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.
In this thesis, we aimed to develop a method to analyze mechanical properties of individual micro and nanofibers using a technique called Atomic Force Microscopy
(AFM). After finding a useful approach to prepare individual islands-in-the-sea form fibers for the further analysis, we also set up AFM and developed an experimental approach to examine individual PET and Nylon-6 micro and nanofibers’ mechanical properties.
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
Dr. Orlando Rojas Dr. Phillip Russell Member of Advisory Committee Member of Advisory Committee BIOGRAPHY
Bilge Hatiboglu was born on July 20th, 1981 in Merzifon, Turkey. On completing her elementary and high school study in Merzifon, Bilge entered college at Istanbul
Technical University, Istanbul, Turkey, in the year 1999 to proceed towards her
Bachelor’s degree in textile engineering. She graduated with distinction in 2004 and, upon graduation she entered the Master of Science program in Textile Engineering, at
North Carolina State University in 2004. While she was working on her masters program, she worked as a research assistant under Dr. Behnam Pourdeyhimi and Dr. Juan P.
Hinestroza on a project funded by Nonwovens Corporative Research Center.
ii 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…
Without all of you, this degree would not be possible.
Thank you all once more…
iii 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
2.5.1.4. Bicomponent Extrusion…………………………...…….15
iv 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
3.3.1. First Approach…………………………………………………….60
3.3.2. Second Approach…………………………………………………61
v 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
7. APPENDIX………………………………………………………………………….115
vi 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 constants because of the size, RF and QF………………………………………..81
vii 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 2.15 AFM images of PA6 nanofibres a) regular fiber b) plasma treated fiber (for 60 seconds) [40]……………………………………………………….35
viii
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 of the membrane. (b) Schematic of the measurement [49]………………………45
ix 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 4.7 3D images of a) PET and b) Nylon6 nanofibers……………………………..75
x 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 7.7 Raw indentation Curve of PET microfiber (φ = 300 ± 20nm )……………...119
Figure 7.8 Raw indentation Curve of PET microfiber (φ = 100 ± 7nm )……………….119
xi
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 Microfiber (φ = 100 ± 7nm )………………………………….…………………128
xii 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 provides a topographic contrast as well as direct height measurements. AFM images are obtained without expensive sample preparation and yield more information than those
1 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 measurements of mechanical properties of micro/nano-sized materials.
2 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 are of finite length and are used directly in textile manufacturing after preliminary cleaning. These fibers are commonly referred to as staple fibers. The utility of natural
3 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 be classified as geometrical, physical, chemical and mechanical [1].
4 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 range. Usually textile fibers are opaque and they have significantly different refractive indexes than those of their surrounding environment.
5 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 ultimate tenacity in units of N/tex or gram-force per denier. Extensibility describes the
6 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 filament in 1940 and a larger scale production a year later [1, 5, 6].
7 Nylon fibers have monomer units joined by amide groups [CONHRNHCOR']n and are
usually prepared from diamines and dicarboxylic acids, or, in the case of []RCONH n , 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].
(CH2)5 O
nHN C O H NH (CH2)5 C OH n
Nylon-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]
Draw Ratio Elastic Modulus [GPa] 11.97
22.74
33.70 44.59 55.77 66.74
8 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 have not been exceeded [3].
9 Table 2.2 Typical Properties of nylon fibers [3]
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
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].
O O
O CH2 CH2 O C C n
PET monomer
10 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
Approx. volumetric swelling in water, % none none
11 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
surface area of all the fibers increases and the spaces between the fibers get smaller.
12 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 technology; systems for energy transport; conversion and storage such as batteries and
13 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 polymers, metals, semiconductors and carbons. The process is simple and requires
14 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 cross section, they appear as one polymer is inserted into a matrix of a second polymer.
Islands-in-the-sea fibers may have a uniform or nonuniform diameter of the island
15 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
Figure 2.1 Cross-sections of bicomponent fibers
16 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.
Figure 2.3 Pipe-in-pipe mixers [12]
17 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 polymer solutions or polymer melts.
18
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 electrospun, long continuous nanofibers can be produced and the production of aligned nanofibers is feasible. [9]. This process can produce nanofibers with diameters as low as
19 50 nm although the collected web usually contains fibers with varying diameters from 50 nm to 2μ . 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,
16].
20 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 improvement in filtration efficiency [18 - 21].
21
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 rate of a specified biomaterial in the in vivo environment. Furthermore, it has been
22 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 with increasing surface area of both the drug and the corresponding carrier if needed [17].
23 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 better reinforcement than conventional glass and carbon microfibers. However, their
24 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 electrospinning [24, 26, 28 - 30], sol-gel [24, 28 - 30], self-assembly [31], and template
25 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 decrease in diameter [28].
26
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].
Figure 2.7 SEM images of the V2O5 fibers [29]
27 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 smooth surfaces; and diameters varying between 100-200nm [30].
28
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 2 μ m; and polyaniline/CeO2 composite nanofibers had an average length of 1.3 μ m [32].
29
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 and 216 ± 72nm, respectively [34].
30 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 embedded multiwalled carbon nanotubes in them. With the help of TEM, they showed
31 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 and it was found out that fibers formed core-shell structure [34].
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]
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 show a smooth but a specific surface topology [37].
33
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 lengths are changing between 5 to 8 nm and 160 to 220 nm, respectively [39].
34 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) Figure 2.15 AFM images of PA6 nanofiber a) regular fiber b) plasma treated fiber (For 60 seconds) [40]
35 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 between 10 to 200 μ m [31].
36
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
Instruments by applying a maximum load of 6nN.
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]
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].
38
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 hardness and the elastic modulus of the silver nanowires and Cu2O nanocubes.
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]
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 and bulk material values which ranges 0.2-5GPa for films and 0.29GPa for highly crystalline bulk materials [44].
41
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 decrease as the fibers diameter increases [45].
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 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
[46].
43
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 maximum load of 479 nN. Results showed that the elastic modulus of PMMA in the
44 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 small ropes [49].
45
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 be 5.4 GPa for soft cantilevers and 6.8 GPa for stiff cantilevers [50].
46 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 the removal of organic residues on a highly electron transparent substrate. Another advantage of the grids is they can be used for TEM, SEM and AFM. For this application
47 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) was used for tip imaging.
48
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 μ m aperture, 1500 picoamps beam current and 40 kV acceleration voltage was used for cross-sectioning the islands in the sea samples and imaging the cross-sections prior to AFM cross-section imaging.
Figure 3.3 Schematic figure of FIB [60]
49 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 operator to control the specimen chamber vacuum level and environment. A non-
50 conductive specimen maybe inserted directly into the VPSEM and observed in its natural state without the need for metallized coatings.
SEM is developed mainly because of the limitations of optical microscopy. Instead of using visible light as optical microscopes do, SEM uses electrons. To summarize the working principle of SEM, it can be said that, in modern SEM instruments electrons are obtained from either a thermal or field-emitting cathode. A beam from these electrons is formed by the electrostatic lens of the Wehnelt-cap and is sharpened usually by two condensers and one objective electromagnetic lens. The electron beam is scanned by electromagnetic coils placed in the back-focal plane of the objective lens, and in most cases the image signal is collected by an Everhart-Thornley secondary electron detector.
The secondary electrons provide an image with the best possible resolution in SEM.
Electrons cannot move very far in air and are easily deflected so it is necessary to remove the air from inside the microscope and form a vacuum. Because of the contrast mechanism and the large depth of field, SEM images look like a 3D image, in spite of the fact that it is two dimensional [48].
51
Figure 3.4 Schematic Diagram of an SEM [49]
3.2.3. Dynamic Mechanical Analyzer (DMA)
A Q 800 with Thermal Advantage for Q Series™ software from TA Instruments
(Ypsilanti, MI) was used for DMA experiments with PET film.
Dynamic Mechanical Analysis is a technique that used to characterize materials. It is mostly used to determine the viscoelastic properties of polymers. DMA can be described as applying an oscillating force to a sample and measuring the resulting displacement of the sample i.e. the response of the sample to the applied force. In this way sample stiffness can determined and sample modulus can be calculated: By measuring both the amplitude of the deformation at the peak of the sine wave and the lag between the stress and the strain sine waves, quantities like the modulus, the viscosity and the damping can be calculated.
52
Figure 3.5 Schematic of how DMA works [49]
(The DMA supplies an oscillatory force, causing a sinusoidal stress to be applied to the sample, which generates a sinusoidal strain. By measuring both the amplitude of the deformation at the peak of the sine weave and the lag between the stress strain sine weaves, quantities like modulus, the viscosity and the damping can be calculated. )
53 3.2.4. Atomic Force Microscopy (AFM)
A Dimension 3000 AFM from Digital Instruments, Veeco (Fremont, CA) was used for all AFM imaging and indentation experiments. Nanoscope©III software was used with the instrument.
Being one of the family members of Scanning Probe Microscopes (SPMs), Atomic Force
Microscope (AFM) developed in 1986 by Binning, Quate and Gerber as collaboration between IBM and Stanford University. AFM relies on the use of a sharp, pyramidal tip mounted on a cantilever, which is brought into close proximity to the surface where intermolecular forces acting between the tip and the surface cause the cantilever to bend.
Images of the surface are obtained by recording the cantilever deflections, as detected by a laser beam focused on the top of the cantilever, as the sample is scanned. Variation of the surface height varies the force acting on the tip and therefore varies the bending of the cantilever. Besides taking images of various surfaces, it is also possible to investigate mechanical properties of many materials. There are three primary modes of AFM:
Contact mode, Non-Contact Mode, Tapping Mode.
In contact mode AFM, a tip attached to the end of a cantilever is scanned across the sample surface while the change in the cantilever deflection is monitored with a photodiode detector. The tip contacts the surface through the absorbed fluid layer on the sample surface [47]. A feed back loop maintains a constant deflection between the cantilever and the sample by vertically moving the scanner. The force between the tip and
54 the sample is kept constant by maintaining a constant cantilever deflection. The force is calculated from Hooke’s law:
Fkx=− (3.1) where, F: Force, k: Spring Constant, x: cantilever deflection.
Force constants usually range from 0.01 to 1.0N/m , resulting in forces ranging from nN to μ N. Operation can take place in ambient and liquid environments.
In tapping mode AFM, different from contact mode, the tip slightly taps on the sample surface during scanning, contacting the surface at the bottom of its swing. In tapping mode cantilever is oscillated at or slightly below its resonant frequency with an amplitude ranging typically from 20nm to 100nm. The feedback loop maintains a constant oscillation amplitude by maintaining a constant RMS of the oscillation signal acquired by the photodiode detector.
In Non-Contact mode AFM, the cantilever is oscillated at a frequency slightly above the cantilevers’ resonance frequency typically with an amplitude of a few nanometers
(<10nm) to obtain an AC signal from the cantilever. During scanning, instead of contacting the sample surface, the tip oscillates above the adsorbed fluid layer on the surface.
55
Figure 3.6 Essential elements of AFM [59]
Probes are one of the most important parts of AFM. Mostly AFM probes are either silicon or silicon nitride. Silicon probes are primarily used for tapping mode applications.
The tip and the cantilever are integrated an assembly of single crystal silicon, produced by etching technique. There is only one cantilever and one tip, integrated with each substrate. These probes are usually much stiffer than the silicon nitride probes and so they have larger force constants and resonant frequencies. On the other hand, silicon nitride probes are usually used for contact mode applications. To use AFM in contact mode, it is necessary to have a cantilever which is soft enough to be deflected by very small forces and has a high enough resonant frequency to not be susceptible to vibrational instabilities. This is accomplished by making the cantilever short, to provide a high resonant frequency, and thin, to provide a small force constant. [47]
56 Besides taking topographic images, AFM provides information about mechanical and chemical properties of the materials. It can also record the amount of force felt by the cantilever as the probe tip is brought close to or indented into a sample surface and then pulled away. By measuring the long range attractive or repulsive forces between the probe tip and the sample surface, local chemical and mechanical properties like adhesion and elasticity, and even thickness of adsorbed molecular layers or bond rupture lengths can be measured. When AFM is used in the force calibration, the X and Y voltages applies to the piezo tube are held at zero and a sawtooth voltage is applied to the Z electrode of the piezo tube, Figure 3.7 a. The force measurement starts with the sample far away and the cantilever in its rest position. As a result of the applied voltage, the sample is moved up and down relative to the stationary cantilever tip. As the piezo moves the sample up and down, the cantilever deflection signal from the photodiode is monitored. The force-distance curve, a plot of the cantilever tip deflection signal as a function of the voltage applied to the piezo tube, is obtained. Figure 3.7 b shows a typical force-distance curve showing the various features of the curve. The arrow heads reveal the direction of the piezo travel. As the piezo extends, it approaches the tip, which at this point in free air and hence shows no deflection. This is indicated by the flat portion of the curve. As the tip approaches the sample within a few nanometers (point A), an attractive force exists between the atoms of the tip surface and the atoms of the sample surface. The tip is pulled towards the sample and contact occurs at point B on the graph. From this point on, the tip is in contact with the surface and as the piezo further extends, the tip gets further deflected. This is represented by the sloped portion of the curve. As the piezo retracts, the tip goes beyond the zero deflection (flat) line because of attractive forces
57 (van der Waals forces and long range meniscus forces), into the adhesive regime. At point C in the graph, the tip snaps free of the adhesive forces, and it is again in free air.
The horizontal distance between point B and C along the retrace line gives the distance moved by the tip in the adhesive regime. This distance multiplied by the stiffness of the cantilevers gives the adhesive force. Incidentally, the horizontal shift between the loading and unloading curves results from the hysteresis in the piezo tube [50].
Z voltage (V) +220 Z scan start
Z scan size
Time
-220 a)
Tip deflection
Retracting Extending
A
B
C Piezo vertical position
b)
Figure 3.7 a) Force calibration Z waveform, b) a typical force-distance curve for a tip in contact with a sample. Contact occurs at point B; tip breaks free of adhesive forces at point C as the sample moves away from the tip [49]
58 In this study, fibers were imaged in tapping mode before and after the indentations. They were imaged before in order to determine the fiber diameter and lock on the fiber surface.
They were imaged after in order to be sure that they did not move and were not damaged during indentations.
After imaging fibers, elastic indentations were done with the AFM tips and raw Tip
Displacement vs. Piezo Movement data was obtained. By the help of the Nanoscope III software the tip radii were determined for each tip after every indentation experiment.
3.3. Experimental Procedures
In order to have better understanding of fibers, they are also cross-sectioned and imaged before the sea component are dissolved. For that purpose, islands-in-the-sea fibers were embedded in an epoxy matrix (Spurr), than cut with microtome for SEM imaging. Then the epoxy matrix is cross-sectioned and also imaged with FIB. The cross-sections, prepared with FIB, were also imaged with AFM.
For AFM imaging and indentation experiments, several sample preparation methods were attempted to dissolve the sea components and get the PET and Nylon-6 nanofibers. To dissolve PE, ACS grade toluene from Sigma Aldrich (Milwaukee, WI) was used as a solvent. To dissolve Evoh (Ethylene-Vinyl Alcohol), a solution of a nonionic surfactant -
Triton (TM) X – 200 - from SPI Supplies (West Chester, PA) was used. Sample preparation was started with the PET/PE islands in the sea fibers. The attempted methods for sample preparation are explained below.
59 3.1.1. First Approach
In this method PET/PE fibers were glued onto glass slides from both ends. Epoxy was used as glue. Then the slides were placed in beakers which contained toluene. The slides were kept in hot toluene (90° C) for 4-6 hours. Then they were taken out and dried at ambient conditions.
Under an optical microscope, it was observed that toluene was able to dissolve the epoxy glue and that the dissolved epoxy glue was redeposited ontop of the fiber surfaces. This recoating prevented the PE from dissolving. An image of epoxy coated fiber can be seen from the Figure 3.4. In order to overcome this problem, methanol and acetone were applied to the fibers after exposure to toluene without satisfactory results.
Figure 3.8 A microscope image of an epoxy coated PET/PE islands in the se form fiber
60 3.3.2. Second Approach
As the second method the fibers were cut in short lengths (2-5mm) and then put in the beaker which is full of toluene and glass slides were placed at the bottom of the beaker.
The toluene kept at 90C until all toluene evaporated. After evaporating toluene completely, it was expected to collect individual nanofibers on the top of the glass slides.
However, it was observed that the fibers agglomerated and formed bundles on the slides preventing the measuring of individual nanofibers.
3.3.3. Third Approach
In a third method we treated PET/PE fibers through perforated aluminum sheets and plates. Aluminum sheets and plates were kept in hot ( ≈ 90C ) toluene for 2 days. With this method some nanofibers were obtained, and it was observed that PE was dissolved better. However, because of the very high specific surface energy of the nanofibers, after dissolving sea component, it became rather difficult to separate individual fibers. Fibers became attached to the tweezers and to the AFM tip. In order to overcome this problem the nanofibers were mounted onto the sample holders using a double sided tape with the aid of a micromanipulator. The manipulator allowed separating bigger fiber bundles into smaller bundles. However the fibers were not successfully separated into individual fibers. Cutting the bundle of nanofibers into short lengths and exposing them to methanol was unsuccessfully attempted in order to decrease the surface energy of the fibers.
Furthermore, the AFM tip was coated with the adhesive from the tape preventing the
61 gathering of clear images of fibers. Some examples of adhesive coated and bundle formed fibers can be seen from Figure 3.5.
Figure 3.9 AFM images of PET nanofibers coated with adhesive
While at first glance the use of aluminum plates seemed to be good choice SEM pictures showed that the surface of the aluminum plates was not smooth enough to allow the engagement of the AFM tip with the fiber.
62
Figure 3.10 SEM images of the PET nanofibers on perforated aluminum plates
63 3.3.4. Final Approach
After all challenges, we found out that we need a smooth and chemical resistant surface.
Instead of gluing we should thread the fibers to the sample holder. Considering these, custom-made TEM grids, provided by Protochips Inc (Raleigh, NC), were started to be used to prepare the samples for AFM analysis.
To dissolve the sea components of the islands in the sea fibers and get the islands – nanofibers - a single I/S bicomponent fiber was threaded through the windows of the external grids. In order to remove the sea of polyethylene and obtain individual PET nanofibers, the grids and the bicomponent nanofibers were submerged in a glass beaker containing 500ml of toluene. The beaker was then heated and the samples were exposed to toluene at 100C for 48 hours. The toluene was then replaced and the fibers were exposed to fresh toluene at 100C for another 24 hours. Similarly, single Nylon6/Evoh islands in the sea fibers and hollow fibers were also threaded with the same way.
Different from the PET/PE I/S fibers, a nonionic surfactant (Triton (TM)X, SPI Supplies
(West Chester, PA)) solution with a concentration of 1.5g/l in distilled water was used to dissolve Evoh. According to the producer, Evoh was supposed to dissolve in warm mater, however we faced some difficulties, and, in order to increase the wettability of the sea component, we used a surfactant solution. Figure 3.11 summarizes the dissolving steps.
64
Heater
a) b) c)
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 4. RESULTS AND DISCUSSION
4.1. Cross-Sectioning and imaging Islands-in-the-Sea Form Fibers
Before the sea component was dissolved, the fibers were imaged using SEM, FIB and
AFM in order to determine their morphology and the organization of the island structures into the sea matrix. (See Figure 4.1-4.3)
66
a)
b)
Figure 4.1 SEM images of a) PET/PE islands-in-the-sea fibers and b) Nylon6/Evoh islands-in-the-sea hollow fibers
67
5µm a)
25µm b)
Figure 4.2 FIB images of a) PET/PE and b) Nylon6/Evoh islands-in-the-sea fibers
68
a)
69
b)
Figure 4.3 AFM images of a) PET/PE b) Nylon-6/Evoh islands-in-the-sea fibers
70 4.2. Method Validation
In order to validate the experimental results, a piece of PET film was indented and the effect of parameters such as the speed of the tip (scan rate), the tip sample distance (ramp size) and the indentation amount were analyzed. For all 5 combinations of operating parameters, it was shown that the obtained values demonstrate the robustness of the method. The same PET film was also analyzed with DMA. The obtained Elastic modulus value from DMA (~7GPa) was lower than the AFM values (~13GPa), however we believe that since the way of testing and the testing conditions are not same for the two instruments, the difference between the results is understandable.
Below some examples of the testing parameters can be seen from Table 4.1.
Table 4.1 Elastic modulus values [GPa] of the PET film, obtained under different testing conditions. Bottom lines: average values of each column
Ramp Size 209.8nm Ramp Size 416.4nm Ramp Size 814.2nm Ramp Size 416.4nm Ramp Size 416.4nm Scan Rate 0.996Hz Scan Rate 0.996Hz Scan Rate 0.996Hz Scan Rate 0.537Hz Scan Rate 0.189Hz 10.72 16.73 10.65 14.65 12.3 10.03 16.99 10.74 12.04 11.56 9.22 17.33 9.78 13.19 12.18 10.24 10.29 12.95 15.24 12.88 14.35 18.6 13.32 14.14 15.41 11.18 8.88 9.04 12.52 16.33 11.05 13.69 9.14 15.13 19.67 9.48 15.59 14.33 14.05 15.9 7.3 12.86 10.36 18.46 15.64 15.32 14.44 11.76 12.33 15.65 15.21 14.8 10.28 9.63 9.18 9.41 12.02 9.74 15.89 11.26 11.96 11.66 12.25 14.08 11.04 9.86 13.05 11.46 15.96 9.73 9.25 14.14 10.59 11.63 15.8 164.58 211.07 166.39 208.94 204.53 10.972 14.07133333 11.09266667 13.92933333 13.63533333
71 4.3 AFM Imaging and Indentations
The following procedure was followed for all experiments:
The cantilever was mounted on the cantilever holder and tuned to find its resonant frequency (RF [kHz]). The quality factor (QF) of the cantilever was also calculated by the software. These two values are used for the calculations of the cantilever’s spring constant. Once the spring constant of the cantilever is known, the sensitivity of the photodetector can be determined from the slope of the force vs. distance curve of an indent made in a material that is, in effect, infinitely hard relative to the cantilever. The material that is typically utilized is quartz. Images of pre- and post-indented quartz are shown below in Figure 4.4. Comparing the two images, it can be observed that no apparent damage occurred during the indent. This suggests that the material is not affected by the indentation forces applied and is a suitable material for determining the sensitivity of the cantilever.
a) b) Figure 4.4 AFM images of quartz sample a) before and b) after indentation
72 Then the S value is used as an input for the calculations of the elastic modulus values of the fibers. Below a raw indentation curve of quartz can be seen.
Figure 4.5 shows a raw indentation curve of quartz.
Approaching Retracting
30
25
20
15 10 5 Tip Deflection [nm] Tip Deflection 0
-5
250 300 350 400 450 Piezo Displacement [nm]
Figure 4.5 A typical Piezo Movement vs. Tip Deflection Curve for a Quartz sample
Fiber diameters were determined using the Nanoscope III© software. Looking at the AFM images of the fibers, they appear to be half circular. This is an AFM artifact since the
AFM tip cannot reach at the very bottom of the fibers as depicted in Figure 4.5. Fiber diameters were determined from the height of the image.
73
AFM Tip
Obtained half- circular image
Height of the image – fiber diameter
Figure 4.6 Schematic of AFM tip imaging the nanofiber
a)
74
Imaged fiber’s cross-section
b)
Figure 4.7 3D images of a) PET and b) Nylon6 nanofibers
Raw Tip Displacement vs. Piezo Movement data was obtained. Figure 4.7 illustrates some of examples of these curves.
75 Approaching Retracting
-10
-30
[nm] Tip Deflection
-50 50 250 450 650 Piezo Displacement [nm]
a)
Approaching Retracting
0 -5 -10 -15 -20 -25 -30
-35 Tip Deflection [nm] Tip Deflection -40 -45 0 100 200 300 400 Piezo Displacement [nm]
b)
76 Approaching Retracting
0
-20
-40
Tip Deflection [nm] Tip Deflection
-60 400 500 600 700
Piezo Displacement [nm]
c)
Figure 4.8 Examples of Tip Displacement vs. Piezo Movement curves of a a)PET and a b)Nylon6 nanofiber c)Nylon6 hollow nanofiber
Some combinations of fiber diameter vs. tip size relation can be seen from Figure 4.8 to illustrate the relative sizes between tip and fiber.
Tip Radius~90nm Tip Radius ~30nm Tip Radius ~20nm Fiber Diameter ~1800nm Fiber Diameter~2500nm Fiber Diameter~700nm
Figure 4.9 Size relations between some of the AFM tips and the fibers
77 4.4. Determination of the tip radius
The tip radius was determined using a ultra sharp silicon calibration grating (TGT01) from NT-MDT Devices (Moscow, Russia). In Figure 4.9 an AFM image and a cross- section graph of one of the used tips can be seen.
a)
Tip Radius Cross Section
500.00 400.00
300.00
200.00 Height [nm] 100.00
0.00 0 50 100 150 200 250 Length [nm]
b)
Figure 4.10 a) An AFM image of the tip obtained imaging calibration gratings and b) Cross-section of the AFM tip
78 4.5. Determination of the Cantilevers’ Sizes
The spring constant ( k ) values of the cantilevers were reported to be 40 N/m by the manufacturer. However as it was reported by Sader et. al. [51] that k values are strongly dependent on the cantilever sizes (width and length), resonant frequencies and quality factors. Considering this, the cantilevers resonant frequencies and the quality factors were noted each time they were used. After each AFM experiment the cantilevers were imaged using a Hitachi S3200 (Japan) SEM and the widths and lengths of the cantilevers were determined, on the SEM images.
35µm
Figure 4.11 SEM images of some of the cantilevers
79 4.6. Determination of the Cantilevers’ Spring Constants
The spring constant ( k ) values were calculated according to Sader’s approach for rectangular cantilevers:
2 2 (4.1) k = 0.1906ρ f b LQ f Γi (ω f )ω f
Where ω f is the frequency of the cantilever, Q f is the quality factor in fluid which is typically used when the cantilevers are placed in air, b and L are the width and the length
of the cantilever, respectively and ρ f is the density of the fluid which is used when the
cantilever is placed in air, and Γi is the imaginary part of the hydrodynamic function
[51]. Quality Factor is related to the damping ratio (δ), which is a measure of how fast the oscillator is losing energy. When the damping is small, the sharpness of the resonany peak is expressed by the quality factor.
1− 2δ 2 Q = (δ<1/ 2 ) (4.2) f 2δ
Or,
Q f = 2π ×(Total energy / Energy loss per cycle) (4.3)
Experiments were conducted in air and the fluid density and the viscosity were taken 1.18 kg/m3 and 1.86x10-5kg/m/s, respectively.
80 The calculations showed that the spring constant values are varying between 17 N/m and
35 N/m. It was also observed that even the same cantilever may have different k values
depending on ω f and Q f values which change from day to day. In Table 3.1 the variation of the values can be seen clearly.
Table 4.2 An example of the variation on the same cantilevers’ spring constants because of the size, RF and QF.
Cantilever Length Width Resonant Quality Spring Constant
Number [microns] [microns] Frequency [kHz] Factor (k) [N/m]
1 112.11 32.42 279.21 479 21.5
1 112.11 32.42 279.07 470 21.1
1 112.11 32.42 279.11 487 21.8
2 112.6 33.86 271.07 387 17.4
2 112.6 33.86 271.29 473 21.3
2 112.6 33.86 271.32 461 20.7
4.7. Data Processing and Obtaining Force vs. Displacement Curves
The actual indentation values cannot be measured directly by the AFM because the data is obtained as a photodiode signal vs. z-piezo displacement curve. For that reason raw data was used to manually calculate Force vs. Displacement (F vs. d) curves. A macro program was developed using Macro Express (Macro Software) in order to gain direct access to the instrument data acquisition system and to normalize the raw experimental data into Force vs. Displacement (F vs. d) curves.
81 The algorithm used by the macro is described as follows:
1. Opens the file selected using the D 3000's software (Nanoscope III©).
2. Chooses the ASCII Export option, and exports the data, for that file, to a
Notepad file.
3. Closes the software of the instrument.
4. Opens the Notepad file, extracts the parameters and copies them. The
copied data are:
i. Z scan sensitivity [nm/V]
ii. Deflection sensitivity [nm/V]
iii. Samples/line
iv. Z range [V]
v. Spring constant [N/m]
vi. LSB (Least Significant Bit) data
5. Closes Notepad file.
6. Opens an Excel file.
7. Pastes the copied information, forms 6 columns by making the first one
LSB data.
8. Then converts that data in the 2nd column in [Volts].
(Column 1 x 10) / 217 = Column 2 [V]
9. Multiplying Column 2 by Deflection Sensitivity [nm/Volt] creates the
Column 3, which is the deflection in [nm].
10. Dividing Z range data by Samples/line number gets the ΔZ value.
82 11. By adding ΔZ to the Z range gets the first row of the Column 4, which is
Piezo Movement in [nm], and by adding to the first row gets the second
and so on.
12. Subtracting Column 3 (deflection of the cantilever [nm]) from Column 4
(PM) gets the Column 5, which is the displacement (elastic indentation
amount) of the fiber in [nm].
13. Multiplying Column 3 (deflection of the cantilever [nm]) by the spring
constant [nN/nm] produces the Column 6, which is the Force acting on the
sample in [nN].
14. Saves the data and stops.
Figures 4.12, 4.13 and 4.14 show some examples of the retracting parts of the Force vs.
Displacement curves for PET and Nylon-6 micro and nanofibers and for Nylon-6 hollow fibers are shown. Hertzian Contact Theory concerns the retracting part is the real response of the fiber to the elastic indentation.
83
600
400
Force [nN] Force 200
0 0 150 300 450 Displacement [nm]
PET microfiber φ = 2.5 ± 0.18μm
700
500 Force [nN] Force 300
100 050100150 Displacement [nm]
PET microfiber φ = 1.8 ± 0.11μm
84 300
200 Force [nN]Force 100
0 0102030 Displacement [nm]
PET nanofiber φ = 700 ± 50nm
900
600
Force [nN] Force 300
0 0 204060 Displacement [nm]
PET nanofiber φ = 400 ± 30nm
85 300
200
Force [nN] Force 100
0 2 9 16 23 Displacement [nm]
PET nanofiber φ = 300 ± 20nm
750
550
Force [nN] 350
150 03060 Displacement [nm]
PET nanofiber φ = 100 ± 7nm
86
Figure 4.12 Some examples of F vs. d curves’ retracting parts of PET micro and nanofibers
87
450
300 Force [nN] Force 150
0 0 35 70 105 140 Displacement [nm]
Nylon 6 Microfiber φ = 1.3 ± 0.09μm
900
600
Force [nN] Force 300
0 0 306090 Displacement [nm]
Nylon 6 Microfiber φ = 1.2 ± 0.08μm
88
1500
1000
Force [nN] Force 500
0 050100150 Displace me nt [nm]
Nylon 6 Microfiber φ = 1± 0.07μm
1500
1000
Force [nN] Force 500
0 0 60 120 180 240 Displace me nt [nm]
Nylon 6 Nanofiber φ = 900 ± 60nm
89
900
600
Force [nN] Force 300
0 0 80 160 240 Displacement [nm]
Nylon 6 Nanofiber φ = 800 ± 55nm
900
600
Force [nN] Force 300
0 0 50 100 150 200 Displacement [nm]
Nylon 6 Nanofiber φ = 700 ± 50nm
90
750
500
Force [nN] Force 250
0 04080120 Displacement [nm]
Nylon 6 Nanofiber φ = 600 ± 40nm
450
300 Force [nN] Force 150
0 0255075100 Displacement [nm]
Nylon 6 Nanofiber φ = 500 ± 35nm
91
750
500
Force [nN] Force 250
0 0 70 140 210 Displacement [nm]
Nylon 6 Nanofiber φ = 300 ± 20nm
1000
500 Force [nN] Force
0 0306090 Displace me nt [nm]
Nylon 6 Nanofiber φ = 200 ±15nm
92
Figure 4.13 Some examples of F vs. d curves’ retracting parts of Nylon-6 micro and nanofibers
93 3000
2000
Force [nN] Force 1000
0 0 50 100 150 200 Displacement [nm]
Nylon 6 hollow Microfiber φ = 1.3 ± 0.09μm
1500
1000 Force [nN] Force 500
0 0 20406080 Displacement [nm]
Nylon 6 hollow Microfiber φ = 1.1± 0.07μm
94 2000
1500
1000 Force [nN] Force
500
0 0 306090 Displacement [nm]
Nylon 6 hollow Microfiberφ = 1± 0.07μm
1500
1000 Force [nN] Force 500
0 0 50 100 150 200 Displacement [nm]
Nylon 6 hollow Nanofiber φ = 500 ± 35nm
95
1000
500
Force [nN]
0 0204060
Displacement [nm]
Nylon 6 hollow Nanofiber φ = 400 ± 30nm
1200
800
Force [nN] Force 400
0 02040 Displacement [nm]
Nylon 6 hollow Nanofiber φ = 300 ± 20nm
96 2000
1500
1000 Force [nN] Force
500
0 010203040 Displacement [nm]
Nylon 6 hollow Nanofiber φ = 200 ±15nm
200
150
100
Force [nN] Force 50
0 01020 Displacement [nm]
Nylon 6 hollow Nanofiber φ = 100 ± 7nm
97
Figure 4.14 Some examples of F vs. d curves’ retracting parts of Nylon6 hollow micro and nanofibers
98 4.8. Calculation of Elastic Modulus Values
Elastic modules of the fibers were calculated from the initial linear parts of the retracting curves according to Hertzian Contact Theory [52].
Hertzian theory attempts to determine the stress at the contact of two elastic solids by assuming that the contact area is generally elliptical. For the purpose of calculating the local deformations, a simplification is added that each body can be regarded as an elastic half-space loaded over a small elliptical region of its plane surface. For these simplifications to be valid two conditions must be satisfied: The significant dimensions of the contact area must be small compared a) with the dimensions of each body and b) with the relative radii of curvature of the surfaces.
The assumptions made in the Hertz theory are as follows:
i) The contact surfaces are continuous and non-conforming; surfaces do not take
each others shape
ii) The strains are small.
iii) Each solid can be considered as elastic half-space.
iv) The surfaces are frictionless.
Two solids of general shape are shown in cross-section after deformation in Figure 4.15
[52].
99 1
x-y plane
2
a)
P .T δ2 2
Z 1 1 .S1 x-y δ1 uz1 δ plane O δ uz2 2 . S2 2 Z aa2 Z
δ1 T . 1 P
b)
Figure 4.15 Cross-section of two solids a) before and b) after deformation [52]
100 Where, δ1 , δ2 : Displacements of the first and the second surfaces, respectively.
2 δ : Total displacement (δ1 +δ2 ) P: Compressive Load (=πapm , pm is the mean pressure)
T1 , T2 : Distant point 1 and 2, respectively a : Radius of the circular contact area
uz1 , uz2 : Displacements of the surface 1 and 2, respectively.
S1 , S2 : Separation between two surface points before deformation
Assuming that the principal radii of curvature of the surfaces at the origin are equal to each other (R), the boundary condition for displacement within the contact can be written as:
2 (4.2) uu+=−δ (1 / 2 Rr ) zz12
where r is the radius of contact and 1/R=1/R1+1/R2 is the relative curvature..
The pressure acting on the second body is equal to that on the first, so that by writing:
1 11−−ν 22ν (4.3) 12 * =+ E EE12
And substituting the expressions for uz1 and uz2 into Equation 4.2 we get:
(4.4) π p0 22 2 * ()21/2ar−=−δ () Rr 4aE
which the radius of contact the circle is given by:
* (4.5) apRE= π 0 /2
101 and the mutual approach of distant points; which is the elastic indentation amount of the surfaces, in the two solids is given by:
* (4.6) δπ= ap0 /2 E
The total load compressing the solids is related to the pressure by:
a 2 (4.7) P ==pr()2π rdr pπ a2 ∫ 0 0 3
Hence the maximum pressure P0 is 3/2 times the mean pressure Pm. By combining the
Equations 4.5, 4.6 and 4.7, E* can be calculated as:
*13/23 −− (4.8) EPR= δ 4
Assuming the tip has fairly larger elastic modulus than the fiber and neglecting the tip effect on the Equation 4.8 for elastic modulus of the fiber we end up with [53]:
*2 (4.9) EEf ≈−(1ν f ) where vf is the Poisson ratio of the fiber.
102 4.9. Results for PET micro and nanofibers
The evolution of elastic modulus values as a function of fiber diameter for PET fibers can be observed in Figure 4.16. The two values marked with green color, are the results for the elastic modulus of commercially available PET fibers. The other values on the graph represent the results for the elastic modulus of nanofibers obtained from islands-in-the- sea fibers. The elastic modulus is below 1GPa for the commercially available PET nanofibers. However, as the diameter of the nanofiber decrease below 1µm, a distinct increase in elastic modulus is noted. As the fiber diameter continues to decrease, their elastic modulus approaches the theoretical ultimate strength of PET [54].
Theoretical Ultimate Value
100
10
1
Elastic Modulus [GPa] 0.1
0.01 0 500 1000 1500 2000 2500
Fiber Diameter [nm]
Figure 4.16 Elastic modulus vs. Fiber diameter relation for PET micro and nanofibers
103 The increase in the elastic modulus of PET fibers, as the fibers diameter goes under 1µm, is a similar behavior with PLLA fibers. Nanoindentation and bending tests performed on
PLLA [44, 45] nanofibers also showed an increase in elastic modulus values with the decreasing fiber diameter as it gets under 500nm.
On the other hand, PET, a semi-crystalline thermoplastic polymer, is a tunable polymer in terms of its modulus. There are studies on increasing its modulus by drawing and applying heat [61, 62] According to published values it is possible to increase the modulus up to 15GPa for PET fibers [62]. Our results show that it is possible to make very strong fibers by making them smaller. Considering the drawing effects, there is an opportunity to draw micro and nanofibers and make them even stronger.
104 4.10. Results for Nylon-6 micro and nanofibers
The variation of the elastic modulus of Nylon 6 fibers as a function of fiber diameter is illustrated in Figure 4. 17.
Theoretical Ultimate Value
100
10
Elastic Modulus [GPa]
1 0 200 400 600 800 1000 1200 1400
Fiber Diameter [nm]
Figure 4.17 Elastic modulus vs. Fiber diameter relation for Nylon-6 micro and nanofibers
Figure 4.17 shows two distinctive regions having 400 nm as their cutoff value. For fibers with diameters larger than 400nm the elastic modulus shows minor variations oscillating around 9GPa. The reported value for bulk Nylon 6 is around 5.1GPA [63]. Compared to the bulk values, making Nylon-6 nanofibers improved the elastic modulus of the material.
But, as the fiber diameter becomes smaller than 400nm, the Elastic modulus values of the fibers show an exponential increase. This may be explained with islands-in-the-sea
105 structure: All of these fibers were obtained from the same islands-in-the-sea fiber. It was noted that middle of the structure contains the smaller nanofibers. Fibers in the middle of the bundle had more time to crystallize during the quenching process. The micro and nanofibers surrounding the small fibers experience higher quenching rates, so for the same draw ratios it is expected that these fibers will have lower elastic modulus.
106 4.11. Results for Nylon-6 micro and nano hollow fibers
In Figure 4.18, how elastic modulus values vary with the diameter for nylon-6 fibers can be seen. Unlike PET and Nylon-6 fibers, Nylon-6 hollow micro and nanofibers do not exhibit size dependence.
Theoretical Ultimate Value 100
80
60
40
20
Elastic Modulus [GPa] 0
-20 0 500 1000 1500 Fiber Radius [nm]
Figure 4.18 Elastic modulus vs. Fiber diameter relation for Nylon-6 micro and nano hollow fibers
The hollow fibers do not show a strong dependence on fiber diameter in terms of their elastic modulus values. Fibers having diameters in µm and in nm have close values to each other. Assuming they have the same wall thicknesses, we may say that wall thickness may be an effect on elastic modulus values. However, to be able to make a conclusion, more experiments needs to be done with different fibers having different wall-thicknesses.
107 5. SUMMARY AND CONCLUSIONS
PET micro and nanofibers showed a similar, increase in elastic modulus with decreasing fiber diameter, behavior with the other polymers reported [44, 45]. While the commercially available PET micro fibers showed lower values in terms of elastic modulus, under 1GPa, as the fiber diameter gets under 1µm, there is a remarkable increase in the modulus values. Similarly to the PET fibers Nylon6 nanofibers also shows an exponential increase in the elastic modulus as the fiber diameter decreases under 400nm.
This behavior of both PET and Nylon 6 nanofibers can be related to the islands-in-the-sea structure. Fibers located in the middle of the structure, which are the fibers having diameters under 500nm, have more time to crystallize during quenching. As a result, smaller fibers have higher elastic modulus values. This behavior can also be related with the fact that as the material size gets smaller, the probability of including flaws and cracks inside also decreases which affects the elastic modulus by increasing it.
The hollow fibers do not show the similar diameter dependent behavior. As a matter of fact we can say that their elastic modulus values are not really `dependent on the fiber diameter. Both the smaller nanofibers and the bigger micro fibers have very close values to each other. These results imply that by judicious changes in the fibers’ wall- thicknesses, their elastic modulus can be tuned to desired values and customized for desired applications. However, to be able to make a conclusion, more experiments needs
108 to be done with different fibers having different wall-thicknesses. On the other hand, since the fibers are hollow fibers, their response to elastic indentation is a very complicated issue which depends on the wall-thickness, tip radius and bending behavior.
Basic rules of mechanics are not enough to explain this type of bending. For that reason, simulating the behavior using a suitable software is necessary in order to have a better understanding.
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114 7. APPENDIX
115 The raw AFM imaging and indentation experiment results are given; from Figure 7.1 to
Figure 7.25.
40000
20000
0
Height [nmx10^2] Height -20000
-40000 100 250 400 550 Length [nmx10^5]
Figure 7.1 3D image of PET nanofiber
25000
10000
-5000 Height [nmx10^2] Height
-20000 0 120 240 Lenght [nmx10^5]
Figure 7.2 3D image of Nylon-6 nanofiber
116 30
20
10
0 Tip Deflection [nm] Tip Deflection
-10 0 200 400 600 Piezo Displacement [nm]
Figure 7.3 Raw indentation Curve of PET microfiber (φ = 2.5 ± 0.18μm )
90
60
30
0
Tip Deflection [nm] Tip Deflection -30
-60 0 200 400 600 Piezo Displacement [nm]
Figure 7.4 Raw indentation Curve of PET microfiber (φ = 1.8 ± 0.11μm )
117 20
0
-20
-40 Tip Deflection [nm] Tip Deflection
-60 300 500 700 Piezo Displacement [nm]
Figure 7.5 Raw indentation Curve of PET microfiber (φ = 700 ± 50nm )
0
-20
-40 Tip Deflection [nm] Tip Deflection
-60 680 710 740 770 Piezo Displacement [nm]
Figure 7.6 Raw indentation Curve of PET microfiber (φ = 400 ± 30nm )
118
-10
-30 Tip Deflection [nm] Tip Deflection
-50 50 250 450 650 Piezo Displacement [nm]
Figure 7.7 Raw indentation Curve of PET microfiber (φ = 300 ± 20nm )
50
30
10 Tip Deflection [nm] Tip Deflection
-10 250 450 650 Piezo Displacement [nm]
Figure 7.8 Raw indentation Curve of PET microfiber (φ = 100 ± 7nm )
119 -5
-10
-15 Tip Deflection [nm] Tip Deflection
-20 400 500 600 700
Piezo Displacement [nm]
Figure 7.9 Raw indentation curve of Nylon 6 Nanofiber (φ = 1.3 ± 0.09μm )
-45
-60
-75 Tip Deflection [nm] Tip Deflection
-90 400 500 600 700
Piezo Displacement [nm]
Figure 7.10 Raw indentation curve of Nylon 6 Nanofiber (φ = 1.2 ± 0.08μm )
120 10
-5
-20
-35 Tip Deflection [nm] Tip Deflection
-50 500 600 700
Piezo Displacement [nm]
Figure 7.11 Raw indentation curve of Nylon 6 Nanofiber (φ = 1± 0.07μm )
20
0
-20 Tip Deflection [nm] Tip Deflection
-40 350 450 550 650 750
Piezo Displacement [nm]
Figure 7.12 Raw indentation curve of Nylon 6 Nanofiber (φ = 900 ± 60nm )
121 -5
-20
-35 Tip Deflection [nm] Tip Deflection
-50 500 600 700
Piezo Displacement [nm]
Figure 7.13 Raw indentation curve of Nylon 6 Nanofiber (φ = 800 ± 55nm )
0
-20 Tip Deflection [nm] Tip Deflection
-40 400 500 600 700
Piezo Displacement [nm]
Figure 7.14 Raw indentation curve of Nylon 6 Nanofiber (φ = 700 ± 50nm )
122 -25
-40
-55 Tip Deflection [nm] Tip Deflection
-70 400 500 600 700
Piezo Displacement [nm]
Figure 7.15 Raw indentation curve of Nylon 6 Nanofiber (φ = 600 ± 40nm )
-5
-20 Tip Deflection [nm] Tip Deflection
-35 500 600 700
Piezo Displacement [nm]
Figure 7.16 Raw indentation curve of Nylon 6 Nanofiber (φ = 500 ± 35nm )
123 10
-5 Tip Deflection [nm] Tip Deflection
-20 500 600 700
Piezo Displacement [nm]
Figure 7.17 Raw indentation curve of Nylon 6 Nanofiber (φ = 300 ± 20nm )
55
40
25 Tip Deflection [nm] Tip Deflection
10 400 500 600 700
Piezo Displacement [nm]
Figure 7.18 Raw indentation curve of Nylon 6 Nanofiber (φ = 200 ±15nm )
124 80
50
20
-10 Tip Deflection [nm] Tip Deflection
-40 300 400 500 600 700 Piezo Displacement [nm]
Figure 7.19 Raw indentation curve of Nylon 6 hollow Microfiber (φ = 1.3 ± 0.09μm )
0
-20
-40 Tip Deflection [nm] Tip Deflection
-60 400 500 600 700
Piezo Displacement [nm]
Figure 7.20 Raw indentation curve of Nylon 6 hollow Microfiber (φ = 1.1± 0.07μm )
125 60
30
0 Tip Deflection [nm] Tip Deflection
-30 250 350 450 550 650 750
Piezo Displacement [nm]
Figure 7.21 Raw indentation curve of Nylon 6 hollow Microfiber (φ = 1± 0.07μm )
0
-20
-40 Tip Deflection [nm] Tip Deflection
-60 400 500 600 700
Piezo Displacement [nm]
Figure 7.22 Raw indentation curve of Nylon 6 hollow Microfiber (φ = 500 ± 35nm )
126 0
-20
-40 Tip Deflection [nm] Tip Deflection
-60 400 500 600 700
Piezo Displacement [nm]
Figure 7.23 Raw indentation curve of Nylon 6 hollow Microfiber (φ = 400 ± 30nm )
60
40
20
0 Tip Deflection [nm] Tip Deflection
-20 400 500 600 700
Piezo Displacement [nm]
Figure 7.24 Raw indentation curve of Nylon 6 hollow Microfiber (φ = 300 ± 20nm )
127 0
-5
-10 Tip Deflection [nm] Tip Deflection
-15 550 650 750 Piezo Displacement [nm]
Figure 7.25 Raw indentation curve of Nylon 6 hollow Microfiber (φ = 100 ± 7nm )
128