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Electrospinning of Nanofibre Yarns using Rotating

Ring Collector

By

Muhammad Nadeem Shuakat

(B.Sc. Textile Engineering,

MS Fibre & Polymer Engineering)

Submitted in fulfilment of the requirements for the degree of

Doctor of Philosophy

Deakin University

(August, 2014)

DEAKIN UNIVERSITY ACCESS TO THESIS - A

I am the author of the thesis entitled

Electrospinning of Nanofibre Yarns using Rotating Ring Collector

Submitted for the degree of

This thesis may be made available for consultation, loan and limited copying in accordance with the Copyright Act 1968.

'I certify that I am the student named below and that the information provided in the

form is correct' Full Name: Muhammad Nadeem Shuakat Signed: ................................. .………………………………………………..

Date: 20 August 2014

DEAKIN UNIVERSITY

CANDIDATE DECLARATIONI certify the following about the thesis entitled

“Electrospinning of Nanofibre Yarns Using Rotating Ring

Collector.”

Submitted for the degree of

Doctor of Philosophy

a. I am the creator of all or part of the whole work(s) (including content and layout) and that where reference is made to the work of others, due acknowledgment is given.

b. The work(s) are not in any way a violation or infringement of any copyright, trademark, patent, or other rights whatsoever of any person.

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I also certify that any material in the thesis which has been accepted for a degree or diploma by

any university or institution is identified in the text.

I certify that I am the student named below and that the information provided in the form is

correct'

Full Name................Muhammad Nadeem Shuakat............................

Signed ......................................................…...................……………

Date..........................20 August 2014...................................…………

Dedications

First and foremost, I thank and dedicate this thesis to my God, my Lord,

The Most Merciful, The Most Kind,

Almighty “ALLAH”

For all the blessings and kindness upon me, (which I knew or not), for giving me inner

strength and strong faith in HIM, to complete this thesis (may HE accept it).

Secondly I would humbly dedicate this work to my parents (Shuakat Ali & Akbari

bibi) who strongly opposed my decision to purse my Ph.D. studies, but later, kindly

consented to fulfil my dream. You are the best parents, you are illiterate but still have

light of knowledge, God bless you, here and hereafter. I am also grateful to the rest of

my family who believed in me and supported on every step in my life with their moral

and monetary encouragement.

Here, I also wish to specially thank to my best friend in Pakistan, Mr. Akram Badshah

Butt, for his time to time emotional and moral support.

Especially I dedicate this thesis to my little angels, my daughters

(Habiba, Haiqa &Yusra)

They suffered the most while I was away and could not be with them during the

festivals where every child had their dad with them. I really value your tears which

you shed in the absence of Daddy. Sorry and thanks for being so good and so loving.

Finally I will dedicate my work to my lovely wife, Gulnaz & her family. She and her

family shared all the bad and good times with me. She is blessing from Allah. She was

brought a smile when I was sad and down, keeping me up and doing my thesis. She

taught me the true meanings of love and sacrifice.

Love you Gul, until I die even after that.

Acknowledgments I am indebted to a number of people who kept me motivated, sane, up and doing my

tasks in the shady and dark times of my Ph.D. I would thank Deakin University,

for giving me the opportunity to pursue my Ph.D., by providing me with a DUPRS

(Deakin University Postgraduate Research Scholarship).

First and foremost, I am thankful to Professor Tong Lin and Professor Xungai

Wang. It is both an honour and pleasure working with you, in completing my .Ph.D.

Professor Wang has been kind enough to give me free hand to work on a topic of

my choice. He has always been keen and eager to listen to my ideas and let me work

on them. Prof. Lin’s continuous support and extra ordinary patience led me to

complete this thesis. He was continuously demanding and challenging to get the

impossible job done. His advice and encouragement has been a valuable asset to me.

He has broadened my knowledge and skills in the field of nanotechnology and

nanofibres. In short, both of my supervisors have been excellent and kind in bringing

the best out of me. Thank you Xungai and Tong for your trust and belief in me.

I would thank Mr. Usman Ali for his help in building the electrospinning setup, Dr.

Jian Fang for his assistance with the Scanning Electron Microscopy analysis, Dr.

Haitao Niu and Mr. Badar Munir Zaidi for their assistance in modelling work. I

especially thank Mr. Alex Orokity, Rob Lovett and Graeme Keating for their

technical support in the design and modification of the yarn electrospinning setup.

Without their support, I would not have been able to develop this setup.

I would also wish to thank all the friends and colleagues in Australian Future Fibres

Research & Innovation Centre (AFFRIC) and Institute for Frontier Materials (IFM)

at Deakin University, for their upkeep and inspiration during my stay here. I enjoyed

the smiling and thanking culture in Australia, which introduced me to the new facet

of life. I loved being here, this is such a wonderful continent.

Finally, I would again express my gratitude to my lovely parents, brothers, sister,

daughters and devoted wife for supporting me every step of my student life.

Abstract

Nanofibres possess remarkable properties. A variety of techniques can produce

nanofibres but electrospinning stands distinct in terms of its simplicity and versatility.

Electrospun nanofibres have gained enormous attention over recent decades. Electrospun

nanofibres are predominantly produced in the form of randomly oriented web, which is

fragile and difficult to tailor into 3D complex structures. It is anticipated that the nanofibre

yarns have improved mechanical properties and can be easily converted into fibrous

structures. A few electrospinning setups have been reported to produce nanofibre yarns,

such as electrospinning over a liquid bath, solid collector plate, or on funnel type

collectors. In these setups, typically they electrospin and twist nanofibres in one place.

As a result, pile up on already twisted yarn on winder often occurs, which results in

beaded, fused or hooked nanofibres. It is a challenge to produce nanofibre yarns without

hooked nanofibres.

In this PhD project, a novel ring collector was used to convert newly electrospun

nanofibres into a yarn. This setup has been designed to separate electrospinning from yarn

drafting/twisting in two distinct zones. Three different types of electrospinning systems,

i.e. needle based, needleless, and needle/needleless hybrid, were employed to produce

nanofibres. Their effect on the nanofibre generation and morphology was examined.

Capability of these generators to produce stable inverted nanofibrous cone and continuous

nanofibre yarn from this cone was also investigated. The effect of processing parameters

such as electrospinning setup, voltage and distance on nanofibre generation, their

morphology, cone development, cone stability, yarn fabrication and continuity were also

studied.

Conjugated dual-needle and needle/needleless hybrid systems demonstrated their

ability to produce nanofibre yarns. Yarns with controllable diameters and twists levels

could be produced. The hybrid system could produce yarn at much higher rates than the

needle based system. The mechanical properties of electrospun nanofibre yarns from

these setups were also assessed. Nanofibre yarns showed improved mechanical properties

when compared to randomly-orientated and aligned nanofibre webs.

Table of contents

Dedications ................................................................................................................... iv

Acknowledgments ........................................................................................................ v

Abstract .......................................................................................................................... vi

List of Figures .............................................................................................................. xi

List of Tables ............................................................................................................ xvii

1 Introduction ........................................................................................................ 1

1.1 Significance of Research ............................................................... 1

1.2 Research Problems and Specific Aims ........................................ 3

1.3 Outline of Thesis.............................................................................. 5

2 Literature Review .............................................................................................. 7

2.1 Nanofibres ........................................................................................ 7

2.2 Electrospinning .............................................................................. 20

2.3 Fibre Alignment.............................................................................. 24

2.4 Discontinuous Nanofibre Bundles .............................................. 26 2.4.1 Non-Twisted Nanofibre Bundles ......................................... 26 2.4.2 Twisted Short Bundles ......................................................... 30

2.5 Non-Twisted Filament Yarns ....................................................... 37

2.6 Nanofibre Yarns ............................................................................. 42

2.7 Potential Applications ................................................................... 49

3 Materials and Characterisation ................................................................... 63

Materials ......................................................................................... 63 3.1.1 Solution Preparation ............................................................. 63

Electrospinning setups ................................................................. 63

Electrospinning of Nanofibre Yarns ............................................ 66 3.3.1 Yarn Formation from Needle Electrospinning .................. 67 3.3.2 Needleless electrospinning of nanofibre yarn .................. 68 3.3.3 Hybrid electrospinning of nanofibre yarns ......................... 72

Finite Element Analysis ................................................................ 74

Characterisations ........................................................................... 75 3.5.1 Scanning Electron Microscopy (SEM) ................................ 75 3.5.2 Mechanical Properties of Nanofibre Yarns ........................ 76

4 Nanofibre Yarn Prepared by Needle Electrospinning Systems ......... 77

4.1 Experimental ................................................................................... 77 4.1.1 Electrospinning of Nanofibre Yarn ...................................... 77

4.2 Preparation of Twisted Nanofibre Yarns .................................... 79

4.3 Influence of Different Factors on Yarn Electrospinning: .......... 81 4.3.1 Nozzle location ....................................................................... 81 4.3.2 Ring collector .......................................................................... 84 4.3.3 Twist feature ........................................................................... 86 4.3.4 Polymer concentration .......................................................... 87 4.3.5 Applied voltage ...................................................................... 90 4.3.6 Winding speed ....................................................................... 92 4.3.7 Flow rate ................................................................................. 94

4.4 Mechanical Properties of Nanofibre Yarns: ............................... 96

4.5 Conclusion ...................................................................................... 98

5 Needleless Electrospinning of Nanofibre Yarns .................................. 100

5.1 Electrospinning of Nanofibres and Nanofibre Yarns .............. 100

5.2 Needleless Electrospinning of Nanofibres ............................... 101 5.2.1 Cylinder Generator .............................................................. 101 5.2.2 Circular Coil Generator ....................................................... 103 5.2.3 Ring Generator .................................................................... 105 5.2.4 Disc Generator ..................................................................... 107 5.2.5 Comparison .......................................................................... 109

5.3 Needleless Electrospinning of Nanofibre Yarns ..................... 113 5.3.1 Cylinder Generator .............................................................. 114 5.3.2 Circular Coil Generator .......................................................1165.3.3 Ring Generator .................................................................... 118 5.3.4 Disc Generator ..................................................................... 120 5.3.5 Modified Yarn Electrospinning System ............................ 125

5.4 Conclusion .................................................................................... 129

6 Nanofibre Yarn Prepared by Hybrid Electrospinning Systems ....... 131

6.1 Experimental ................................................................................. 131 6.1.1 Preparation of Nanofibres .................................................. 131

6.2 Yarn Formation in the Hybrid System ...................................... 134

6.3 Factors Affecting Yarn Electrospinning ................................... 137 6.3.1 Effect of Positions of Different Elements ......................... 137 6.3.2 Disc Generator Parameters ............................................... 145 6.3.3 Polymer Concentration ....................................................... 153 6.3.4 Applied Voltage ................................................................... 158 6.3.5 Flow Rate ............................................................................. 161

6.4 Twist Level .................................................................................... 163

6.5 Yarn Production Rate ................................................................. 165

6.6 Mechanical Properties of Nanofibre Yarns ............................. 166

6.7 Conclusion .................................................................................... 168

7 Conclusions and Future Works ................................................................ 171

7.1 Main Conclusions of Thesis ....................................................... 171

7.2 Future Works ................................................................................ 174

8 References ...................................................................................................... 176

List of Figures

Figure 2.1 Present and future trend of global nanofibre consumption. ................. 8

Figure 2.2 Classification of nanofibre bundles. ....................................................... 11

Figure 2.3 (a) Setup for generating polymer spikes from a solution on a ferro-

magneto fluid [196], (b) Nanofibre generator of the Nanospider® (c) Bubble

electrospinning, [198] and (d) Needleless conical wire coil generator [199]. ..... 21

Figure 2.4 Needleless generators and their electric field profiles (a) and (d) disc

generator, (b) and (e) cylinder generator, and (c) and (f) circular coil generator

[207]. .............................................................................................................................. 23

Figure 2.5 Fibre alignment using (a) high speed rotating collector [212], (b) sharp

edge collector [214], (c) frame electrode [215] and (d) parallel electrode [216]. 24

Figure 2.6 Intensified electric field using (a) sharp needle encompassed in a drum

cylinder [213], (b) parallel sharp edge plates [217] and (c) sharp edge needle

placed at an angle [217]. ............................................................................................. 25

Figure 2.7 Photograph showing short nanofibre bundles over sharp blade edges

[217]. .............................................................................................................................. 27

Figure 2.8 Schematic of short nanofibre bundles over a rotary fin collector [228].

........................................................................................................................................ 29

Figure 2.9 (a) SEM image of short PVDF-HFP nanofibre bundle prepared by

twisting a nanofibre strip, (b) & (c) effect of twist on (b) bundle strength and (c)

bundle diameter[232] . ................................................................................................. 32

Figure 2.10 (a) Schematic for twist insertion and (b) SEM images of twisted

nanofibre bundle [218]. ............................................................................................... 33

Figure 2.11 Twisted single PEO nanofibre produced by auxiliary electrode. [40].

........................................................................................................................................ 34

Figure 2.12 (a) Schematic diagram for producing short nanofibre bundles[41], (b)

hollow hemi-sphere and sharp rod setup for nanofibre bundle collection, and (c)

SEM image of collected semi conducting metal oxide nanofibre bundle [236]. . 35

Figure 2.13 (a) Schematic setup consisting two needle nozzle generators over a

sharp needle collector for micro-rope manufacturing. (b) SEM image of twisted

micro-ropes [39]. ........................................................................................................... 36

Figure 2.14 Photograph of continuous non-twisted PLA/PAN and SWCNTs

filament yarn [42] .......................................................................................................... 38

Figure 2.15 (a) Schematic for conjugated needle and roller system. (b) SEM

image of non-twisted nanofibre filament yarn [44]. ................................................. 38

Figure 2.16 (a) Schematic of self-bundling by needle. (b) PAN nanofibre filament

yarn using organic salt [238]. ...................................................................................... 39

Figure 2.17 Non-twisted filament yarn from (a) single nozzle and circular disc

[242], multiple nozzles electro-spinning over, (b) disc and (c) conveyer belt. .... 40

Figure 2.18 (a) Schematic for collecting continuous nanofibre filament on liquid

bath [243], (b) SEM image of nanofibre filament yarn [244]. ................................. 41

Figure 2.19 (a) Schematic of bar manipulated electric field. (b) SEM image of

PAN nanofibre yarn [246]. ........................................................................................... 42

Figure 2.20 (a) Schematic double disc setup for simultaneously twist insertion

and winding. (b) SEM image of twisted nanofibre yarn [248]. ............................... 43

Figure 2.21 (a) Schematic of liquid vortex, (b) resultant twisted nanofibre yarn.

[251]. ............................................................................................................................... 45

Figure 2.22 (a) Schematic diagram of twisted nanofibre yarn developed by

vertical funnel and single needle generator, (b) SEM image of twisted nanofibre

yarn. [46] ........................................................................................................................ 46

Figure 2.23 Effect of funnel rotation speed on yarn twist level of nanofibre yarns.

......................................................................................................................................... 47

Figure 2.24 (a) Schematic diagram of horizontal funnel and double needle

generators. (b) Highly twisted PAN nanofibre yarn [47]. ........................................ 47

Figure 2.25 Photograph showing change of fibrous cone shape using tube with

(a) large diameter and (b) small diameter [252]. ..................................................... 48

Figure 2.26 (a) Continuous twistless PLLA-Zein/Zein nanofibre filament yarn [45],

(b) schematic of tapered and twisted yarn. (c) twisted NiO/ZnO nanofibre yarn.

[236, 237]. ...................................................................................................................... 49

Figure 2.27 (a) Biscrolled nanofibre single nanofibre yarn, plied nanofibre yarn

and nanofibre braided structure. (b) Solid state supercapacitor woven into glove.

[255] ................................................................................................................................ 51

Figure 2.28 (a) A paddle attached to biscrolled yarn to visualize torque in the

nanofibre yarn. (b) Niobium twisted yarn [257]. ...................................................... 52

Figure 2.29 (a) Woven PCL fabric [243], (b) Zein/PLLA composite nanofibre

yarns based plain weave fabric [45], (c) double layer plain weave composite fabric

of PAN (upper layer) and PVDF-HFP (bottom layer) nanofibre yarn. (d) single

jersey knitted fabric using PVD F-HFP nanofibre yarn and polypropylene

multifilament 251. ............................................................................................................ 54

Figure 3.1 Schematic for basic needle based electrospinning setup. ................. 64

Figure 3.2 Schematic for needleless generator based electrospinning setup. .. 65

Figure 3.3 Schematic illustration of needle based nanofibre yarn electrospinning

setup. .............................................................................................................................. 67

Figure 3.4 Needleless electrospinning setups for preparation of nanofibre yarns

(a) cylinder generator (b) circular coil generator (c) ring generator and (d) disc

generator. ...................................................................................................................... 69

Figure 3.5 Yarn electrospinning setup with an auxiliary electrode. ...................... 70

Figure 3.6 Yarn electrospinning setup with air jet enhancement. ........................ 71

Figure 3.7 Hybrid electrospinning setup for nanofibre yarn fabrication. .............. 73

Figure 3.8 Photograph of Neoscope JCM 5000 bench top SEM machine. ........ 75

Figure 3.9 Photograph of FAVIMAT with Robot 2 creel, showing pneumatic jaws

in magnified view. ......................................................................................................... 76

Figure 4.1 a) Schematic illustration of yarn electrospinning setup, b) photograph

of fibrous cone formed on ring collector, and c) inset shows nanofibre yarn

collected on a spool. .................................................................................................... 79

Figure 4.2 Schematic showing details of nozzle placement. ................................. 82

Figure 4.3 Effect of ring collector speed on a) fibre and yarn diameters, b) twist

level features. ................................................................................................................ 85

Figure 4.4 Effect of polymer concentration on nanofibre and yarn diameter

(Applied voltage = 13 kV, Ring collector speed 1200 rpm; total volume flow rate

3.0 ml/h, flow rate ratio for positive/negative nozzles 2.1:0.9). ............................. 87

Figure 4.5 Effect of Voltage on nanofibre and yarn diameter (polymer

concentration 17.0 %, ring collector speed 1200 rpm; total volume flow rate 3.0

ml/h, flow rate ratio for positive/negative nozzles 2.1:0.9). .................................... 91

Figure 4.6 Effect of winder speed on nanofibre and yarn diameter, (polymer

concentration 17.0 %, 13kV applied voltage , Ring collector speed 1200 rpm; total

volume flow rate 3.0 ml/h, flow rate ratio for positive/negative nozzles 2.1:0.9).

......................................................................................................................................... 93

Figure 4.7 Effect of flow rate on nanofibre and yarn diameter, (polymer

concentration 17.0 %, applied voltage = 13kV, Ring collector speed 1200 rpm;

flow rate ratio for positive/negative nozzles 2.1:0.9). .............................................. 95

Figure 4.8 a) Typical stress ~ strain curves of randomly-orientated nanofibre web,

aligned nanofibre web and nanofibre yarn and b) effect of twist level on stress

and strain of typical nanofibre yarns. ......................................................................... 97

Figure 5.1 SEM images of nanofibres produced from cylinder generator at

different magnification levels. ................................................................................... 102

Figure 5.2 SEM images of nanofibres produced using circular coil as fibre

generator. ..................................................................................................................... 104

Figure 5.3 SEM images for nanofibres generated by ring generator at different

magnification levels. ................................................................................................... 106

Figure 5.4 SEM images for nanofibres produced by disc generator. ............................ 108

Figure 5.5 Effect of generator geometry on fibre diameter. ................................ 110

Figure 5.6 Effect of generator geometry on nanofibre productivity. ................... 112

Figure 5.7 Schematic for nanofibre yarn production using cylinder as nanofibre

generator. .................................................................................................................... 115

Figure 5.8 Schematic for nanofibre yarn production using circular coil as

nanofibre generator. .................................................................................................. 117

Figure 5.9 Schematic for nanofibre yarn production using ring as nanofibre

generator. .................................................................................................................... 119

Figure 5.10 Schematic for nanofibre yarn production using disc as nanofibre

generator ..................................................................................................................... 121

Figure 5.11 Effect of needleless nanofibre generator on fibre and yarn diameter.

...................................................................................................................................... 122

Figure 5.12 Modified electrospinning setup with auxiliary metal electrode. ..... 126

Figure 5.13 Air jet enhanced yarn electrospinning setup. ............................................ 127

Figure 5.14 SEM images of nanofibre yarns produced by using electrospinning

setup with modification (a) auxiliary metal electrode and (b) pressurised air. . 128

Figure 6.1 Schematic illustration of hybrid electrospinning setup for making

nanofibre yarn. ............................................................................................................ 133

Figure 6.2 (a) Fibrous cone formed on top of ring collector, (b) SEM image of an

electrospun nanofibre yarn and (c) shows nanofibre yarn collected on a spool.

...................................................................................................................................... 135

Figure 6.3 Schematic diagram showing the position of different electrospinning

elements in hybrid nanofibre yarn setup. ............................................................... 137

Figure 6.4 Effect of disc-ring distance on fibre and yarn diameters. (Disc

generator at 50 kV, Needle fibre generator at 9 kV, 48 mm from ring, winder at

55 mm from ring generator. ...................................................................................... 139

Figure 6.5 Photographs of jets formed on a) HDP disc, b) copper disc c) steel

disc and d) aluminium disc. ...................................................................................... 145

Figure 6.6 Simulated electric field profile of polymer layer on a) conductive disc

and b) non-conductive disc ....................................................................................... 146

Figure 6.7 Calculated electric field profile of polymer layer on a) conductive disc

and b) non-conductive disc. ...................................................................................... 148

Figure 6.8 Effect of disc diameter on fibre and yarn diameters. (Applied voltage

for disc and needle fibre generators was 50 kV and 9 kV, disc, needle and winder

were placed at a distance of 60 mm, 48 mm and 55 mm, from the ring collector

respectively. ................................................................................................................. 150

Figure 6.9 Effect of disc thickness on fibre and yarn diameters. (Disc and needle

fibre generator at 50 kV and 9 kV respectively; disc, needle and winder were

placed from ring collector at a distance of 60 mm, 48 mm and 55 mm

respectively). ............................................................................................................... 152

Figure 6.10 Effect of polymer concentration on fibre and yarn diameters. Applied

voltage for the disc- and the needle- spinning was at 50 kV and 9 kV respectively.

Disc, needle and winder were placed from ring collector at a distance of 60 mm,

48 mm and 55 mm, resp ............................................................................................ 155

Figure 6.11 Effect of applied voltage on fibre and yarn diameters. (Needle fibre

generator at 9 kV; polymer concentration for needle system was 17.0% and

polymer concentration in polymer bath was 15.0%, disc, needle and winder were

placed from ring collector at a distance of 60 mm, 48 mm and 55 mm respectively.

....................................................................................................................................... 159

Figure 6.12 Effect of ring speed on twist levels and twist angles in electrospun

nanofibre yarns. .......................................................................................................... 164

Figure 6.13 (a) Typical stress ~ strain curves of random nanofibre web, aligned

nanofibre web and nanofibre yarn, (b) Effect of twist levels on stress and strain

of nanofibre yarns. ...................................................................................................... 167

List of Tables

Table 2.1 Different nanofibre making techniques ................................................... 12

Table 2.2 Commonly used polymer materials in electrospinning ......................... 13

Table 4.1 Effect of nozzle distances on yarn formation*........................................ 83

Table 4.2 Effect of polymer concentration on yarn electrospinning ..................... 89

Table 5.1 Effect of generator geometry on fibre morphology, cone stability and

yarn formation. ............................................................................................................ 123

Table 6.1 Effect of nanofibre generators and winder position on cone stability and

yarn formation. ............................................................................................................ 141

Table 6.2 Effect of polymer concentration on yarn electrospinning ................... 157

- 1 -

C H A P T E R O N E

1 Introduction

1.1 Significance of Research

Nanofibres have a diameter in the range of nanometres, and possess large

surface to mass ratio. They exhibit superior physiochemical properties to their

micrometre counterparts. The unique characteristics together with the

functionalities from the material itself have offered huge potential for nanofibres

to be used in a number of areas e.g., filters, sensors, drug delivery, biomedical,

protective clothing, catalysis energy generation and storage. Although the

concept of electrospinning was observed a century ago, it has not received any

attention until very recently.

Nanofibres are predominantly produced in the form of randomly oriented

fibrous web, which are fragile and require sophisticated technology to process

them. By converting nanofibres into continuous twisted nanofibre yarns, their

strength improves and they can be easily tailored into complex 3D structures.

These complex 3D structures are anticipated to exhibit novel functionalities. It

is anticipated that the fabrics developed from nanofibre yarns would be softer,

lighter, tougher, flexible and more absorbent than traditional fabrics. Owing to

interconnected pores, these fabrics would possess increased number of

capillaries; helping in better wicking, superior drug release and enhanced

proliferation of cells. Therefore, fabrics produced from nanofibre yarns could be

a good candidate in wound dressings, scaffolds, sanitary products, composite

reinforcement, high efficiency filters and wearable electronics.

Chapter 1: [Introduction and Research Aims]

In some instances, the formation of short twisted nanofibre bundles or

cut strip webs after electrospinning have been reported. Continuous non-twisted

nanofibre yarns have also been produced using non-solvent liquid baths.

Nanofibres were directly deposited on the liquid surface and wound into yarns.

Using vortex at the bath base resulted in twisted nanofibre yarns. Although liquid

bath helped in neutralizing nanofibres, it adversely affected nanofibre

morphology, orientation and ingredients. It was also very difficult to control

twist and uniformity in as-prepared yarns, and only insoluble polymers could be

processed. Direct deposition of nanofibres over solid collectors and twisting

them into nanofibres without utilizing liquid bath is extremely encouraging. The

yarns produced from these setups demonstrated well defined twists and superior

mechanical properties.

Recently, usage of conical funnel or rotary tube as transient collector had

greatly increased yarn production rates. During electrospinning nanofibres were

manually dragged to the winder to form a fibrous cone over collector periphery.

The funnel rotation inserted twists, while cone apex was continuously drafted to

form nanofibre yarn. This setup could produce nanofibre yarns with controllable

fibre/yarn diameters and twist levels. However, the nanofibre generation and

yarn forming were carried out in the same zone, the already twisted yarns was

contaminated with later electrospun nanofibres. Consequently, fused, beaded

and hooked nanofibres were observed, deteriorating yarn quality.


1.2 Research Problems and Specific Aims

The electrospinning setups reported so far generate nanofibres and yarn

in the same region. Consequently, the yarns produced from these systems show

beads, fused and hooked nanofibres, which deteriorate fibre morphology,

alignment and overall yarn quality. It is still a challenge to produce nanofibre

yarn without beads, fused or hooked nanofibre structure. Therefore, the overall

objective of this PhD study is to find a new way to electrospin nanofibre yarn. It

is preferable that nanofibre and yarn are formed in two distinct zones, such that

the above mentioned problems are eliminated. The new setup will prevent

beaded, fused or hooked nanofibre formation and exclude these defects in final

nanofibre yarn. Accordingly, the research focuses on three specific aims as

described below:

Specific Aim 1: To develop a novel needle electrospinning setup

capable of electrospin nanofibres and convert nanofibres into a yarn in a

zone beyond fibre generating area; and study the structure-property

relationship.

Needleless electrospinning is anticipated to have higher nanofibre

production rate than needle electrospinning. Although considerable work has

been carried out in nanofibre yarn fabrication, most of nanofibre yarns are

prepared using needle based electrospinning technique. These yarn

electrospinning techniques typically have a low yarn electrospinning rate.

Nanofibre yarns prepared by needleless electrospinning technique is highly

desired to have higher yarn production rate, but needleless yarn electrospinning


technique has not been reported in research literature. The second specific aim

of this PhD thesis is:

Specific Aim 2: Production of nanofibre yarn using needleless

electrospinning setup, and elucidating the effect of fibre generator on fibre

and yarn morphology.

As nanofibre yarn manufacturing is still in its early stages, it is not

surprising to note that no nanofibre yarn is prepared from a hybrid system

combining both needle and needleless electrospinning system. It is interesting to

see whether hybrid electrospinning system can be used to produce nanofibre

yarns. This leads to the third specific aim of this research:

Specific Aim 3: To use a combination of needle and needleless

electrospinning systems for nanofibre yarn production, and evaluate effects

of processing parameters on yarn formation.

In short, this project mainly revolves around the development of novel

electrospinning setup for production of nanofibre yarns, and effects of

electrospinning parameters on yarn formation.


1.3 Outline of Thesis

Apart from this introduction chapter, this thesis includes further six

chapters as detailed below:

Chapter 2 is a literature review summarizing the recent progress in the

nanofibres, nanofibre-making techniques, polymer materials and their solvents

used for electrospinning, applications of nanofibres, nanofibre alignment,

techniques used for formation of nanofibre bundles and nanofibre yarns, and

potential applications of nanofibre yarns.

Chapter 3 details polymer materials and solvents used in this research,

basic electrospinning for nanofibre production, novel needle, needleless and

hybrid yarn electrospinning setups, and characterisation techniques.

Chapter 4 deals with the development of a novel needle yarn

electrospinning system. Effect of material and processing parameters on

nanofibre morphology, fibrous cone and nanofibre yarn were investigated.

Mechanical properties of nanofibre yarn was also studied and compared with

aligned nanofibre web and nanofibre nonwoven. Effect of twist levels on yarn

tensile property was also examined.

Chapter 5 describes experiment results about needleless electrospinning

using cylinder, circular coil, ring and disc as fibre generator. Nanofibre

production, fibre morphology, fibrous cone formulation and possibility of

nanofibre yarn production using needleless generators was assessed. Effect of


auxiliary electrode and compressed air system on quick drying of nanofibres and

yarn fabrications were also evaluated.

Chapter 6 reports the usage of hybrid electrospinning system in

combination of needle and needleless electrospinning to produce nanofibre yarn.

Impact of different processing parameters on nanofibres, cone stability and

continuous yarn assembling was studied. Mechanical properties of nanofibre

yarns and effect of twist levels on yarn tensile properties were also evaluated.

Chapter 7 The chapter sums up the entire work done in this project. This

chapter also proposes future works in this specific area.

- 7 -

C H A P T E R T W O

2 Literature Review

2.1 Nanofibres

Nanofibres also known as superfine fibres have diameter smaller than 1

micron 1 and aspect ratio (length to diameter ratio) at least 100:1 2. Nanofibres

show considerably improved physiochemical properties in comparison to fibres

of greater diameter. They possess high surface-to-mass ratio and excellent pore

inter-connectivity. Nanofibres have shown enormous potential for applications

in areas as diverse as filtration 3, sensor 4, drug delivery 5-7, biomedical 8-11,

protective clothing 12, catalysis 13,14, energy storage 15 and generation 16-18.

Several techniques have been developed to produce nanofibres, such as drawing

19,20, template-synthesis 21, phase-separation 22, centrifugal-spinning 23, bi-

component extrusion 24, solution blowing 25, chemical vapour deposition 26, and

hydrothermal 27-29. Table 2.1 describes different nanofibre making techniques,

materials and fibre dimensional features.

Electrospinning is a simple but efficient technique to produce nanofibres. This

technique involves drawing a polymer fluid under a strong electric field into fine

filaments, which deposit randomly on the collector to form a nonwoven web in

the most cases. During electrospinning, “Taylor-cone” is developed from the

fluid surface, and jet ejects from Taylor-cone when the electric force is strong

enough to overcome the surface tension of the fluid. The charged jet typically

undergoes a whipping movement due to the intensive interaction with the

Chapter 2: [Literature Review]

external electric field, stretching itself thinner. With this technique, nanofibres

are prepared from a number of polymers. Table 2.2 enlists polymer materials

mainly used for electrospinning of nanofibres.

Electrospinning has become a popular fibre-making technique since

1995, although the concept of drawing liquid with electric force was known as

early as 16th century. Practical success of using electrostatic force to spin a liquid

into fibres was first patented by Formhals et al. 30,31 in 1930s. In the patents, the

formation of yarns from polymer solutions with a motive to replace expensive

silk fibres was also claimed. The invention of nylon (polyamides) and eruption

of World War II subdued electrospinning as it could not compete with the bulk

production of nylon and later polyester.

Since the revival of electrospinning research in recent decades, the

slowly creeping global market of nanofibres has suddenly gained enormous

momentum. As revealed in Figure 2.1 the nanofibre market is expected to

Figure 2.1 Present and future trend of global nanofibre

consumption.


increase 1,200% in less than a decade. It was predicted that chemical and

mechanical sectors will utilize 70% of total nanofibre production, while energy

and electronics sectors will encompass a major share of remaining nanofibres 32.

Ever increasing consumption of nanofibres will definitely require scaling

up of nanofibre production. However, low production, frangibility and random

arrangement of nanofibres have been identified as bottle-necks in nanofibre

commercialization. Most of the initial researches in electrospinning were

focused on preparation and characterisation of randomly-oriented nanofibre

webs 33. A recent study conducted on tissue engineering has revealed that cells

show better proliferation on aligned nanofibrous structures 34. In addition,

aligned nanofibre bundles have been demonstrated to have better mechanical

strength than randomly-oriented nanofibre webs35.

The pursuit for enhanced strength and bulk production of nanofibres

encouraged researchers to find new methods to produce aligned nanofibre yarns.

While a lot of papers reported on non-twisted 36-38 or twisted 39-41 nanofibre

bundles and yarns 42-48, not a single study was found covering all the aspects of

nanofibre mass production, alignment and formation of nanofibre bundles/yarns.

In this chapter, two major electrospinning techniques, needle and

needleless electrospinning, and fibre alignment are briefly described. Detailed

accounts on different techniques for producing aligned short fibre bundles (in

either twisted or non-twisted form), continuous filament yarn, and nanofibre

yarns are elaborated. It is expected that this literature review provides overall

understanding of underlying concepts for producing nanofibres on a mass scale,


electrospun nanofibre bundle and yarn formation, and assistance in planning

future research in the field of electrospun nanofibre yarns and their subsequent

structures.

Owing to the enormous application potential, electrospun nanofibres

have attracted wide interests from researchers with diversified backgrounds, in

both textile and non-textile fields. Fibre and yarn terminologies are occasionally

used interchangeably, which create misunderstanding. For example, yarn was

used in some papers to describe fibre filaments, while nanofibre filaments were

used to describe nanofibre yarns in some others. To eliminate this confusion, it

is necessary to clarify these terms.

Filament: A fibre of indefinite length.

Fibre bundle: An assembly of small fibres aligned in a specific direction.

Filament yarn: A long non-twisted array of one or more filaments.

Yarn: A long continuous length of interlocked (twisted) fibres.

According to the structural characteristics, nanofibre bundles can be categorized

in few classes as shown in the Figure 2.2.


Figure 2.2 Classification of nanofibre bundles.

- 12 -

Techniques Materials Diameter range Length continuity Refs.

Drawing Inorganic salt, ionic solid 2 nm–100 nm Discontinuous 19

Phase separation Polymer, metal, metal oxide 40 nm–280 nm Discontinuous 22,49-51

Hydrothermal Metallic oxide 2 nm–200 nm Discontinuous 27-29

Self-assembly Polymer 6 nm–100 nm Discontinuous 52-54

Chemical vapour deposition (CVD) Graphitic nanofibres, metal oxide 50 nm–200 nm Discontinuous 55,56

Microfibre extrusion Thermoplastic polymer 39 nm–1.8 μm Continuous 24,57,58

Solution blow spinning Polymer 80 nm–260 nm Discontinuous 25,59,60

Centrifugal spinning Polymer melt or solution Average 300 nm Continuous 23,61

Hydro-entanglement Flax fibres 10 nm–50 nm Discontinuous 62

Solvent precipitation Solution processable polymers 200 nm–2.0 μm Discontinuous 63

Electrospinning

Polymers, inorganic, organic,

biomaterial, nanoparticles, ceramics

solution or melt

16 nm–800 nm Continuous 63-65

Table 2.1 Different nanofibre making techniques

- 13 -

Polymer Solvent Fibre diameter Applications Refs.

CA (Cellulose acetate) DMF, Acetone, DMAc,

AcOH 30 nm–1 μm Biosensors 66-69

Chitosan HCl, AcOH, FA, DCM,

TFA 200 nm–1000 nm

Biomedical, drug delivery, tissue

engineering 70-72

Poly[(9,9-di-n-octylfluorenyl-2,7-diyl)alt-

(benzo[2,1,3] thiadiazol-4,8-diyl)]

THF, DMSO, Toluene,

CHCl3 100 nm–700 nm

Photonics, optoelectronics, lab-

on-chips 73,74

P(LLA-CL) Poly(L-lactide-co-

epsiloncaprolactone) Acetone 200 nm–1.5 μm

Tissue engineering, scaffold,

blood vessel 75,76

PA (Polyamide) DMF, HFP, FA 50 nm–500 nm Nano-composite, optical-sensors 77-79

PAA (Poly acrylic acid) DMF, H2O 100 nm–400 nm Optical Sensors, gas sensors 78,80,81

PAN (Polyacrylonitrile) DMF 50 nm–300 nm Nano-electronics, carbon

nanofibres, composites 65,82-84

PANi (Polyaniline) CHCl3, SA, HFP 96 nm–500 nm

Nano-electronics, p–n junctions,

chemical sensors and micro-

electrodes

65,85

Table 2.2 Commonly used polymer materials in electrospinning

- 14 -


PBI (Poly benzimidazole) DMAc 100 nm–500 nm

Carbon nanofibres, strain sensor,

electromagnetic shield, nano-

composites

86,87

PBT (Polybutylene terephthalate) HFP, TFA 125 nm–2500 nm Tissue engineering 88-90

PC (polycarbonate) THF, CHCl3, DMF 100 nm–3 μm Nano-electronics 91-93

PCL (Poly(-caprolactone)) Acetone, CHCl3 200 nm–5 μm Scaffold, tissue engineering 86,87

PCU (polycarbonate urethane) DMAc 1 μm–3.5 μm Tissue engineering, scaffold 94,95

PDS (Polydioxanone) HFP, DMSO 180 nm–1.4 μm Biomedical applications, scaffold, 96-98

PEDOT

(Poly(3,4-ethylenedioxythiophene))

CHCl3,1-propanol,

1-butanol 40 nm–7 μm

Biosensors, medical implants,

flexible capacitors 99,100

PEI (Polyether imide) HFP, CHCl3, H2O 50 nm–15.1 μm Affinity membranes,

nano-sensors 101

PEO (Poly ethlyene Oxide) H2O, EtOH 80 nm–350 nm Biomedical applications, tissue

engineering 102,103

PES (Polyethersulfone) DMF 80 nm–424 nm Filtration, tissue engineering,

biomedical 104,105

PEVA (Polyethylene-co-vinyl acetate) CHCl3, MeOH 1.2 μm–22 μm Drug delivery, tissue engineering 106-108

- 15 -


PFO-PBAB (Poly[(9,9-dioctylfluorenyl-2,7-

diyl)co-(N,N -diphenyl)-N,N -di(p-butyloxy-

phenyl)-1,4-diaminobenzene])

CHCl3 DMSO, THF 180 nm–1.5 μm Sensors, photonic circuits

optoelectronic 109

PFS (Poly(ferrocenylsilane)) DMF, THF, 450 nm–2 μm Manipulators, switches, sensors 110,111

PGA (polyglutamic acid)) TFA, H2O 150 nm–600 nm Biomedical, tissue engineering,

drug delivery 112,113

PGCA (Poly(glycolic acid)) HFP 50 nm–1.5 μm

Tissue engineering, surgical

sutures, implant materials and

drug delivery

114,115

PGS (Poly(glycerol sebacate)) HFP 450 nm–1.4 μm Tissue engineering, regenerative

medicine 116,117

PHBV (Poly(hydroxybutyrate-

cohydroxyvalerate)) CHCl3 up to 800 nm

Tissue engineering, regenerative

medicine, drug delivery,

biodegradable implants

118,119

PHB (3-hydroxybutyric acid) HFP, CHCl3 250 nm–1000 nm Biodegradable implant materials,


PHMA (Poly(hexamethylene adipate)) TFA, DCE 100 nm–700 nm

Medicine, pharmacy, agriculture,

biodegradable implants, drug

delivery

122,123

- 16 -


PIB (Polyisobutylene) TCE, EtOH, Toluene,

THF 200 nm–800 nm Biomedical, tissue engineering 124-126

PLA (Poly(L-lactide)) DCM, DMF 50 nm and 3.5 μm Scaffolds 127,128

PLA-PEG (D,L-Lactide-polyethylene glycol) DCM, DMF 250 nm and 875 nm Biomedical, tissue engineering 129,130

PLGA (Poly(lactic-co-glycolic acid)) DMF, THF, HFP, CHCl3 100 nm–865 nm Drug deliver, wound healing,

tissue engineering 131,132

PLLA (Poly(L-lactide)) DCM, DMF, CHCl3,

Acetone, DCE 200 nm–3 μm Biomedical 133-135

PLLA-CL (Poly(L-lactide-co-caprolactone)) Acetone, DCM, HFP 200 nm–7.0 μm Tissue engineering, surgery and

drug delivery, Biomedical 136-139

PMAPS (poly(2-acrylamido-2methyl-1-

propanesulfonic acid)) EtOH, H2O 30 nm–155 nm – 140,141

PMMA (Poly(methyl methacrylate)) THF, Acetone, CHCl3,

DMF, FA 150 nm–13 μm

Biomedical, tissue engineering,

scaffolds 142,143

- 17 -


PMSQ (Polymethylsilsesquioxane)

Acetone, n-Butyl acetate,

CHCl3,

Cyclohexanone, EtOH,

Ethyl acetate, MeOH,

Methyl acetate, DCM,

1-Pentanol, THF

400 nm–3.5 μm _ 144,145

PNIPAM

(Poly(N-isopropylacrylamide)) DMF, H2O, MeOH 165 nm–1.2 μm

Thermo-responsive actuators,


PPC (Poly Propylene carbonate) CHCl3, DCM 900 nm–5 μm Scaffolds, implant materials,

biomedical 148,149

PPTA (Poly(p-phenylene terephthalamide)) SA 40 nm–80 nm Filtration, composites, suture 150

PPy (Polypyrrole) CHCl3, DMF, H2O 70 nm–3 μm Battery, super capacitor, nano-

electronic devices 151,152

PS (Polystyrene) DMF, THF 1 nm–3 μm – 153,154

PSBMA (Poly(sulfobetaine methacrylate)) H2O, MeOH 200 nm–1.1 μm Filtration and wound dressing 155-157

PU (Poly urethane) DMF, THF, EtOH, 7 nm–1.5 μm Filtration, scaffolds, wound

dressing 158-160

- 18 -


PVA (Poly Vinyl Alcohol) H2O 50 nm–2 μm

Regenerative medicine, scaffold,

tissue engineering, contact lenses,

cartilage skin

160-163

PVAc (Poly(vinyl acetate)) DMF, CHCl3, AcOH 300 nm–1000 nm

Electronic devices, solar cell,

tissue engineering, blood vessel,

wound dressing

164-168

PVC (Polyvinyl Chloride) DMAc, THF, DMF 170 nm–1.2 μm OLED devices, energy transfer,

battery 169,170

PVCz (Poly(N-vinylcarbazole)) DCE, DCM 2 μm–14 μm Energy transfer, photo-induced

charge transfer 171-173

PVDF (Polyvinylidene fluoride) Acetone, DMAc, DMF 70 nm–250 nm Battery, super capacitor, fuel cell 174,175

PVDF-HFP (Poly(vinylidene fluoride-co-

hexafluoropropene))

EC, EMC, DMF,

Acetone 100 nm–625 nm Fuel cell, battery, capacitor 176-178

PVDF-TrFE

Poly(vinylidenefluoride-cotrifluoroethylene) MEK 550 nm–3.5 μm

Piezoelectric scaffolds, tissue

engineering 179,180

PVP (Poly(vinylpyrrolidone)) EtOH, AcOH, DCM,

DMF 20 nm–500 nm Tissue engineering, catalytic 181,182

- 19 -

Note:

1,2-dichloroethane DCE Ethylmethyl carbonate EMC 1,1,1,3,3,3-hexafluoro-2-propanol HFP Formic acid FA Acetic acid AcOH Methanol MeOH Chloroform CHCl3 Methylethylketone MEK Di chloroacetic acid DCA Sulfuric Acid SA Dichloromethane DCM Tetrahydrofuran THF Diluted hydrochloric acid HCl Trichloroethylene TCE Dimethyl sulfoxide DMSO Trifluoroacetic acid TFA Dimethylacetamide DMAc Water H2O Dimethylformamide DMF Ethylene carbonate EC Ethanol EtOH Ethylene dichloride EDC


- 20 -

2.2 Electrospinning

Electrospinning has a number of advantages over other nanofibre making

techniques. This technique can produce long continuous fibres with controllable

diameter ranging from nanometers to microns. A wide variety of materials

including polymers, inorganic compounds, biomaterials, nanoparticles and

ceramics can be utilized to produce electrospun nanofibres. Fibrous membranes

made of electrospun nanofibres are highly porous with excellent pore

interconnectivity. Modified electrospinning techniques can produce nanofibres in a

number of different configurations, including sheath-core, side-by-side, islands-in-

a-sea and hollow nanofibres. Electrospinning parameters can be controlled easily

to produce nanofibres with desired surface morphology, fibre structure, component

and fibrous architecture.

Although electrospinning has proved its supremacy over other nanofibre

producing techniques, conventional needle electrospinning can only produce a

limited amount of nanofibres ranging from 0.1 to 1.0 gram per hour 183 or process

polymer solution with a flow rate of 1.0–5.0 mL/h 184. Some researchers increased

nanofibre production using gas-blowing 185-187, multi-jets 188-191 or multiple needles

192-195. Even with these techniques, the fibre productivity could only be increased to

a certain extent beyond that electrospinning process was adversely affected.

Needleless electrospinning was developed as an alternative for production

of nanofibres on much larger scale than conventional needle electrospinning. It not

only breaks the production limitation of electrospinning from needle nozzle 183, but


- 21 -

also eliminates problems of inter-jet repulsion, needle-clogging and huge-space

requirement to setup a multi-needle system 192.

Yarin et al. 196 reported upward electrospinning of nanofibres from a

polymer solution without using needle like nozzle. They placed a ferromagnetic

fluid underneath a polymer solution (as shown in Figure 2.3a). Although nanofibres

could be electrospun from the upper solution because of spikes created by magnetic

field, little information was provided on the control of nanofibre generation sites,

polymer flow rate and fibre diameter.

Figure 2.3 (a) Setup for generating polymer spikes from a solution on a ferro-

magneto fluid [196], (b) Nanofibre generator of the Nanospider® (c) Bubble

electrospinning, [198] and (d) Needleless conical wire coil generator [199].

Jirsak et al. 197 were the first to report using metallic roller which was

partially immersed in polymer solution as fibre generator for producing nanofibres.

The rotation of the roller loaded polymer solution on the top surface from which a

number of jets/nanofibres were generated (Figure 2.3b). This design was

subsequently commercialized by Elmarco Ltd under the trade name of

Nanospider®.

Gas bubbles 198 were reported to initiate electrospinning from an open

solution surface in 2007 (Figure 2.3c). Nitrogen gas was blown into polymer


- 22 -

solution to form multiple jets and then nanofibres. In 2009, a conical wire coil was

used as a fibre generator to prepare nanofibres. In this setup, the polymer solution

was conveyed to the spinning sites under the action of gravity and guided by the

coil structure (Figure 2.3d). This system can produce high-quality nanofibres with

a significantly increased production rate compared with needle electrospinning 199.

Later on, a few fibre generators, for example, metal plate 200, splashing spinneret

201, rotary cone 202, cylinder 203, and bowel edge 204 were reported for needleless

electrospinning.

The influences of spinneret geometry on needleless electrospinning process

and fibre quality were examined by Niu et al. 205 who also proved the crucial role

of spinneret shape on needleless electrospinning. They found that a disc spinneret

formed higher intensity electric field thus exhibiting a better electrospinning

performance when compared with a cylinder spinneret (Figure 2.4d & e) 206.

Inspired by these results, the same group also invented a spiral coil setup

and proved that spiral coil had higher fibre production rate and better control toward

the fibre morphology compared to disc and cylinder electrospinning (Figure 2.4e)

207. More recently, a moving bead chain has been used as a spinneret to electrospin

nanofibres needlelessly 208.

Subsequently, wire electrode 209-211 rotating in polymer bath solution has been

utilized to produce nanofibres. Although this system has been described to produce

variety of nanofibres (simple nanofibres, core-shell and fibres containing

nanoparticles) yet its production rate is very low and it is very difficult to control

volume flow of polymers being electrospun. The considerably-increased nanofibre

production has allowed needleless electrospinning to produce nanofibres for


- 23 -

practical applications. However, issues still remain in precisely controlling fibre

diameter, morphology and deposition, as well as polymer concentration during

needleless electrospinning.

Figure 2.4 Needleless generators and their electric field profiles (a) and

(d) disc generator, (b) and (e) cylinder generator, and (c) and (f) circular

coil generator [207].


- 24 -

2.3 Fibre Alignment

Early attempts to produce aligned nanofibres were carried out by collecting

nanofibres on high speed rotating collectors (Figure 2.5a & b) 212-214. The fibres

were taken away by rotating surface, forcing nanofibre deposition in much aligned

manner. Stationary collectors such as metal frame and parallel electrode were also

used for fibre alignment (Figure 2.5 c & d). A high degree of fibre alignment was

achieved between the electrodes. During electrospinning toward the parallel

electrodes, nanofibres tend to deposit on the surface where the electrical potential

is lowest. The attachment of the fibres on one of the electrodes leads to immediate

increase in the potential and the generation of a local field pushes the later deposited

fibre segment to align vertically on the electrode.

Figure 2.5 Fibre alignment using (a) high speed rotating collector [212], (b)

sharp edge collector [214], (c) frame electrode [215] and (d) parallel

electrode [216].

The fibres also preferably approach the second electrode, resulting in

bridging the fibre between the electrodes. In this way, up-coming nanofibres layer

up in aligned fashion. Apart from frame and parallel electrode stripe, blades, rings

and discs were also used for collecting aligned nanofibres. Such a gap-collection

resulted in aligned nanofibre bundles 215-223.


- 25 -

Some papers reported on preparation of aligned nanofibres using a rotating

roller in front of grounded sharp-edge counter electrode (Figure 2.6). The sharp-

edged configuration presents strong pointed electric field and attracts nanofibres

toward surface 213,217. The alignment direction of fibres on the roller collector can

be adjusted through the relative angle between the axis of the roller and the

electrospinning direction.

Figure 2.6 Intensified electric field using (a) sharp needle encompassed

in a drum cylinder [213], (b) parallel sharp edge plates [217] and (c)

sharp edge needle placed at an angle [217].


- 26 -

2.4 Discontinuous Nanofibre Bundles

Recently, considerable efforts have been made to prepare yarns from

electrospun nanofibres. Early papers have focused on the production of short fibre

bundles, either twisted or non-twisted. Although some authors named these fibrous

structures as yarns, they are actually beyond the scope of yarns due to the lack of

basic features defined (section 2.1).

2.4.1 Non-Twisted Nanofibre Bundles

Fong et al. 224 produced aligned nanofibre bundles by swinging a grounded-

frame between nozzle and collector electrode. The vibrating frame forced

nanofibres to align on the frame. Teo et al. 217 used two parallel blades to deposit

nanofibres in bundle form on sharp blade edges (Figure 2.7). A fibre bundle formed

in this way often had a short length. When blade distance was longer than 10 cm,

no nanofibre bundle was formed between the blades.

Although the majority of the fibres were aligned along bundle axis, many

stray fibres were still seen. As these nanofibres had similar charges, they repelled

each other, to form compact structures; these bundles were dipped in de-ionized

water to remove charges. They also examined effect of blade polarity on deposition

pattern, and indicated that negative polarity produced more aligned and compact

nanofibre bundles than neutral polarity. Positively charged fibres were attracted by

negatively charged blades. A dense nanofibre bundle was therefore formed in

confined area


- 27 -

Figure 2.7 Photograph showing short nanofibre bundles over

sharp blade edges [217].

Jalili et al. 225 analysed the effect of gap between two collector plates on the

formation of nanofibre bundles. It was indicated that the overall nanofibre density

decreased as distance between parallel plates increased, while the degree of

nanofibre alignment increased first with the increase in distance, and then declined

with a further increase in distance.

Some of the studies have mentioned the formation of self-assembled

bundles directly on the electrode collector 37,226. Intrinsic ionic conduction 37 or

conductivity induced by the addition of salt to polymer solution 226 were reported

to cause nanofibres to deposit perpendicularly to the collector, and further

electrospinning of fibres resulted in formation of loosely-packed bundles.

Most of electrospinning experiments have utilized DC (direct current)

power supply to produce nanofibres. Maheshwari et al. 227 used AC (alternate

current) power supply to electrospin nanofibre bundles. By using AC power supply

during electrospinning, polymer jet was split into multiple jets. Before reaching

collector electrode, these jets bundled together. It was suggested that change of

electric polarity from positive to negative weakened the electric field in single cycle


- 28 -

of AC, thus hindering jet stretching. In next AC cycle, change of polarity, caused

jet split into multi-filament from previously inadequately stretched jet. As both

negative and positive charges were induced in multi-jets by polarity change in

single cycle, the fibres attracted each other to form a bundle in mid-air. In addition,

combination of negative and positive charges neutralized overall fibre bundle, fibre

bundles could be easily removed even with a small puff of air.

Afifi et al. 228 prepared aligned nanofibre bundles on a special rotary

collector with fins as illustrated in Figure 2.8. Needle nozzle was placed in front of

a rotating collector. Nanofibres aligned over the fins were removed as nanofibre

bundles. It was reported that collector rotary speed showed little effect on nanofibre

bundle formation. However, applied voltage and nozzle-collector distance were

important parameters affecting the formation of nanofibre bundle. It was also

indicated that heating and drafting improved mechanical properties and crystal

orientation of these nanofibre bundles.

Apart from needle electrospinning, needleless electrospinning has also been

reported to form nanofibre bundle on a stationary target. For example Chvojka et

al. 38 reported using Nanospider® with double comb-like target to collect nanofibre

bundles. Nanofibres had better alignment and more compact structure on the spikes

of same comb whereas nanofibres bridging between the two combs were randomly

arranged. These bundles had beaded-structures along with nanofibres, a typical

problem often seen in needleless electrospinning systems, as there is no control over

polymer flow rate, nanofibre generation sites and solvent evaporation from

needleless generators.


- 29 -

Figure 2.8 Schematic of short nanofibre bundles over a rotary

fin collector [228].


- 30 -

2.4.2 Twisted Short Bundles

In conventional fibre and textile areas, twists were often inserted into a fibre

bundle with the purpose of improving the mechanical strength, because twists can

compress the bundle and increase adhesion and friction between fibres. Moreover,

twists improve the bundle uniformity. Several works have been reported to insert

twists into nanofibre bundles either through a post-spinning process or during

electrospinning.

2.4.2.1 Post-electrospinning Twisting

A few groups have reported the preparation of twisted nanofibre bundles by

cutting electrospun web into narrow strips and subsequently twisting them 229-231.

Fennessey et al. 229 used fast rotating collector electrode to produce aligned PAN

nanofibre webs. These webs were utilized to prepare fibre bundles. After twisting,

these bundles were rinsed with de-ionized water to remove remaining charges. They

observed an initial increase in ultimate strength and modulus with increasing twist

angle. However with further increase in twist, the mechanical properties decreased.

Stainless-steel annular collector has been used to collect highly aligned

PAN nanofibres on its periphery 232. Circular edge condensed electric field on the

annular collector circumference, resulting in aligned deposition of nanofibres as

thick fibre bundle. These aligned nanofibre bundles were stretched and twisted by

careful removal from the collector to form short twisted bundles.

He et al. 233 used co-axial electrospinning to load a drug into fibre bundles.

Highly aligned PLLA nanofibres were electrospun and deposited on a fast rotating

wheel. These bundles were twisted and hot-stretched on a custom made stretching


- 31 -

assembly to improve mechanical properties. Loading of drug into fibre showed

improvement in fibre mechanical properties. Drug particles in the fibre can act as a

filler to reinforce the fibres. The composite structure stiffened the fibres and

reduced the overall plasticity of the fibre bundle.

Moon et al. 230 studied the effect of carbonization temperatures, heating

rates, heat-exposure time and drawing ratio on ultimate strength of PAN nanofibre

bundles converted into carbon nanofibre bundles. The ultimate tensile strength of

PAN cut-strip-bundles after carbonization was 1.0 GPa, whereas the twisted PAN

nanofibre bundles after carbonization showed improved ultimate tensile strength of

1.7 GPa.

Uddin et al 234 added MWCNTs (multi walled carbon nanotubes) into PAN

nanofibres through electrospinning. They observed improvement of mechanical

performance when MWCNTs were well dispersed into the PAN nanofibres in the

cut-strip-twisted bundles.

Zhou et al. 231 twisted short nanofibre bundles from PVDF-HFP nanofibre

web collected on a high-speed drum collector, and studied the effect of fibre

orientation, fibre diameter, morphology and twist level on the tensile strength

properties of bundles (Figure 2.9). It was found that fibre quality, diameter and

orientation played important roles in the final strength. Finer uniform fibres gave

better mechanical properties. Higher level of twist can be inserted in finer bead-free

fibres than beaded fibres. Yarn produced from strips cut parallel to fibre direction

had much higher tensile strength and modulus than strips cut perpendicular or at an

angle of 45o to the fibre orientation 231.


- 32 -

Figure 2.9 (a) SEM image of short PVDF-HFP nanofibre bundle prepared by

twisting a nanofibre strip, (b) & (c) effect of twist on (b) bundle strength and (c)

bundle diameter[232] .


- 33 -

2.4.2.2 Direct Electrospinning

Dalton et al. 218 utilized two grounded discs to collect aligned nanofibres

(Figure 2.10). Twist was then inserted in the nanofibre bundles collected when one

of the discs was rotated while the second remained stationary. They succeeded in

producing a 5 μm diameter nanofibre bundle. However, the bundle length was short,

only in the range of 40 mm ~100 mm. Adjusting disc distance caused crossover

nanofibres and deteriorated alignment.

Figure 2.10 (a) Schematic for twist insertion and (b)

SEM images of twisted nanofibre bundle [218].

Gu et al.40 reported a process to obtain short twisted single nanofibre

mimicking collagen fibrils and DNA as shown in Figure 2.11. In this modified

electrospinning, an auxiliary four-sided electrode with relay system was employed.

An electric field was sequentially applied in each face of auxiliary electrode. This

continuous change of electric field from one face to other yielded continuous

revolving electric field. By manipulation of relay time for cyclic-electric-field

twisted single PEO nanofibres with periodic twist length of 2.6 μm, 5.5 μm, 11 μm

and 16 μm were obtained.


- 34 -

Liu et al. 41 reported a unique design using a disc and a needle as collectors.

In this design, the disc collector was motionless, while the needle collector rotated

during electrospinning in Figure 2.12a. By rotating and moving needle they

developed poly (methyl methacrylate) (PMMA)/ MWCNTs twisted bundles of 30

to 40 cm length with diameter ranging from 13 to 23 μm as shown.

Pure PMMA nanofibre bundles were reported to exhibit better mechanical

properties than composite nanofibre bundles, while composite nanofibre bundles

showed improved toughness (energy to break). This method could produce almost

10 times longer bundle than Dalton’s method 218.

A hollow hemi sphere and a sharp metallic rod were reported to be used as

collectors 235,236 to produce twisted semi-conducting metal oxide nanofibre bundles

(Figure 2.12b & c). The hollow hemi-sphere was rotated between 100 to 1000 rpm

to insert twist, while sharp rod was slowly moved away to get shorter length of

twisted bundle. By rotating the hemi-sphere, twists were inserted into the fibre

bundle during electrospinning. This method was reported to produce twisted fibre

bundles of length up to 10 cm.

Figure 2.11 Twisted single PEO nanofibre produced by auxiliary

electrode. [40].


- 35 -

Figure 2.12 (a) Schematic diagram for producing short nanofibre

bundles[41], (b) hollow hemi-sphere and sharp rod setup for nanofibre

bundle collection, and (c) SEM image of collected semi conducting

metal oxide nanofibre bundle [236].

Chang et al. 39 reported the preparation of poly(vinyl pyrrolidone) (PVP)

micro-ropes using vertically-rotating thin needle collector underneath two needle

electrospinning nozzles. Micro-rope with diameter less than 10 μm and length up

to 5 cm can be produced as shown in Figure 2.13. The diameter of needle collector

was reported to play a vital role in the formation of micro-rope. Needle collector of

0.5 mm diameter resulted in smooth micro-ropes, while 5 mm diameter needle

resulted in random fibre web.

Paneva et al. 226 reported self-organization of nanofibres when polycation

and polyanion polyelectrolytes were electrospun. The nanofibres accumulated on a

disc collector in the form of fibre bundle. Twist was inserted through disc rotation

during electrospinning. A combination of single needle nozzle and two twisting-

tubes (each capable of independent rotation) was described by Yan et al. 48.

Nanofibre web was collected within the gap of these rotating tubes. When these

tubes were rotated in reverse direction to each other, twist was inserted in the web.

Rotation of a third roller perpendicular to rotating tube system was used to draw the


- 36 -

fibres into yarn and wind on the roller. By changing the collector speed, yarn with

different twist angles were produced. It was indicated that the twist angle of 35o can

produce yarn with maximum strength. Yarns with twist angle values below this and

higher than 35o showed inferior mechanical properties. The nanofibre yarn was

spun at 0.078 m/min.

Figure 2.13 (a) Schematic setup consisting two needle

nozzle generators over a sharp needle collector for micro-

rope manufacturing. (b) SEM image of twisted micro-

ropes [39].


- 37 -

2.5 Non-Twisted Filament Yarns

Two types of continuous nanofibre bundles were produced from electrospun

nanofibres. Non-twisted nanofibre bundles here also referred to as filament yarns,

while twisted continuous fibre bundles actually meet all the yarn characteristics. A

number of researchers have produced such nanofibre structures.

Ko et al. 42 reported a setup to prepare continuous nanofibre filament yarns.

They co-electrospun polylactic acid (PLA) and polyacrylonitrile (PAN) solution

containing SWCNTs (single wall carbon nanotubes) on a rotating cylindrical

collector. A continuous web was electrospun from a vertically placed

electrospinning nozzle. Fibres formed a suspended web in the air, which was

manually taken to the winder (Figure 2.14). Upcoming fibres deposited on the

previously deposited fibres. In this way, a continuous strand of nanofibres was

obtained. Although a schematic of twisted nanofibre yarn assembly was presented

in the paper, little detail was given.

Pan et al. 44 reported a conjugated system that can produce both positively

and negatively charged nanofibres simultaneously (Figure 2.15). Two needle

nozzles were independently connected to the positive and negative electrodes of

high voltage power supply. Oppositely charged nanofibres were electrospun from

the nozzles, which met in an intermediate area where charges were neutralized.

Upon drawing fibres from the fibre accumulation zone and transferring to a rotating

drum, aligned fibre filament yarn can be wound continuously


- 38 -

Figure 2.14 Photograph of continuous non-twisted

PLA/PAN and SWCNTs filament yarn [42]

Figure 2.15 (a) Schematic for conjugated needle and roller system. (b)

SEM image of non-twisted nanofibre filament yarn [44].


- 39 -

Wang et al. 237 reported a process to produce filament yarns (Figure 2.16).

Grounded needle-tip was used to initiate the formation of fibre bundle. They found

that polymer solution conductivity played a vital role in the formation of fibre

bundle. When the conductivity was 10 μS/cm, self-bundling of fibres only took

place occasionally, while a metal needle was required for initiation of the self-

bundling process when conductivity was in the range of 10 ~ 400 μS/cm.

When the solution conductivity was greater than 400 μS/cm, self-bundling

occurred automatically without initiation with a metal needle. It was also indicated

that the addition of a small amount of salt rendered steady, trouble-free self-

bundling process. Another needle induced self-bundling was reported by Mondal et

al. 238 This process was reported to be highly dependent on the electric field strength

and polymer concentration.

Figure 2.16 (a) Schematic of self-bundling by needle. (b) PAN

nanofibre filament yarn using organic salt [238].


- 40 -

Kim et al. 239-241 patented methods of producing non-twisted continuous

nanofibre filament yarns. A fibre web strip was prepared on the edge of a thick

rotating disc, which was taken up continuously using a winder system

(Figure 2.17a). In another patent, the fibre production rate was improved using

multiple needle nozzles array (Figure 2.17b). Not only rotating disc, but also a

narrow conveyor belt can also be used as collector. Twist can even be inserted into

to nanofibre bundle during taking up (Figure 2.17c).

Apart from collecting nanofibres on solid collectors, liquid bath (as shown

in Figure 2.18) was also used for collecting nanofibres into continuous filament

yarns. In this technique, nanofibres collected over a liquid-bath were converted to

continuous non-twisted filament yarn at a relatively high production rate 240,242,243.

When these deposited nanofibres were lifted up for winding, surface tension of

liquid bundled these nanofibres into continuous strands. Water also assisted in

removing charges from the nanofibres, reducing inter-fibre repulsion. Evidently,

polymers which are soluble to the coagulation solution could not be processed by

this technique.

Figure 2.17 Non-twisted filament yarn from (a) single nozzle and

circular disc [242], multiple nozzles electro-spinning over, (b) disc and

(c) conveyer belt.


- 41 -

Teo and co-workers 244 improved yarn strength by introducing vortex in

liquid bath. A whirlpool was created in water-bath by introducing a hole in the bath

base. Nanofibres were deposited near the whirlpool and were carried down by water

through the hole at bottom. A continuous nanofibre filament yarn was wound on a

mandrel. The take up speed of the yarn was reported to be 60 m/min.

Figure 2.18 (a) Schematic for collecting continuous nanofibre filament on

liquid bath [243], (b) SEM image of nanofibre filament yarn [244].


- 42 -

2.6 Nanofibre Yarns

Recently, great efforts have been made to produce nanofibre yarns directly

from electrospinning process. Dabirian et al. 245 reported electrospinning of

nanofibre yarns using a modified needle electrospinning setup as shown in

Figure 2.19. A negative charged metal bar was placed in-between the collector and

winding assembly. The positively charged jet was attracted in mid-way due to

multi-polar electric field generated by negative plate collector and negative bar.

Initially fibres were deposited on the plate collector, which were drawn toward the

winder. Twists were inserted by rotation of the whole winder system. Negatively

charged bar functioned to balance the overall electric field between positively-

charged nozzle and the negatively charged collector plate. Nanofibres were

relatively free to move to winding and twisting assembly to formulate twisted

nanofibre yarns.

Figure 2.19 (a) Schematic of bar manipulated electric field. (b) SEM image

of PAN nanofibre yarn [246].


- 43 -

Lee et al. 246 patented a modified electrospinning system which can

simultaneously collect, draw, twist and wind nanofibre yarn. Electrospun

nanofibres were deposited on a grounded-circular-plate. When the fibre deposition

was sufficient, some of the nanofibres were withdrawn and wound on a rotating

drum. During drawing, twist was inserted through circular plate rotation.

Bazbouz et al. 247 reported a setup to make twisted nanofibre yarn

(Figure 2.20). The setup consisted of two perpendicularly placed circular disc

collectors, both capable of independent rotation. Aligned nanofibres were collected

within the vicinity of these two discs. On enough deposition of aligned nanofibres,

one disc was rotated to insert twist in the collected nanofibres, while the rotation of

other disc wound twisted nanofibre yarn. After twist insertion, second disc was

rotated to wind yarn. Although it has been claimed that a continuous production

speed of 8 m/min can be achieved using this system but at the same time it was

reported that electrospinning was done for only 2 minutes. This contradicts claim

for continuous production of nanofibre yarn from this system.

Figure 2.20 (a) Schematic double disc setup for simultaneously twist

insertion and winding. (b) SEM image of twisted nanofibre yarn [248].


- 44 -

Dabirian et al. 248,249 utilized conjugated needle nozzles along with a round

plate as collector to produce nanofibre yarn. A post-heat treatment system was set

between the winder and the collection disc, to improve mechanical properties of as-

spun yarn. It was reported that heat treated yarn showed more than twice initial

modulus and almost double the tensile strength to that of untreated one, whereas

the elongation at break was reduced to half. In situ heat treatment also improved

yarn structure and reduced yarn diameter.

Yousefzadeh et al. 250, used an auxiliary electrode attached to the needle

nozzle to enhance the nanofibre deposition in liquid-bath (Figure 2.21). Instead of

going through the hole at the bottom of the liquid bath, the nanofibres were taken

up from water surface. Nanofibre yarns with different twist angles ranging from

4.93o to 19.76o were produced in this way. The nanofibre yarn did not show similar

modulus to non-twisted nanofibre filament yarn, but elongation at break were 4.5

times and 6.5 times higher, respectively.

Although the liquid-bath technique facilitates to eliminate charges from

fibre and compact the yarn, it requires an additional drying process. Spinning fibres

in liquid bath also causes contamination issue and change of fibre morphology due

to the diffusion of solvent from the fibres in which solvent has not been evaporated

completely before immersion. Also, the vortex does not assure uniform and

consistent twist.


- 45 -

The yarn collection on a solid surface not only improved yarn twist, but

fibre morphology also remained intact, which is better than the liquid-bath

collectors. However, the deposited nanofibres remained affixed on solid surface

while they were pulled and twisted to form yarn. No doubt, some friction and

tension is necessary to insert twist and draw nanofibres into proper nanofibre yarns,

but this restricted high speed yarn production.

The concept of intermediate collectors has been recently extended by some

researchers46,47,251,252. The intermediate collectors proved better alternative to

increase yarn production speed, but minimize nanofibre sticking issue. Afifi et al.

46 utilized single needle nozzle placed at an angle to vertical grounded funnel to

produce twisted nanofibre yarn (Figure 2.22).

Initially nanofibres deposited on funnel surface. With fibre accumulation,

web was formed on the vicinity of funnel. Manually drawing the web toward the

winder inserts twist. The effect of drawing on mechanical properties of nanofibre

yarn was assessed in this work. Electrospun yarn initially exhibited better

mechanical properties in comparison to un-stretched yarn with drawing, but further

drawing reduced the strength. The production speed of yarn from this system was

Figure 2.21 (a) Schematic of liquid vortex, (b) resultant twisted

nanofibre yarn. [251].


- 46 -

0.063 m/min. As nanofibres were electrospun on a grounded collector from

positively charged single needle nozzle, residual charges were present in the

electrospun web. Thus, raw electrospun nanofibre yarn produced by this technique

showed dis-oriented fibres, irregular twist and some under-stretched nanofibres.

Ali et al. 47 produced highly twisted nanofibre yarn using conjugated needle

nozzles to deposit oppositely-charged nanofibres on an non-earthed funnel

(Figure 2.24). The combination of positively and negatively charged fibres rendered

overall yarn a neutral charge. The nanofibre yarn formed in this way did not stick

to non-grounded collector, leading to an improved nanofibre yarn production of 5

m/min. The fibrous web formed at the end of the funnel was drawn initially to form

a fibrous cone, which can be simply taken up with a winder. Twists were inserted

by the rotation of the funnel.

Figure 2.22 (a) Schematic diagram of twisted nanofibre yarn

developed by vertical funnel and single needle generator, (b) SEM

image of twisted nanofibre yarn. [46]


- 47 -

Figure 2.23 Effect of funnel rotation speed on yarn twist level of

nanofibre yarns.

Twist levels were controlled by changing funnel rotation speed

(Figure 2.23). Up to 7000 twists per metre can be inserted into the yarn through the

system. The authors also studied the effect of twist level on the yarn tensile

properties. Increasing twist level in the yarn resulted in compacting nanofibres and

overall yarn diameter was significantly reduced at higher twist levels. Increasing

twist level increased the strength up to a certain limit. However, further increasing

the twist showed decrease in the tensile strength. When compared with the aligned

nanofibre bundle, nanofibre yarns showed improved mechanical strength.

Figure 2.24 (a) Schematic diagram of horizontal funnel and double needle

generators. (b) Highly twisted PAN nanofibre yarn [47].


- 48 -

More recently, the same group also reported the use of rotating tube as a

collector to produce nanofibre yarns 251. They employed PVDF-HFP as model

polymer to elucidate the effect of spinning distance, tube rotation speed, polymer

flow rate and winding speed on yarn formation. This new design shows

improvement in yarn production rate and control of twist levels. The yarn diameter

can be adjusted by changing the tube diameter. Under the same yarn electrospinning

condition, when the outer diameter of the tube collector decreases from 11 to 8 cm

(Figure 2.25), fibrous cone was decreased in size and yarn diameter reduced from

178μm to 140μm.

Figure 2.25 Photograph showing change of fibrous cone shape using

tube with (a) large diameter and (b) small diameter [252].


- 49 -

2.7 Potential Applications

Electrospun nanofibre yarns offer great opportunities to incorporate

nanofibre assemblies in advanced fibrous structures. Zein and Zein/poly-L-lactide

(PLLA) composite yarns have been produced using conjugate electrospinning

technique (Figure 2.26a). These yarns showed improved fibre alignment and

mechanical properties 45. In a similar work, poly-L-lactide (PLLA)/nano-b-

tricalcium phosphate (n-TCP) composite nanofibre filament yarns have been

produced for biomedical applications 43. It was reported that nanofibre yarns

prepared in this way, showed improved tensile strength and enhanced cell growth.

It was indicated that these nanofibre yarns can be utilized in formation of knitted or

woven fabrics for complex biomedical scaffolds.

Figure 2.26 (a) Continuous twistless PLLA-Zein/Zein nanofibre

filament yarn [45], (b) schematic of tapered and twisted yarn. (c) twisted

NiO/ZnO nanofibre yarn. [236, 237].


- 50 -

Lotus et al. 236 fabricated ZnO and NiO ceramic nanofibre bundles showing

p-n junction rectification. They showed that nanofibre yarn p-n junction can be

developed in two ways, one masking and electrospinning other ceramic material on

top of it. Secondly, preparing nanofibre yarns individually and then twisting them

to form p-n junction (Figure 2.26b and c).

Lee et al. 253 reported highly flexible nanotube bundles by a bi-scrolling

method from MWNTs/PEDOT sheets. These nanotube bi-scrolled bundles showed

improved super capacitor properties. It was shown that these nanofibres can be

converted into single or plied nanofibre yarns or nanofibre braids (Figure 2.27a).

These flexible nanofibre yarns solid state capacitors could also be woven into

gloves. These nanofibre solid state super capacitors show high life cycle as

indicated in Figure 2.27b. It was reported that these nanofibre yarns are so flexible

that they can be used as thread in embroidery or sewing process to power electronic

devices. Otherwise, these nanofibre yarns can form woven or knitted structure to

store energy and reinforce fabric as well.

Kamiyama et al. 254 utilized nanofibres in golf gloves to improve their

frictional properties. They observed that using nanofibres in the gloves not only

improved frictional properties of the gloves but also reduced the required force to

hit a golf ball by reducing muscle activity.


- 51 -

Figure 2.27 (a) Biscrolled nanofibre single nanofibre yarn, plied

nanofibre yarn and nanofibre braided structure. (b) Solid state

supercapacitor woven into glove. [255]


- 52 -

Mirvakili et al. 255 produced niobium (Nb) nanofibre short twisted bundles

using severe plastic deformation (SPD). These bundles possess higher mechanical

strength (0.4 to 1.1 Gpa) almost similar to carbon multi wall nanotube yarns and

showed improved electrical conductivity (3x106 S/m) 100 times that of carbon

multi wall nanotube yarns (3x104 S/m). It was also found that the thermal torsional

actuation of these nanofibre yarns required very low voltage (Figure 2.28). The low

voltage requirement was indicated to be due to high conductivity of Nb (niobium)

nanofibre yarns. These nanofibre yarns were proposed to be good candidate for heat

or electric field activated drug delivery, catheters, toys and miniature valves.

Figure 2.28 (a) A paddle attached to biscrolled yarn to visualize torque in the

nanofibre yarn. (b) Niobium twisted yarn [257].


- 53 -

Khil et al. 242 developed woven structures from electrospun

polycaprolactone (PCL) nanofibre filament yarns for tissue engineered scaffolds

Figure 2.29a. Yao et al. 45 and Ali et al. 251 have also produced woven and knitted

fabrics utilizing nanofibre yarns Figure 2.29 b, c & d. Similarly, 3D fibrous

structures can be developed from nanofibre yarns using well-established fibrous

weaving or knitting technology. The dream of producing human organs in

laboratories might become a reality 8,256-262.

Fabrics produced from nanofibre yarns will be breathable yet lightweight,

robust and more lustrous. Well-defined porosity of fabrics made from nanofibre

yarns can be utilized in filters, protective clothing and wound dressings. The

interconnected pores in nanofibre yarns can assist in wicking and improved dye

ability. Nanofibres have diameter below the wavelength of visible light spectra,

making them transparent in normal light. This unique property can be utilized to

produce transparent protective composite sheets made from nanofibre yarns for

security and defence applications 263.


- 54 -

Figure 2.29 (a) Woven PCL fabric [243], (b) Zein/PLLA composite

nanofibre yarns based plain weave fabric [45], (c) double layer plain

weave composite fabric of PAN (upper layer) and PVDF-HFP (bottom

layer) nanofibre yarn. (d) single jersey knitted fabric using PVD F-HFP

nanofibre yarn and polypropylene multifilament 251.

Chapter 3: [Materials, methods and characterization]

-63-

C H A P T E R T H R E E

3 Materials and Characterisation

This chapter provides details about the materials, electrospinning setups,

and characterizing instruments employed in this research work.

Materials

Poly (vinylidene fluoride-co-hexaflouropropylene) (PVDF-HFP, density

1.78 g/cm3), N-N-dimethylformamide (DMF), acetone are all reagent grade

purchased from Sigma-Aldrich. They were used without any further purification.

3.1.1 Solution Preparation

PVDF-HFP solutions with different concentrations were prepared by

dissolving PVDF-HFP pellets in a solvent mixture consisting of acetone and

DMF (50:50 vol/vol) at room temperature. The mixtures were stirred overnight

to get homogeneous solutions.

Electrospinning setups

3.2.1.1 Basic Needle Electrospinning

A basic needle electrospinning system consisted of a 10 ml plastic

syringe (Terumo) connected to a metallic needle (20G, Outer Diameter = 0.8

mm, Inner Diameter = 0.5 mm), syringe pump (for continuous polymer supply)

a metallic collector (placed 10 cm apart from needle), and high voltage supply


- 64 -

(ES30P, Gamma High Voltage Research, 30 kV). The electrospinning setup is

illustrated in Figure 3.1.

When the polymer solution in the syringe was fed to the needle tip, it

adopted a spherical drop shape due to the effect of surface tension. When high

electric voltage was applied to the needle, an electric force compelled the

spherical drop to deform into a cone shape. The jet attenuated several thousand

times before hitting the collector. During jet flight solvent evaporated, resulted

in solidified fibres on the collector.

Figure 3.1 Schematic for basic needle based electrospinning setup.


- 65 -

3.2.1.2 Needleless Electrospinning

Four different needleless electrospinning generators were employed

separately to generate nanofibres. The fibre generator was partially immersed in

a polymer solution bath. A conductive wire was embedded in the bath to electrify

the polymer solution. A thin polymer layer was coated onto the generator surface

when it rotated. On applying a high electric voltage, numerous jets were formed

on generator surface as shown in Figure 3.2.

.

Figure 3.2 Schematic for needleless generator based electrospinning

setup.


- 66 -

Electrospinning of Nanofibre Yarns

A purpose-built electrospinning setup was used to collect and twist

nanofibres into a continuous nanofibre yarn. This setup includes a nanofibre

generator, a ring collector and a simple yarn winder. The ring collector was 40

mm in width, with an outer diameter of 100 mm and inner diameter of 88 mm.

It served as intermediate collector to maintain fibrous cone, which was crucial

in yarn formation. The ring collector was placed horizontally on a high density

polypropylene base fitted with ball bearing to reduce friction in rotation. A

variable high speed motor was connected to drive the ring collector through an

O-ring belt.

Three types of fibre generators were evaluated for nanofibre generation,

cone formation and formation of continuous nanofibre yarn:

Needle based electrospinning nozzle

Needleless fibre Generators

Hybrid (both needle nozzle and needleless fibre generator) fibre

generators


- 67 -

3.3.1 Yarn Formation from Needle Electrospinning

A pair of oppositely charged needle nozzles was used to prepare

nanofibres. A fibrous cone was developed on the ring collector, and a yarn was

then drawn from the apex of the cone. The fibre generators were separately

connected to the positive and the negative electrodes of a high voltage power

supply (ES30P-5, Gamma High Voltage Research 0~50 kV). Polymer solution

was supplied through a plastic syringe attached to a syringe pump. Each of the

plastic syringe was fitted with a 20G syringe needle. The needle nozzles were

placed close to the ring collector exactly underneath the ring itself as shown in

the Figure 3.3.

Figure 3.3 Schematic illustration of needle based nanofibre yarn

electrospinning setup.


- 68 -

3.3.2 Needleless electrospinning of nanofibre yarn

Needleless fibre generators such as cylinder, circular coil, ring and disc

were used for electrospinning of nanofibre yarns. Each fibre generator was

driven by a motor to vary the rotation speed. A conductive wire was embedded

into the polymer solution to apply high electric voltage to the polymer solution.

The needleless generator was partially immersed in polymer solution.

When it rotated, the polymer solution was loaded over the generator surface

(Figure 3.4). When the generator was rotated too slowly, i.e., 12 rpm, the amount

of polymer was not sufficient to coat entire spinneret surface. In this case, the

polymer solution thickness on the spinneret was not even. At a speed of 20 rpm,

the entire fibre generator was covered with a thin layer of polymer solution on

the surface, and the solution coating was smooth. A speed higher than 30 rpm

resulted in over coating of polymer solution due to drag and generator started

throwing away polymer solution.


- 69 -

Figure 3.4 Needleless electrospinning setups for preparation of nanofibre yarns (a) cylinder generator (b) circular coil generator (c) ring generator

and (d) disc generator


- 70 -

3.3.2.1 Modified needleless generators

Two modifications in the electrospinning setup were done to see their

effect on the electrospinning of nanofibres, their (nanofibres) morphology, cone

development formation and its stability and non-stop nanofibre yarn winding.

These modifications are described in details as follows:

3.3.2.1.1 Auxiliary Metal Electrode

To improve the fibre production and inclusion of all the fibres into the

yarn manufacturing process, an auxiliary metallic electrode was placed just

above the polymer bath having same electric potential as that of polymer bath

Figure 3.5. By using auxiliary electrode, more and continuous jets could be

produced on needleless generator surface.

Figure 3.5 Yarn electrospinning setup with an auxiliary electrode.


- 71 -

3.3.2.1.2 Air jet enhanced yarn electrospinning

Another approach to increase fibre productivity was applying pressurised

air (10 psi) along spinning direction. The pressurised air was applied through 8

gas nozzles placed around the polymer bath to get an even air flow around the

spinneret Figure 3.6. The air jets are expected to help in quick drying of

nanofibres and pushing more fibres into yarn twisting zone.

Figure 3.6 Yarn electrospinning setup with air jet enhancement.


- 72 -

3.3.3 Hybrid electrospinning of nanofibre yarns

A combination of needle and needleless electrospinning systems was

used to produce nanofibre yarns. Needleless generator was placed below the ring

collector, while a needle nozzle was placed above the ring collector. The

needleless generator was positively charged using a positive high-voltage power

supply (RR100-2P/240, Gamma High Voltage Research 0~100 kV), whereas the

needle nozzle was connect to a separate negative terminal of a high voltage

power supply (ES30P-5, Gamma High Voltage Research 0~50 kV), as shown in

Figure 3.7

The main concept behind using a combination of two oppositely charged

nanofibre generators was to get a neutralized web which could be dragged across

the ring collector. Once the neutral web gets across the ring and continuously

pulled by the winder, the fibrous cone develops which accommodates the newly

generated fibres. The needle supplies negatively charged fibres which attract and

counterbalance the positive charges in the fibrous cone. The positively charged

fibres coming from the needleless generator contributes the formation of fibrous

cone from the lower side of this ring collector. In this way, fibres are supplied in

a continuous manner, prohibiting crack appearance in fibrous cone.


- 73 -

Figure 3.7 Hybrid electrospinning setup for nanofibre yarn

fabrication.


- 74 -

Finite Element Analysis

Electric field intensity profiles were calculated using a finite element

method with the aid of the COMSOL Multiphysics 3.5a Software in 3D

electrostatic mode. The yarn electrospinning setup (including ring collector, disc

generator, polymer solution and solution bath) were developed in 3D

SolidWorks and transformed into geometrical objects using a bi-interface for

COMSOL. The dimension and relative position of geometrical objects replicated

the real electrospinning setups. The sub-domain settings for relative permittivity

for different materials was set according to the values described in literature

264,265. The relative permittivity for air was set to 1. For polymer solution and

polymer bath, it was 84 and 2, respectively. For conductive disc relative

permittivity was set to -1300, while for non-conductive disc, it was fixed at 2.4.

The voltage boundary conditions were set by applying 50 kV electric potential

to the polymer solution. The voltage for ring collector and the rest of the

boundaries placed at infinity distance was set to V=0, and for remaining interior

boundaries, the condition of continuity n. (D1-D2) = 0 was applied. The

geometries were converted to mesh and calculations were performed in the

COMSOL. The disc with different materials (conductive and non-conductive)

was simulated using this software to evaluate their electric field intensity. The

constitutive relation D = ε0. εr. E was used along with E = - V to obtain the

equation, - .ε0. εr. = dρ. Here “E” represents electric field relative, while “ρ”

and “εr” stand for space charge density and permittivity of air, respectively.


- 75 -

Characterisations

3.5.1 Scanning Electron Microscopy (SEM)

The morphology and diameter of electrospun nanofibres and yarns were

observed with a scanning electron microscope (SEM Neoscope JCM 5000) as

shown in Figure 3.8. Conductive carbon tape was used to cover aluminium stubs

(SEM sample holder) surface. Nanofibre webs and yarns were carefully placed

on carbon tape covered stubs. These nanofibre web and yarn samples were gold

coated using Baltec SCD 50 sputter coater. SEM images were taken from

different places at 10 kV voltage and different magnification levels. Image

analysis software (Image J 1.46 r) was utilized to calculate fibre and yarn

diameters, and twist angles from 100 different places in a single SEM image.

Figure 3.8 Photograph of Neoscope JCM 5000 bench top SEM

machine.


- 76 -

3.5.2 Mechanical Properties of Nanofibre Yarns

The tensile properties of electrospun nanofibre webs and nanofibre yarns

were evaluated with FAVIMAT + AI Robot 2 single fibre tester as shown in

Figure 3.9. All the samples were placed in standard atmospheric conditions

(temperature 20 ± 2 °C & relative humidity 65 ± 2%) overnight and the testing

was performed in standard atmospheric conditions. A gauge length of 10 mm

and 10 mm/min jaw speed were maintained in all the tests. Five samples of 20

mm length from each category were cut and placed in the creel of Robot 2. The

machine was set to perform tests automatically to reduce human error in sample

mounting. Any sample showing breakage near the jaw edge was rejected.

Figure 3.9 Photograph of FAVIMAT with Robot 2 creel, showing

pneumatic jaws in magnified view.

- 77-

C H A P T E R F O U R

4 Nanofibre Yarn Prepared by Needle

Electrospinning Systems

In this chapter, a novel electrospinning technique for continuous

electrospinning of nanofibre yarns has been described. A pair of positive and

negative needle generators along with intermediate ring collector and drum

winder were employed to fabricate nanofibre yarns. Nanofibres and their

electrospun yarns were examined under scanning electron microscope (SEM) to

observe their morphologies, diameters, and twist angles. Mechanical properties

of electrospun nanofibre yarns were tested to evaluate improvement in nanofibre

strength in yarn structure. The effect of different twist levels on tensile strength

and strain percentage of yarns thus produced was also investigated.

4.1 Experimental

4.1.1 Electrospinning of Nanofibre Yarn

A purpose-built electrospinning setup was used to prepare nanofibre

yarns, which comprised of two needle-based electrospinning systems, an

aluminium ring collector, and a simple drum winder as schematically illustrated

in Figure 4.1. The ring collector (40 mm in width, outer diameter = 100 mm,

inner diameter = 88 mm) was laid horizontally on a plastic plate fitted with a ball

bearing to reduce friction in rotation. A motor was connected to the ring collector

through an O-ring belt.

Chapter 4: [Nanofibre Yarn Prepared by Needle Electrospinning Systems]

- 78 -

The two needle nozzles were connected separately to the positive and the

negative electrodes of a high-voltage power supply (ES30P, Gamma High

Voltage Research 0~50 kV). Both needles were linked with a plastic syringe for

continuous feeding of polymer solutions. Initially, newly electrospun nanofibres

were deposited on the ring collector to form a flat fibrous web. This web was

drawn manually using a plastic rod to form an inverted fibrous cone. The cone

apex was continuously drawn to form a nanofibre yarn.


- 79 -

4.2 Preparation of Twisted Nanofibre Yarns

Figure 4.1a schematically illustrates the setup for electrospinning of

nanofibre yarns. Two electrospinning systems, one positively charged and

another negatively charged, were employed to produce oppositely charged

nanofibres. Initially, when the ring collector was not moving, a fibrous web was

formed by deposition of nanofibres on the inside surface of the ring collector.

As soon as, the ring started rotating, flat web was formed on the periphery of

ring collector. A fibrous cone was then formed through drawing the fibrous web

at the central part along electrospinning direction (Figure 4.1b). By further

drawing nanofibres from the apex of the fibrous cone, a nanofibre yarn resulted,

which was taken up easily by the drum winder. It was noticed that the later-

formed fibres deposited inside the fibrous cone, and the ring collector separated

fibre twisting and yarn collection from electrospinning zone, which is

completely different from previously reported setups46,47,245,249,251,266,267.

Figure 4.1 a) Schematic illustration of yarn electrospinning setup, b)

photograph of fibrous cone formed on ring collector, and c) inset shows

nanofibre yarn collected on a spool.


- 80 -

Under the scanning electron microscope (SEM) a number of fibres were

observed in the nanofibre yarn collected and all fibres looked uniform and

without binding or fusing with each other. These fibres had an average diameter

around 592 nm. The twist feature was clearly seen on the bundle surface, and the

nanofibres were oriented at an angle 54.4o along the yarn length, typical

characteristics of nanofibre yarns.

The deposition of both positively and negatively charged nanofibres on

the rotary ring collector was essential to form a neutral fibrous cone. To achieve

this, the two electrospinning systems must work simultaneously, and the

threshold voltage to ensure smooth running of both electrospinning systems was

found to be 13 kV. When just one of the nozzle systems worked, though

nanofibres were produced, however, nanofibres either stuck to the ring collector

surface or flew in all directions off the collector and no yarn was formed.


- 81 -

4.3 Influence of Different Factors on Yarn Electrospinning:

4.3.1 Nozzle location

The nozzle location in the yarn electrospinning setup is an important

factor affecting electrospinning. When the two needle nozzles were placed

equidistant from the ring collector (30 mm), nanofibres produced by the positive

nozzle tended to deposit on the negative nozzle, and this led to end of

electrospinning from the negative nozzle. Thus in this case, only positively

charged nanofibres were produced and they affixed to the ring surface and

consequently yarn fabrication ceased.

Moving the negative nozzle close to the ring collector facilitated

deposition of nanofibres in the central region of the ring collector. In this case,

the negatively charged nanofibres reached middle of the ring collector prior to

positively charged fibres, resulting in charge neutralization of nanofibrous web

in central periphery of the ring collector. In the latter case, the negative and the

positive nozzles were set at 10 mm and 30 mm away from the ring collector,

respectively as shown in Figure 4.2.

Table 4.1 shows yarn electrospinning results for two nozzles locating at

different distances. When the needle nozzles were arranged in style I, no yarn

was formed. Most of the nanofibres deposited just on the inner surface of the

ring collector. This was presumably due to the electrostatic interaction between

the ring and the nanofibres was so strong to retain fibres on the ring surface.


- 82 -

Figure 4.2 Schematic showing details of nozzle

placement.

Schematic showing details of nozzle placement. In style II, a stable

fibrous cone with a continuous nanofibre yarn was formed. However, coiled

nanofibres were formed on the yarn surface. Quality nanofibre yarn was

prepared in style III. In this case, a very stable fibrous cone was formed during

electrospinning. Continuous nanofibre yarn was produced under this condition.

Nanofibres formed were preferably aligned along the yarn axis. Style IV only

resulted in a continuous filament, due to merging of the jets during

electrospinning. The distance between the two nozzles was so close that the

attractive force between oppositely charged jets did not allow them to split and

dry to form nanofibres; instead, the jets coalesced into a single thick jet

formulated a non-twisted monofilament.

- 83 -

A (mm)

B (mm)

C (mm) Result Morphology

I < 3 > 72 < 4 No yarn (fibres deposition just on ring surface)

II 3 ~ 5 69 ~ 72 5 ~ 6 Yarn with coiled nanofibres

III 5 ~ 10 64 ~ 68 > 7 Quality yarn

IV > 10 < 63 ≥ 8 Mono filament

* Negative and positive nozzles were placed 10 mm and 30 mm away from the ring collector, respectively.

Table 4.1 Effect of nozzle distances on yarn formation*


- 84 -

4.3.2 Ring collector

The Ring collector is the essential part of this yarn electrospinning setup,

and the rotation of the ring plays a key role in the formation of fibrous cone and

twist in nanofibre yarns. Without rotation, the ring can just collect nanofibres

along the inner surface, and no fibrous cone was formed.

The rotation of the ring collector affected yarn morphology. When ring

speed was below 430 rpm, only a few nanofibres went across the ring collector

without formation of yarn. A continuous fibrous web was formed when the ring

speed was above 500 rpm, but still no yarn was formed. When the speed was

higher than 660 rpm, a steady fibrous cone was generated and a continuous

strand of nanofibre yarn was wound. Nanofibre yarn was prepared without any

interruption when the speed was in the range of 660~1300 rpm. However, when

higher ring speed was employed, 1350 rpm for example, the fibrous cone became

unstable, and yarn breakage often took place.

The ring rotation speed also affected the fibrous cone height, the diameter

of fibre and yarn, and yarn twist feature. With the increase in ring speed from

100 to 430 rpm, the fibre and yarn diameters decreased dramatically (Figure 4.3).

When the ring speed was higher than 660 rpm, increasing the ring speed had no

significant effect on fibre/yarn diameter. Yarn produced at higher ring speed was

more compact and consisted of finer fibres. At 1350 rpm, a very short cone (just

half the produced at 880 rpm) was produced, which was not sufficient enough to

be outstretched into a continuous yarn.


- 85 -

Figure 4.3 Effect of ring collector speed on a) fibre and yarn diameters,

b) twist level features.


- 86 -

4.3.3 Twist feature

Twist is an important feature of yarns. Twist not only binds fibres

together, it also changes yarn’s physical appearance. The twist level (twist per

metre) of nanofibre yarns can be estimated by equation (1):

(1)

Where NR is the rotation speed of the ring collector in rpm (revolutions

per minute) and Sw is winder speed in m/min (metres per minute). By varying

the ring speed, twist levels can be adjusted. Figure 4.3 b shows effect of ring

speed on yarn twist feature. At higher ring collector speeds, yarn with larger

twist levels resulted, and twist angles of fibres along yarn axis increased as well.

The maximum level and twist angle prepared by continuous yarn production

were found to be 4086 twists per metre (tpm) and 54.4°, respectively.


- 87 -

4.3.4 Polymer concentration

Polymer concentration had a significant effect on nanofibre morphology

and yarn spinning. To illustrate the effect of polymer concentration on fibre and

yarn morphologies, solutions with PVDF-HFP concentration of 14.0% ~ 20.0%

were electrospun. When polymer concentration was low, beaded nanofibres

were prepared, which is similar to normal electrospinning. By increasing the

polymer concentration, beads diminished, yet fibre diameter was increased.

Figure 4.4 Effect of polymer concentration on nanofibre and yarn

diameter (Applied voltage = 13 kV, Ring collector speed 1200 rpm;

total volume flow rate 3.0 ml/h, flow rate ratio for positive/negative

nozzles 2.1:0.9).


- 88 -

When the concentration was lower than 15.0%, an unstable cone was

formed and continuous yarn could not be collected, while higher polymer

concentration, such as 16.0% and 17.0%, led to a stable cone and continuous

yarn. Further increasing the polymer concentration, e.g. 18.0% ~ 20.0%, resulted

in coarser nanofibres and discontinuous nanofibre yarns

Figure 4.4 shows the effect of polymer concentration on nanofibre and

yarn diameters. Fibre diameter increased by increasing the polymer

concentration. The yarn diameter initially showed an increasing trend when

polymer concentration was gradually increased, while when further increasing

the polymer concentration beyond a certain point, a decline in yarn diameter was

observed. At lower polymer concentration, fibres with entrapped solvent were

produced. Some of fibres stuck on ring collector and remaining fibres went

through ring collector. Cone produced from lesser number and coalescence of

nanofibres resulted in yarn with lower diameter. At higher polymer

concentrations, the solution viscosity increased, fibres produced from such

concentrations were comparatively dry and well separated. These dry and well

separated fibres formed larger diameter yarn. Whereas, further increase in

polymer concentration, presented difficulty in electrospinning process. On such

higher concentrations coarser and fewer nanofibres were produced, thus the

number of fibres going through the ring collector were substantially decreased

and yarn formed showed decrease in diameter.

Table 4.2 summarizes the effect of polymer concentration on threshold

voltages, nanofibre morphology and yarn continuity.

- 89 -

Concentration

(% wt)

Velectrospinning

(kV)

Vworking

(kV)

Fibres Yarns

14.0 ~ 15.0% 6 kV 12 kV Beaded fibres Discontinuous

16.0% 7 kV 13 kV Beaded fibres Discontinuous

17.0% 7 kV 13 kV No-bead nanofibres Continuous

18.0 ~ 20.0% 10 kV 15~17 kV Coarse fibres Discontinuous

*(Ring collector speed 1200 rpm; total volume flow rate 3.0 ml/h, flow rate ratio for positive/negative nozzles 2.1:0.9).

Table 4.2 Effect of polymer concentration on yarn electrospinning


- 90 -

4.3.5 Applied voltage

Applied voltage plays a vital role in the formation of nanofibres. The

threshold voltages for electrospinning (Velectrospinning) and for the formation of

fibrous cone (Vworking) using solution of different polymer concentrations are

listed in Table 4.2

When applied voltage was lower than 7 kV, wet fibres were often

electrospun. Fibrous cone formed by wet fibres was unstable and yarn broke

occasionally during drawing due to the fused fibrous structure. Increasing the

voltage decreased the number of fused fibres. When the voltage was 13 kV,

sufficient number of dry nanofibres were produced, which formed a steady

fibrous cone and continuous nanofibre yarn.

Higher voltage than 13 kV also made the fibrous cone unstable.

Nanofibres produced under this condition stuck to the ring collector firmly. No

many nanofibres went through ring, and the lack of sufficient nanofibres to build

fibrous cone resulted in breakage of yarn. High applied voltage also led to fleeing

of nanofibres from the fibre generating zone.

The applied voltage also affected the fibre and yarn diameters. As shown

in Figure 4.5, a steady reduction in nanofibre diameter and diameter distribution

was observed when increasing the applied voltage. Whereas, the yarn diameter

initially increased with increasing the applied voltage until 13 kV, and then

showed a decreased trend at higher voltage.


- 91 -

Figure 4.5 Effect of Voltage on nanofibre and yarn diameter (polymer

concentration 17.0 %, ring collector speed 1200 rpm; total volume flow

rate 3.0 ml/h, flow rate ratio for positive/negative nozzles 2.1:0.9).


- 92 -

4.3.6 Winding speed

Winding speed was also found to influence the stability of fibrous cone

and nanofibre/yarn diameters. At a winding speed lower than 60 m/h, the fibrous

cone jerked, slackening the yarn, sometimes caused the fibrous cone to break.

When winding speed was higher than 72 m/h, nanofibre yarn was wound with a

proper tension, and a good balance between cone formation and yarn winding

achieved ensuring the continuous withdrawal of nanofibres into yarn. However,

when the winding speed was beyond 200 m/h, vigorous drafting caused cone

breakage.

Figure 4.6 shows the effect of winding speed on nanofibre and yarn

diameters. Increasing the winding speed led to finer nanofibres and yarns. This

can be explained as that nanofibres were stretched at a higher winding speed. In

addition, the winding speed also affected yarn twist feature. When the ring was

kept at a constant speed, increasing the winding speed lowered twist levels in the

yarn. The change in twist can be referred to amount of time spent by yarn

spinning. It took more time in spinning when winding speed was low leading to

larger twist.


- 93 -

Figure 4.6 Effect of winder speed on nanofibre and yarn diameter,

(polymer concentration 17.0 %, 13kV applied voltage , Ring collector

speed 1200 rpm; total volume flow rate 3.0 ml/h, flow rate ratio for

positive/negative nozzles 2.1:0.9).


- 94 -

4.3.7 Flow rate

The flow rates of polymer solutions being processed by the

electrospinning systems are another parameter determining the formation of

nanofibre yarn. It was found that a ratio of flow rate between the positive and

the negative nozzles at 7:3 favoured yarn formation. When this ratio was fixed,

the overall flow rate affected yarn electrospinning process as well as the fibre

and yarn diameters.

The flow rate at 1.0 ml/h was insufficient to produce incessant

nanofibres. Quite often only one of the needle nozzle produced fibres, and

neither fibrous cone nor nanofibre yarn was formed in this case. When the

overall flow rate was increased to 2.0 ml/h, nanofibres were generated

discontinuously. A thin fibrous cone was formed, and the cone broke frequently

due to the low thickness. The yarn obtained was discontinuous and varied in

structure.

Adequate number of nanofibres were generated when the overall flow

rate was 3.0 ml/h, and a stable fibrous cone was therefore formed leading to a

continuous nanofibre yarn. The overall flow rate beyond 3.0 ml/h allowed

electrospinning of more nanofibres. However, nanofibres produced at such a

high flow rate formed a wet fibrous cone, which resisted dragging of nanofibres

during yarn formation. Continuous nanofibre yarn with dry fibres was formed

only at the overall flow of 3.0 ml/h. Figure 4.7 shows the effect of overall flow

rate on nanofibre and yarn diameter. Nanofibre and yarn diameters increased

with increasing the overall flow rate.


- 95 -

Figure 4.7 Effect of flow rate on nanofibre and yarn diameter,

(polymer concentration 17.0 %, applied voltage = 13kV, Ring

collector speed 1200 rpm; flow rate ratio for positive/negative

nozzles 2.1:0.9).


- 96 -

4.4 Mechanical Properties of Nanofibre Yarns:

The mechanical properties of nanofibre yarns were tested on FAVIMAT

(AI) + Robot2. For comparison, random and aligned nanofibre webs fabricated

using conventional electrospinning setup under same electrospinning parameters

were also tested. Nanofibre yarn showed a tensile strength and elongation at

break of 93.6 MPa and 242.6%, respectively. Whereas the randomly-orientated

nanofibre web and aligned nanofibre web showed tensile strength of 9.1 MPa

and 18.5 MPa with elongation at break of 68.3% and 103.8%, respectively

(Figure 4.8a).

Twists in nanofibre yarn helped in binding fibres within yarn and

improved overall fibre to fibre friction. This improved packaging of nanofibres

not only enhanced strength bearing properties of individual fibres but also

improved overall mechanical performance of nanofibre yarns. The tensile

properties were also affected by yarn twist level. As shown in Figure 4.8b, by

increasing the twist level, tensile strength and elongation at break both increased.

It can be anticipated from the fact that at higher the twist levels, higher packaging

of fibres enhanced fibre to fibre contact/friction. At higher twist levels,

individual fibres gained support from neighbouring fibres and showed higher

load bearing capacity with increased strained percentage.


- 97 -

Figure 4.8 a) Typical stress ~ strain curves of randomly-orientated nanofibre

web, aligned nanofibre web and nanofibre yarn and b) effect of twist level

on stress and strain of typical nanofibre yarns.


- 98 -

4.5 Conclusion

In this chapter, a novel electrospinning system capable of producing

twisted, continuous nanofibre yarns has been demonstrated. The ring collector

in the system played a key role in formation of fibrous cone and nanofibre yarn.

The ring collector also separated nanofibre generation from yarn

formation/winding zone. By increasing the rotation speed of the ring collector

nanofibre and yarn diameter decreased, whereas twist angle was increased up to

54.4°.

The nozzle location, polymer concentration, applied voltage, flow rate

and winding rate also play roles in the formation of nanofibre yarn and yarn

dimension. The nozzles location was a crucial factor in the manufacturing of

nanofibre yarn. When nozzles were placed closed to ring collector, all the fibres

were deposited on ring surface and no yarn was formed. A coiled fibre yarn was

formulated when nozzles were placed at 69 to 72 mm. A properly dried and

twisted nanofibre yarn was fabricated when nozzle distance was 64 to 68 mm;

whereas, a monofilament yarn was created when nozzles were placed too close

to each other (i.e., less than 63 mm apart).

An increase in polymer concentration and polymer flow rate directly

affected the nanofibre diameter; however, increasing applied voltage, ring

collector and winder speed helped in reducing nanofibre diameter. Yarn

diameter initially increased by increasing polymer concentration and applied

voltage until reaching an optimum value of 17.0 % and 13 kV respectively and

then showed a declining trend. Higher ring collector and winder speeds showed


- 99 -

a continuous decrease in yarn diameter, whereas larger polymer flow rates

produced coarser yarns. Ring collector and winder speed also directly affected

twist levels in nanofibre yarns. Higher twists could be obtained by keeping

winder speed constant and increasing the ring speed up to 1200 rpm beyond that

it was difficult to properly formulate fibrous cone and thus discontinuous

nanofibre yarn was obtained.

The nanofibre yarn showed much higher tensile strength and elongation

at break (93.6 MPa and elongation of 242.6%) than both aligned and randomly

orientated nanofibre webs (9.1 MPa and 18.5 MPa with elongation at break of

68.3% and 103.8%, respectively). Twist factor in nanofibre yarns also showed

direct influence on yarn strength and strain percentage. Higher twists in

nanofibre yarns helped in nanofibres compaction within yarn. This compaction

and increased fibre to fibre friction facilitated in achieving higher tensile strength

with increased strain percentage.

- 100 -

C H A P T E R F I V E

5 Needleless Electrospinning of Nanofibre

Yarns

In this chapter, four different nanofibre generators (i.e., cylinder, coil,

ring and disc) were used in combination with a transient ring collector and a

simple drum winder to prepare nanofibre yarns. The effect of the fibre generators

on nanofibres generation, morphology and yarn formation was examined.

5.1 Electrospinning of Nanofibres and Nanofibre Yarns

A stationary collector plate covered with aluminium foil was employed

to collect nanofibres during needleless electrospinning. The needleless

generators were immersed in a polymer bath solution, and the slow rotation of

the generators loaded polymer solution uniformly on the generator surface to

coat a uniform polymer layer on spinneret surface. A conductive wire was

embedded in the bottom of polymer solution bath. Electric field was applied to

the polymer solution through this conductive wire. On applying high electric

voltage to polymer solution and rotating the needleless generator, polymer jets

were formed on the fibre generator surface. These jets attenuated and deposited

on stationary collector plate. To properly electrospin nanofibres, needleless fibre

generator was placed at a distance of 60 mm; voltage was changed from 0 kV to

65 kV, and polymer concentration was varied from 9.0 % to 19.0%.

A novel electrospinning system comprising of a needleless generator, a

transient ring collector and a winder system was utilized to electrospin nanofibre

yarn, and the effect of nanofibre generators on yarn formation was evaluated.

Chapter 5: [Needleless Electrospinning of Nanofibre Yarns]

- 101 -

5.2 Needleless Electrospinning of Nanofibres

5.2.1 Cylinder Generator

Needleless electrospinning using cylinder as nanofibre generator has

already established by Elmarco Co. and many other researchers. A cylinder of

70 mm width and 50 mm diameter was used to produce nanofibres. Cylinder was

slowly rotated at a speed of 20 revolution per minute (rpm) in polymer bath to

evenly coat polymer over the cylinder surface. When high electric voltage was

applied, perturbations started appearing on cylinder surface mainly over cylinder

edges. At polymer concentrations lower than 13.0 %, jets could be seen even as

low voltage as 30 kV but these jets produced wet nanofibres and forming a film

instead of fibrous web.

When polymer concentration was 15.0 %, no jets were observed, unless

the electric voltage was above 37 kV (Table 5.1). Initially at this voltage, jets

appeared only from the cylinder edges; while flat part of cylinder showed no jet

formation. When electric voltage was increased up to 45 kV jets were randomly

formed from flat part. At 58 kV, nanofibres could be produced from overall

cylinder surface still, more jets were formed on cylinder edges. The average fibre

diameter was 1.147 μm, with a large variation in diameter ranging from 4.589

μm to 123 nm (Figure 5.1). Whereas at concentrations higher than 17.0 %,

threshold voltage increased to 47 kV, yet number of jets decreased and fibre

formation was greatly reduced from cylinder generator.


- 102 -

Figure 5.1 SEM images of nanofibres produced from cylinder

generator at different magnification levels.

* (applied voltage = 50 kV, 15.0% polymer concentration, generator to

target distance = 60 mm, generator rotation speed = 20 rpm).


- 103 -

5.2.2 Circular Coil Generator

Circular coil spinneret was constructed by winding a 2 mm thick wire

giving four turns around a mandrel of 50 mm diameter. The circular coil was

partly immersed in the polymer solution. Polymer solution was charged by

connecting the conductive wire embedded at the bottom of polymer bath to high

electric voltage.

At lower concentrations (below 13.0%), jets were observed from top the

coil surface at applied voltage of 32 kV, variably from outer and inner coils, but

these fibres showed entrapped solvent. Even increasing applied voltage, solvent

evaporation was not enhanced and still wet fibres were obtained.

At polymer concentration of 15.0%, threshold voltage to start nanofibre

generation was 34 kV (Table 5.1). Still increasing applied voltage to 45 kV,

Outer coils produced consistently more number of jets whereas jets formed on

inner coils were intermittent and random. While, jets were formed equally from

all over the coil tops when applied voltage was increased to 55 kV, yet, inner

coils suffered jet deflection and life of these jets was shorter than outer coil jets.

Increasing voltage helped in amplifying number of jets from coil surface, till 65

kV beyond that corona discharge was observed. Fibres from 15.0%

concentration showed a diameter range of 2.645μ to 698 nm (Figure 5.2).

At higher concentrations (17.0% and beyond) the threshold voltage

shifted to 37 ~39 kV and jet stability on such higher polymer concentrations was

adversely affected, even increasing voltage did not help. Fibres were evenly


- 104 -

deposited on collector plate by circular coil rotation. Although thinner and more

jets were produced from circular coil, yet these jets showed deflection from

neighbouring jets. Which caused jet mixing and occasional wet areas in fibre

matt.

Figure 5.2 SEM images of nanofibres produced using circular coil

as fibre generator.

* (applied voltage = 50 kV, 15.0% polymer concentration, generator to target

distance = 60 mm, generator rotation speed = 20 rpm


- 105 -

5.2.3 Ring Generator

A 2 mm thick wire was folded around 50 mm mandrel to form a round

ring collector. On applying electric field, at lower concentrations (> 13.0%) ring

surface showed jet formation at low electric field i.e., 28 kV; moreover, jets were

produced from overall outer periphery of ring spinneret. Jets were also observed

to emerge from inner ring surface and interfered with jets from outer ring

surface.

The lower concentrations (≥ 13.0%) showed huge number of nanofibres

emerging from disc top at 27 kV, but similar to other generators, the fibres from

such lower concentrations were wet and fused. As described in Table 5.1, the

threshold voltage for 15.0% polymer concentration was greatly reduced to 31

kV. Numerous jets appeared on ring surface at an applied voltage of 49 kV. Fibre

diameter in web from 15.0% varied from very fine nanofibres 184 nm to 1.039

μm coarse fibres as shown in Figure 5.3.

The higher polymer concentrations (17.0 % ≤) it was difficult to produce

jets and nanofibres from ring surface. The jet interference influenced jet stability.

Jet positions variably moved from one place to another on ring surface.

Occasionally, partially, interconnected and wet nanofibres were observed in the

final nanofibre web.


- 106 -

Figure 5.3 SEM images for nanofibres generated by ring generator

at different magnification levels.

*(applied voltage = 50 kV, 15.0% polymer concentration, generator

to target distance = 60 mm, generator rotation speed = 20 rpm).


- 107 -

5.2.4 Disc Generator

A disc of 2 mm thickness and 50 mm diameter was employed as

needleless generator to produce nanofibres. After coating polymer solution by

disc rotation, high electric voltage was applied to polymer bath. At lower

concentrations (≤ 13.0%), enormous jets were observed emerging from disc top

even at 25 kV, but these fibres formed a uniform film of fused nanofibres.

Although the fibres to film ratio gradually increased from 9.0% to 13.0% yet,

fibres produced from these lower concentrations could not produce properly

dried nanofibres.

At polymer concentration of 15.0%, disc could produce nanofibres even

at 30 kV. Polymer jets were continuously produced from disc all over the top

disc surface at a voltage of 48 kV (as shown in Table 5.1). Jets emerging from

disc had longer life span than any other needleless generator. These jets were

only produced from top of the disc surface, no jets were formed from side or

lower part of disc surface. The jets position and distribution was also pretty much

fixed. The fibrous web produced from 15.0% concentration was comparatively

denser and composed of completely dry nanofibres as seen in Figure 5.4

At higher polymer concentrations (17.0% ≥), though jets were still

produced but size and number of jets substantially reduced. The threshold

voltage on such higher concentrations ranged 35~38 kV, yet discontinuous

electrospinning was observed on such higher concentrations. Similar to other

needleless generators, the fibres from disc produced at shorter distances (≤ 55

mm) were wet and coarse. 60 mm distance was suitable to produce dry


- 108 -

nanofibres, whereas higher electrospinning distance (65 mm ≥) affected fibre

deposition over ring collector.

Figure 5.4 SEM images for nanofibres produced by disc generator.

*(applied voltage = 50 kV, 15.0% polymer concentration, generator

to target distance = 60 mm, generator rotation speed = 20 rpm).


- 109 -

5.2.5 Comparison

Nanofibres produced from different generators were compared in terms

of their diameters and productivity.

Fibre Diameter

Disc and ring generators were found to produce finer nanofibres with

narrower diameter distribution than other fibre generators. Fibres from the coil

generator had narrower diameter distribution than those from cylinder generator

(Figure 5.5). The web produced from disc generator was dense. The fibres were

completely dry without any traces of solvent or fused fibres. Nanofibres from

ring generator were also dry but less dense when compared with those from disc

spinning. Nanofibre webs from coil had a few wet areas and showed

interconnected nanofibres. The fibres produced from cylinder had a wide variety

in diameter distribution, ranging from micron to submicron. The electric field

profile from central part of the cylinder differed to the cylinder edges in intensity.

The edges showed highly intensified electric field which led to continuous, finer

and higher number of jets. The fibres produced from cylinder edges were finer

with narrow diameter distribution. Whereas, central cylinder part had lower

electric field intensity. The jets produced from this specific area were

discontinuous, fewer and varying in sizes, consequently the fibres produced from

central part show a broad range of diameters.


- 110 -

Figure 5.5 Effect of generator geometry on fibre diameter.


- 111 -

Productivity of Nanofibres

Efficiency of fibre spinnerets was evaluated by electrospinning of fibres

for 10 minutes and then calculating fibre density rate (g/hr) as shown in

Figure 5.6. Although coil spinneret proved to be the most efficient in generating

nanofibres, yet nanofibres produced from it had frequent wet areas. This could

be due to the jets merger due interference within different coils. It was interesting

to note that disc spinneret was the almost as efficient as cylinder though its

surface area was much smaller than cylinder spinneret. The fibres from disc were

completely dry and formed a dense web whereas fibres from cylinder were wet

due to un-evaporated solvent and vague web. The disc had comparatively small

area for jet initiation and nanofibre generation yet it had the strongest electric

field profile 207. This strong electric field helped it in producing equally

distributed jet with longer life. The ring spinneret proved to be next to disc

spinneret. Although it had very similar electric profile to disc but its jet initiation

and stability was disturbed by secondary jets coming from inner ring surface.

Nanofibres from both disc and ring spinnerets were deposited on a very narrow

area.


- 112 -

Figure 5.6 Effect of generator geometry on nanofibre

productivity.


- 113 -

5.3 Needleless Electrospinning of Nanofibre Yarns

The novel ring was used as transient collector to evaluate different

needleless generators for formation of fibrous cone and yarn manufacturing. The

needleless generators were partially immersed in a polymer bath embedded with

a conductive wire. The wire was connected to a high electric voltage power

supply (Gamma High Voltage Research RR100-2P/240, 0 ~ 100 kV).

Polymer concentration for electrospinning of nanofibre yarn was fixed to

15.0%, as fibres generated from all the needleless generators showed good

electrospinning at this concentration. When needleless generator was slowly

rotated (20 rpm) in polymer bath, a thin polymer film was coated on it. On

applying voltage, the polymer solution was charged; a number of jets were

produced and fibres were deposited on ring collector. A web was formed on ring

opening and it was manually dragged to other side of ring. On transferring the

web to a simple drum winder, the fibrous web adopted a conical shape. The ring

rotation imparted twist in the web to form twisted nanofibre yarn. The freshly

generated fibres were deposited on inner fibrous cone. In this way, a continuous

nanofibre yarn was produced.


- 114 -

5.3.1 Cylinder Generator

The cylinder was used for electrospinning nanofibres to formulate

fibrous cone and yarn (Figure 5.7). The nanofibres were variably produced from

centre and edge of cylinder. The jets varied greatly in shape and size from both

areas. Although the jets from outer cylinder area had longer life, they were

mostly diverted away from target. The jets emerging from central cylinder part

were few and did not last longer. The fibres from this part were generally focused

towards ring collector, but their production was intermittent. The web formed on

the ring opening was not uniform and showed wet areas. On dragging this web

to other side, most of the fibres stuck to ring collector. It was very difficult to

form a fibrous cone. The reason for discontinuous fibrous cone could be due to

the lack of fibres and inclusion of wet fibres.

Distance between cylinder and ring collector was varied between 30 mm

to 70 mm to see its effect on nanofibres and yarn formation. At lower distance

(≤ 45 mm) wet fibres were observed sticking to ring surface and no web was

formed on ring opening. At a distance of 60 mm, invariably wet and dry fibres

were seen. The web formed by fibres was distorted with thick and thin places.

On dragging this web to other side of ring collector, cone was formed

momentarily which resulted in small piece of nanofibre yarn. This could be

anticipated to the charge present on the web. As this web, was highly charged, it

diverted to ring collector and collapsed, ending yarn formation. Further

increasing distance beyond 60 mm, did not help in drying nanofibres rather fibres

flew away from target and no web was formed on ring opening.


- 115 -

Keeping generator at a distance of 60 mm, applied voltage was changed

from 20 kV to 60 kV to see its effect on fibres and yarn fabrication. Lower

voltage (≤ 37 kV) could not produce any fibres. Gradually increasing, the voltage

beyond 45 kV, intermittent nanofibres were produced from flat area, while edges

produced continuous nanofibres. Figure 5.11 shows the diameter of yarn

produced from cylinder spinneret averaging around 41 m with average fibre

diameter about 1.133 m. At 58 kV, the fibres were produced from both cylinder

edge and flat surface yet edges showed increased number of jets. Non uniform

web was formed on ring opening and yarn formation was not possible. On further

amplifying voltage, no further improvement was observed, instead increased

fibre wastage was seen.

Figure 5.7 Schematic for nanofibre yarn production using cylinder as

nanofibre generator.


- 116 -

5.3.2 Circular Coil Generator

The circular coil generator was also used to generate nanofibres and

convert them in to nanofibre yarn. The fibres were produced from top edges of

all coil turns. The jets produced from outer coil were comparatively finer and

had longer life than inner coil turns. The fibres from outer coils were partially

flying away from target. Although fibres from inner coils were focused towards

ring collector, yet inner coil jets were instable and frequently distracted by outer

jets. Despite that the web formed on ring opening was better than cylinder

generator; it still contained fused fibres, which resisted dragging to other side

similar to cylinder generator. The fibrous cone was hard to form, resulting in

short pieces of yarns as shown in Table 5.1b.

The effect of distance, in the case of coil generator was similar to

cylinder. The wet fibres were electrospun when spinning distance was shorter

than 42 mm. Fibres morphology slowly improved with increasing distance to 55

mm, yet fibre loss occurred on higher distances (65 mm ≥). At lower applied

voltages (≥ 34 kV) the coil, behaved like a cylinder generator, i.e., outer coils

produced more jets and fibres, whereas, inner coils produced intermittent

nanofibres. Increasing applied voltage to 55 kV, all the coil tops started

generating fibres, still outer coils were more productive. Although increasing

voltage (60 kV ≥) augmented fibre production, the fibres produced on such high

voltage from outer coils went astray and could not reach ring collector. Whereas,

central coils suffered interference from outer coils and jets initiation was largely


- 117 -

disturbed. Consequently, the jet and fibre production was greatly decreased from

central coils.

In short, circular coil helped in improving fibre morphology by

increasing jet initiation sites and finer fibre with average diameter of 699 nm

were produced as shown in Figure 5.11. Numerous fibres with improved

morphology were produced but due to interference of jets from neighbouring

coil turns, the fibres moved away from target. Small portion of fibres reaching

target were highly charged and clung to ring collector, thus temporary cone was

formed and small yarn pieces having average diameter around 56 m could only

be obtained.

Figure 5.8 Schematic for nanofibre yarn production using circular

coil as nanofibre generator.


- 118 -

5.3.3 Ring Generator

When ring generator was utilized to produce nanofibre to formulate

fibrous cone and nanofibre yarn (Figure 5.9), initially fairly distributed jets were

produced on ring surface. These jets were consistent and stayed longer on ring

surface in comparison to cylinder and coil generators. Secondary jets were

observed emerging from inner ring surface. The secondary jet initiation from

inner ring surface distracted jets on upper ring surface. Ultimately, disturbing jet

position and reducing jet stability. As majority of the fibres were dry and mainly

deposited on a narrow area on the ring, they formed a denser web and covered

most of ring area. To form a fibrous cone this web was manually pulled to other

side of ring collector. This web was highly charged and stuck to ring collector.

The fibrous cone was formed temporarily and cracked immediately after. A short

strand of twisted nanofibres formed as shown in Table 5.1 c.

The smaller ring to generator distances (≤ 40 mm) proved to be

inefficient and frequent wet fibrous area formed. When spinneret was beyond a

distance of 55 mm, nanofibres produced were dry with occasional wet and fused

fibres. The distances farther than 62 mm, did not help in further drying of

nanofibres, rather fibres movement away from ring collector was observed.

Keeping the spinneret distance at 60 mm, effect of voltage on fibre

morphology and movement was studied. When the applied voltage was lower

than 31 kV, hardly any jet could be seen ejecting from ring surface. Increasing

applied voltage to 40 kV, numerous jets started appearing on the ring top surface,

with few random jets issuing from inner ring surface. Increasing voltage to 49


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kV helped in reducing wet fibre formation. The web formed on ring opening was

denser and consistent, than previous two generators. Yet occasional wet fibres

were seen. These fibres retained huge amount of charge and it was really hard to

get a stable cone as web was either stuck to ring or diverted away from winder.

Only short pieces of yarns were obtained with an average fibre diameter of 539

nm and yarn diameter of 32 μm as shown in Figure 5.11. Amplifying voltage

was useless as no improvement in fibre passage through ring or cone formation

could be achieved.

Figure 5.9 Schematic for nanofibre yarn production using ring as

nanofibre generator.


- 120 -

5.3.4 Disc Generator

When using disc as generator, varying the disc to ring distance affected

fibre morphology as shown in Figure 5.10. Lower distances (≤ 36 mm) produced

fibres with fused morphology and wet areas. By increasing the distance to 44

mm, the fibre morphology was improved. At a distance of 56 mm the fibres were

completely dry and formed a very stable web on ring opening. The web and cone

thus produced were thickest in comparison to rest of the needleless generators,

yet the web showed increased adhering to ring collector. The manual dragging

to form fibrous cone was still not successful. Only intermittent fibrous cone

could be achieved. Consequently, continuous fibrous cone and nanofibre yarn

fabrication was not possible. While increased distances, resulted in reduction of

fibre inclusion in yarn formation process.

When disc was used as generator, the nanofibres could be produced even

at a voltage of 30 kV. The fibres were mainly produced from disc top, neither

disc flat surface nor bottom curve produced any fibres. On gradually increasing

voltage to 40 kV, more number of jets were produced from the disc surface. At

47 kV, enormous jets were produced from disc; these jets were very steady and

showed longer jet life in comparison of all the needleless generators. Nanofibre

yarns with average fibre and yarn diameter of 472 nm and 24 μm respectively

could be produced on 50 kV Figure 5.11. On further amplifying voltage beyond

55 kV, fibres were diverted away from target.


- 121 -

Although the disc generator showed improved results in term of fibre

morphology, deposition and thick web formation; yet the nanofibre yarn

collection was not continuous. The main reason seems to be the remainder

charge in nanofibre web. The positively charged nanofibres were extremely

unstable and they tend to fly towards neutral surface.

Figure 5.10 Schematic for nanofibre yarn production using disc as

nanofibre generator


- 122 -

Figure 5.11 Effect of needleless nanofibre generator on fibre and

yarn diameter.

- 123 -

Table 5.1 Effect of generator geometry on fibre morphology, cone stability and yarn formation.

Generator type* Velectrospinning VWorking Fibres Cone Yarn Yarn morphology*

a Cylinder 37 kV 58 kV Wet fibres Highly unstable Discontinuous

b Circular Coil 34 kV 55 kV Semi wet fibres Highly unstable Discontinuous

- 124 -

Generator type* Velectrospinning VWorking Fibres Cone Yarn Yarn morphology*

c Ring 31 kV 49 kV Semi dry fibres Unstable Discontinuous

d Disc 30 kV 48 kV Dry fibres Intermittent Discontinuous

*Applied voltage = 50 kV Generator to ring collector distance = 60 mm

Polymer concentration = 15.0 % Ring collector speed = 1200 rpm Generator rotating speed = 20 rpm


- 125 -

5.3.5 Modified Yarn Electrospinning System

Auxiliary Metallic Electrode

Disc generator showed greater possibility to produce nanofibre yarn than

other needleless fibre generators, yet number of fibres forming fibrous cone were

not enough to produce a stable cone. Consequently nanofibre yarn fabrication

was not continuous. To provide sufficient fibres for yarn production, an auxiliary

metallic electrode was placed just above the polymer bath. The metal electrode

had the same electric potential as that of polymer bath (Figure 5.12).

It was noticed that by using auxiliary electrode, more and thicker jets

were produced on disc top. These jets could not properly stretch and dry, the

cone and as-prepared yarn had larger diameter but with fused fibre morphology

as shown in Figure 5.14. Continuous and proper yarn formation using auxiliary

electrode was not possible.


- 126 -

Figure 5.12 Modified electrospinning setup with auxiliary metal

electrode.


- 127 -

Air jet enhanced yarn electrospinning

Another approach to produce more fibres in to yarn formation zone was

adding an air jet (10 psi) along spinning direction as shown in Figure 5.13. The

air jet was applied through 8 nozzles placed around polymer bath to get even air

flow around the disc generator. Though, the pressurized air was applied to

improve solvent evaporation and quick drying of nanofibres, it disturbed overall

jet formation and distribution over disc generator. The air flow shortened jet life

reducing number of fibres entering into cone formation and yarn manufacturing

zone. The small pieces of yarns so obtained showed large number of coalesced

nanofibres (Figure 5.14).

Figure 5.13 Air jet enhanced yarn electrospinning setup.


- 128 -

Figure 5.14 SEM images of nanofibre yarns produced by using

electrospinning setup with modification (a) auxiliary metal electrode

and (b) pressurised air.


- 129 -

5.4 Conclusion

In this chapter, use of needleless generators to produce nanofibre yarn

has been described. Four needleless generators (cylinder, coil, ring and disc)

were accessed to fabricate nanofibre yarn from the new developed

electrospinning setup. The needle generators were replaced with different

needleless generators.

The nanofibres produced from cylinder generators showed huge amount

of solvent entrapped inside the fibre. The majority of fibres were fused together

to form film on ring surface. The reasons for this include overall lower electric

field of cylinder generator, variations of electric field from edge to flat

cylindrical part, enormous amount of solution over cylinder surface. All these

factors produced wet fibres and distorted web on ring collector opening. The jet

sizes and positions varied from central cylinder to cylinder edge. The edges

produced frequent jets with long jet life, while, jets from central part were

scattered and discontinuous. The nanofibre yarn produced from cylinder

generator, was composed of almost all fused nanofibres. Due to wet and fused

fibres, it was very difficult to get web accumulation over ring opening;

consequently, no cone or yarn could be fabricated from cylinder generator.

The fibres produced from coil generator showed improved morphology

from cylinder yet it still contained fused fibres. The fibres were produced from

each coil individually, but at the same time, the jets from inner coils were

distracted by outer coils. Thus the jet interference caused fusing of jets from

different coils. The yarn produced from coil generator showed fused morphology


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of fibres. The web formed at the ring opening was distorted and had wet patches.

The cone could not be formed under these conditions and only very small piece

of yarn could be obtained.

The ring and disc generators both showed comparatively improved

nanofibre morphology in final nanofibre yarn. Although the outer jets were

slightly distracted by inner ring jets but overall web showed dry nanofibres. The

webs formed by both ring and disc generators on ring opening were thicker than

coil and cylinder generators. On dragging the web to fabricate yarn, the web

showed resistance. The highly charged web either stuck to ring collector or

drifted away from drum winder. The cone was momentarily formed and ruptured

from bottom near ring collector or top near drum winder. The disc generator

setup was modified to enhance nanofibre production by using auxiliary metal

electrode or enhanced air jets. The both modifications did not work. Instead, the

jet stability was affected and wet fused nanofibres were collected.

In conclusion, it was difficult to produce continuous nanofibre yarns

from needleless generators. Stable cone could not be achieved in any of the four

needleless generator systems. The cone stability was mainly disturbed either by

wet fibres or by large remaining charges in the final web. Therefore, needleless

generators alone could not be used to produce properly dried continuous

nanofibre yarn.

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C H A P T E R S I X

6 Nanofibre Yarn Prepared by Hybrid

Electrospinning Systems

In this chapter, novel technique of using a ring collector and a hybrid

electrospinning system consisting of both needle and needleless fibre spinneret

was developed to prepare nanofibre yarns. By combining a disc-based fibre

generator with a needle generator, the system could produce nanofibre yarns at

considerably increased production rates. Effects of electrospinning setup, disc

parameters from the disc generator and processing factors on nanofibre

fibre/yarn morphology, diameter, yarn formation, twists levels and mechanical

properties were examined. A finite element method was also employed to

analyse the effect of different disc generator materials on electrospinning

capabilities.

6.1 Experimental

6.1.1 Preparation of Nanofibres

A purpose-built electrospinning setup was used to prepare nanofibre

yarns. The modified system comprised of one needle-based electrospinning, a

disc based needleless electrospinning, an intermediate ring collector, and a drum

winder, as schematically shown in Figure 6.1. The disc and the needle generators

were separately connected to the positive high-voltage power supply (RR100-

2P/240, Gamma High Voltage Research 0~100 kV) and negative high-voltage

power supply (ES30P, Gamma High Voltage Research 0~50 kV), respectively.

Chapter 6: [Nanofibre Yarn Prepared by Hybrid Electrospinning System]

- 132 -

The ring collector was 40 mm wide, with outer and inner diameter of 100 mm

and 88 mm, respectively. The ring collector was placed horizontally on a HDP

(high density polyethylene) base mounted with a ball bearing to lower friction

in rotation. A variable speed motor was connected to ring collector through an

O-ring belt to control rotation speed of ring collector.

The disc spinneret produced nanofibres upwards, while the needle

generator which was placed near the ring collector produced nanofibres from

top. The nanofibres were initially deposited on the ring collector surface. A

fibrous web was formed on ring aperture when ring rotated. An inverted fibrous

cone was formed by manually dragging this web using a plastic rod. The edge

of cone was drawn endlessly into nanofibre yarn.


- 133 -

Figure 6.1 Schematic illustration of hybrid electrospinning setup

for making nanofibre yarn.


- 134 -

6.2 Yarn Formation in the Hybrid System

Figure 6.1 schematically illustrates the modified setup for

electrospinning of hybrid nanofibre yarns. The polymer solution was fed through

a syringe to the needle nozzle, while a solution was loaded on disc generator

from a solution bath underneath it. The needle generator was connected to a

negative high power supply whereas, polymer solution bath was electrically

charged by a positive high power supply. Two electrospinning systems produced

oppositely charged nanofibres.

Initially, when ring collector was not moving, most of the fibres got

deposited on ring surface, but no web was formed on ring opening. A fibrous

web was formed on ring opening when it rotated. An inverted fibrous cone was

then formed through drawing the fibrous web from the central part (Figure 6.2a).

The cone apex was continually drawn by the winder, while the ring collector

rotated at a high speed. In this way, a twisted nanofibre yarn was produced from

the system as shown in Figure 6.2b & c.


- 135 -

Figure 6.2 (a) Fibrous cone formed on top of ring collector, (b) SEM image of

an electrospun nanofibre yarn and (c) shows nanofibre yarn collected on a

spool.

It was noted that when only one of the nanofibre generators worked, no

web was formed on the ring collector. All the fibres produced clung on the inner

walls of the ring collector. To form a fibrous web on the ring collector, it was

necessary to intermix positively and negatively charged fibres in mid of the ring

collector and both electrospinning systems must work concurrently. A voltage

of 9 kV was found strong enough to run the needle electrospinning system, while

the disc electrospinning continuously generated nanofibres only when the

applied voltage was above 50 kV.

The fibres from needle nozzle evenly covered the upper side of fibrous

cone, while lower side of cone was augmented by nanofibres produced by the

disc generator. The cone formed by hybrid system was rich in nanofibres on

every part of cone. The cone thus formed was thick and extremely stable. The


- 136 -

cone apex could be drawn and drafted on higher winding speeds resulting in

increased production rates. The hybrid system was also capable of processing a

high volume (approximately 6.5 ml/h) of polymer solution, which was higher

than that of the previously designed needles generator (3.0 ml/h) and needleless

generator systems (4.5 ml/h).


- 137 -

6.3 Factors Affecting Yarn Electrospinning

6.3.1 Effect of Positions of Different Elements

The position of different elements in the yarn electrospinning setup greatly

affected nanofibre disposition, formation of fibrous cone, cone stability and

nanofibre yarn fabrication. The position of different elements has been depicted

in Figure 6.3 and effect of their position has been discussed in detail in following

sections.

Figure 6.3 Schematic diagram showing the position of different

electrospinning elements in hybrid nanofibre yarn setup.


- 138 -

6.3.1.1 Disc generator position

The position of fibre generators was crucial for this yarn electrospinning

process. Table 6.1 summarizes the effect of nanofibre generators on the stability

of fibrous cone and yarn formation. It was important to position the disc

generator beneath the centre of the ring collector. In this way, the nanofibres

were deposited evenly on the ring collector. Asymmetrical distribution of

nanofibres resulted when the disc generator was move away from the central line

of the ring collector, which led to uneven deposition of nanofibres on ring open

and destabilized cone.

The distance between the disc and the ring collector played a key role in

determining the yarn formation. When the distance was less than 30 mm, wet

fibres formed, which struck inside the ring. As a result, no fibrous cone and

nanofibre yarn were prepared. Thin fibrous cone and discontinuous yarn could

be obtained when the disc-ring distance was between 32 mm to 49 mm. In this

case, most of the fibres were found to be fused together.

When the distance was in the range of 50 ~ 60 mm, a continuous cone

and incessant yarn was formed. Further increasing the distance to 62 ~ 68 mm

resulted in thinner cone with frequent yarn breakage. At such a high distance,

lots of nanofibres flew into the air, leaving only a small amount of nanofibres to

add onto the fibrous cone. However, no cone or yarn was produced when the

disc-ring distance was larger than 72 mm.


- 139 -

Figure 6.4 Effect of disc-ring distance on fibre and yarn diameters.

(Disc generator at 50 kV, Needle fibre generator at 9 kV, 48 mm from

ring, winder at 55 mm from ring generator.

The effect of disc-ring distance on fibre and yarn diameter is shown in

Figure 6.4. There was gradual decrease in fibre diameter till 60 mm, beyond this

point fibre diameter showed slight increase. The fibres at lower disc generator

distances had lesser time to elongate and stretch. At a lower distance, the solvent

was also not fully evaporated and consequently coarse wet fibres were obtained.

When the disc distance was between 50 mm to 60 mm, fibres had enough time

and space to stretch and elongate, resulting in dry separated nanofibres. Further

increasing disc distance beyond 62 mm, jet size, distribution and direction

started varying. This jet instability and randomness produced fibres with varying

diameter and higher diameter distribution.


- 140 -

With increasing the distance, the yarn diameter increased initially until

the distance reached 60 mm. The yarn diameter then decreased rapidly with

further increasing the distance. At lower distances, wet fibres resulted which

fused together, leading to yarns with smaller diameter. Gradually increasing the

disc-generator distance from 50 mm and above facilitated to produce dry, well

separated nanofibres. Yarns produced on these distances showed larger

diameters. On further increasing disc distance beyond 62 mm, fibres flew off the

target. Consequently the cone lacked nanofibres and the number of fibres in final

yarn dramatically reduced. The yarn produced on such high disc distances

showed a declined trend in diameter.

- 141 -

Conditions Disc-ring*

(mm)

Needle-ring**

(mm)

Winder-ring***

(mm) Cone Yarn

A 1 <30 48 55 No cone No yarn

A 2 31 ~ 49 48 55 Unstable, wet Discontinuous

A 3 50 ~ 60 48 55 Stable, dry Continuous

A 4 61 ~ 68 48 55 Unstable, dry Discontinuous

A 5 > 72 48 55 No cone No yarn

B 1 60 <21 55 No cone No yarn

B 2 60 22 ~ 40 55 Unstable, wet Discontinuous

B 3 60 41 ~ 50 55 Stable, dry Continuous

B 4 60 51 ~ 60 55 Unstable, dry Discontinuous

B 5 60 > 61 55 No cone No yarn

Table 6.1 Effect of nanofibre generators and winder position on cone stability and yarn formation.

- 142 -

Conditions Disc-ring*

(mm)

Needle-ring**

(mm)

Winder-ring***

(mm) Cone Yarn

C 1 60 48 <25 No cone No yarn

C 2 60 48 26 ~ 49 Unstable, wet Discontinuous

C 3 60 48 50 ~ 56 Stable, dry Continuous

C 4 60 48 57 ~ 66 Unstable, dry Discontinuous

C 5 60 48 > 67 No cone No yarn

* Disc was placed perpendicularly below ring collector

** Needle generator was positioned at angle of 30° and 20 mm from inner ring edge.

*** Drum winder was placed in centre directly above the ring collector

Chapter 6: [Nanofibre Yarn Manufacturing Using Hybrid System]

- 143 -

6.3.1.2 Needle generator location

The position of the needle generator was also a very important parameter

affecting the yarn formation. Here, the needle nozzle was placed at angle of 30°

and 20 mm from ring horizontal edge. The vertical distance was varied to

observe its effect on cone and yarn formation.

Table 6.1 summarizes the effect of needle-ring distance on fibre

morphology, cone stability and yarn formation. When the distance was below 20

mm, all the fibres generated by needle deposited on ring edge, and no web was

formed on ring opening. When the distance was in the range of 22 ~ 40 mm,

intermittent wet cone was formed, but still no continuous yarn was prepared.

When the distance was larger than 42 mm, a stable cone and continuous yarn

were formed. At this point, the jets from the needle nozzle moved towards the

gap between the winder and the ring collector.

When the distance was beyond 50 mm, the majority of fibres were

deposited on the winder, and only quite a few fibres went to gap between the

winder and the ring collector. Consequently, the cone became unstable. When

the distance was larger than 63 mm, the fibres started depositing on winder and

no cone or yarn was formed.

Chapter 6: [Nanofibre Yarn Prepared by Hybrid Electrospinning Systems]

- 144 -

6.3.1.3 Winder location

The ring-winder distance also affected cone and yarn formation. When

winder was placed less than 25 mm away from the ring collector, the wet

nanofibres generated from the needle nozzle were deposited on the winder, and

in this case, neither fibrous cone nor yarn was produced. When the distance was

in the range between 27 mm and 48 mm, fibres deposited partly on ring and

partly on winder, still no cone or yarn was formed.

At a distance of 50 mm, fibres from needle generator accumulated on gap

between the winder and the ring collector. At this stage, a very stable cone was

formed and a continuous yarn could be drawn. When the winder-ring distance

was larger than 58 mm, the cone frequently ruptured and discontinuous yarn was

produced. Larger distance beyond 68 mm disturbed the balance between

nanofibre generation, cone formation and yarn drawing. As result, the cone was

not formed and no yarn could be produced.


- 145 -

6.3.2 Disc Generator Parameters

6.3.2.1 Disc Material

Discs made of conductive (steel, aluminium, copper) and non-conductive

(high density polyethylene (HDP)) materials were investigated for the nanofibre

generation and yarn formation. At a low applied voltages (≤ 40 kV) all the metal

discs showed random jets on their surface and nanofibres of variable diameters.

The jets produced on non-conductive disc were thinner, numerous, continuous

and equally distributed on disc surface (Figure 6.5a)

Figure 6.5 Photographs of jets formed on a) HDP disc, b) copper disc

c) steel disc and d) aluminium disc.


- 146 -

Conversely, the jet formation on the metal discs was thicker, fewer,

discontinuous, and irregular mainly on disc top (Figure 6.5 b ~ d). When the

applied voltage was above 50 kV, a corona discharge took place on all the metal

discs. In contrast, the HDP disc could produce nanofibres without corona

discharge even up to 65 kV of applied voltage.

Figure 6.6 Simulated electric field profile of polymer layer on a) conductive

disc and b) non-conductive disc

Electric field analysis for different disc materials was performed using a

finite element method as shown in Figure 6.6. At the same applied voltage, the

electric field intensity around the disc top in case of conductive disc was greater

than that on the non-conductive disc. The electric field intensity at bottom in the

conductive disc was same as that at top. However, in the case of non-conductive

disc, the electric field at disc bottom was higher than that of disc top. This shows

that the conductive disc behaved as equipotential surface from one edge to other

edge.


- 147 -

The charge flows without requiring any extra force, from disc bottom

(which is immersed in charged polymer liquid) to disc top. While, non-

conductive disc does not permit charges to transfer through disc and holds them

on disc periphery. This was exactly what was observed in experiments, when

non-conductive disc was employed as nanofibre generator. Numerous number

of evenly distributed jets were seen on non-conductive disc, while, the metal

discs showed fewer thicker and random jets.

For conductive disc, the electric field intensity was found to gradually

increase from disc part immersed in polymer bath to the disc top as shown in

Figure 6.7. The non-conductive disc showed a reverse pattern of electric field

profile. This suggests that electric field in the polymer solution and near the disc

bottom was stronger in case of non-conductive disc. In other words, more

charges were retained in polymer solution and polymer layer on non-conductive

disc surface.

It was also found that when a polymer solution layer covered the non-

conductive disc (HDP), the non-conductive disc showed higher electric

polarization along lower disc circumference in comparison to conductive disc.

When high applied voltage was applied, it creates polarization within the disc

material.


- 148 -

Figure 6.7 Calculated electric field profile of polymer layer on a) conductive

disc and b) non-conductive disc.


- 149 -

6.3.2.2 Disc Diameter

The diameter of disc generator was found to affect fibre and yarn

diameters. To examine the effect of disc size on nanofibre and yarn diameter,

HDP (1 mm thick) discs with four different diameters (e.g. 50 mm, 60 mm, 70

mm and 80 mm) were employed. When disc size was small (e.g. 50 mm), fewer

jets were generated, leading to wet coarse fibres. These wet fibres were sticky

and hard to form a continuous cone, therefore, no continuous yarn was produced.

As the disc diameter was increased, number of jets on disc surface increased and

dry fibres were produced, which led to the formation of a stable cone and

continuous yarn production.

Figure 6.8 shows the effect of disc size on fibre and yarn diameters. By

increasing disc generator diameter, a declining trend was observed in nanofibre

diameter with maximum attenuation on 80 mm diameter disc. These results

comply with the work of other researchers who reported similar trends while

using discs of different diameters sizes 264,265. It was described in their work that

larger diameter discs showed higher electric field intensity in comparison to

small size discs. This high electric field intensity in non-conductive disc led to

more charges being accumulated on disc surface, increasing number of equally

spaced jets. The fibres were thus produced in higher productivity with lower

diameter. The yarn consisting of these finer well separated finer fibres showed

larger yarn diameters. Therefore, increasing the disc diameter led to steady

increase in the yarn diameters. The yarns produced from 80 mm disc had almost

4 times larger diameter than that fabricated from 50 mm disc.


- 150 -

Figure 6.8 Effect of disc diameter on fibre and yarn diameters. (Applied voltage

for disc and needle fibre generators was 50 kV and 9 kV, disc, needle and

winder were placed at a distance of 60 mm, 48 mm and 55 mm, from the ring

collector respectively.


- 151 -

6.3.2.3 Disc Thickness

Disc thickness was investigated for its effect on nanofibre and yarn

diameters. When HDP disc diameter was fixed at 80 mm and disc thickness was

changed from 1 mm, 2 mm, to 4 mm and 6 mm, both yarn and fibre diameter

showed noticeable change. As shown in Figure 6.9 when disc thickness

increased, the fibre diameter increased, while the yarn diameter decreased. This

can be explained by the effect of disc dimension on electric field intensity. It

has already been established that thinner discs show higher electric field

intensities at top of disc. While thicker discs showed lower electric field intensity

on disc top 264,265. It was also mentioned that the electric field intensity was

mainly focused on thicker disc edges not on flat part of discs. The electric profile

for thicker discs resembles to cylinder generator profile in which only cylinder

edges showed increased electric intensity and produced jets. While flat cylinder

area lacked electric intensity which could not produce well-spaced continuous

jets.

Disc with 1 mm thickness produced thinner and increased number of jets,

therefore, nanofibre generation form thinner disc was steady and continuous.

The fibres thus produced were dry and smooth. While, thicker discs (4 mm or 6

mm) had random discontinuous jets, resulting in discontinuous production of

coarser and rough fibres.


- 152 -

Figure 6.9 Effect of disc thickness on fibre and yarn diameters. (Disc

and needle fibre generator at 50 kV and 9 kV respectively; disc,

needle and winder were placed from ring collector at a distance of 60

mm, 48 mm and 55 mm respectively).

The yarn continuity was also largely affected by disc thickness. The

thinner disc resulted in highly stable cone. Although fibres produced by thin disc

were fine, they formulated a yarn with large diameter. This could be due to the

fact that the electric field intensity on thin disc edge was higher when reducing

the disc thickness.


- 153 -

6.3.3 Polymer Concentration

Similar to conventional electrospinning, modified yarn electrospinning

was also affected by polymer concentration. To elucidate the effect of polymer

concentration on fibre and yarn morphologies, PVDF-HFP solutions with

concentration varying from 9.0% to 19.0% were electrospun. Table 6.2

summarizes the effects of polymer concentration on threshold voltage for

electrospinning, fibre morphology, cone stability and continuity in yarn

formation.

When polymer concentration was in the range of 9.0% ~ 11.0%, polymer

viscosity was fairly low and a thin polymer layer thoroughly covered the disc

surface. A large number of jets could be seen at a voltage range of 25 ~ 28 kV.

Although it required 38 ~ 42 kV voltage to run electrospinning smoothly, wet

beaded fibres were generated and it was extremely difficult to produce fibrous

cone and nanofibre yarn.

As the polymer concentration was gradually increased to 13.0%, polymer

viscosity increased, the solution layer on disc was slightly thick. On applying

voltage above 45 kV, semi-continuous jets were produced from disc surface and

dry fibres with smaller beads were observed. At this point, cone stability showed

a little improvement, still yarn fabrication was not continuous. The 15.0%

polymer solution showed increased viscosity and uniform thick polymer

coverage on disc. At a voltage of 50 kV, the jet protrusion was distributed

thoroughly on disc surface. The nanofibres produced from this solution were dry

and bead-free. These dry and well separated fibres led to a steady cone and

incessant yarn production.


- 154 -

At higher polymer concentrations (17.0% and above), polymer viscosity

was such high that the polymer coating on the disc surface became uneven,

leading to random and discontinuous jet initiation. Moreover, higher threshold

voltages (34 to 37 kV) were required to generate nanofibres. For steady

production of nanofibres, still higher voltages (54 to 57 kV) were required. Due

to increased polymer viscosity, the fibres thus produced were coarser and had a

lot of solvent rich areas. These highly charged, wet fibres were hard to go

through ring collector. As there were not enough fibres to support cone, no

continuous yarn was prepared.

Figure 6.10 shows the effect of polymer concentration on fibre and yarn

diameters. At a polymer concentration lower than 11.0%, beaded fibres were

produced. Increasing the polymer concentration to 13.0%, solution viscosity was

enhanced. As a result, reduced fibre beads and fused morphology were noticed.

Yet on such viscosity, coarse semi wet fibres were produced which could not

form a stable cone and yarn.

Bead free fibres could be obtained when the polymer concentration was

above 15.0%. At this point, the viscosity was high enough to produce dry and

well defined fibres, leading to a steady fibrous cone and continuous nanofibre

yarn. However, still when the polymer concentration was higher than 17.0%, the

viscosity was too high to electrospin the solution into fibres.


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Figure 6.10 Effect of polymer concentration on fibre and yarn diameters.

Applied voltage for the disc- and the needle- spinning was at 50 kV and

9 kV respectively. Disc, needle and winder were placed from ring

collector at a distance of 60 mm, 48 mm and 55 mm, resp

Yarn initially showed an increasing trend with increasing the polymer

concentration, and then a declining trend was observed (Figure 6.10). At a

polymer concentration less than 13.0%, wet fibres were produced. These wet

fibres fused together leading to smaller yarn diameter. Solution viscosity

gradually increased at higher polymer concentration higher than 15.0%, which

helped in reducing portion of wet and fused fibres. Dry, bead-free fibres were

produced from 15.0% polymer solution and these fibres were well separated

from each other. These dry and well separated fibres formulated yarns with

higher diameters. Whereas, the decrease in yarn diameter at still higher polymer


- 156 -

concentrations (19.0%) can be associated to the difficulty in electrospinning. The

high viscosity made it difficult to electrospin nanofibres, as a result few and

coarse fibres were produced. These fibres were not enough to form cone and

yarn.

- 157 -

Table 6.2 Effect of polymer concentration on yarn electrospinning

Concentration

(% wt)

Velectrospinning

(kV)

Vworking

(kV)

Fibres Cone stability Yarn Formation

9.0 23 kV 38 kV Fused fibres Occasional cone Discontinuous

11.0 25 kV 42 kV Round beaded fibres Highly unstable Discontinuous

13.0 27 kV 45 kV Spindle beaded fibres Unstable Discontinuous

15.0 30 kV 50 kV Bead free fibres Stable Continuous

17.0 34 kV 54 kV Coarse fibres Unstable Discontinuous

19.0 37 kV 57 kV Coarser fibres Highly unstable Discontinuous

*Applied voltage for disc and needle electrospinning systems were 50 kV and 9 kV, respectively. A polymer solution for needle electrospinning

was kept at 17.0 %. Disc, needle and winder were placed from ring collector at a distance of 60 mm, 48 mm and 55 mm, respectively.


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6.3.4 Applied Voltage

Applied voltage plays a pivotal role in nanofibre and yarn production.

The threshold voltage for electrospinning (Velectrospinning) and for the formation of

fibrous cone (Vworking) are listed in Table 6.2. The effect of voltage on nanofibre

generation and yarn formulation was determined by varying the applied voltage

from 0 to 70 kV, keeping polymer concentration at 15.0%. At this condition,

stable cone, bead-free nanofibres, and continuous yarn were produced.

No jets or fibres were produced till the voltage reached 30 kV. At this

point, only a few jets appeared, but the fibres formed were wet and coarse.

Occasionally, wet cone was formed which led to discontinuous yarn production.

The solvent enriched fibres continued to produce even up to 42 kV. Fibrous cone

formed by wet fibres was unstable and yarn broke occasionally during drawing

due to the fused fibrous structure. Once the voltage was above 45 kV, dry fibres

were prepared. Increasing the voltage gradually decreased the number of beaded

and fused fibres.

When applied voltage was increased to 50 kV, plenty of dry nanofibres

were produced which formed steady fibrous cone and continuous nanofibre yarn.

Further increasing the voltage to 60 kV, a number of highly charged nanofibres

were produced. However, a large number of these highly charged fibres flew in

the air other than the target. The fibres which reached target firmly affixed to the

ring collector due to high charges on fibres. As a result, insufficient supply of

nanofibres caused cracked cone and discontinuous yarn.


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Figure 6.11 Effect of applied voltage on fibre and yarn diameters.

(Needle fibre generator at 9 kV; polymer concentration for needle

system was 17.0% and polymer concentration in polymer bath was

15.0%, disc, needle and winder were placed from ring collector at a

distance of 60 mm, 48 mm and 55 mm respectively.

The applied voltage also affected the fibre and yarn diameters as shown

in Figure 6.11. With increasing the voltage, nanofibre diameter decreased

initially until the voltage reached 50 kV, then the diameter showed an increment.

While a steady increase in yarn diameter was observed till the applied voltage

reached 55 kV and then yarn diameter decreased. Initially at applied voltages

lower than 40 kV, the jets produced by such a low electric force could not

properly stretched and wet fibres were thus obtained. When the applied voltage

was higher than 45 kV, the electric force was strong enough to draw jets and

form dry fibres. On further increasing the voltage to a value higher than 55 kV,


- 160 -

the electric force was so strong that it disturbed the stability and direction of jets.

The fibres produced on such a high voltage were coarse.

Yarn diameter was directly affected by the applied voltage. At voltages

lower than 40 kV, wet fibres fused together and formed smaller diameter yarns.

Yarn produced on higher applied voltages (45 kV≥) constituted of dry well

separated nanofibres. The yarns on such voltages showed gradual increase in

diameter. Although, applied voltages above 55 kV produced dry and well

separated nanofibres, it disturbed jet stability. The fibres produced on such high

voltages went astray. At this point, reduced amount of nanofibres were included

in fibrous cone and yarn diameter showed a declining trend. Moreover, cone

stability was highly unbalanced and yarn fabrication was not steady.


- 161 -

6.3.5 Flow Rate

The amount of polymer solution processed into nanofibres through both

of the electrospinning system was another very critical factor in fibrous cone

development and yarn formation. The polymer flow in needle generator was

controlled through micro-syringe pump, while polymer flow rate from polymer

bath was difficult to control. The polymer coating over disc was partially

controlled by disc generator rotation and applied voltage, yet exact amount of

polymer converted into nanofibres could not be calculated. Disc rotation speeds

from 20~25 rpm helped in evenly loading of polymer solution on disc surface.

The jets produced at this point were evenly distributed and produced dry

nanofibres. At lower speeds (≤ 15 rpm) uneven coating was observed, while

higher speeds (30 rpm ≥) forced more polymer solution on the disc and started

throwing polymer away from disc. Speeds above or below 20 ~ 25 resulted in

irregular jet initiation and wet nanofibre formation.

For a stable cone formation it was necessary to get neutral web over ring

opening, when only disc or needle was producing fibres no proper web was

formed. When polymer flow rate from needle generator was ≤ 1.5 ml/h, not

enough fibres were produced from needle generator to neutralize the positive

fibres coming from disc generator. The web was highly charged and could not

formulate fibrous cone or nanofibre yarn. As the polymer flow rate was increased

to 2.5 ml/h, enough amount of fibres formed a smooth and neutral web on ring

opening. On manually dragging this web to drum winder a very stable cone was

achieved, which resulted in continuous nanofibre yarn formation. Further

increasing polymer flow to 3.0 ml/h and above, polymer dripping was observed.


- 162 -

The fibres produced from needle on such higher polymer flow contained

entrapped solvent and fused fibres. The cone from these fibres, frequently

suffered splitting and adversely affected yarn formation.


- 163 -

6.4 Twist Level

The ring collector rotation and winder speed were responsible for stable

cone formulation, twist insertion and proper winding of nanofibre yarn. At lower

ring rotation speeds (≤ 310 rpm), the number of nanofibres passing through the

ring collector were not enough to support steady cone formation. On gradually

increasing the ring speed to 440 rpm, the fibres coming from disc and needle

generator were evenly distributed on cone surface. At this point, cone could be

continuously dragged and drafted into nanofibre yarn.

Cone stability and continuous yarn winding were not disturbed till the

ring collector speeds were between 440 rpm to 1430 rpm. However, ring

collector speeds beyond 1500 rpm caused vibrations in overall electrospinning

system leading to frequent cone cracking and ceasing nanofibre yarn production.

With increasing ring collector speed within 440 to 1430 rpm, a steady increase

in yarn twist levels was observed (as shown in Figure 6.12). A maximum twist

level of 4766 twists per metre could be achieved from this modified nanofibre

yarn electrospinning system. At this maximum twist level, highest twist angle of

fibres along yarn was noticed to be 48.8°.


- 164 -

Figure 6.12 Effect of ring speed on twist levels and twist angles

in electrospun nanofibre yarns.


- 165 -

6.5 Yarn Production Rate

Production rate of nanofibre yarns was determined by calculating surface

speed of winding drum. The drum winder was mounted on a small variable speed

motor. The winder speed was varied from 60 to 260 m/hr, to see its effect on

cone stability and y arn formation. It was noted that when winder speed was

lower than 65 m/h, the cone apex was slightly loose, causing jerks and yarn

rupturing at cone apex. Once the speed was increased beyond 90 m/h, a smooth

thick cone was formed and incessant yarn could be produced.

Yarn could be produced without any interruption at maximum winder

speed of 240 m/h. The improvement in yarn production was achieved by

additional nanofibre deposition from both upper and lower sides of the cone. The

disc generator supplied dry nanofibres from disc spinneret, which were evenly

and continuously spread over lower fibrous cone side by ring rotation. While

upper side was augmented by nanofibres coming from the needle generator. A

delicate balance between fibre deposition on cone and drafting of yarn by winder

existed at this stage. At a speed higher than 245 m/h, a lot of attenuation in yarn

and frequent cracks appeared on cone surface, thereby disturbing the balance

between fibre deposition and yarn drafting. On such high speeds, nanofibre

fabrication was not possible.


- 166 -

6.6 Mechanical Properties of Nanofibre Yarns

The mechanical properties of nanofibre yarns were evaluated using

FAVIMAT (AI) + Robot 2. A gauge length of 10 mm and vertical jaw speed of

10 mm/min were employed to test at least 5 samples for each category. Samples

produced from random and aligned web produced under same conditions from

this modified system were also tested to compare overall improvement in

mechanical strength. Typical nanofibre yarn produced by the hybrid system

showed a tensile strength and strain percentage of 128.9 MPa and 222.1%

respectively. Randomly orientated and aligned webs showed a tensile strength

of 9.9 MPa and 22.5 MPa with strain percentage of 98.7 % and 84.6%,

respectively (Figure 6.13a).

Effect of twist levels on nanofibre yarns was also studied. Yarns with

different twist levels were prepared under the same operating condition. It was

found that yarn strength and strain percentage gradually increased with adding

more twists into the yarns, as shown in Figure 6.13b. The increment in twist

levels leads to firmly attached nanofibres and enhanced tensile strength.

Moreover, the number of nanofibres per yarn cross section increased. This

superior fibre to fibre adherence and higher number of nanofibres per yarn cross

section helped in achieving better mechanical properties in nanofibre yarns with

higher twist levels.


- 167 -

Figure 6.13 (a) Typical stress ~ strain curves of random nanofibre

web, aligned nanofibre web and nanofibre yarn, (b) Effect of twist

levels on stress and strain of nanofibre yarns.


- 168 -

6.7 Conclusion

In this chapter, hybrid electrospinning system was used to fabricate

nanofibre yarns. This hybrid system consisted of a needle and a needleless

nanofibre generators, a ring collector, and a simple drum winder. Effect of

different parameters, such as needle and needleless nanofibre generators and

processing factors on fibre morphology, cone stability and yarn fabrications

were studied.

The distance of different electrospinning elements (nanofibre generator,

needle nanofibre generator and drum winder) was found to be very crucial.

Positions of these electrospinning elements greatly affected the nanofibre

morphology, cone stability and yarn fabrication. At lower distances, there was

not enough time for to get completely dry nanofibres. These wet, coarse fibres

fused together on ring surface, thus no cone nor yarn were formed. Increasing

distance to a suitable point led to dry, well separated fibres. These dry and well

separated fibres helped in achieving highly stable cone and continual withdrawal

of nanofibre yarn. At higher distances, though dry fibres were obtained, they

went off the ring collector.

Parameters from disc fibre generator, including material, diameter and

thickness, were also found to have effects on fibres and yarn diameters. The disc

materials was evaluated for its conductivity and non-conductivity. Non-

conductive HDP (High Density Polyethylene) disc showed evenly distributed

thinner jets with longer life on overall upper disc surface. While conductive discs

(steel, copper and aluminium) produced fewer, thicker jets mostly on disc top.


- 169 -

The disc diameter and thickness affected the nanofibre and yarn

diameters significantly. As the disc diameter was decreased, the number and size

of jets also varied. The smaller the disc, fewer, thicker and irregular were the

jets, which caused diameter of nanofibres increased. The cone stability and yarn

formation were adversely affected by thicker jets producing coarser fibres. The

larger diameter disc produced finer equally spaced jets and nanofibres with lower

diameters resulted in smooth cone and yarn formation.

At a polymer concentrations below 11.0%, fibres with beaded and fused

morphology were produced which ended up on ring surface and no cone or yarn

could be obtained. An increase in polymer concentration to 13.0%, gradually

reduced beads and fused fibres, yet coarser fibres were produced, still no

continuous cone or yarn was produced. At polymer concentration of 15.0%,

plenty of dry and well separated nanofibres rendered to a smooth cone and

continuous fabrication of nanofibre yarn. Beyond 15.0% concentration, the

threshold voltage for nanofibre generation was dramatically increased. On such

concentrations, the fibres still retained solvent and coarser wet fibres were

produced. These wet and fused fibres were generated at higher voltages and

contained higher charges. These fibres either flew off target or stuck to ring

surface. The cone suffered serious shortage of fibres and fibres included were

mostly wet, the cone could not exist and yarn formation ceased.

As at 15.0% polymer concentration, dry and well separated nanofibres

formed a stable cone and incessant yarn, keeping it fixed, the applied voltage

was varied to observe its effect on cone and yarn production. At lower applied

voltages, the electric force was not strong enough to properly stretch jets and wet


- 170 -

fused nanofibres were obtained. These wet fibres did not form cone or yarn. An

applied voltage of 50 kV, demonstrated better fibre morphology, higher cone

stability and increased yarn productivity. Higher voltages did not favour steady

jets and balanced cone formation and yarn manufacturing was discontinuous.

Yarn produced from hybrid system was compared with random and

aligned nanofibre web produced under same conditions. The hybrid yarn showed

improved tensile strength of 128.9 MPa in comparison to aligned (22.5 MPa)

and random web (9.9MPa). The strain % of yarns under test were 222%, 84.6%

and 98.7% for hybrid yarn, random web and aligned nanofibre web. An

increment in twist levels was found to augment tensile properties of nanofibre

yarns. In hybrid electrospinning system, nanofibres were added on both lower

and upper cone surface from disc and needle generator respectively. The cone

formed from hybrid system was thickest and could be drafted at higher speeds

i.e., 240 m/h. This system proved to be more productive and continuous.

The yarn spinner has been developed and tested on laboratory scale.

However, further work on improvement of productivity will allow its application

in industry. With the development of commercial machine for yarn

electrospinning, nanofibre yarn could be used to prepare various fibrous

products for various purposes.

- 171 -

C H A P T E R S E V E N

7 Conclusions and Future Works

7.1 Main Conclusions of Thesis

In this Ph.D. research, a novel ring collector was developed to form

fibrous cone in a separate zone beyond the electrospinning nanofibre generation

site. This not only helped in improving yarn twisting and winding, also restricted

inclusion of beaded, hooked nanofibres. Effect of processing parameters and

material properties on yarn formation were studied. The main conclusions drawn

from this research are as follows:

1) The novel ring collector developed proved to be efficient to isolate

nanofibre generations from yarn twisting and winding. This helped in removing

fused and un-stretched nanofibres from nanofibre yarn. A pair of oppositely

charged needle nanofibre generators, placed near the lower edge of ring

collector, was utilized to produce nanofibres. To get continuous nanofibres, the

negative needle was place at 10 mm from ring collector while positive needle

was kept at 30 mm. The horizontal needle positioning greatly affected the fibre

morphology and continuation in yarn fabrication. Too far distance from needle

tips to ring collector resulted in nanofibre deposition over ring surface and no

cone or yarn was produced. Yarn with coiled nanofibres was obtained when

distance between the needles was further decreased. At an appropriate distance,

a high quality yarn consisting of well aligned and twisted nanofibres was

obtained. Further decreasing needle distance resulted in non-twisted

monofilament yarn.

Chapter 7: [Conclusions And Future Works]

- 172 -

2) By keeping a balance between the ring rotation speed and the winder

speed, a very stable cone could be formed. On continuously twisting and drafting

cone apex, a twisted nanofibre yarn was produced. The yarn twist level could be

varied within a range either by changing ring collector speed or by varying

winding speed. A continuous yarn with maximum twist level of 4086 tpm and

twist angle of 54.4o was obtained from this needle based electrospinning setup,

without disturbing the balance between nanofibre generation and cone

formation. This yarn showed improved tensile properties (93.6 MPa stress and

242.6% strain) in comparison with random-orientated nanofibre nonwoven (9.1

MPa stress and 103.8% strain) and aligned nanofibre web (18.5 MPa stress and

68.3% strain). The increment in twist level helped in achieving yarns with higher

mechanical properties.

3) Coarser nanofibres resulted from increasing polymer solution flow

rate increased polymer concentration, or decreasing winding rate. While

increasing the applied voltage, ring speed and winding rate assisted in reducing

fibre diameter, an optimum polymer concentration and applied voltage were

found to be 17.0% and 13 kV, respectively, while ring rotation. Winding rate of

120 m/h proved to be good enough for collecting smooth, twisted nanofibre yarn.

4) Needleless electrospinning using fibre generators, cylinder, circular

coil, ring and disc was used to produce nanofibres. Cylinder and coil produced

nanofibres had a fused fibrous morphology, while ring collector greatly reduced

fused fibres. The disc generator was found to be the only generator to produce

completely dry, non-fused nanofibres. Moreover, the disc and ring collector


- 173 -

produced finer fibres with narrow diameter distribution, while coarser fibres

with broader diameter distribution were obtained from cylinder and coil

generators. Coil generator seemed to be the most efficient in producing

nanofibres, but fibre mat contained wet and fused nanofibres. Although, cylinder

and disc productivities were comparable, the fibres from cylinder generator were

coarser and wet.

5) Needleless electrospinning was used to produce nanofibre yarns. A

polymer concentration of 15.0%, generator to ring distance of approximately 60

mm and applied voltage in the range of 48~58 kV were found to be appropriate

for needleless electrospinning of nanofibres. However, no needleless fibre

generator could formulate stable cone and continuous yarn. By introducing

auxiliary electrode or air jet, only led to small success in nanofibre yarn

preparation.

6) Hybrid electrospinning system composed of both needle and

needleless electrospinning was found to be suitable for production of nanofibre

yarns. The disc/needle system showed the best performance among the hybrid

system. When the applied voltage for the needle and the disc was respectively 9

kV and 50 kV, continuous nanofibres were produced. Nanofibre yarn could be

produced at appropriate positions. The conductivity of disc, disc size and disc

thickness were also important parameters for jet initiation, and determining the

diameters of nanofibres and yarns. Non-conductive disc produced numerous

well-distributed jets with longer life, while conductive disc resulted in fewer,

and thicker jets. The nanofibres formed from non-conductive disc were


- 174 -

completely dry and evenly covered the whole ring, whereas fibres from

conductive disc were wet and just deposited unevenly on the ring periphery. The

thinner and larger diameter discs produced larger diameter yarns with finer

nanofibres.

7) The combination of needle and disc electrospinning systems helped in

neutralizing charges over nanofibres and increased amount of nanofibres

inclusion in cone passing through the ring collector. The cone produced from

this hybrid electrospinning system was thicker than other setups. The cone apex

could be dragged and twisted at a substantially high speed. Consequently highly

twisted (4766 tpm) yarns were produced at a large production rate (up to 240

m/h).

7.2 Future Works

Future research in this area could be conducted in the following

directions:

1) Evaluation of the applicability of the yarn electrospinning technique

to other polymers. The work so far carried out in this project mainly

focused on PVDF-HFP. Different materials may lead to difference in

nanofibre yarn formation and mechanical properties.

2) This system is capable of electrospinning different materials

simultaneously. Nanofibre yarn consisting of different polymer

materials can easily be prepared by the yarn electrospinning system

developed.


- 175 -

3) Ring collector of different materials, shapes, and sizes should have

effect on nanofibre and yarn formation. How the parameters from

ring collector affect these properties is worthwhile to investigate.

4) An additional air suction or air propulsion system can be installed in

novel electrospinning setup. Such modification might assist in

quickly drying nanofibres. This modification may further improve

the nanofibre morphology and increase yarn productivity.

- 176 -

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Electrospinning of Nanofibre Yarns using Rotating Ring ...dro.deakin.edu.au/eserv/DU:30072950/shuakat-electrospinningof-2014... · Electrospinning of Nanofibre Yarns using Rotating