is a laboratory scale process limited to the conversion of

specific polymers directly into nanofibers structures [9].

2.5.1.3. Phase Separation

The phase separation technique is based on thermodynamic demixing of a homogenous

polymer solvent solution into a polymer-rich phase and a polymer-poor phase, usually

either by exposure of the solution to another immiscible solvent or by cooling of the

solution below a bimodal solubility curve. Thermally induced phase separation uses

thermal energy as a latent solvent to induce phase separation. The polymer solution

quenched below the freezing point of the solvent is freeze-dried to produce a porous

structure. Various porous structures including porous nano fiber matrices can easily be

obtained with this technique by adjustment of the thermodynamic and kinetic parameters

[9].

2.5.1.4.Bicomponent Extrusion

Bicomponent fibers can be defined as extruding two polymers from the same spinneret

with both polymers being contained within the same filament [10]. Some examples of

bicomponent fibers include sheath-core, eccentric, islands-in-the-sea and segmented pie

fibers. Islands-in-the-sea form fibers are also called matrix-filament fibers because in

portion. Basically, these fibers are spun from the mixture of two polymers in the required

proportion; where one polymer is suspended in droplet form in the second’s melt. An

important feature in production of matrix-fibril fibers is the necessity for artificial cooling

of the fiber immediately below the spinneret orifices. Different spinnability of the two

components would almost disable the spinnability of the mixture, except for low

concentration mixtures (less than 20%). One of the fiber components can be removed by

the use of heat, a solvent or a chemical; or using mechanical devices [10, 11].

Sheath-core Side-by-side Eccentric

Islands-in-the-sea Segmented-pie

In bicomponent extrusion two polymers are delivered to a simple spinneret hole, split by

a knife edge or septum, which channels the two components into side by side

arrangements. This same principle can also be applied to multi-ring/multi-knife edge

arrangements and to non circular arrays of holes and knife edges [11, 12].

Figure 2.2 Classical bilateral bicomponent spinning (A, B: Polymers) [12]

The pipe in pipe method is one of most used methods to manufacture bicomponent fibers.

As it is seen from the Figure 2.3 one of the component streams, A, envelopes the other

component stream, B, at the end of the tube containing the inner stream.

The islands-in-the-sea form fibers, used for this project, were produced with bicomponent

extrusion technique and provided by Hills Inc. (Melbourne, FL).

2.5.1.5. Self-Assembly

Self assembly is the autonomous organization of components into patterns or structures

without human intervention. Self assembly processes are common throughout nature and

technology and they involve components from the molecular (crystals) to the planetary

(weather systems) scale and many different kinds of interactions. The concept of self

assembly is used increasingly in many disciplines with a different flavor and emphasis in

each field. The process requires standard laboratory equipment. However, it is

laboratory-scale process limited to the conversion of specific polymers directly into nanofibers

structures [9].

2.5.1.6. Electrospinning

Electrospinning is a process that creates nanofibers from an electrically charged jet of

Figure 2.4 Schematic figure of Elecrtospinnig process [13]

The electrospinning process, in its simplest form, consists of a pipette or a syringe to hold

the polymer solution, a collector, a DC voltage supply with one electrode attached to the

collector and the other to the syringe. The charged jet is ejected from the tip at a critical

voltage when the repulsive electrostatic force overcomes the surface tension. The charged

causes the jet to bend in such a way that every time the polymer jet loops, its diameter is

reduced. The solvent in the polymer jet evaporates and the jet diameter is reduced to

nano-dimensions before the jet reaches the collector. The polymer jet finally solidifies

and the fiber is collected as a web of fibers on the surface of a collector. The diameters of

fibers electrospun from polymer melts are larger than the electrospun fibers from polymer

solutions. The process is simple and cost effective. A large variety of polymers can be

50 nm although the collected web usually contains fibers with varying diameters from 50

nm to2μ. However, since the production of this process is very slow, measured in grams

per hour, the cost of nanofibers production is very high to be used as an industrial

production technique [8].

2.5.1.7. Other Techniques

Another technique to produce polymeric nanofibers has recently been introduced by

Nanofiber Technology Inc, NC. They created nanofibers by melt blowing a fiber with a

modular die. The produced fibers are a mixture of both micron and submicron sizes. This

technique lends to use of thermoplastic fibers in a relatively inexpensive spinning

process. This technique appears to have the potential to make larger quantities of

polymeric nanofibers with lesser costs. However there are still concerns, such as the

broad range of fiber diameter, which is also, can be an advantage for some applications

and the cost of the spinning versus the production rate. Despite these concerns, this

technique can take the nanofiber production form laboratories to commercial futures [15,

2.5.2. Applications of nanofibers

Polymeric nanofibers are finding uses in filtration, biomedical applications, protective

clothing, sensors, and reinforcement for composite materials. Some other applications are

solar sails, light sails and mirrors for use in space. Some of the most popular applications

of nanofibers are explained below.

2.5.2.1. Filters

Freudenberg Nonwovens, has been producing electrospun filter media from a continuous

web feed for ultra high efficiency filtration markets for more than 20 years [17].

Filtration efficiency or capture efficiency of filter media has been shown to be inversely

proportional to the diameters of the fibers in filters. Because of the very high surface

area-to-volume ratio and the resulting high surface cohesion of nano fibers, particles on

the order of less than 0.5micrometer are easily trapped in the nano fiber mats.

Electrospun nano fibers on substrates such as glass, polyester and nylon have also proved

to enhance the life of filters in pulse-clean cartridges for dust collection and increase the

efficiency of filters used in cabin air filtration of mining vehicles. Polymer nanofiber

mats can also be electrostatically charged to provide them with the ability to capture

particles by electrostatic attraction without an increase in pressure drop, leading to an

Figure 2.5 Fractional efficiency (Filtration Efficiency vs. particle size) for a standard cellulose media and nanofiber filter media [22].

2.5.2.2. Biomedical Applications

From the biological viewpoint almost all of human tissues and organs are deposited in

nanofibrous forms or structures. Examples include bone, dentin, collagen, cartilage, and

skin. All of them are characterized by well organized hierarchical fibrous structures

re-aligning in nanometer scale. Because of this analogous behavior, it can be seen that

nanofiber webs have a promising potential in various biomedical areas [17].

For example, tissue engineering requires the design of ideal scaffolds from synthetic or

natural materials to provide temporary templates for cell seeding, invasion, proliferation

and differentiation, resulting in regeneration of biologically functional tissue. Mats made

of nano fibers from biodegradable polymers may be helpful in adjusting the degradation

theorized that cells attach and organize well around fibers with diameters smaller than the

diameter of the cells. Hence, researchers have tried to convert biopolymers into nano

fiber mats that mimic biological structures [18 - 21].

It was also shown that electrospun biocompatible polymer nanofibers can also be

deposited as a thin porous film onto a hard tissue prosthetic device designed to be

implanted into the human body. This coating film with gradient fibrous structure works

as an interphase between the prosthetic device and the host tissues, and is expected to

efficiently reduce the stiffness mismatch at the tissue/device interphase and hence prevent

the device failure after the implantation [17].

As another biomedical application, polymer nanofibers can be used for the treatment of

wounds or burns of a human skin, as well as designed for haemostatic devices with some

unique characteristics. With the aid of electric field, fine fibers of biodegradable

polymers can be directly sprayed/spun onto the injured location of skin to form a fibrous

mat dressing, which can let wounds heal by encouraging the formation of normal skin

growth and eliminate the formation of scar tissue which would occur in a traditional

treatment [17].

Drug delivery with nanofiber capsules is another promising biomedical application of

nanofibers. It is based on the principle that dissolution rate of a particulate drug increases

2.5.2.3. Protective Clothing

Because of their great surface area, nanofiber fabrics are capable of the neutralization of

chemical agents and without impedance of the air and water vapor permeability to the

clothing. Preliminary investigations have indicated that compared to conventional textiles

the electrospun nanofibers present both minimal impedance to moisture vapor diffusion

and extremely efficiency in trapping aerosol particles, as well as show strong promises as

ideal protective clothing [17, 23, 24]. Researchers, developing polymer nanofibers for

various protective clothing applications, have found that compared with conventional

textiles, electrospun nanofibers mats provide minimum impedance to moisture vapor

diffusion and maximum efficiency in trapping aerosol particles [18 - 21].

2.5.2.4. Reinforcement for Composite Materials

The majority of work in the current literature on nanofiber composites is concerned with

carbon nanofiber or nanotube reinforcements. Publications on polymeric

nanofiber-reinforced composite materials are quite limited. Kim and Reneker investigated the

reinforcing effects of nanofibers in an epoxy and in a rubber matrix using electrospun

nanofibers of PBI (polybenzimidazole). They observed that nanofiber reinforcement

toughened the brittle epoxy matrix and the composite also showed better performance in

terms of fracture toughness and modulus than the composites reinforced with

whisker-like particles. Nanofiber reinforcement improved the Young’s modulus of the rubber

matrix, as well [17, 25]. It may be too early to conclude that polymer nanofibers provide

higher surface-to-volume ratio may improve the inter-laminar toughness and interfacial

adhesion in nanofiber-reinforced composites [18 - 21].

2.5.2.5. Sensors

Results of studies on sensors indicate that the sensitivities of nanofiber films to detect

ferric and mercury ions, and a nitro compound are two to three orders of magnitude

higher than that obtained from thin film sensors [18 - 21, 26]. Polymeric nanofibers could

also be used in developing functional sensors with the high surface area of nanofibers to

facilitate the sensitivity. Poly(lactic acid co glycolic acid) (PLAGA) nanofiber films were

employed as a new sensing interface for developing chemical and biochemical sensor

applications [22, 27].

2.5.3. Analytical Techniques

The techniques mostly used to analyze nanofibers are Scanning Electron Microscopy

(SEM), Transmission Electron Microscopy (TEM) and Atomic Force Microscopy

(AFM).

2.5.3.1. Scanning Electron Microscopy (SEM)

SEM technique is mostly used to observe morphological, structural and surface properties

of nanofibers. On some of the recent studies, nanofibers were produced by using

synthesis [32, 33]. The produced nanofibers were analyzed in terms of their diameters

[26, 28 - 33], lengths [32], and surface properties [24, 26, 28 – 30].

Silva et al. prepared electrospun nanofibers with different quantities of a colloidal

dispersion of graphite particles, blended with polyacrylonitrile (PAN) and N,N

dimethylfromamide (DMF). They prepared a series of solutions with carbon

concentrations ranging from 0 to 25%. By using SEM they observed that the electrospun

fibers have an irregular shape, and the variations in the diameter of their smooth sections

decrease with the increase of the carbon concentration in the blend [26].

Dharmaraj et al. prepared Nickel titanate/poly (vinyl acetate) composite nanofibers by

sol-gel processing and electrospinning and they observed the structural and

morphological properties with SEM. It was seen that the composite nanofibers have

cylindrical diameters and smooth surfaces due to the amorphous nature of PVAc and

nickel titanate composites. After calcinations, fibers kept their cylindrical shapes with a

a) b)

Figure 2.6 SEM images of nickel titanate fibers: (a) as-prepared composite fibers, (b) fibers calcinated at 1273 K [28]

Viswanathamurthi et. al. produced vanadium pentoxide (V2O5) nanofibers with

electrospinning and determined the fiber microstructure using SEM. It was observed from

the images that the fibers smooth, uniform surfaces and uniform diameter in whole length

[29].

Gibson et. al. applied electrospun nanofibers coatings directly to an open cell

polyurethane foam. Then they stretched the nanofiber membranes in biaxial tension to

strain levels of 100%. Using Environmental SEM it was confirmed that elastomeric

nanofiber membranes are deformed and when elastic fibers were under an increasing

tension while inter-fiber pore space increased [24].

a) b)

Figure 2.8 SEM images of elastomeric nanofiber membranes under two different levels of biaxial strain a) 100%, b) 0 % [24]

Yu et al. prepared poly(vinyl alcohol (PVA))/ lithium chloride / manganese acetate

composite nanofibers through sol-gel processing and electrospinning techniques. SEM

observations showed that due to the amorphous nature of the nanofibers, they have

Figure 2.9 SEM photograph of PVA/lithium chloride/manganese acetate composite fiber samples [30]

Liu et. al., self-assembled polyphenylene dendrimer, in different organic solvents, into

nanofibers, on various substrates, upon drop-casting under a saturated solvent

atmosphere. The investigation of fiber morphology with SEM showed that the

morphology was dependent on the substrate, the solvent and the preparation method [31].

He Y. synthesized polyaniline nanofibers by a methylene chloride/water emulsion and

used CeO2 nanoparticles as stabilizer. The fiber diameter and length were measured with

SEM and it was shown that the polyaniline nanofibers had an average diameter of 65nm

and an average length of 2μm; and polyaniline/CeO2 composite nanofibers had an

a) b)

Figure 2.10 SEM images of a) polyaniline nanofibers b) polyaniline nanofibers and polyaniline/CeO2 composite microspheres [32]

King et al. synthesized polyaniline nanofibers with two different methods: via a template

free procedure and via using small amount of carbon nanotubes as a seed template. The

morphology and diameter of the nanotubes were observed with SEM. It was observed

that the diameter of the nanofibers obtained with the first method have diameters ranging

from 38nm to 76nm and the nanofibers obtained with the second method have diameters

ranging from 67nm to 87nm, these fibers also showed needle-like morphology [33].

Zhang et. al. produced poly(caprolactone) (PCL) nanofibers with and without

collagen-coating by coaxial electrospinning technique and observed the fiber diameters with SEM.

The data showed that the coated and non-coated nanofibers have diameters 318±131nm

2.5.3.2. Transmission Electron Microscopy (TEM)

TEM is also used to observe morphological and structural properties of nanofibers. On

some of the recent studies, nanofibers were produced by using electrospinning [34, 35,

37], sol-gel [39], self-assembly [36, 38], and template synthesis [32]. The produced

nanofibers were analyzed in terms of their diameters [36, 37, 39], lengths [36, 39], and

morphologies [32, 34, 35, 38].

He Y. synthesized polyaniline nanofibers by a methylene chloride/water emulsion and

used CeO2 nanoparticles as stabilizer. He used TEM to prove that the synthesized

polyaniline structures were not nanotubes, but nanofibers [32].

Figure 2.11 TEM image of polyaniline nanofibers [32]

Dror et. al. produced poly(ethylene oxide) (PEO) nanofibers via electrospinning and

that the embedded nanotubes were mostly aligned along the fiber axis however they also

showed some twisted or bent structures in the nanofibers [35].

Figure 2.12 TEM images of a) Twisted nantubes; b) and c) Aligned, nanotubes in PEO nanofibers [35]

Matsumura et. al. fabricated peptide nanofibers using the self assembly method. Fiber

morphology was observed with TEM and it was shown that homogenous straight

nanofibers were produced with 80-130nm in diameter and 10µm length [36].

Zhang et. al. produced collagen-coated poly(caprolactone) (PCL) nanofibers by coaxial

electrospinning technique. The morphology of the nanofibers was observed with TEM

Figure 2.13 TEM image of an individual Collagen-r-PCL composite nanofiber (a) collagen as the shell material, and PCL the support (b) is the TEM image of a pure

PCL nanofiber [34]

Dersch et. al. prepared electrospun nanofibers from polyamide-6 and polylactide with a

diameter of about 50nm. TEM images shown that for the polymaode-6 nanofibers; the

fiber diameter along the fiber axis was almost constant. However PLA fibers did not

a) b)

Figure 2.14 Transmission electron micrographs of a) PA6 fiber, a segment almost constant in diameter b) PLA

Liu G., prepared nanofibers from diblock poly(2-cinnamoylethylmethacrylacts)

(PCEMA) by self-assembly technique. Morphology of the prepared nanofibers was

observed with TEM and it was shown that the diblock polymer nanofibers had a

core-shell structure [38].

Liu et. al. prepared hydroxyapatite nanofibers by using calcium chloride and sodium

phosphate, separately. By the help of TEM, they showed that nanofiber diameters and

2.5.3.3. ATOMIC FORCE MICROSCOPY (AFM)

AFM can provide both imaging and mechanical property determination. Studies, done

with AFM have mostly focused on either assessing the surface properties of small

materials [28, 29, 31, 36, 40] or determining their mechanical properties such as elastic

modulus and hardness [41- 50].

Wei et. al. modified the polyamide-6 nanofiber surfaces, which were prepared by

electrospinning, with cold gas plasma treatment. Using a Topometrix TMX 2000

Explorer (TM Microscopes) they observed the changes on the fibers surfaces. Surface

roughness of the fibers was found to be increased after plasma treatment [40].

a) b)

Dharmaraj et al. prepared Nickel titanate/poly (vinyl acetate) composite nanofibers by

sol-gel processing and electrospinning and they observed the fiber morphology with an

AFM from Nanoscope(R)-III A. They verified the data obtained with SEM. It was found

that the fibers have cylindrical structures with diameters changing between 150 and

200nm [28].

Viswanathamurthi et. al. after producing vanadium pentoxide nanofibers via

electrospinning they studied the surface topography of the fibers with AFM (XE-100,

Psia Co.). AFM data demonstrated that the produced fibers are homogenous, smooth and

uniform [29].

Liu et. al. produced polyphenylene dendrimer nanofibers by self-assembly method, in

different organic solvents on various substrates, upon drop-casting under a saturated

solvent atmosphere. Using a Discoverer TMX2010 AFM system (ThermoMicroscopes,

San Francisco, CA) and operating it in non-contact mode and using Si probes

(ThermoMicroscopes, San Francisco, CA) with a spring constant of 34-47 N/m and a

resonance frequency of 174-191 kHz; they verified the data obtained with SEM and

showed that the fiber lengths were tens of micrometers and diameters were varying

a) b) c)

Figure 2.16 a) NCAFM image (25 ím _ 25 ím) of dendrimer 1 nanofibers prepared by drop-casting a 2.0 _ 10-6 M dendrimer 1 solution in THF on a silicon surface in a

saturated environment of THF, b) NCAFM image (25 ím _ 25 ím) of dendrimer 1 nanofibers prepared by drop-casting a 2.0 _ 10-6 M dendrimer 1 solution in THF in a

saturated environment of THF:H2O ) 90:10 (v/v) on a silicon surface, c) NCAFM image (50 ím _ 50 ím) of dendrimer 1 nanofibers prepared by drop-casting a 2.0 _ 10-6 M dendrimer 1 solution in THF in a saturated environment of THF:H2O ) 80:20 (v/v)

on a silicon surface [31]

Matsumura et. al. fabricated peptide nanofibers by self assembly method. Fiber

morphology was observed with AFM to verify the TEM data and it was shown that

homogenous straight nanofibers were produced with 80-130nm in diameter and 10

microns in length [36].

Lee et. al. synthesized TiO2 and TiO2/PVP nanocomposite nanofibers on porous supports

via sol-gel chemistry and electrospinning. Elastic modulus of the composite nanofibers

were determined via 3 Point bending tests using a Dimension 3100AFM from Digital

Figure 2.17 a) Schematic diagram of three-point bending test and b) actual AFM scanning data on fiber (i) and pore (ii) [41]

It was found that the mean elastic moduli of TiO2 and TiO2/PVP nanofibers have elastic

moduli 75.6 and 0.9 GPa, respectively [41].

Figure 2.18 Young’s modulus (E) vs. diameter of TiO2–PVP and TiO2 nanofibers [41]

Li et. al. indented silver nanowires and Cu2O nanocubes by using a Hysitron Triboscope

nanoindenter in conjunction with a Veeco Dimension 3100 and they measured the

Figure 2.19 (a and b) AFM images of indents on a silver nanowire (c) height profile of an indent on the wire, and (d) a representative nanoindentation load-displacement

Figure 2.20 (a) Indentation load-displacement curves made on a solid Cu2O nanocube and (b) a hollow Cu2O nanocube [43]

It was found the hardness and the elastic modulus of silver nanowires and Cu2O

nanocubes; 0.87±0.4 and 88±5 GPa, and 0.61±0.2 (for the hollow cubes) and 82±12

GPa (for the solid cubes), respectively [42, 43].

Bellan et. al. determined the Young’s moduli of individual PEO nanofibers using a

Dimension 3000 from Digital Instruments, operated in contact mode with tips having

0.58N/m spring constant. Three Point bending test results showed that PEO nanofibers

have an average Young’s modulus of 7±0.5GPa, which is higher than the published film

Figure 2.21 Schematic of AFM tip depressing suspended nanofiber [44]

Figure 2.22 (a) Average Young’s modulus versus diameter for several PEO nanofibres produced by electrospinning. (b) Average Young’s modulus versus diameter for several

polysiloxane and glass nanofibres produced by electrospinning [44]

Tan et. al. determined the elastic modulus of a single PLLA nanofibers, extracted from a

nanofibrous scaffold via Nanoscope IIIa from Digital Instruments The used cantilevers

had a spring constant of 0.15N/m and the maximum load applied was 15nN. Three point

bending test results showed that single PLLA nanofibers with diameters less than 350 nm

typically have an elastic modulus value of 1.0±0.2 GPa. However this value tends to

Figure 2.23 Nanofibers suspended over etched grooves of silicon wafer: a) Electron micrograph of PLLA nanofibers deposited onto the silicon wafer; b) AFM contact mode image of a single nanofiber (300 nm diameter) suspended

over an etched groove; c) schematic diagram of a nanofiber with mid-span deflected by an AFM tip [45]

They also measured the elastic modulus of single PLLA nanofibers, produced by phase

separation method, by indentation tests with the AFM tip. A Dimension 3100 from

Digital Instruments and cantilevers having spring constants 0.7 to 8 N/m were used by

applying a maximum load of 40-100nN on the fiber surface. The elastic modulus value

was found to be 1.0 GPa which is in good agreement with the 3 point bending test results

Figure 2.24 Variation of elastic modulus with fiber diameter for nanoindentation of PLLA nanofiber [46]

Sugawara et. al. determined the Young’s modulus of guinea pig outer hair cells by

conducting AFM indentation experiments. The AFM studies were performed with a

commercial instrument (NVB100, Olympus. A V-shaped silicon nitride cantilever

(OMCL-TR400PSA-2, Olympus) with a spring constant of 0.09 N/m was used. The

typical radius of curvature of the cantilever tip was less than 20 nm. Hertzian modal was

used to calculate the Young’s modulus values and it was found that that it decreases with

increase in cell length [47].

Reynaud et. al. studied on the determination of elastic modulus of a biphase system

composed of polymethylmethacrylate (PMMA) and polyacrylate. AFM indentation tests

were done with Nanoscope III from Digital Instruments. Silicon, rectangular-shaped

microfabricated cantilevers (Nanosensors) were used for indentation experiment with a

resonance frequency of 276.2 kHz and stiffness of 31.98±3:15 N/m by applying a

biphase system is weaker than the value for pure PMMA. The reason for this was the thin

acrylate layer coated the PMMA surface during sample preparation [48].

Salvetat et. al. measured the elastic and shear moduli of single-walled carbon nanotubes

(SWCN) ropes using AFM with Si3N4 cantilevers having force constants of 0.05 and 0.1

N/m operated in the contact mode; and conducting 3 point bending tests.

Figure 2.25 (a) AFM image of a SWCT rope adhered to the polished alumina ultrafiltration membrane, with a portion bridging a pore of the membrane. (b) Schematic of the measurement: the AFM is used to apply a load to the nanobeam and

to determine directly the resulting deflection. A closed loop feedback ensured an accurate scanner positioning. Si3N4 cantilevers with force constants of 0.05 and 0.1

N/m were used as tips in the contact mode [49]

Elastic and shear moduli were found to be 1 TPa and 1 GPa, respectively. Large ropes

showed lower moduli and this was explained with containing more imperfections than

Figure 2.26 Measured reduced modulus, Er , for ten different SWNT ropes with diameters between 3 and 20 nm (circles). The points corresponding to different ropes of

equal diameters have been shifted by 60.2 nm for better legibility. (Shear modulus for large ropes (D > 4 nm) extracted for the experimental data by assuming E =600 GPa

[49]

Stark et. al. indented single aerogel powder particles with two different kinds of AFM

cantilevers; soft and stiff, with spring constants 0.2N/m and 54N/m, respectively. After

analyzing the data with Hertz model, they found out that the data obtained from both

cantilevers are in good convenience. The calculated elastic modulus values were found to

3. EXPERIMENTAL APPROACH

3.1. Materials

Polyester (PET) and Polyamide-6 (Nylon-6) nanofibers were obtained from Hills Inc

(Melbourne, FL). The nanofibers were produced via bicomponent extrusion using

islands-in-the-sea (I/S) spinning blocks. PET and Nylon-6 were the islands and low

molecular weight polyethylene (PE) and ethylene vinyl alcohol (Evoh) were the sea

components, respectively. A PET film of known elastic modulus was obtained from

Goodfellows (Boston, MA). 200 and 265 denier PET fibers were obtained from Goulston

Technologies (Monroe, NC).

ACS grade toluene from Sigma Aldrich (Milwaukee, WI) and solution of a nonionic

surfactant -Triton (TM) X – 200 - from SPI Supplies (West Chester, PA) were used for

sample preparation.

Custom-made TEM grids, provided by Protochips Inc (Raleigh, NC), were used to

prepare the samples for AFM analysis. The Protochips DuraSiNTM Film provided a

durable, non-organic, low scattering substrate for quantitative TEM and X-ray analysis.

DuraSiNTM Film substrates were fabricated from high quality, low-stress silicon nitride

and supported on a rigid silicon substrate. The DuraSiNTM Mesh is robust to most

cleaning procedures, including acetone, alcohol and oxygen plasma/UV ozone, enabling

three grids were used as shown in Figure 3.1 The first and the third grids have square

holes of 0.5mm in size. The grid in the middle has circular holes of 2 micrometers in size.

Figure 3.1 Custom-made TEM grids

Tapping mode, aluminum reflex coated BS-Tap300Al AFM tips from Budget Sensors

(Sofia, Bulgaria) were used for all AFM imaging and elastic indentation experiments.

An ultra sharp calibration grating (TGT01) from NT-MDT Devices (Moscow, Russia)

Figure 3.2 SEM image of the calibration grating used for tip radius determination

3.2. Instruments

3.2.1. Focused Ion Beam (FIB)

A Hitachi FB – 2100 (Japan) FIB system with 150 μm aperture, 1500 picoamps beam

current and 40 kV acceleration voltage was used for cross-sectioning the islands in the

The system is similar to that of SEM, the major difference is it uses a gallium ion (Ga+)

beam instead of an electron beam. The ion beam is generated in a liquid-metal ion source

(LMIS), and the application of a strong electric field causes emission of positively

charged ions from a liquid gallium cone, which is formed on the tip of a tungsten needle.

As illustrated in the Figure 3.3, modern FIB systems involve the transmission of a

parallel beam between two lenses. The beam is raster-scanned over the sample, which is

mounted in a vacuum chamber at pressures of around 10-7mbar. When the beam strikes

the sample, secondary electrons, secondary ions and neutral atoms are emitted from its

surface. The electron or ion intensity is monitored and used to generate an image of the

surface. Secondary electrons are generated in much greater quantities than ions and

provide images of better quality and resolution; consequently the secondary electron

mode is used for most imaging applications. Ion beams can also be used to remove

material from the surface of the sample. This process, called milling, which is a major

advantage of FIB as much of the constructional analysis and failure analysis of

semi-conductor devices is performed on cross-sections [60].

3.2.2. Scanning Electron Microscopy (SEM)

A Hitachi S3200N (Japan) SEM was used at 5 kV voltage. Prior to observations all

samples were coated with gold and palladium. SEM images were grabbed by using

Pinnocle Studio software. The Hitachi S3200 is a Variable Pessure Scanning Electron

Microscope. This is a conventional high resolution thermionic SEM which allows the