studies on carbon nanotube and graphene oxide …...abstract the latest quest of polymer...

STUDIES ON CARBON NANOTUBE AND

GRAPHENE OXIDE REINFORCED POLYIMIDE

ELECTROSPUN NANOFIBER COMPOSITES

A THESIS

Submitted by

DHAKSHNAMOORTHY M.

Under the guidance of

Dr. R. VASANTHAKUMARI

in partial fulfillment for the award of the degree of

DOCTOR OF PHILOSOPHY

in

POLYMER ENGINEERING

B.S.ABDUR RAHMAN UNIVERSITY

(B.S. ABDUR RAHMAN INSTITUTE OF SCIENCE & TECHNOLOGY) (Estd. u/s 3 of the UGC Act. 1956)

www.bsauniv.ac.in

June 2015

ii

B.S.ABDUR RAHMAN UNIVERSITY (B.S. ABDUR RAHMAN INSTITUTE OF SCIENCE & TECHNOLOGY)

(Estd. u/s 3 of the UGC Act. 1956) www.bsauniv.ac.in

BONAFIDE CERTIFICATE

Certified that this thesis STUDIES ON CARBON NANOTUBE AND GRAPHENE

OXIDE REINFORCED POLYIMIDE ELECTROSPUN NANOFIBER

COMPOSITES is the bonafide work of DHAKSHNAMOORTHY. M (RRN. 0982205)

who carried out the thesis work under my supervision. Certified further, that to the best

of my knowledge the work reported herein does not form part of any other thesis or

dissertation on the basis of which a degree or award was conferred on an earlier

occasion on this or any other candidate.

SIGNATURE SIGNATURE

Dr. R. Vasanthakumari Dr. S.S.M. Abdul Majeed

RESEARCH SUPERVISOR HEAD OF THE DEPARTMENT

Professor & Director (PNTC) Professor & Head Department of Polymer Engineering Department of Polymer Engineering B. S. Abdur Rahman University B. S. Abdur Rahman University Vandalur, Chennai – 600 048 Vandalur, Chennai – 600 048

iii

ACKNOWLEDGEMENT

At the very onset, I am humbly indepted to my supervisor

Prof. Dr. (Mrs) R. Vasanthakumari, Director, Polymer Nanotechnology

Centre, Department of Polymer Engineering, B. S. Abdur Rahman University,

Vandalur, Chennai – 600 048 for her invaluable guidance and supervision. She

inspired me to initiate and complete the work with dynamic and dedication under

adverse situation. I have been blessed by her kind support and valuable inputs

in my research work.

I express my sincere thanks to the Doctoral Committee Members Dr. V.

Subrahmanian, Professor, Department of Rubber Technology, Madras Institute

Technology (MIT Campus), Anna University and Dr. R. Raja Prabu, Professor

and Head, Department of Electrical and Electronics Engineering, B. S. Abdur

Rahman University, Vandalur, Chennai for their critical comments and

suggestions on my research work.

I would like to thank Dr. S. S. M. Abdul Majeed, Professor and Head,

Department of Polymer Engineering, B. S. Abdur Rahman University,

Vandalur, Chennai for providing facilities in the department.

I am very much thankful to Dr. Murali Rangarajan, Associate

Professor, Department of Chemical Engineering and Materials Science, Amrita

University, Coimbatore for extending his kind support and suggestions at all

times.

I am truly grateful to Dr. Nikhil K. Kothurkar, Associate Professor,

Department of Chemical Engineering and Materials Science, Amrita University,

Coimbatore for his valuable comments on my research studies and also

providing lab equipment facilities to carry out polymer testing in his laboratory.

I had the pleasure to work with Mr. S. Vikram and Mr. S.

Ramakrishnan for their encouragement, valuable help and useful suggestions

iv

throughout the course of this study. My sincere thanks go to my friends S.

Mukundhan, G. Shanmuganathan, Mrs. Shahitha Parveen, Mrs. D. Nandhitha,

Mrs. Jaya chitra and Mrs. Gayathri for their support and help on different

occations.

A special credit goes to my wife Mrs. S. Revathi for being patient,

supportive, understanding, and caring at all times. Also, I would like to thank

my son Sri. D. Tharun kumar, my parents and family members for their

profound affection, caring and constant source of energy for me.

I am most grateful to the Department of Science & Technology

(DST), New Delhi for rendering financial help (DST-Nano mission project)

during my Ph.D.

I owe and dedicate everything to the Almighty GOD for the strength

that keeps me standing and for the hope that keeps me believing.

I also thank those who indirectly contributed in this research work.

I thank all of you.

(M. Dhakshnamoorthy)

v

ABSTRACT

The latest quest of polymer technologist in the field of polymer nanofibers is to

modify/improve their properties to achieve the desired performance. In the

current work, research is aimed at preparing polyimide nanofiber composites by

reinforcing the fillers namely carbon nanotubes and graphene oxide using

electrospinning technique. Polyimide was synthesized by a conventional two-

stage method, the poly (amic acid) (PAA) obtained in the first stage was

thermally treated up to 300°C in the second stage to obtain the polyimide film

by solution casting. The effect of isopropylidine and ether bridging groups in the

polyimide chain on the mechanical properties of polyimide film was studied.

Polyimide nanofiber webs were prepared by electrospinning of PAA solution

with and without the reinforcements.

Polyimide nanofiber composites were prepared by reinforcing the nano fillers

such as functionalized single walled carbon nanotubes (f-SWCNTs),

functionalized multi walled carbon nanoubes (f-MWCNTs), and graphene oxide

(GO) up to 2 wt % of loading in polyimide matrix using electrospinning set up.

The prepared nanofiber composites were characterized using Fourier

Transform Infrared spectroscopy (FTIR), Raman spectroscopy, X-Ray

diffractometer, X-Ray photoelectron spectroscopy, Scanning electron

spectroscopy, Transmission electron microscopy, Atomic force microscopy,

thermogravimetric analysis, dynamic mechanical analysis, Universal testing

machine, and Four-probe electrical conductometer.

The improvement in thermal, mechanical, and electrical properties of PI

nanofiber webs due to the reinforcements was achieved even at lower loading

level. The carbonization of polyimide/f-MWCNTs nanofiber composites was

carried out at 1000°C temperature. The electrical conductivity increased

significantly for carbonized PI/CNTs nanofiber compared to neat PI nanofiber.

vi

TABLE OF CONTENTS

CHAPTER NO. TITLE PAGE NO.

ACKNOWLEDGEMENT iii

ABSTRACT v

LIST OF TABLES x

LIST OF FIGURES xi

LIST OF SYMBOLS AND

ABBREVIATIONS

xvi


1.0 INTRODUCTION 1

1.1 POLYIMIDES 1

1.1.1 Polymer nanofibers by electrospinning method 4

1.1.2 Carbon nanotubes 9

1.1.3 Graphene oxide 12

1.1.4 Polyimide nanofiber composites 14

1.1.5 Carbonization of polyimide nanofibers 15

1.2 THESIS ORGANIZATION 19

2.0 LITERATURE OVERVIEW 22

2.1 INTRODUCTION 22

2.2 ELECTROSPINNING OF POLYMERIC

MATERIALS

24

2.3 POLYIMIDE NANOFIBERS BY ELECTRO-

SPINNING TECHNIQUE

25

2.4 POLYIMIDE NANOFIBER COMPOSITE WITH

CARBON NANOTUBES

26

2.5 POLYIMIDE NANOFIBER COMPOSITE WITH

GRAPHENE OXIDE

29

vii


2.6 CARBONIZATION OF POLYIMIDE

NANOFIBER COMPOSITES

32

2.7 APPLICATIONS OF POLYIMIDE NANOFIBER

COMPOSITES

34

2.8 OBJECTIVES OF THE THESIS 35

3.0 EXPERIMENTAL WORK 37

3.1 SYNTHESIS AND CHARACTERIZATION OF

POLYIMIDE AND ITS NANOFIBERS

37

3.1.1 Materials and Chemicals 37

3.2 PREPARATION OF POLYIMIDE FILM 37

3.3 FUNCTIONALIZATION OF SWCNT AND

PREPARATION OF POLYIMIDE

NANOFIBERS WITH F-SWCNT

42

3.4 PREPARATION OF GRAPHENE OXIDE

FROM GRAPHITE BY ACID TREATMENT

45

3.5 PREPARATION OF POLYIMIDE NANOFIBER

WITH GRAPHENE OXIDE

47

3.6 CARBONIZATION OF POLYIMIDE NANOFIBER WITH f-MWCNTs

49

3.7 CHARACTERIZATION AND TESTING 50

4.0 SYNTHESIS AND CHARACTERIZATION OF

PMDA-ODA AND PMDA-ODA- IDDA

POLYIMIDE FILMS

52

4.1 INTRODUCTION 52

4.2 RESULTS AND DISCUSSION 53

4.2.1 Structural studies 53

4.2.2 Thermal properties 55

4.2.3 Mechanical properties 56

viii


4.2.4 Optical properties 58

4.3 CONCLUSION 59

5.0 PREPARATION AND CHARACTERIZATION

OF POLYIMIDE/f-SWCNTS NANOFIBER

COMPOSITES

60

5.1 INTRODUCTION 60


5.2.1 Structural and morphological studies 61



5.2.4 Electrical properties 71

5.3 CONCLUSION 72

6.0 PREPARATION AND CHARACTERIZATION

OF POLYIMIDE/GRAPHENE OXIDE

NANOFIBER COMPOSITE

73

6.1 INTRODUCTION 73




6.2.3 Dynamic mechanical properties 86


6.3 CONCLUSION 88

7.0 CHARACTERIZATION OF f-MWCNT

REINFORCED POLYIMIDE NANOFIBERS

AND CARBONIZED NANOFIBERS

89

7.1 INTRODUCTION 89


7.2.1 Carbon nanofiber yield 91

ix





7.3 CONCLUSION 100

8.0 CONCLUSIONS 101

9.0 SCOPE FOR FUTURE WORK 103

REFERENCES 104

TECHNICAL BIOGRAPHY 123

x

LIST OF TABLES

TABLE NO. TITLE PAGE NO.

3.1 List of different monomer combinations used in

preparation of polyimide film

38

4.1 Tensile properties of PMDA-ODA and PMDA-ODA-

IDDA PI films

58

5.1 Diameter of the PAA and PI nanofiber composites

with f-SWCNT contents of 0% (neat PAA/PI), 0.5%,

1.0% and 2.0% (by weight)

67

5.2 Tensile properties of PI/f-SWCNTs nanofiber

composites

71

5.3 Electrical conductivity of PI/f-SWCNTs nanofiber

composites

72

6.1 Diameter of the PAA and PI nanofiber composites

with GO contents of 0% (neat PAA/PI), 0.1%, 1.0%

and 2.0% (by weight)

83

6.2 DMA analysis of PI/GO with GO contents of 0% (neat

PI), 0.1%, 1.0% and 2.0% (by weight)

87

6.3 Electrical properties of PI/GO nanofiber webs with GO

contents of 0% (neat PI), 0.1%, 1.0% and 2.0% (by

weight)

88

7.1 Tensile properties of polyimide/f-MWCNT nanofibers 98

7.2 Electrical conductivity of PI nanofiber and CNF with f-

MWCNT (0, 1 and 2 % w/w)

99

xi

LIST OF FIGURES

FIGURE NO. TITLE PAGE NO.

1.1 Chemical structure of polyimide backbone chain 1

1.2 Chemical structure of Kapton polyimide 2

1.3 Schematic of three types of fiber spinning: (a). Dry

spinning, (b). Wet spinning, and (c). Melt spinning.

5

1.4 Schematic diagram of Fiber drawing process 6

1.5 Schematic diagram of Phase separation process 7

1.6 Schematic diagram of Self-assembly process 7

1.7 Schematic diagram of an Electrospinning setup 8

1.8 The 3D structure of Single walled and Multi walled

carbon nanotubes

11

1.9 The layered structure of (a). Graphite and (b).

Graphene oxide

13

1.10 Schematic diagram of Catalytic chemical vapour

deposition

16

1.11 Schematic diagram of Induction Heating coil 17

1.12 Schematic of Radio-Frequency Induction Heating

setup

18

3.1 Chemical structure of monomers 40

3.2 Schematic of PMDA-ODA polyimide preparation 41

3.3 Schematic of PMDA-ODA-IDDA polyimide

preparation

41

3.4 Flow chart for functionalization of p-SWCNTs 42

3.5 Schematic of acid treatment of p-SWCNTs 43

3.6 Graphical abstract for the preparation of PAA/

f-SWCNT nanofiber

44

xii


3.7 Flow chart for preparation of PI/f-SWCNTs nanofibers 44

3.8 Photographic Images of (a). poly(amic acid) and (b).

polyimide nanofiber webs with f-SWCNTs (0, 0.5, 1,

and 2 % w/w)

45

3.9 Flow chart for preparation of graphene oxide 46

3.10 Flow chart for preparation of PI/GO nanofibers 47

3.11 (a). Poly amic acid (PAA) solution with different levels

of graphene oxide (GO) loading, (b). Electrospun

webs of poly amic acid (PAA) with different levels of

graphene oxide (GO), loading, before imidization, (c)

after imidization

48

3.12 Flow chart for carbonization of PI/f-MWCNT

nanofibers

50

4.1 FT-IR spactra of PMDA-ODA PAA-1 & PMDA-ODA-

IDDA PAA-2 powders

53

4.2 FT-IR spactra of PMDA-ODA PI-1 & PMDA-ODA-

IDDA PI-2 films

54

4.3 XRD pattern of PAA-1 & PAA-2 powders and PI-1 &

PAA-2 films

55

4.4 TGA curve of PMDA-ODA PI-1 & PMDA-ODA-IDDA

PI-2 films at nitrogen atm

56

4.5 DMA curve of PMDA-ODA PI-1 film 57

4.6 DMA curve of PMDA-ODA-IDDA PI-2 film 57

4.7 UV-Visible spectra of PI 1 & 2 films 58

5.1 FT-IR spectrum of (a). p-SWCNT and (b). f-SWCNT 61

5.2 FTIR spectra of PAA/f-SWCNT nanofiber web with f-

SWCNT contents of 0% (neat PAA), 0.5%, 1.0%, and

2.0% (by weight).

62

xiii


5.3 FTIR spectra of PI/f-SWCNT nanofiber web with f-

SWCNT contents of 0% (neat PI), 0.5%, 1.0%, and

2.0% (by weight)

62

5.4 Raman spectra of (a). p-SWCNT and (b). f-SWCNT 63

5.5 Raman spectra of PI nanofiber webs with f-SWCNTs

(0, 0.5, 1, and 2 % w/w)

63

5.6 Wide X-ray photoelectron spectra of (a) f-SWCNT and

(b) PAA/f-SWCNTs (1 % w/w) powder

64

5.7 Deconvoluted XPS (a) C1(s) peak and (b) O1(s) peak

of f-SWCNTs

65

5.8 Deconvoluted XPS (a) C1(s) peak, (b) N1(s) peak,

and (c) O1(s) peak of PAA/f-SWCNT 1 % w/w

65

5.9 HRSEM micrograph of (a) & (b) neat PAA nanofiber

web, (c) & (d) neat PI nanofiber web

66

5.10 HRTEM micrograph of (a) p-SWCNTs bundle, (b) f-

SWCNTs bundle

68

5.11 HRTEM micrographs of PAA nanofibers containing

0.5 % w/w f-SWCNTs (a) & (b) at low magnification,

(c), & (d) at high magnification

68

5.12 TGA curve of PI nanofiber webs with and without f-

SWCNT at nitrogen atm

69

5.13 DMA curve of PI nanofiber webs with and without f-

SWCNT in air atm.

70

6.1 FTIR spectra of (a) Graphite, and (b) Graphene oxide 75

6.2 FTIR spectra of PAA/GO with GO contents of 0%

(neat PAA &PI), 0.1%, 1.0%, and 2.0% (by weight)

76

6.3 FTIR spectra of PI/GO with GO contents of 0% (neat

PAA &PI), 0.1%, 1.0%, and 2.0% (by weight)

76

xiv

6.4 Raman spectra of (a) Graphite, and (b) Graphene

oxide

77

6.5 Raman spectra of PI/GO with GO contents of 0%

(neat PI), 0.1%, 1.0% and 2.0% (by weight)

78

6.6 X-ray diffraction pattern of (a) Graphite and (b)

Graphene oxide

78

6.7

AFM micrograph of graphene oxide 79

6.8 The height profile of the graphene oxide in AFM

micrograph

79

6.9 SEM micrographs of (a) and (b) graphite, (c) and (d)

graphene oxide

80

6.10 Scanning electron micrographs of PAA/GO with GO

contents of (a) 0% (neat PAA), (b) 0.1 % (GO), (c)

1% (GO), and (d) 2% (GO)

81

6.11 Scanning electron micrographs of PI/GO with GO

contents of (a) 0% (neat PAA), (b) 0.1 % (GO), (c) 1%

(GO), and (d) 2% (GO)

82

6.12 High resolution transmission electron micrographs of

PAA/GO 1 % w/w nanofiber

84

6.13 TGA curves of PI/GO nanofier webs with GO contents

of 0% (neat PI), 0.1%, 1.0%, and 2.0% (by weight) at

nitrogen atm

85

6.14 DMA curves of PI/GO nanofiber webs with GO

contents of 0% (neat PI), 0.1%, 1.0% and 2.0% (by

weight)

86

7.1 FT-IR Spectra of (a) PAA and PI with f-MWCNT (0, 1

% w/w) nanofibers

92

7.2 FT-IR Spectra of CNF/PI-f-MWCNT (0, 1 % w/w) at

900°C & 1000°C

92

xv

7.3 Raman spectra of (a) Neat PI, (b) PI/f-MWCNT-1 %

w/w, (c) CNF/PI at 900°C, (d) CNF/PI at 1000°C, (e)

CNF/PI-f-MWCNT 1 % w/w at 1000°C, and (f)

CNF/PI-f-MWCNT 2 % w/w at 1000°C nanofiber webs

93

7.4 X-ray diffraction pattern of (a) Neat PI, (b) PI/f-

MWCNT-1 % w/w, (c) neat CNF/PI at 1000°C, and (d)

CNF/PI-f-MWCNT 1 % w/w at 1000°C

94

7.5 Scanning electron micrographs of (a) PAA, (b) PI, (c)

PAA/1 % w/w f-MWCNTs, and (d) PI/1 % w/w f-

MWCNTs nanofiber web

95

7.6 Scanning electron micrographs of (a) and (b) Neat

CNF, (c) and (d). CNF PI/1 % w/w f-MWCNT webs at

different magnification

96

7.7 Transmission electron micrographs of PAA-1 % w/w f-

MWCNT nanofibers at various magnifications (a-d)

97

xvi

LIST OF SYMBOLS AND ABBREVIATIONS

° - Degree

°C - degree celsius

Å - Angstrom

AC - Alternate current

AFM - Atomic force microscopy

Al - Aluminium

BPDA - Bicyclo (2.2.2) oct-7-ene-2,3,5,6-tetracarboxylic

dianhydride

C - Carbon

C=O - Carbonyl group

–CH3 - Methyl group

C-N - Cyanide group

CNF - Carbon nanofiber

CNT - Carbon nanotube

conc. - concentration

–COOH - Carboxyl group

Co-PI - Co-polyimide

CT - Charge transfer

Cu - Copper

CVD - Chemical vapour deposition

xvii

D-band - Defect band

DC - Direct current

deg - degree

DMA - Dynamic mechanical analyzer

DMF - N, N’ - Dimethyl formamide

DMS - Dynamic mechanical spectrometer

E’ - Storage modulus

E” - Loss modulus

eV - Electron volt

f-MWCNT - Functionalized multiwalled carbon nanotube

f-SWCNT - Functionalized single walled carbon nanotube

FT-IR - Fourier Transform Infrared Spectroscopy

g - gram

G - Graphite

G-band - Graphitic band

GO - Graphene oxide

GPA - Giga pascal

h - hour

H2SO4 - Sulfuric acid

HCl - Hydrochloric acid

HNO3 - Nitric acid

xviii

Hz - Hertz

IDDA - 4,4’-(4,4’- isopropylidene diphenyl- 1,1’-diyldioxy) dianiline

IDPA - 4,4’-(4,4’-isopropylidene diphenoxy) bis phthalic anhydride

kg - Kilogram

kHz - kilo hertz

KMnO4 - Potassium permanganate

kV - kilo volt

Kα - K alpha

mA - milli ampere

MBA - 4,4’ methylene bis (2-chloro aniline)

mg - milligram

min - minute

mL - milli litre

mm - millimeter

MPa - Mega pascal

mV - milli volt

MWCNT - Multiwalled carbon nanotube

N - Nitrogen

NaNO2 - sodium nitrate

NF - Nanofiber

–NH2 - Amine group

xix

Ni - Nickel

O - Oxygen

ODA - 4, 4’ – Oxy dianiline

–OH - Hydroxyl

Pa - Pascal

PAA - Poly (amic acid)

PI - Polyimide

PMDA - Pyromellitic dianhydride

p-MWCNT- Pristine multiwalled carbon nanotube

p-SWCNT- Pristine single walled carbon nanotube

RF - Radio-frequency

rpm - Rotations per minute

s - Second

SEM - Scanning electron microscopy

SWCNT - Single walled carbon nanotube

Tan δ - Damping factor

TEM - Transmission electron microscopy

TGA - Thermogravimetric analyzer

UTM - Universal tensile machine

V - Volt

v/v - volume/volume

xx

% w/w - Weight percentage

XPS - X-ray photoelectron spectroscopy

XRD - X-ray diffractometer

δ - delta

θ - Theta

- Lambda (wavelength)

A - micro ampere

m - micrometer

1

1. INTRODUCTION

1.1 POLYIMIDES

Polyimide is one of the excellent high performance polymers and has

higher thermal stability along with good mechanical strength, chemical resistance

and better electrical properties. It has become an important polymer in the

materials used in high temperature and flexible conditions. Polyimide nanofibers

have received great attention among the researchers for their applications in

different fields such as membrane, fuel cell, solar cell, sensor devices, filtration

assemblies etc,. The performance of polyimide nanofibers are being improved by

incorporating fillers such as carbon nanotubes, graphene, metal nanoparticles,

etc. and utilized in various applications.

Polyimide (PI) is a polymer having imide group linkage in its backbone.

Since 1955, polyimides have been prepared as mass production. The common

chemical structure of polyimide backbone is shown in Figure. 1.1, in which, R1 &

R2 denotes alkyl & aryl groups [1].

Figure. 1.1: Chemical structure of polyimide backbone chain.

Polyimides (PI) are commonly synthesized using dianhydride and diamine

monomers. The polycondensation reaction of dianhydride and diamine leads to

formation of poly(amic acid) (PAA) the precursor of polyimide. The thermal

imidization of amic acid results in forming polyimide with water molecule. The first

synthetic polyimide namely, Kapton (Figure. 1.2) is a commercial polyimide which

2

was synthesized by Dupont. This polyimide was synthesized using pyromellitic

dianhydride and 4, 4’ - oxy dianiline monomers [2].

Figure. 1.2: Chemical structure of Kapton polyimide.

Polyimides are classified into mainly three types based on the composition of their main backbone chain. They are

(i) Aliphatic (linear polyimides).

(ii) Semi-aromatic (combination of aliphatic and aromatic polyimides).

(iii) Aromatic: these are the most widely used polyimides because of their higher thermal stability compared to aliphatic and semi-aromatic polyimides.

Polyimides are further classified into two types based on the type of interaction between the main chains. They are

(i) Thermoplastic polyimides: They are often called pseudo thermoplastic.

(ii) Thermosetting polyimides: They are available in the form of uncured resins, laminates, thin sheets, and machined parts.

There are many methods to synthesis polyimides, among them two methods are being used [3]

(i) The reaction of a dianhydride withd a diamine (widely used method).

(ii) The reaction of a dianhydride with a diisocyanate.

A few examples of dianhydrides include pyromellitic dianhydride (PMDA),

benzophenone tetracarboxylic dianhydride, naphthalene tetracarboxylic

dianhydride. Some of the diamines include 4, 4'-oxy dianiline (ODA), 3, 3’-

diamino diphenyl methane, and m-phenylene diamine (MDA). There have been

several dianhydrides and diamines examined to improve the physical, chemical,

and processing characteristics of these monomer materials to obtain polyimides.

3

The charge-transfer interactions between the polymeric planar structures cause

the polyimides to become insoluble [3].

Polyimides have excellent heat resistance, high mechanical properties,

and good chemical resistance. Due to these properties, polyimides have become

a superior high performance polymer and widely utilized in many fields such as

membranes in fuel cell, solar cell, gas filtration, sensors and reverse osmosis of

water. They have also been used in aerospace components, printed circuit

boards, adhesives, and matrix material in composite fabrication because of their

excellent thermal stability, mechanical properties, chemical resistance, and

electrical properties. However, the properties and application areas of polyimides

are generally determined by the molecular structures of the monomers and also

the number of different monomers used in the polymerization process [4].

Generally, dianhydride and diamine monomers are reacted and form the

precursor poly(amic acid) and then the thermal imidization of PAA gives

polyimide film. There have been different monomers systems selected and

developed different types of polyimides to improve their thermal, mechanical and

electrical properties. The researchers have also tried to develop copolyimides by

choosing the combination of monomers such as two or more different

dianhydrides with one diamine, otherwise one dianhydride with two or more

different diamines [4, 5].

A slight modification in the chemical structure may often result in a

significant change in polymer properties. Only some of the investigators have

studied the relationships between monomer structure and polyimide properties.

However, considering some limitations in experimental conditions, it is not

economical to synthesize all the possible polyimides. There have been a lot of

literatures on different types of polyimides and manipulation of properties based

on the requirements. However, there are only a few literatures on more flexible

copolyimides. The flexibility and processability of polyimide can be improved by

introducing bulky and flexible groups such as iso-propylidine, hexa fluoro

propylidine, etc. into copolyimide structure. The prepared flexible copolyimides

4

can be used as flexible substrate, membrane and electrodes in solar cells, fuel

cells, and sensor applications respectively.

1.1.1 Polymer nanofibers by electrospinning method

A material is considered to be a fiber when it has an aspect ratio (L/D)

more than 100 i.e., length to diameter ratio. There are several polymeric fiber

materials produced by various methods namely dry spinning, wet spinning, melt

spinning etc,. In dry spinning [6], polymers must be dissolved in solvent and

solidification is done by solvent evaporation. This can be achieved by a steam of

air or inert gas sending through the chamber, from which, the polymer comes out

as fiber form. The advantages of this method include no precipitation of liquid

involved, the fiber does not need to be dried, and the solvent is more easily

recovered. Some of the fibers being manufactured are acetate, acrylic, and poly

benzimidazole fibers. Figure. 1.3 (a) is attributed to the schematic diagram of dry

spinning process.

In Wet spinning [6], polymers are to be dissolved in a suitable solvent. The

spinneret is submerged in a chemical bath and this causes the fiber to solidify, as

it emerges. Figure. 1.3 (b) shows the schematic diagram of Wet spinning

process. Acrylic, rayon, and aramid, are manufactured using this process. Melt

spinning [6] is used for the polymers which must be melted to process it. The

molten polymer solidifies in water bath by cooling after being extruded

immediately from the spinneret. The schematic diagram of Melt spinning is

shown in Figure. 1.3 (c) Nylon, polyolefin, and polyester are produced via this

process. When the diameter of the polymer fiber materials is reduced from

micrometer to nanometer level, there may be many excellent characteristics

appeared such as high aspect ratio, large surface area to volume ratio, flexibility

and superior mechanical properties compared with conventional polymer

materials. These properties of polymer nanofibers make them optimal candidates

for many applications [7].

5

Figure. 1.3: Schematic of fiber spinning processes: (a) Dry spinning, (b) Wet

spinning, and (c) Melt spinning (After Carraher, C. E., Jr. 2002. Polymer News ,

27, 3, 91).

There are a number of processing techniques used to prepare polymer

nanofibers such as fiber drawing, phase separation, template synthesis, self

assembly of fibers, electrospinning, etc. The fiber drawing process is similar to

dry spinning of fibers, which is only used for viscoelastic materials (Figure. 1.4).

The phase separation process (Figure. 1.5) involves the dissolution of polymer in

different solvents, formation of gel by freezing and drying for extraction of

polymer materials resulting in nano-porous foams. However, this process takes

long time to convert polymer into nano-porous foams [8].

6

Figure. 1.4: Schematic diagram of Fiber drawing process (Adapted from [8]).

In the template synthesis [9, 10], the nanoporous membrane is used as

template to prepare nanofibers. This method can be utilized for polymers, metals,

and carbon materials. However, this technique cannot be used to prepare one-

by-one continuous nanofibers. The preparation of polymeric porous

nanostructure is shown in Figure. 1.5.

7

Figure. 1.5: Schematic diagram of Phase separation process (Ref. [10]).

Molecular self-assembly is a powerful approach being explored for novel

supramolecular nanostructures and bio-inspired nanomaterials. The process

(Figure. 1.6) contains the pre-existing materials which organize themselves into

the patterns. Recent researches concern the self-assembly of de novo designed

artificial peptides and peptidomimetics into nanofiber structures, specifically

towards developing a new class of soft-materials. These nanofiber architectures

have potential use not only in biomedical applications, such as 3D-matrix

scaffolds for tissue engineering and biomineralization, but also in nanotechnology

such as nano-templates and dimension-regulated functional nano-objects. But,

this process is also a time-consuming in producing polymer nanofibers. Thus, the

electrospinning is a process to be the only technique to produce continuous

nanofibers for mass production from different polymers [11].

Figure. 1.6: Schematic diagram of Self-assembly process (Ref. [11]).

8

Electrospinning was first introduced by Formalas et al. in 1934 and he

published many patents during 1934 to 1944 on production of polymer filaments

by developing experimental setup using an electrostatic force [12]. Recently,

polymer nanofiber technology is growing rapidly as nanofiers find in several

applications in different fields. The electrospinning technique involves the use of

electric field on to a polymer solution. The opposite electric charges are applied

to polymer solution and the collector. Once the polymer jet is produced and

ejected from the spinneret, the solution jet gets evaporated and forms fibers

which are collected on the rotating drum collector [13]. The schematic of an

Electrospinning setup is shown in Figure. 1.7.

Figure. 1.7: Schematic of an Electrospinning set up (Adapted from [14]).

Electrospinning process mainly depends on the polymer characteristics

such as structure, solubility, molecular weight, melting temperature etc, and

applied electrical voltage. These parameters interact and results in the creation

of electrified polymer jets from a polymer solution. When a strong electrostatic

field is applied, the charges are induced and the ions carry the charges, the

9

charged polymer moves towards the metal collector having opposite charge. The

pendant drop emerging from the spinneret is prevented by the surface tension of

the polymer solution at low electric field. When the electric field is increased, the

charges induced on the liquid surface repel each other and forms extension of

the drop into conical shape. The charges of polymer jet emanating from the

conical drop lead to continuous jet during the continuous applied high voltage.

The solvent of the polymer solution jet gets evaporated and forms solid polymer

nanofibers before being deposited on the drum collector [13].

There are many other parameters involved in the electrospinning process

such as polymer viscosity, flow rate, applied voltage, distance between the

spinneret and collector, rotating drum collector speed, etc. [15]. These

parameters have to be optimized to produce smooth, continuous and uniform

nanofibers. The polymer nanofibrous membranes prepared by electrospinning

method, can be effectively used as optical sensors, tissue-engineering scaffolds,

protective clothing, and nanocomposites.

1.1.2 Carbon nanotubes

Carbon nanotube (CNT) is an allotrope of carbon with a cylindrical

nanostructure. The nanotubes have very high length-to-diameter ratio [16], which

is specifically larger than the other existing materials. Carbon nanotube was first

discovered by Iijima (1991) and it has been a potential candidate for many

applications such as advanced nanocomposites, sensors, optoelectronics, gas

storage devices etc,. The CNTs possess excellent properties including strength

and stiffness to weight ratio. In particular, owing to their extraordinary thermal

conductivity, mechanical, and electrical properties, carbon nanotubes find

applications as additives to various structural materials [17].

A carbon nanotube, the name is derived from its very long, hollow

structure with the wall formed by one atom thick graphene sheet. These

graphene sheets are rolled at specific angle, and the combination of the rolling

angle and radius decides the properties of nanotubes. Individual nanotubes

10

naturally align themselves into ropes held together by van der Waals forces,

more specifically, π-stacking. The chemical bonding of nanotubes is composed

of sp2 bonds, similar to those of graphite. These bonds, which are stronger than

the sp3 bonds found in alkanes and diamond, provide nanotubes with their

unique strength [17].

Many research papers were published by the researchers on mechanical

and electrical properties of CNTs. They proved that the mechanical strength of

CNTs is 100 times higher than that of steel and the density is six times lower [5].

There exists mainly two types of CNTs, namely single walled carbon nanotube

(SWCNT) and multi walled carbon nanotube (MWCNT) shown in Figure.1.8.

Single-walled carbon nanotubes (SWCNTs) have nearly 1 nm diameter and a

length that can be a few millions of times longer. The one atom thick graphene

layer wrapped up into a cylinder shape which forms the structure of a SWCNT.

The SWCNTs have been an expensive material for wide applications; however,

they are forecast to make a huge impact in electronics by 2020 according to The

Global Market’s report on Carbon Nanotubes. Multi-walled carbon nanotubes

(MWCNTs) comprise of multiple rolled graphene layers. The interlayer distance

in multi-walled carbon nanotubes is approximately 3.6 Å. The individual shells of

Russian Doll structure could be described as SWCNTs [16].

Recent developments focus on the incorporation of CNTs into polymers to

produce new composite materials with improved thermal, mechanical and

electrical properties. The dispersion of CNTs in polymer matrix is a major

challenge in composite fabrication. Therefore, good dispersion of CNTs and

interfacial bonding between CNTs and polymer matrix are essential to improve

the mechanical properties of CNTs reinforced polymer nanocomposites [18]. Due

to their nature, CNTs have poor solubility in most of the organic solvents and also

chemically not compatible with polymer matrix. The chemical inertness and

formation of bundles of CNTs present difficulties in fabrication of composite

materials. The three dimensional structure of single walled and multi walled

carbon nanotubes is shown in Figure. 1.8.

11

Figure. 1.8: The 3D structure of Single walled and Multi walled carbon nanotubes

(Adapted from (Ref.Nos.16)).

The modification of CNTs structure by the oxidation of CNTs has acquired

a lot of attention in purification and enhancing the chemical reactivity. The

oxidation of CNTs can be done by acid treatment, oxygen plasma, photo-

oxidation, or gas phase treatment. The acid treatment is a simple method and

has been utilized widely to oxidize both SWCNTs and MWCNTs. The oxygen-

containing functional groups such as carboxyl and hydroxyl found to be attached

on the surface of the carbon nanotubes. The presence of carboxyl (-COOH) and

hydroxyl (-OH) groups reduce the Van der Waals forces between the CNTs,

which strongly facilitate the exfoliation of CNT bundles and separate into

individual tubes [19].

The oxidation of CNTs is carried out by refluxing the CNTs in a mixture of

concentrated sulfuric acid and nitric acid treatment. This process leads to the

opening of carbon nanotube caps and also holes formation in the side walls. The

acid mixture etches the side walls with evolution of carbon dioxide and the final

nanotube fragments have length between 100 and 300 nm. The oxygen

containing functional groups such as carboxyl and hydroxyl groups are attached

to the side walls of carbon nanotubes [20].

12

The dispersion of acid functionalized CNTs in a polymer matrix is highly

effective due to the exfoliation of CNTs and also the enhancement of interaction

between functionalized CNTs and polymer matrix. This interfacial adhesion

results in the transfer of unique properties of CNTs into polymer CNT based nano

composites [21]. It has also been described that the acid functionalization of

CNTs affects the rope size and results in exfoliation into smaller nanotubes.

These challenges are important in the fabrication of molecular electronics, high

sensitive sensor devices.

The chemical modification of CNTs by less vigorous acid treatment leads

to the formation of functional groups on the side walls. The acid groups

functionalized carbon nanotubes from this treatment mostly retain their pristine

CNTs’ mechanical and electrical properties [19].

1.1.3 Graphene oxide

Graphene is a single atom thick layer of graphite with 2-dimensional

structure of sp2 bonded carbon atoms. This single layer of graphite has superior

properties such as high thermal and electrical conductivities, excellent

mechanical strength, good transparency, flexibility and large surface area. Due to

these outstanding properties, many research works are being carried out in

recent years in different fields such as microelectronics, energy storage devices,

opto-electronics, and polymer nanocomposite materials. Graphene based

materials are used as active materials or transparent conductive electrodes in

solar cells, counter electrodes in organic dye-sensitized solar cells,

electrocatalysts in fuel cell for oxygen reduction, photo catalysts in water splitting,

and high-performance electrodes in super capacitors and lithium ion batteries

[22].

13

Figure. 1.9: The layered structure of (a) Graphite and (b) Graphene oxide

(Adapted from C. E. Hamilton, PhD Thesis, Rice University (2009) [23]).

The layered structure of graphite exhibits 3-dimensional (3D) order, which

is shown in Figure. 1.9 (a) and schematic of graphene oxide layer in Figure. 1.9.

(b). In graphite, the adjacent layer of graphene sheets are separated each other

by 0.335 nm and held together by weak van der Waals forces. Thus, the

graphene layers can easily slide off from one another and this provides graphite

layers with soft and lubricating nature [23]. The graphene sheets incorporated

polymer matrix composites have poor adhesion between the graphene layers

and polymer matrix. Hence, there have been many approaches carried out to

improve the adhesion of graphene sheets by modifying the structure.

There are three modified graphitic structure which are termed as graphene

oxide (graphite oxide), expanded graphite and graphite intercalated compounds.

The first form, graphene oxide is prepared by simple and inexpensive method of

acid treatment of graphite. During this treatment, graphite is oxidized and

producing graphene oxide sheets. The functional groups such as carboxyl,

hydroxyl and epoxy groups present on the graphene layers prevent the

agglomeration of graphene sheets. This improves the interaction between the

graphene oxide and polymer matrix in polymer composite fabrication [24-25].

The properties of graphene oxide are transferred to the polymer matrix

and hence, the mechanical, thermal, and electrical properties are improved for

14

the GO-polymer composites. The manufacturing of polymer composites requires

graphene sheets to be homogenously distributed into the matrix. However, the

graphene sheets do not readily exfoliate from the graphite to yield individual

graphene sheets. It is essential to prevent the agglomeration of individual

graphene sheets and also improve the adhesion between the graphene sheet

and polymer matrix. The interfacial interaction between graphene sheet and

matrix can be improved by modifying the structure of graphene sheets [26]. This

modification or attaching functional groups such as carboxyl, hydroxyl groups on

the surface of graphene layers can be achieved by oxidation of graphite using

acid treatment. The organic functional groups intercalated between the graphene

sheets. The gap between the adjacent layers is increased due to the steric

hinderance of same groups present. This leads to reduce the wander waals

forces acting on the adjacent layers, thereby, exfoliation of graphene oxide layers

occur. These functional groups also interact with the polymer matrix and form

strong adhesion between graphene oxide layers and matrix.

1.1.4 Polyimide nanofiber composites

Most of the aromatic polyimides (PI) are highly rigid and more conjugation

of backbone structure. Due to this property, aromatic polyimides have relatively

poor processability in fabrication and may have limited applications especially in

fiber forms used in high temperature resistant separation technology. This is due

to their excellent thermal, chemical and mechanical properties. In the

conventional wet spinning process, polyimide fibers can be produced up to a few

micrometer ranges in diameter. The reduction of the diameter of the fibers from

micro level to nanoscale range may be utilized greatly in improvement of the

properties and performance of the fibers. This is because of the large surface

area to volume ratio of nanoscale diameter of the fibers [27, 28].

In the past two decades, the electrospinning technique has been used to

produce non-woven fabrics of nano-scale fibers. Electrospinning is a low-cost but

effective method to continuously produce polymer nanofibers. Several polymer

electrospun nanofibers have been reported, only a few literature has been found

15

in polyimide nanofibers [29-31]. Polyimide nanofiber properties can be improved

by reinforcing nanofillers such as carbon nanotubes, graphene oxide, and nano

particles in polyimide. There are two methods namely, solution mixing and in-situ

synthesis of nanofillers with polymer to prepare polymer composites. In the case

of in-situ synthesis, one can achieve the effective dispersion of nanofillers in

polymer compared to mere mixing of nano fillers in polymer solutions [32].

Carbon nanotubes (both SWCNT and MWCNT) can be used as

reinforcements to prepare polyimide nanofiber composites by the combination of

in-situ synthesis of polyimide with CNTs and electrospinning of prepared CNT-

polyimide solution. The pristine SWCNTs and MWCNTs have poor dispersion in

polyimide solution due to the agglomeration of CNTs. Hence, surface

functionalized CNTs are being used to improve homogenous dispersion and

interfacial interaction between CNTs and polyimide matrix. Especially, acid

treated CNTs have good adhesion and dispersion in polyimide matrix and the

resulting polyimide nanofiber composites have improved mechanical, thermal

and electrical properties [33].

1.1.5 Carbonization of polyimide nanofibers

Carbon fibers were first produced from rayon fibers in 1959 by the US Air

Force Materials Laboratory. Due to the development of nanotechnology, carbon

nanofibers (CNFs) are being prepared from polymer nanofibers. In recent days,

CNFs are being used in many fields, namely rechargeable batteries, high

temperature filters, templates for nanotubes, supporting material for high-

temperature catalysis, nanoelectronics, super capacitors, gas adsorption

materials, and mainly as reinforcement in composite materials due to their good

high temperature resistance, excellent electrical and thermal conductivities.

There are many conventional methods used for preparation of CNFs such as

vapour growth method, substrate method, spraying method and plasma-

enhanced chemical vapour deposition method [34, 35].

16

The chemical vapor deposition [36] (CVD) is the dominant commercial

method in the preparation of vapour grown carbon fiber (VGCF) and carbon

nanofiber (VGCNF). In this method, the reactant gas is passed in a quartz tube at

high temperature. Due to high temperature, the gas phase molecules

decomposed and deposited on the substrate in the presence of metal catalyst,

where the fibers are grown around the catalyst particles. This process consists of

several steps such as the decomposition of gas, carbon deposition, growth and

thickening of fibers. Then, the graphitization and purification results in carbon

nanofibers and their diameter mainly depend on the catalyst size. The schematic

diagram of catalytic chemical vapour deposition process is shown in Figure. 1.10.

These methods are very complicated and costly and therefore, the simple and

inexpensive method called electrospinning process followed by, carbonization

has been currently utilized to produce continuous CNFs. The heat treatment of

polymeric nanofibers can also be done by Induction heating method.

Figure. 1.10: Schematic diagram of Catalytic chemical vapour deposition

(Adapted from [36]).

Induction heating is a process of heating an electrically conducting

material by electromagnetic induction, through which heat is generated in the

object by Eddy current. An induction heating material consists of metal wire coil

(electromagnet), and an electronic oscillator which passes the alternating current

17

through the electromagnet. The rapid penetration of alternating magnetic field in

the conducting material, which generates electric current in the conductor called

Eddy currents. The flow of eddy currents through the material generates heat

[37].

Figure. 1.11: Schematic diagram of Induction Heating coil (adapted from [37])

An important advantage of the induction heating method is that the heat is

generated inside the object itself and it does not need an external heating

source. Thus, the material can be heated rapidly. Induction heating is used in

metallurgy, crystal growth, semiconductor industry, and in melting of refractory

metals which require very high temperatures. The schematic representation of

Induction-heating coil is sown in Figure. 1.11.

The Radio-frequency induction heating setup contains three important

parts namely, a radio-frequency (RF) generator with induction coil, a quartz

chamber and a high vacuum system. Figure. 1. 12 shows the schematic diagram

of Radio Frequency Induction heating setup. The efficiency of the induction

heating depends mainly on coil current, number of turns in the coil, and the

shape of the specimen to be heated [38-40]. Graphite boat can be used for

carbonizing the polymer nanofiber web. Heating power can be varied by

increasing the RF generator oscillator plate current. The level of heating

requirement can be achieved within the maximum input current in the radio

frequency power stage. The advantages of RF-induction heating method are

18

pollution free source of heating, ability of producing heat to very high level

efficiently, and absence of thermal Inertia (i.e. rapid start up).

Figure. 1.12: Schematic of Radio-Frequency Induction Heating setup (Adapted

from [40]).

Carbonization of polymer involves the heat treatment of a material which

leads to crosslinking, coalescence of cyclized polymer chains, and structural

transformation of polymer material from a ladder structure into a graphite one.

The morphology of the material also changes from smooth to wrinkle texture.

This process is carried out at high temperature around 1000°C, and hence it

needs dynamic inert gas atmosphere such as nitrogen, argon, helium, etc., to

prevent oxidation and also remove the volatile molecules such as H2O, NH3, H2,

CO2 and CO. This carbonization process can also be carried out in vacuum

condition, but the carbonization degree is lower than that in inert gas

atmosphere.

19

Polymer nanofibers have been recently carbonized to produce carbon

nanofiber. The fiber shrinkage and sometimes fusion may occur during

carbonization of polymer nanofibers. Polyimide nanofiber webs can be

carbonized at high temperature around 1000°C [41]. In addition, CNTs reinforced

polyimide nanofiber composites can also be carbonized which has been tried in

this work. Carbon nanofibers, with their lower cost and outstanding electrical

conductivity, are promising fillers in polymer nanocomposites compared to CNTs.

1.2 THESIS ORGANIZATION

This thesis consists of nine chapters

Chapter 1. Introduction

This chapter deals about the fundamentals of polyimide and its structure

modifications. A brief description about polymer nanofibers and electrospinning

technique is discussed. The polyimide nanofiber composites with nanofillers such

as carbon nanotubes and graphene oxide are explained along with carbonization

process of polymer nanofibers briefly.

Chapter 2. Literature overview

A review of literature on preparation of polyimide films, nanofiber

composites is carried out. This literature review presents the previous

publications related to existing experimental technique, electrospinning of

polymer nanofibers, polyimide nanofiber composites with functionalized carbon

nanotubes and graphene oxide. The objectives and scope of the present work

have been given at the end of this chapter.

Chapter 3. Experimental work

This chapter deals about the detailed experimental work carried out in

synthesis of polyimide from different dianhydride and diamine monomers. The

working principle and optimization of electrospinning technique to get uniform

polyimide nanofibers are explained. Acid functionalization of carbon nanotubes

20

and oxidation of graphite into graphene oxide (GO) are elaborately discussed. A

detailed description about in-situ synthesis of polyimide with SWCNTs,

MWCNTs, and GO and preparation polyimide nanofiber composites is given. The

preparation of functionalized CNTs and GO reinforced polyimide nanofiber

composites is explained in detail. The preparation of carbon nanofiber from

polyimide nanofiber composite by carbonization process is also discussed.

Chapter 4. Synthesis and Characterization of PMDA - ODA and PMDA – ODA

- IDDA polyimide films

This chapter outlines the synthesis and properties of polyimide and

copolyimide films. The characteristics of these polyimide films using FT-IR, X-ray

diffraction, thermogravimetry analysis, dynamic mechanical analysis, UV-Visible

spectroscopy are explained in detail.

Chapter 5. Preparation and Characterization of f-SWCNT reinforced polyimide

nanofiber composites

In this chapter, the acid functionalization of single walled carbon nanotube

and its incorporation in polyimide nanofiber web by in-situ preparation of PAA

with f-SWCNTs (0, 0.5, 1 and 2 % w/w) are discussed in detail. The

characterization and properties of PI/f-SWCNTs nanofiber composite are studied

and discussed.

Chapter 6. Preparation and Characterization of Polyimide - Graphene oxide

nanofiber composites

The conversion of graphite into graphene oxide by Hummer’s method is

carried out and the properties of graphene oxide are determined using FTIR,

XRD and Raman spectra. The studies carried out on graphene oxide reinforced

polyimide nanofiber composites with different weight percentages of GO are

explained. The morphological studies of PI/GO nanofibers and formation of

beads in nanofibers are discussed. The properties such as thermal and electrical

conductivity of PI/GO nanofiber composites are also studied and reported.

21

Chapter 7. Carbonization of Polyimide/f-MWCNTs nanofiber composites and

their properties

In this chapter, the carbonization of polyimide nanofibers has been carried

out to get carbon nanofiber. The comparative studies on electrical conductivity of

CNF prepared from neat polyimide nanofiber web and PI/f-MWCNTs nanofiber

composites are reported. Also, the effect of carbonization temperature on

electrical conductivity is discussed.

Chapter 8. Conclusions

This chapter concludes the findings of this research work on polyimide

modifications, properties of their nanofiber composites and the effect of carbon

nanotubes and graphene oxide fillers on polyimide nanofiber composites.

Chapter 9. Scope for future work

The further requirement of this present work on polyimide nanofiber

composites for future is mentioned here.

22

2. LITERATURE OVERVIEW

2.1 INTRODUCTION

Polyimides exhibit outstanding thermal stability, reasonable chemical

resistance, and mechanical properties. Der et al. investigated the advanced

polyimide materials. They also described the synthesis, properties and their

applications in various fields [3]. There have been many articles published on

understanding the basic characteristics of polyimides [2, 42]. Hiroshi et al.

prepared polyimide powder by imidizing low molecular weight poly(amic acid)

powder obtained from dianhydride and diamine monomers [43]. PIs are relatively

rigid and have reduced solubility and processability. There have been many

attempts made by researchers to overcome these difficulties in the development

of flexible polyimides with improved solubility and processability. Yexin et al. [44]

investigated the properties such as glass transition temperature (Tg) and density

of various polyimides prepared from different combinations of dianhydride and

diamine monomers. They concluded that two important factors such as chain

flexibility and strength of inter chain interaction influence the Tg of polyimide.

Further, the inter chain interaction strongly affects Tg more than chain flexibility.

There are a few papers published on the modification of polyimide

structures by introducing bulky groups in backbone of polyimide chain. Chin et al.

[45] synthesized polyimides having both ether and fluorine group in the polymer

backbone. The fluorinated polyimides exhibited good thermal stability with higher

Tg, better solubility, and lower moisture absorption compared to non-fluorinated

polyimides. The introduction of bulky groups into polyimide structure or

copolymerization of dianhydride and diamine monomers can improve the

solubility and processability of polyimides. The back bone chains in copolyimide

decrease the stereo-regularity and increase the space between the main chains.

This reduces the inter-chain interactions and thus, the solvent molecules can

23

easily diffuse and dissolve the polymers. Yang et al. successfully synthesized

soluble copolyimides by reacting Pyromellitic dianhydride (PMDA) with two

different diamines such as 4, 4’ p-methylenebis-(2-tertbutylaniline) (MBTBA) and

p-phenylene diamine in different ratios [4].

Benlin et al. prepared organosoluble polyimides from unsymmetrical

dianhydride monomers [46]. Yan et al. investigated the solubility effect during the

chemical imidization of PMDA-ODA poly(amic acid). The homogenous poly(amic

acid) is converted into heterogeneous poly(amic acid imide); which resulted in

phase separation. They reported that the degree of imidization increases and it

leads to improvement in the molecular chain arrangement, thereby, the

mechanical properties of films are also increased [47]. Naiheng et al. studied the

polyimides having acid and base functionality with many other substances. They

prepared multilayer thin films by rapid spin-coating process [48].

Yong et al. prepared fluorinated polyimide membranes and studied their

applications in gas separation process [49]. David et al. synthesized the co-

polyimides by poly condensation of two different dianhydride namely 4,4’-

(hexafluoro isopropylidene) diphthalic anhydride and 3,3’-diphenyl sulfone

tetracarboxylic dianhydride and diamine monomers such as 2,3,5,6-tetramethyl-

1,4-phenylene diamine and 4,4’-diaminodiphenyl sulfide. They exhibit good

solubility and high optical transparency of 88% at 589.6 nm even at the thickness

of 180 µm [50].

Chenyi et al. investigated the solubility and optical properties of novel

polyimide prepared from the monomers containing tetramethyl pendant groups

and tertiary butyl toluene moiety [51]. Tung et al. prepared high temperature

resistant, flexible, and colorless polyimide from 4, 4’-(hexa fluoro isopropylidene)

diphthalic anhydride monomers and 3, 3’-diaminodiphenylsulfone (3, 3’-DDS).

They coated Indium-Tin-Oxide (ITO) on the polyimide substrate and studied their

properties [52].

24

2.2 ELECTROSPINNING OF POLYMERIC MATERIALS

Polymer nanofibers are the fibers having diameter of less than 100

nanometers. There are many techniques being used in the preparation of

polymer fibers such as melt spinning, interfacial polymerization, anti solvent-

induced polymer precipitation, electrospinning etc. In particular, electrospinning is

a simple and versatile technique to produce polymer nanofibers [7]. Andry et al.

reported the recent advances in the synthesis and characterization of conjugated

polymeric nanofibers [53].

Zheng et al. published a review article on the development of electrospun

polymer nanofiber materials including processing, structure - property

relationship, and applications. They also discussed about the research

challenges and future trends in nanofiber technology [12]. Several attempts have

been made to understand the electrospinning technique of producing polymer

nanofibers. Polymer nanofibers are formed from the electrified fluid jet which

comes through an orifice, forming tailor cone and finally solidifying into a

continuous fiber by evaporation of solvent. Darrel et al. reported in their feature

article that the inclusion of other materials such as different polymers, chemical

reagents, dispersed particles, proteins with polymer electrospun fibers made

them useful for different fields [13].

Avinash et al. published a review article on the effect of fiber diameter and

collector distance on the structural morphology of nanofibers and their tensile

strength. The incorporation of CNT in the nanofibers leads to additional interface

for orientation of polymer chains. This oriented chains due to CNT loading,

increase the strength and stiffness of the fibers. They also reported the

advancement in polymer nanofiber composites with magnetic particles. Such

type of composites can be utilized in miniaturized electronics and

electromagnetic interference (EMI) shielding [54]. Deitzel et al. reported the

processing variables specifically spinning voltage and polymer solution

concentration on the morphology of electrospun nanofibers. They found that the

correlation between voltage and bead formation in the fiber, with increasing

25

solution concentration which increases the fiber size and diameter according to a

power law [55]. The surface chemistry in electrospun nanofibers has been

utilized in reinforced composites and biomedical applications [56].

Seema et al. reported the chemistry of nanofibers have been utilized in

catalysis, enzymes, biomedical, coatings and bulk modifications [57]. Zuefen et

al. developed polyacrylo nitrile (PAN) electrospun nanofibers using a new

technique called self-bundling followed by stretching and annealing. They also

compared the tensile properties of self-bundled electrospun PAN nanofibers with

wet-spun PAN fibers and found that the electrospun PAN fibers exhibit increased

tensile strength [58]. Lisa et al. have achieved highly aligned mat from single

fibers of polyimide and poly glycolic acid. They investigated the effect of auxiliary

electrode on the elimination of jet whipping during electrospinning process to

obtain aligned fibers. They also reported the effect of opposing electric field on

the fiber alignment [59].

2.3 POLYIMIDE NANOFIBERS BY ELECTROSPINNING TECHNIQUE

Polyimide nanofibers have been produced by electrospinning technique

for past two decades. Changwoon et al. produced polyimide sub-micron fibers

first time using electrospinning technique. Polyimide ultrafine fibers were

obtained by electrospinning of poly (amic acid), as the precursor followed by

thermal imidization process. They also studied the morphology of polyimide

ultrafine fibers [28]. Chuyun et al. developed novel polyimide nanofibers with a

flexible diamine, 2, 2-bis [4-(4-aminophenoxy) phenyl] hexa fluoropropane (6F-

BAPP) and 3, 3’, 4, 4’-biphenyl tetracarboxylic dianhydride. The obtained

polyimide had high tensile strength and high elongation at break [29]. Yuuki et al.

fabricated fluorinated polyimide nanofibers were uniaxially aligned to the collector

and deposited on a plate. They found the aligned fibers have narrow nanopores

in the membrane due to regular accumulation of the nanofibers [30].

High strength polyimide electrospun nanofibers were developed by

chaobo et al. They prepared the nanofibers from the monomers of 3, 4, 3’, 4’-

26

biphenyltetracarboxylic dianhydride (BPDA) and p-phenylene diamine (PDA)

[60]. Satoshi et al. prepared bead-free ultrafine nanofibers from fluorinated

polyimide by electrospinning. Bead-free ultrafine fibers were prepared by

addition of salt and at controlled humidity condition [31]. Woo et al. prepared

estrone imprinted polyimide nanofiber which showed specific recognition ability

and fast kinetic absorption for estrone [61]. Polyimide nanofibers are being

produced by electrospinning of poly(amic acid) precursors, followed by

imidization. However, imidization resulted in fusion of nano filaments at contacts

due to heat treatment. Alexander et al. investigated residual solvent effect on

nanofiber fusion in electrospun nanofiber, which bundles together during

imidization process. They found that the addition of dodecyl ethyl dimethyl

ammonium chloride (DEDAC) in polyimide reduced the fusion after removal of

residual solvent by aging in vacuum [62].

There have been few attempts made to prepare copolyimide nanofibers

such as porphyrinated polyimide by the addition of monomer having porphyrin

structure with pyromellitic dianhydride (PMDA) and oxy dianiline (ODA)

monomers [63]. Polyimide nanofibers have been modified by incorporating filler

particles to prepare polyimide nanofiber composite. Some researchers have

been prepared polyimide nanofiber composites with metal/metal oxide such as

Fe-FeO nanoparticles [64], silica [32], and europium powder [65] to improve their

thermal, mechanical and electrical properties of polyimide nanofibers. Also, the

glass transition temperature increased with the increase in reinforcement loading.

2.4. POLYIMIDE NANOFIBER COMPOSITE WITH CARBON NANOTUBES

Polyimides are used for high temperature resistant applications due to

their excellent thermal stability. Literature is available for polyimide nanofiber

research. The polyimide nanofibers are being used for membrane application in

filtration. More research is required for the investigation of polyimide nanofiber

composites by incorporating nanofillers such as carbon nanotubes and graphene

oxide in polyimide matrix. Carbon nanotubes are man-made materials which

have higher stiffness and strength. The electrical and mechanical properties of

27

these nanomaterials will be highly advantageous to embed in polymer materials

homogeneously. Zdenko et al. reviewed the preparation and properties of

carbon nanotubes reinforced polymer composites [66].

The dispersion of CNTs in polymer matrix is a major challenge in the

polymer composite fabrication. This is due to the agglomeration of the CNTs in

the matrix during dispersion and results in poor properties of obtained composite.

Several attempts such as surface treatment of CNTs, ultra-sonication, etc have

been made to overcome this disadvantage. The surface functionalization by acid,

amine, or ester results in strong interaction between CNTs and polymer matrix.

Kannan et al. reviewed the chemical functionalization of CNTs and mentioned

that covalent modification allows the alteration in CNTs properties [67]. Shuhui et

al. investigated the functionalization of single walled carbon nanotube with

polystyrene by grafting method. They found that the functionalized SWCNTs

dissolved in organic solvent, the SWCNT bundles were broken into small ropes

and even individual tube [68].

Chemical functionalization [69] of carbon nanotubes has been carried out

to modify its surface and to disperse in the solvent. Acid treatment is a widely

used method to attach functional groups such as –COOH on CNT surface. A

mixture of nitric acid and sulfuric acid is being used in the acid treatment [70 -

72]. Maxim et al. carried out the surface modification by mild acid treatment using

nitric acid and they reported the increase in dispersability by sonication in

addition to acid treatment [73]. Donavon et al. investigated the aliphatic/aromatic

polyimides as dispersant materials for single walled carbon nanotube. They used

polyimides for debundling and dispersion of SWCNTs in N, N’-Dimethyl

acetamide solvent. The polyimide systems containing up to 1 % w/w of SWCNT

were used to prepare films and electrospun fibers. The dispersion and orientation

of SWCNTs within the nano composite films and fibers were studied [33].

Dan et al. prepared highly aligned polyimide nanofiber composites

containing up to 2 % w/w CNTs. They used these nanofiber mats as

reinforcement in the fabrication of polyimide nanocomposite films. The tensile

28

strength and elongation at break of the composites increased with increasing

loading of CNT % w/w [74]. Wolfgang et al. reviewed the electrical percolation

analysis in carbon nanotube reinforced polymer composites. They discussed

about the factors such as CNT type, polymer type, CNT treatment, and

dispersion method and their impact on electrical percolation threshold and

conductivity of the composites [75]. Wael et al. investigated the SWCNTs

embedded in oriented polymeric nanofiber by electrospinning method. They used

oxygen plasma etching process to expose the carbon nanotubes in nanofibers.

The separated and well-dispersed nanotubes aligned in a straight form, while

non-separated and entangled nanotubes were reinforced as dense aggregated

nanotubes [76]. Therefore, special attention is required for the dispersion method

to achieve the alignment of nanotubes in the nanofiber during electrospinning

process.

Siu et al. investigated acid and amine modified multi walled carbon

nanotube (MWCNT) reinforced polyimide composites. Unmodified, acid, and

amine modified MWCNTs separately incorporated to poly(amic acid) and heated

up to 300°C to prepare polyimide/MWCNT composite. The volume electrical

resistivity of acid and amine modified MWCNT reinforced polyimide was

investigated and it decreased dramatically compared to neat polyimide. The

glass transition temperature and mechanical properties were improved by adding

functionalized MWCNTs in polyimide [77]. Hyang et al. prepared

MWCNT/polyimide composite by adding up to 3 % w/w of MWCNTs and studied

the mechanical and electrical properties [78]. Wei et al. reported the process of

incorporating MWCNT incorporated in polyimide composite film by solution

casting method. The highest electrical conductivity of 38.8 S cm-1 and also

tensile strength of 179 MPa were achieved by loading up to 30 % w/w of

MWCNTs in polyimide [79].

Dan et al. investigated the polyimide nanofibers with and without

MWCNTs loading level up to 10 wt %. The surface functionalized MWCNTs were

dispersed in polyimide and nanofiber composites were prepared. They found that

29

there was a highly aligned nanotubes in fiber along axis, but the pristine

MWCNTs aggregated and protruded out of nanofibers. This alignment of CNTs

was achieved by electrospinning of in-situ prepared poly(amic acid)/MWCNT

nanofiber composite. The tensile strength and elongation at break increased for

CNT loading up to 3.5 % w/w in polyimide nanofiber, and then decreased up to

10 % w/w. This indicated that agglomeration of CNTs starts above 3.5 % w/w

and lead to poor alignment of CNTs and decreased mechanical properties. The

thermal stability also increased drastically up to 3.5 % w/w CNTs and then only

slight increase up to 10 % w/w due to the aggregation of CNTs at high level

loading. In dynamic mechanical analysis, the storage modulus was steadily

increased with the increase in concentration of CNTs [80].

2.5 POLYIMIDE NANOFIBER COMPOSITE WITH GRAPHENE OXIDE

Graphene is one of the most potential nano materials discovered in 2004.

It is a monolayer of graphite with 0.34 nm thickness and consisting of carbon

atoms in sp2 hybridization. Each carbon atom is covalently bonded with three

other carbon atoms [22]. In the past two decades, several graphene based

materials have been prepared and utilized in various applications such as solar

cells, fuel cells, lithium ion batteries and super capacitors due to their excellent

properties [24, 81]. Graphene can be easily functionalized by the absorption of

molecules such as oxygen, or hydrogen, etc. Otherwise, graphite can be oxidized

into graphene oxide (GO) by chemical treatment and then reduced into graphene

by heat treatment. Graphene oxide is being prepared by modified Hummers

method. Viet et al. successfully prepared functionalized graphene oxide and

reduced by solvo-thermal reduction method. They achieved superior dispersibility

in various organic solvents which was attributed to steric-effect of functional

groups present on the surface of graphene oxide [82].

Polymer nanocomposites have been produced by dispersing

nanoparticles in polymer matrix. Tapas et al. reported a review article on recent

development in modification of graphene and graphene based polymer

nanocomposites. The modification of graphene, graphene oxide and reinforced in

30

different polymers have been explored [26]. Raja tendu et al. reviewed the

mechanical, electrical properties of graphite and modified graphite reinforced

polymer composites. They reported the various modifications of graphite and

their utilization in polymer nanocomposites [25]. In the development of high

performance lightweight polymer composites, low nanofiller content is required

with improved properties. However, the cost of nanoparticles, availability and

dispersion in the matrix raise a big challenge to achieve the goal. Ramanathan et

al. prepared polymer nanocomposites with functionalized graphene sheets to

overcome these difficulties and also provide good polymer-graphene interaction.

Thermal and mechanical properties were improved by effective dispersion of

functionalized graphene sheets. The percolation threshold was achieved at very

low level of modified graphene in polymer composites [83].

Wei et al. developed polymer nano composites with graphene sheets and

utilized their superior thermal properties. They also studied the enhancement in

electrical conductivity of graphene nanosheets [84]. Hyungu et al. studied the

thermal conductivity of graphene oxide and multi-walled carbon nanotube hybrid

epoxy nanocomposites [85]. In lithium ion rechargeable batteries, graphene

sheets with polyimide are used as a cathode. The highly dispersed graphene

sheet in polymer composite enhanced the electrical conductivity and allowed the

electrochemical activity of polymer cathode material to be utilized effectively [86].

Graphene oxide reinforced with poly vinylidene fluoride (PVDF) showed purely

piezoelectric behavior due to the strong interaction between the fluorine group

and graphene oxide nanosheets [87].

Hua et al investigated graphene oxide (GO) reinforced poly vinyl alcohol

(PVA) nanocomposite film. They achieved uniform dispersion of GO nanosheets

in PVA polymer and determined the strong H-bonding interaction between GO

and PVA by differential scanning calorimeter. The obtained film had high oxygen

and water barrier properties and can be utilized in packaging industry [88]. Ok ja

et al. prepared the poly (D, L-lactic-co-glycolic acid) (PLGA)/GO nanofiber

composite by electrospinning technique. They found that the hydrophilicity of

31

nanofiber composite increased by the presence of GO nanosheets in the

nanofibers. Due to the biocompatibility, these nanofibers can be used in

biomedical applications such as scaffolds, drug delivery, etc., [89]. Mitra et al.

prepared graphene nanosheets incorporated polycarbonate nanocomposite by

emulsion mixing and solution blending methods. They achieved excellent

electrical conductivity by loading of 2.2 % v/v of graphene in polycarbonate [90].

Dan et al. developed graphene oxide reinforced polyimide nanocomposite film.

Graphene oxides prepared by modified Hummers method were exfoliated in a

polar aprotic solvent by sonication treatment. Polyimide nano composites were

prepared by in-situ method of dispersing up to 2 % w/w of functionalized

graphene sheets in a solvent and then addition of monomers into the solution.

The resulted poly(amic acid) was thermally imidized into polyimide

nanocomposite [91].

Hun et al. prepared polyimide nano composite films with different loading

level of reduced graphene oxide (rGO). They achieved 12 fold increase in tensile

strength by loading of up to 5 wt % GO in polyimide. Also, thermo-oxidative

stability of PI was also improved by the addition of both GO and rGO [92]. Hsiang

et al. developed colourless polyimide films by incorporating only 0.001 wt % of

graphene oxide in PI matrix. The nanocomposites exhibited improved

mechanical and dimensional stability. They also exhibited low co-efficient of

thermal expansion (CTE) which was achieved by homogeneous dispersion of

graphene oxide [93]. Li et al. synthesized polyimide nanocomposite with

graphene oxide having isocyanate functional groups. The tensile strength and

modulus were increased about 60% with only 0.75 wt % loading of graphene

oxide and thermal stability was also slightly improved [94]. Ok et al. prepared

highly electrical conductive reduced graphene oxide reinforced polyimide nano

composites by in-situ thermo-chemical synthesis method and found 10 times

higher electrical conductivity compared to typical reduced graphene oxide

incorporated polyimide composites [95].

32

2.6 CARBONIZATION OF POLYIMIDE NANOFIBER COMPOSITES

Carbonization is a process in which the material is heated to around

1000°C to make the material having only carbon atoms by allowing other atoms

to get evaporated. There are many literatures which contain reports on

carbonization of polymeric materials such as poly acrylo nitrile (PAN), polyimide

(PI), etc. Later, polyimide nanofibers were also carbonized into carbon

nanofibers.

Hidetaka et al. carried out the carbonization and graphitization of PMDA-

ODA polyimide film containing boron functional groups. During heat treatment,

>B-N< bonds start to form around 800 °C temperature and these bonds broken

at 1200 °C. They found that the carbon atoms were substituted by boron atoms

which resulted in more disordered arrangement of atoms at 2600 °C [96].

Yuezhen et al. investigated the effect of nickel catalyst under carbonization of

polyimide films. They prepared the graphitic film at temperature lower than

3000°C by mixing nickel particles in poly(amic acid) solution. Three layers of

polyimide films (150 µm thickness) were stacked and carbonized up to 1600 °C.

The stable form of carbonized film was achieved with nickel particles present in

the three layers [97]. Kapton type (PMDA-ODA) polyimide films were carbonized

under the heavy ion irradiations and the effect of such process is also

investigated [98, 99]. Jincai et al. studied carbonization of Kapton film at different

atmospheres such as nitrogen, argon, helium flow, and vacuum, which resulted

in different effects on the structure and properties polyimide [100].

In the past decade, there have been electrospun polymer nanofibers

carbonized into carbon nanofibers by heat treatment. Cheng et al. reviewed the

carbon nanofibers prepared from thermoplastic and thermosetting electrospun

polymer precursor nanofibers. They reviewed polyacrylo nitrile based nanofibers

and other nanofibers such as poly(amic acid), polyvinyl alcohol, etc. and their

carbonization [101]. Zhengping et al. developed carbon nanofibers from

33

polyacrylo nitrile aligned nanofibers by electrospinning process and also studied

their electrical and mechanical properties. They found that the carbon nanofibers

formed had more ordered graphitic structure with increase in carbonization

temperature from 1000 °C to 2200 °C. The carbon nanofiber bundles had 20

times higher electrical conductivity in bundle axes than in the perpendicular

direction. It is also reported that the tensile strength and moduli of the carbon

nanofibers were in the range of 300-600 MPa and 40-60 GPa respectively [102].

Naoko et al. prepared polyimide nanofibers with narrow fiber diameter distribution

using electrospinning process. They developed a novel approach of ion-beam

irradiation and focused the effect of ion species on the electrical properties. It is

reported that the electrical conductivities of the ion-irradiated carbon nanofibers

increased in the order He+< Ne+<Ar+ [103].

Yang et al. prepared carbon fiber web from electrospun PMDA-ODA poly

(amic acid) nanofiber web. The polyimide nanofiber web was heated up to 2200

°C and the carbonization decreased from 64 % at 700 °C to 53 % at 1000 °C. It is

reported that the electrical conductivity increased with increase in temperature

and exhibited 2.5 and 5.3 S/cm at 1000 °C and 2200 °C, respectively [41].

Chung et al. investigated the crystal dimension of carbon nanofibers prepared

from electrospun polyimide nanofiber web. They incorporated Iron (III)

acetylacetonate into PI nanofibers up to 3 % w/w which increased the crystal

dimension from 1.06 to 4.18 nm as examined by X-ray diffraction [104]. Chan et

al. studied the super capacitor behavior of activated carbon nanofiber web

prepared from PMDA-ODA electrospun polyimide nanofibers. The carbon

nanofiber web was activated under steam at temperature of 850 °C. The specific

capacitance of PI electrode of electrical double layer capacitor (EDLC) was 175

F/g even at higher current current density of 1000 mA/g [105]. The electrical

conductivity of carbon nanofiber increased with decreasing the diameter of the

nanofibers. The enhancement is due to increase in number of cross-junctions

between the nanofibers in non-woven web [106].

34

Michael et al. developed polyimide nanocomposite by in-situ synthesis of

polyimide with carbon nanofiber (CNF). They incorporated pristine CNF (PCNF)

and amine functionalized CNF (ACNF) in polyimide matrix. They observed direct

formation of graft on to ACNF, whereas, PCNF had weak interface between the

CNF and PI matrix [107]. David et al. developed pristine and amine

functionalized vapour-grown carbon nanofibers (VGCNF) reinforced polyimide

composite films. They studied their effect on molecular weight (Mw) and glass

transition temperature (Tg) [108]. Radio-frequency Induction heating method has

been used for heating the conducting materials such as metals, graphite, etc.

Hartshorn et al. published the basic principles of radio-frequency heating method

[109]. Brown et al. reported the theory and applications of radio-frequency

heating [110].

2.7 APPLICATIONS OF POLYIMIDE NANOFIBER COMPOSITES

Feng et al. reported a review article on the recent development in the

synthesis, characterization, and applications of polymeric nanofibers via

electrospinning method. The electrospun nanofibers have been utilized in several

applications such as high performance air filters, sensors, protective textiles,

medicine, and scaffolds in tissue engineering, advanced composite materials,

photovoltaic cells, and membranes in separation technology [111]. Takuya et al.

prepared the novel sulfonated polyimide nanofibers and utilized as proton

exchange membrane for fuel cell. It is found that the proton conductivity of the

membrane was higher in the parallel direction compared to perpendicular

direction [112]. Yuan et al. fabricated a novel sensor from porphyrinated

polyimide nanofibrous membrane for rapid detection of hydrogen chloride gas.

Due to higher surface area, the porphyrinated polyimide nanofibrous membrane

provides good gas accessability, high sensitivity, and fast response time in

sensor applications [113]. Sim et al. reported application of carbon-nanofibers

prepared from polymeric nanofibers in gas sensor devices on plastic substrate.

They found, if CNF is used as both anode and cathode in the diode, there is

35

negligible breakdown voltage of air, but the voltage fluctuation increased up to

10% [114].

Timothy et al. developed carbon nanofiber electrodes at macroscopic (5

mm) and microscopic (250µm) dimensions and characterized their

electrochemical activity. The nanofiber forest electrode provides buffering

capacity against surface activation/inactivation [115]. Zexuan et al. published a

review article on electrospinning materials in energy-related devices and

applications. They reviewed mainly four fields such as fuel cells, dye-sensitized

solar cells, Li-ion batteries, and super capacitors. In fuel-cell, electrospun

polymer nanofiber webs are used as polymer membrane and supporting

materials. Polymer nanofibers are utilized as electrolyte in dye-sensitized solar

cells, Li-ion rechargeable batteries. In super capacitor, carbon nanofibers

(produced by electrospinning and carbonization processes) are used as

electrode material [116].

Wenguo et al. reviewed the electrospun nanofiber applications in tissue

engineering and drug delivery. It is noted that different biocompatible materials

have been used to fabricate electrospun nanofibers and utilized in blood vessels,

bone tissue engineering, nervous system and also drug delivery [117]. Zhaohui

et al. developed a novel high-flux and low-fouling thin-film nanofibrous composite

membranes, which contain a top-layer of thin hydrophilic coating, a mid-layer of

nanofibrous scaffold and a supporting material of non-woven microfibrous mat.

These membranes have been used in nanofiltration applications [118]. Amit et al.

reported the application of polyimide nanofiber membrane in solid-liquid

microfiltration process [119].

2.8 AIM AND OBJECTIVES OF THE THESIS

Ø To synthesize and prepare polyimide films by solution casting and

characterize for structural, thermal and mechanical properties.

Ø To develop polyimide nanofiber webs using electrospinning technique by

optimizing various parameters such as polymer concentration, high

36

voltage, collector drum speed, distance between spinneret and collector,

etc.

Ø To carry out the acid functionalization and characterization of Single

Walled Carbon Nanotube (SWCNT), Multi Walled Carbon Nanotube

(MWCNT)

Ø To prepare and characterize Graphene oxide from Graphite by using

modified Hummer’s method

Ø To analyze the effect of low quantity loading of f-SWCNTs, f-MWCNTs,

and graphene oxide with polyimide nanofiber by In-situ Polymerization

technique and characterize their thermal, mechanical, and electrical

properties

Ø Carbonization of polyimide nanofiber webs into carbon nanofiber webs to

improvise the electrical conductivity

37

3. EXPERIMENTAL WORK

3.1 SYNTHESIS AND CHARACTERIZATION OF POLYIMIDE AND ITS

NANOFIBERS

3.1.1 Materials and chemicals

Pyromellitic dianhydride (PMDA, 97 % ), 4,4’-(4,4’-isopropylidene

diphenyl-1,1’-diyl dioxy) dianiline (IDDA, 99 %), N, N’-Dimethyl formamide (DMF)

with boiling temperature of 153 °C (99 %), Graphite powder (<20µm) were

supplied by Sigma Aldrich chemicals Ltd. 4,4’-Oxydianiline (ODA) (Alfa Aesar, 98

%), single walled carbon nanotubes (SWCNTs) & multi walled carbon nanotubes

(MWCNTs) (Yunnan Great (Group) Co., Ltd.), were purchased and used without

any further purification. Sulphuric acid, nitric acid, hydrochloric acid, potassium

permanganate, hydrogen peroxide, and sodium nitrite were supplied by Merk

chemicals Ltd. Distilled and deionized water was obtained by a ‘Milli-Q’ water

purification system (Millipore Milli Q185 plus system, USA). These materials

were used to prepare polyimide and nanofiber composites.

3.2 PREPARATION OF POLYIMIDE FILM

Polyimide films were prepared by two stage method of poly condensation

of dianhydride and diamine monomers. The polyimide synthesis was carried out

as follows: In the first stage, diamine monomers 4, 4’- oxy dianiline (ODA) (0.234

g, 0.5 mmol) and 4, 4’ -(4,4’-isopropylidine diphenyl-1,1’-diyldioxy) dianiline

(IDDA) (0.479 g, 0.5 mmol) were dissolved in N-Methyl 2-pyrrolidone solvent

(NMP) solvent (3 mL) in 50 mL round bottom flask fitted with a magnetic stirrer.

Then, the dianhydride, pyromellitic dianhydride (PMDA) (0.510 g, 1.0 mmol) was

added to the solution very slowly and stirred for 12 h to form a viscous solution of

PAA. Then, the solution was poured slowly into methanol and PAA precipitate

was formed. The pale yellow precipitate was filtered and dried at 60 °C for 6 h

under vacuum and the yield was 88 % [120].

38

In the second stage, PAA powder was dissolved in 5 ml of NMP solvent to

form viscous PAA having 30 % polymer concentration. The solution was poured

on to a glass plate (solution casting) and kept in a hot air oven. Then, the PAA

solution poured glass plate was heated up to 300 °C in three steps such as 100,

200, and 300 °C and maintained soaking time of 1 h at each step and then

cooled to room temperature slowly. The PI film was obtained and removed from

the glass plate. The preparation procedure was carried out with different

dianhydride and diamine monomer combination listed in Table 3.1.

Table 3.1 Different monomer combinations used in preparation of polyimide film.

S.No. Monomer combination Remarks

1. Pyromellitic dianhydride [PMDA] + 4, 4’-Oxy dianiline [ODA]

Continuous film with 88% yield

2. [PMDA] + 4,4’-(4,4’- isopropylidene diphenyl-

1,1’-diyldioxy) dianiline [IDDA]


3. Bicyclo (2.2.2) oct -7-ene- 2,3,5,6-tetracarboxylic dianhydride

[BPDA] + [IDDA]

Brittle film with 71% yield

4. [BPDA] + [ODA]


O

NH2NH2

O O

O

O

O

O

O O

CH3 CH3NH2 NH2

O O

O

O

O

O

O O

CH3 CH3NH2 NH2

OO

O

O O

O

O

NH2NH2O

O

O

O O

O

39

S.No. Monomer combination Remarks

5. [BPDA]+ 4,4’ methylene bis (2-chloro aniline) [MBA]


6. 4,4’-(4,4’-isopropylidene diphenoxy) bis (phthalic anhydride) [IDPA] +

[IDDA]


7. [IDPA] + [ODA]


8. [PMDA] + [ODA] + [IDDA]


Different combinations of dianhydride and diamine monomers were

selected and polymerized to get poly(amic acid) and then imidized into polyimide.

These were tested for film forming ability. The fine polyimide films were obtained

NH2 NH2

Cl Cl

OO

O

O O

O

O O

CH3 CH3

O O

O

O

O

O

O O

CH3 CH3NH2 NH2

O

NH2NH2

O O

CH3 CH3

O O

O

O

O

O

O O

CH3 CH3NH2 NH2

O

NH2NH2

O O

O

O

O

O

40

from a few monomer combinations. The selected dianhydride and diamine

monomers such as pyromellitic dianhydride (PMDA), oxy dianiline (ODA), and 4,

4’ - (4,4’-isopropylidine diphenyl-1,1’-diyldioxy) dianiline (IDDA) are shown in

Figure. 3.1.

Figure. 3.1: Chemical structure of monomers.

Based on the results obtained, the monomers that gave better films are

PMDA, ODA and IDDA. Therefore all the further trials were focused only with

these monomers and their combinations with and without the presence of nano

additives such as CNTs, graphene oxide. The commercial polyimide Kapton

(PMDA-ODA) film was prepared and Figure. 3.2. shows the schematic of

synthesis of polyimide. We prepared copolyimide by adding IDDA with PMDA-

ODA polyimide film to improve the flexibility. Figure. 3.3. shows schematic

preparation of the copolyimide (PMDA-ODA-IDDA) film.

41

Figure. 3.2: Schematic of PMDA-ODA polyimide preparation.

Figure. 3.3: Schematic of PMDA-ODA-IDDA polyimide preparation.

The effect of iso-propylidene group and aryl ether linkage in diamine

monomer such as 4, 4’ -(4,4’-isopropylidine diphenyl-1,1’-diyldioxy) dianiline

(IDDA), 4, 4’ oxydianiline (ODA) on flexibility of polyimide films was studied.

42

3.3 FUNCTIONALIZATION OF SWCNT AND PREPARATION OF

POLYIMIDE NANOFIBERS WITH f-SWCNT

Van et al. investigated the surface modification of carbon nanotubes by

acid treatment and achieved the attachment of functional groups on carbon

nanotube surface [70]. 1.0 g of pristine SWCNTs (p-SWCNT) was added to

150mL mixture of HNO3 and H2SO4 with 1:3 weight ratios, v/v and the mixture

was ultra-sonicated for about 4 h in an ultrasonication bath (40 kHz) [121, 122].

The mixture was refluxed at 90 °C for 9 h with vigorous stirring and then cooled

to 25 °C. The mixture was vacuum filtered using a 0.2 m Millipore polycarbonate

membrane filter and the filtered solid mass was washed several times with

distilled water until the pH of the filtrate attains 7.0.

Figure. 3.4: Flow chart for functionalization of p-SWCNTs.

43

The obtained solid mass of functionalized SWCNTs (f-SWCNT) was dried

at 60 °C under vacuum for 24 h. The flow chart of acid functionalization of p-

SWCNTs into f-SWCNTs is shown in Figure.3.4. The schematic representation of

acid treatment of p-SWCNT is shown in Figure. 3.5.

Figure. 3.5: Schematic of acid treatment of p-SWCNTs (adapted from [121]).

The precursor of polyimide, poly(amic acid) was synthesized by in-situ

method from a dianhydride (PMDA) and diamines (ODA, IDDA) with a molar ratio

of 1:0.5:0.5 respectively [123]. The diamines were first dissolved and dianhydride

was added slowly in N,N’-Dimethyl formamide (DMF) solvent at 25 °C, in which f-

SWCNT had been ultrasonically dispersed for 1 h. The different weight

percentages (0, 0.5, 1.0, and 2.0 %) of f-SWCNTs in PAA solutions were

prepared for electrospinning. Figure. 3.6 shows the graphical abstract of PAA/f-

SWCNT nanofiber preparation.

44

Figure. 3.6: Graphical abstract for the preparation of PAA/f-SWCNT nanofiber

(adapted from [122]).

Figure. 3.7: Flow chart for the preparation of PI/f-SWCNTs nanofibers.

45

Electrospinning of PAA/f-SWCNTs solution was carried out using an

electrospinning unit (Espin-Nano, Physics Instruments, Chennai, India; 50 kV

power supply) and a non-woven nanofiber web was obtained. The polymer

solution with 45 % concentration was taken in a syringe (2 mL) with a needle

inner diameter of 0.80 mm was connected to a positive electrode to which high

voltage was applied and aluminum foil mounted on a collector connected to a

negative electrode was rotated at 2000 rpm and the collector was kept 11 cm

away from the needle spinneret. The applied voltage to needle was 20 kV and

flow rate was maintained at 0.25 mL/h. The different steps involved in the

preparation of PI/f-SWCNTs nanofibers are given by a flow chart (Figure. 3.7).

Figure. 3.8: Photographic Images of (a) poly (amic acid) and (b) polyimide

nanofiber webs with f-SWCNTs (0, 0.5, 1, and 2 % w/w).

The prepared nanofiber webs on aluminum foil were first dried at 60°C for

10 h to remove residual solvent and then thermally imidized in a hot air oven by a

three-step heat treatment at 100 °C for 1 h, 200 °C for 1 h, and 300 °C for 1 h.

During heat treatment, heating rate of 5 °C/min was maintained and thus, the

electrospun PI/ f-SWCNTs nanofiber web was developed. The photographic

images of PAA and PI nanofiber webs are shown in Figure. 3.8 (a) & (b).

3.4 PREPARATION OF GRAPHENE OXIDE FROM GRAPHITE BY ACID

TREATMENT

Graphene oxide was prepared from graphite power using a modified

Hummer’s method [91]. Graphite powder (2.5 g), KMnO4 (7.5 g), and NaNO2 (2.5

46

g) were added slowly into 50 mL concentrated H2SO4 at below 5 °C under

vigorous stirring using magnetic stirrer. The entire solution mixture was

continuously stirred at 35 °C for 1 h to oxidize the graphite powder. Then, 100 mL

de-mineralized water was added and heated up to 100 °C, and maintained for 15

min. After that, the suspension was poured into 300 mL of deionized water and

20 mL of H2O2 was added.

Figure. 3.9: Flow chart for preparation of graphene oxide.

Then, the mixture was cooled to room temperature and filtered using

polycarbonate membrane filter. The obtained solid product was washed several

times with 5 % aqueous HCl solution and deionized water, and finally dried to

obtain graphene oxide powder. The flow chart of preparation of graphene oxide

from graphite powder is given in Figure. 3.9.

47

3.5 PREPARATION OF POLYIMIDE NANOFIBER WITH GRAPHENE

OXIDE

PAA/GO solution was prepared by in-situ polymerization of dianhydride

and diamine. The in-situ synthesis of PAA/GO involves, the desired amount of

GO was dispersed in DMF solvent (1.0 mg mL-1) by ultrasonication for 1 h using

an ultrasonicator bath. The diamine ODA was added dissolved in the solvent

completely. Then, the equimolar quantity of PMDA was added and the solution

was stirred at room temperature for 12 h, forming a viscous PAA/GO solution (35

% conc.) in DMF solvent.

Figure. 3.10: Flow chart for the preparation of PI/GO nanofibers.

The PAA/GO solution was electrospun into a nonwoven nanofiber mat

using an electrostatic spinning unit. It had a 2 mL syringe with a needle inner

diameter of 0.40 mm. The viscous PAA/GO solution was extruded from the

48

syringe at an applied voltage of 15 kV and a flow rate of 0.2 mL/h onto a piece of

aluminum foil, which was wrapped around a negatively-charged drum collector

rotating at 2000 rpm, placed 12 cm away. Figure. 3.10. shows the flow chart of

different steps involved in the preparation of PI/GO nanofibers [124].

Figure. 3.11: (a) Poly(amic acid) (PAA) solution with different levels of graphene

oxide (GO) loading, (b) Electrospun webs of poly(amic acid) (PAA) with different

levels of graphene oxide (GO), loading, before imidization, (c) after imidization.

49

The imidization of the PAA/GO nanofiber mats on the aluminium foil was

conducted in a 3-step heat treatment in a hot air oven at 100 °C for 1 h (solvent

removal), 250 °C for 1 h (dehydration and cyclization) and 350 °C for 1 h

(imidization). The heating rate was 5°C/min between the steps. Thus, a PI/GO

nanofiber web was obtained. The photographic images of poly(amic acid solution

with graphene oxide (0, 0.1, 1, and 2 % w/w) and electrospun PAA and PI webs

are shown in Figure.3.11.

3.6 CARBONIZATION OF POLYIMIDE NANOFIBER WITH f-MWCNTS

Poly(amic acid) solution was prepared by polymerizing PMDA, ODA and

IDDA in DMF solvent with and without f-MWCNT (1 & 2 % w/w). Acid

functionalization of MWCNTs was carried out using a procedure as same as

reported for f-SWCNTs. Figure. 3.12. shows the preparation procedure of PMDA-

ODA-IDDA polyimide system and carbonization of nanofiber webs. The viscous

PAA solution with 35 % concentration was taken in a syringe and kept in the

electrospinning unit. For electrospinning, PAA solution was filled in 2 mL syringe,

the distance between the needle tip and drum collector was 12 cm, 25 kV voltage

was applied to syringe needle (positive terminal), and the drum was connected to

negative terminal and the flow rate of PAA solution extruding from needle

capillary was maintained at 0.25 mL/h.

The formed continuous nanofibers were collected on aluminium foil

mounted drum collector and rotating speed of drum was around 2000 rpm. The

obtained poly(amic acid) nanofiber web was imidized by heating up to 300 °C.

The heat treatment carried out by stepwise heating the mat about 100 °C for 1 hr

to remove the residual solvent, 200 °C for 1 h, and 300 °C for 1 h in hot air oven

to form imide structure from amic acid. The obtained PI/f-MWCNTs nanofiber

webs were carbonized by using Radio-Frequency induction heating method. The

PI nanofiber web (0, 1 and 2 % w/w of f-MWCNT) was sandwiched between two

parallel polished graphite plates and kept in quartz tube which was mounted in a

radio-frequency induction coil. Nitrogen gas flow was maintained inside the

quartz tube at 600 mL/min. The nanofiber webs were heated to different preset

50

temperatures at the heating rate of 20 °C/min under nitrogen atmosphere. The

carbonization yield was determined after the thermal treatment up to 1000 °C.

Figure. 3.12: Flow chart for carbonization of PI/f-MWCNT nanofibers.

3.7 CHARACTERIZATION AND TESTING

Fourier-transform infrared spectroscopy (FTIR) analysis was carried out

using Thermo Nicolet iS10 FTIR spectrometer (ATR mode) in the wave number

range of 650 - 4000 cm-1. Raman spectroscopy was carried out on a LABRAM

HR UV-VIS-NIR Raman microscope from HORIBA Jobin Yvon (633 nm laser

source). X-ray photoelectron spectroscopic (XPS) analysis was carried out using

KRATOS Axis Ultra (Kratos Analytical, Manchester, United Kingdom). A mono

chromated Al Kα radiation (1486.6 eV) was focused on a sample surface area of

0.7mm ´ 0.3mm. The XPS survey scans were obtained by three passing sweeps

of 15.0 eV and a step size of 0.1 eV was employed. Wide-angle XRD

51

measurements were performed at room temperature (25 °C) on a Bruker D8

Focus x-ray diffractometer having a Ni-filtered Cu Kα radiation ( =1.541 Å) and a

scintillator detector (40 kV, 40 mA). The measurements were done in a 2θ range

of 5–90° at 0.04°/step and 1 s/step.

High-resolution scanning electron microscopy (SEM) observations of

nanofibers were performed in a HRSEM Hitachi S4800 with EDAX instrument at

an acceleration voltage of 5.0 kV and the fiber diameter was calculated using

ImageJ software. Transmission electron microscopy (TEM) observations of

nanofibers were performed with a JEOL JEM 2100 HRTEM, and samples were

prepared for TEM analysis by depositing the nanofibers onto copper grids during

electrospinning process. Atomic Force Microscopy (AFM) carried out by using

NTEGRA Prima - NT-MDT.

Dynamic mechanical analysis (DMA) of the nanofiber webs were

conducted on Seiko Instruments DMS 6100 at 1 Hz and 2 °C/min from 30 to 400

°C under tensile mode in air atmosphere. Thermogravimetry analysis (TGA)

results were obtained from Seiko Instruments TG/DTA6200 EXSTAR 6000 under

a nitrogen gas flow of 140 mL/min and a heating rate of 20 °C/min. The thickness

of the nanofiber webs were in the range of 200-300 mm.

Tensile properties of the nanofiber webs were carried out on DAK System

Inc.’s Universal Testing Machine with 250 kg load cell and the test samples were

mounted to clamps (grips) and stretched the clamps at a speed of 50 mm/min.

The tensile testing values reported in this thesis are average values of the results

obtained from the tests run on at least four samples.

Electrical conductivity was measured by Four-Probe set up consisting of

DC micro-voltmeter with the voltage range of 1 mV – 10 V and low current source

with the range of 0.2 A – 2 mA, DFP-RM model from SES Instruments, India.

52

4. SYNTHESIS AND CHARACTERIZATION OF PMDA-ODA AND PMDA-

ODA- IDDA POLYIMIDE FILMS

4.1 INTRODUCTION

In recent years, polyimides have become most demanding high

performance polymers due to their excellent thermal stability, improved

mechanical and electrical properties. Among these polyimides, aromatic

polyimides are well known for their excellent mechanical and chemical

properties, like resistance to acids as well as their solubility in polar aprotic

solvents. The modifications in this type of polyimides have been made and used

in aerospace, optoelectronics, and liquid crystal display (LCD) and other needful

industries. In-addition, polyimides have also been used in sensors, membranes

in fuel cell, gas separation, ultrafiltration and nanofiltration applications due to

their flexibility, toughness and thermo-oxidative resistance. In general, aromatic

polyimides have high melting temperature and poor solubility in most of the

aromatic solvents due to the rigid backbone and strong interaction between the

polymer chains. Many modifications have been made to improve the flexibility

and solubility with reservation of other advantageous properties by introducing

flexible linkages, non-coplanar units or bulky substituent.

Most of the polyimides are light brown in colour which is due to inter and

intramolecular charge transfer (CT) interaction between alternating electron-

donating diamine and electron accepting dianhydride components. It has been

shown that the polyimides with unsymmetric structure of diamine prevent the

close packing of chains and also the CT interactions. It is also found that the

introduction of bulky groups with unsymmetric structure into the polyimides

rendered the polymer to have better solution processability, less colour and

improved flexibility.

53

In the present study, the synthesis and characterization of two polyimides

namely pyromellitic dianhydride with 4, 4’-oxydianiline (PMDA - ODA) (PAA1 &

PI-1) and PMDA-ODA with 4, 4’- (4, 4’ isopropylidine diphenyl 1, 1’-diyl dioxy)

dianiline (PMDA-ODA-IDDA) (PAA-2 & PI-2) have been studied with their

comparative properties such as thermal stability, transparency and flexibility.

More experimental determinations are provided in chapter 3 (section 3.2). The

effect of phthalic iso-propylidine group and ether linkage (PMDA-ODA-IDDA) on

thermal stability, optical property and flexibility is discussed. In addition to bulky,

packing-disruptive –CH3 groups, the unsymmetrical structure of the bis(ether

amine) component will prevent the extended close packing of chains and also the

CT interactions.

4.2 RESULTS AND DISCUSSION

4.2.1 Structural studies

Figure. 4.1: FT-IR spactra of PMDA-ODA PAA-1 & PMDA-ODA-IDDA PAA-2

powders.

54

The conversion of the PAA to the fully cyclized polyimide was determined

by Fourier Transform Infrared spectroscopy.

Figure. 4.2: FT-IR spactra of PMDA-ODA PI-1 & PMDA-ODA-IDDA PI-2 films.

Figure. 4.1 shows the FTIR spectra of PAA-1 & PAA-2 and Figure. 4.2

shows PI-1 & PI-2 films. The conversion of carboxylic amide groups to the imide

ring was evidenced by the disappearance of bands at 2500 cm-1 - 3500 cm-1

corresponds to the amic acid with the appearance of bands at 1775 cm-1

(asymmetrical C=O stretch), 1713 cm -1 (symmetrical C=O stretch), 1382 cm-1 (C-

N stretch), 745 cm -1 (C=O bending).

Polyimide films were characterized by wide-angle XRD studies and the

diffraction patterns are shown in Figure. 4.3. A broad diffraction peak intensity

was observed around 2θ = 10° - 25° for PI films which showed amorphous nature

with broad diffraction intensities peak due to the presence of flexible iso-

propylidene group. There were two broad peak intensities observed around 10°

and 20° (2 θ) for PAA-1 & 2 and it indicates the semi crystalline behavior of PAA.

55

Figure. 4.3: XRD pattern of PAA-1 & PAA-2 powders and PI-1 & PI-2 films.

4.2.2 Thermal properties

Typical thermogravimetry curves for polyimides are shown in Figure. 4.4.

as determined at nitrogen atmosphere. The PI-1 is found to be more thermally

stable compared to PI-2 and decomposition temperature of PI films at 10 % w/w

decomposition was found to be 541.1 °C and 531.1 °C respectively for PI-1 & PI-

2. It is clear that the higher decomposition temperature of polyimide is due to the

target imide ring decomposition. The nature of cleavage of the imide ring is same

for all the polyimides. However, the difference in decomposition temperature is

due to the presence of electron releasing or electron withdrawing groups in the

polyimide chain. The slight shift in the decomposition temperature for PI-1 & PI-2

is attributed to the influence of the electronic factors (electron releasing or

electron withdrawing) associated with phenoxide oxygen and the iso-propylidine

group of the dianhydride and diamine.

56

Figure. 4.4: TGA curve of PMDA-ODA PI-1 & PMDA-ODA-IDDA PI-2 films at

nitrogen atm.

4.2.3 Mechanical properties

The visco-elastic relaxation behavior of polyimide films was measured by

dynamic mechanical analyzer (DMS 6100). The specimen dimensions for DMA

measurements were about 40 mm × 5 mm × 0.03 mm (l × w × t). The storage

modulus E’, loss modulus E’’ and dissipation or damping factor tan d

(tan δ = E’/E’’) at 1 Hz oscillatory deformation were recorded in the temperature

range from 25 °C to 300 °C. Figure. 4.5 shows the DMA curve of PMDA-ODA PI-

1 film and Figure. 4.6 shows PMDA-ODA-IDDA PI-2 film. The glass transition

temperatures were determined as the temperatures of the maxima of tan δ which

showed 238.8 °C and 216.7 °C for PMDA-ODA and PMDA-ODA-IDDA polyimide

films respectively. The damping factor (tan δ) value decreased from 1.265 to

1.1324.

57

Figure. 4.5: DMA curve of PMDA-ODA PI-1 film.

Figure. 4.6: DMA curve of PMDA-ODA-IDDA PI-2 film.

58

Tensile strength of polyimide films is listed in Table. 4.1. The tensile

strength and modulus of PMDA-ODA-IDDA polyimide film are lower than PMDA-

ODA film. On the other hand, elongation at break % of PMDA-ODA-IDDA film

increased compared to PMDA-ODA film. This is due to the flexible iso-propylidine

group present in IDDA monomer.

Table 4.1 Tensile properties of PMDA-ODA and PMDA-ODA-IDDA PI films.

S.No. PI film

sample

Tensile

strength (MPa)

Tensile

Modulus (GPa)

Elongation at

break

1. PMDA-ODA 89.1± 1.2 2.16±0.01 9.2±0.3 %

2. PMDA-ODA-

IDDA

85.4±1.5 2.12±0.02 9.7±0.2 %

4.2.4 Optical properties

Figure. 4.7: UV-Visible spectra of PI 1 & 2 films.

59

The UV-Visible spectra of the polyimides with film thickness of 70 µm -

90 µm are shown in Figure. 4.7. Both the polyimides show high transmission

(85 % - 88 %) in the wavelength range of 800 nm - 1600 nm. The iso-propylidine

group and phthalic ether groups present in dianhydride and diamine moieties

were effective in decreasing the charge transfer complex between polymer

backbone chains through steric-hindrance leading to increase in the

intermolecular distance and thus decreased the interaction between the

polyimide chains resulting in a good optical transparency.

4.3 CONCLUSION

The two different polyimide films were prepared by two stage process. The

first stage involves the formation of poly(amic acid) and second stage consists of

casting the PAA solution on to the glass plate and heated to 300 °C in hot air

oven to convert PAA into polyimide film. The polyimide films were characterized

by FT-IR spectroscopy, X-ray diffractometer, thermogravimetry analyzer,

dynamic mechanical analyzer, and UV-Visible spectrophotometer.

The PMDA-ODA polyimide film showed higher value in thermal stability

and tensile strength but lower elongation at break % which indicates the poor

flexibility. The PMDA-IDDA-ODA polyimide has more flexibility and transparency

than PMDA-ODA polyimide film which is due to the presence of iso-propylidine

group and ether linkage in the dianhydride and diamine monomers.

60

5. PREPARATION AND CHARACTERIZATION OF f-SWCNT

REINFORCED POLYIMIDE NANOFIBER COMPOSITES

5.1 INTRODUCTION

Many polymers have been used to prepare polymeric nanofibers for

several applications. In addition to films, polyimide nanofibers are being

produced by electrospinning technique, and several works have been reported

for obtaining ultrafine, uniform and non-woven or aligned nanofibers.

Nanoparticles dispersed polymer fiber composites are known to possess

improved thermal and mechanical properties compared to neat polymer fibers.

Carbon nanotubes (CNT) have been an attractive material for such enhancement

even at low loading levels, due to their high aspect ratio (greater than 1000), low

density, and high strength, alignment and the large interfacial areas between

them and the polymer matrix.

Donavon et al. prepared single-walled carbon nanotubes (SWCNT)/PI

composite by adding polyimide to the SWCNTs/DMAc solution and fabricated

nanofiber web by electrospinning [33]. Wael Salalha et al investigated SWCNTs

incorporated in polymeric nanofibers by electrospinning [76]. However, polyimide

nanofibers consisting of aligned SWCNTs have not yet been reported to the best

of our knowledge. Further, there is as yet no clear understanding of the nature of

interactions between the polyimide nanofibers and the functionalized CNTs.

In this study, poly(amic acid) with f-SWCNT solution was prepared by in

situ polymerization from pyromellitic dianhydride (PMDA), 4,4’-oxydianiline (ODA)

and 4,4’-(4,4’ isopropylidine diphenyl 1,1’-diyl dioxy) dianiline (IDDA) in f-

SWCNT-dispersed DMF solvent, followed by electrospinning into nonwoven

poly(amic acid) nanofiber mats. The nanofiber mats were imidized by thermal

treatment to obtain a polyimide nanofiber mat. The morphological, thermal and

61

mechanical properties of PAA and PI with f-SWCNT nanofiber webs were

determined and analyzed. More experimental details are explained in chapter 3

(section 3.3 and 3.7).


5.2.1 Structural and morphological studies

Figure. 5.1 shows the Fourier Transform Infrared spectrum of (a). p-

SWCNT and (b). f-SWCNT. Compared to p-SWCNT spectrum, the f-SWCNT

spectrum showed new bands at 1634 cm-1 for –COOH stretching and 3446 cm-1

for –OH stretching. These bands indicate the new functional groups attached to

SWCNTs by acid treatment. Figure. 5.2 shows the FTIR spectra of PAA

nanofibers with and without f-SWCNT. The characteristic peaks, –OH and –NH2

stretching bands appears in the region of 2500 cm-1 - 3500 cm-1, while C=O in –

CONH and –C-NH stretching bands can be seen at 1656 cm-1 and 1537 cm-1

respectively. In the PI nanofiber webs (Figure. 5.3), new bands were observed at

1774 cm-1 corresponds to C=O symmetric stretch, at 1724 cm-1 for the C=O

asymmetric stretch. The bands at 1377 cm-1 and 726 cm-1 are attributed to the

C-N stretch and C=O bending respectively. These bands confirmed the

formation of imide linkage.

Figure. 5.1: FT-IR spectrum of (a) p-SWCNT and (b) f-SWCNT.

62

Figure. 5.2: FTIR spectra of PAA/f-SWCNT nanofiber web with f-SWCNT

contents of 0% (neat PAA), 0.5%, 1.0%, and 2.0% (by weight).

Figure. 5.3: FTIR spectra of PI/f-SWCNT nanofiber web with f-SWCNT contents

of 0 % (neat PI), 0.5 %, 1.0 %, and 2.0 % (by weight).

63

Raman spectrum of the p-SWCNT (Figure. 5.4 (a)) showed a peak

observed at 1583 cm-1 (G-band) due to sp2 hybridization of carbon atoms. In

addition, f-SWCNT (Figure. 5.4 (b)) spectrum showed an additional peak at 1338

cm-1 (D-band) due to sp3 hybridization caused by the formation of defects on f-

SWCNT surface due to acid treatment [19].

Figure. 5.4: Raman spectra of (a) p-SWCNT and (b) f-SWCNT.

Figure. 5.5: Raman spectra of PI nanofiber webs with f-SWCNTs (0, 0.5, 1, and 2

% w/w).

64

Raman spectra of the electrospun PI nanofiber webs (Figure. 5.5) showed

no characteristic peaks observed for neat PI nanofiber in the range of 1300 cm-1

- 1600 cm-1, while the PI/SWCNT nanofibers showed peak at 1578 cm-1 (G-

band) corresponding to in-plane vibrations of SWCNTs and peak at 1343 cm-1

(D-band) corresponding to the defect-induced region of the SWCNTs. These two

peaks confirmed the incorporation of SWCNTs in the PI matrix.

Figure. 5.6: Wide X-ray photoelectron spectra of (a) f-SWCNT and (b) PAA/f-

SWCNTs (1 % w/w) powder.

The wide X-ray photoelectron spectra of f-SWCNT and PAA/f-SWCNT are

shown in Figure. 5.6 (a) & (b) respectively. The wide spectrum of PAA/f-SWCNT

shows the oxidized SWCNTs analyzed by X-ray photoelectron spectroscopy to

study the bonding nature of oxygen on the surface of SWCNT [20].

Figure. 5.7 (a) shows the deconvoluted spectrum of the C (1s) peak of f-

SWCNTs. The component peak 1 at 284.3 eV corresponds to sp2 C=C/sp3 C-C,

and the component peak 2 at binding energy 287.6 eV is attributed to C-O and –

COO. In the deconvoluted XPS O (1s) (Figure. 5.7 (b)), component peak 1 at

530.6 eV corresponds to O-physically absorbed on the surface of SWCNT, peak

2 at 531.6 eV corresponds to C=O, and peak 3 at 532.6 eV corresponds to

O-C=O, C-O-C. The quantification of C and O on the oxidized SWCNT surface

65

provides the atomic concentration % for C is 70.19 and O is 29.81. These results

confirm the formation of carboxyl functional groups on the surface of the

SWCNTs during the acid treatment.

Figure. 5.7: Deconvoluted XPS (a) C1(s) peak and (b) O1(s) peak of f-SWCNTs.

Figure. 5.8: Deconvoluted XPS (a) C1(s) peak, (b) N1(s) peak, and (c) O1(s)

peak of PAA/f-SWCNT 1 wt %.

66

In the case of PAA-SWCNT composite powder, the deconvoluted C (1s)

(Figure. 5.8 (a)) component peaks 1 and 2 are shifted to 282.6 eV and 286.1 eV

respectively. N (1s) (Figure. 5.8 (b)) component peak 1 at 397.7 eV appears,

corresponding to phenylene nitrogen, and peak 2 observed at 398.9 eV

corresponds to C-N amide nitrogen. For oxygen O (1s) (Figure. 5.7 (c)), the

binding energies of component peaks are shifted to 528.8, 529.8 and 531.1 eV.

These shifts indicate possible hydrogen bonding interactions between the PAA

and SWCNT. The XPS spectra results of functionalized carbon nanotubes were

satisfied with the results found in literature [122].

Figure. 5.9: HRSEM micrograph of (a) & (b) neat PAA nanofiber web, (c) & (d)

neat PI nanofiber web.

Figure. 5.9 (a) & (b) show the HRSEM micrographs of neat PAA nanofiber

mats at different magnification. It can be seen that the non-aligned and uniform

nanofibers were obtained. Figure. 5.9 (c) & (d) represent the HRSEM images of

67

neat PI nanofibers at different magnification. There were some fibers entangled

and fused, this may be due to thermal imidization of PAA nanofiber mats up to

300 °C. The diameter of the PAA and PI nanofiber composite with f-SWCNT

contents of 0 % (neat PAA/PI), 0.5 %, 1.0 % and 2.0 % (by weight) were

calculated by ImageJ software and given in Table 5.1. The average diameter of

nanofibers was in the range of 50 nm - 150 nm and it is seen that there are no

beads in the obtained nanofibers.

Table 5.1 Diameter of the PAA and PI nanofiber composites with f-SWCNT

contents of 0 % (neat PAA/PI), 0.5 %, 1.0 % and 2.0 % (by weight).

S.No. Sample with f-

SWCNT (by weight)

PAA nanofiber

(nm)

PI nanofiber (nm)

1. Neat (0 %) 98±15 87±13

2. (0.5 %) 121±18 119±21

3. (1 %) 132±25 117±23

4. (2 %) 145±21 139±24

Figure. 5.10 (a) shows the HRTEM micrograph of bundled p-SWCNTs.

The distorted rough surface observed in Figure. 5.10 (b) shows the defects on

the SWCNTs due to acid functionalization, which are potential sites of hydrogen

bonding with the polymer matrix.

HR-TEM micrograph of PAA/0.5 % w/w f-SWCNT nanofibers at low and

high magnification are shown in Figure.5.11(a-d). It is seen that the carbon

nanotubes are dispersed inside the fibers and aligned with the fiber along

direction. Since the PAA is polymerized in situ, acid-functionalized surface sites

of the CNTs may interact with the monomers via hydrogen bonding interactions,

facilitating polymerization around the CNTs themselves. These interactions are

preserved and probably enhanced during the process of electrospinning.

68

Figure. 5.10: HRTEM micrograph of (a) p-SWCNTs bundle, (b) f-SWCNTs

bundle.

Figure. 5.11: HRTEM micrographs of PAA nanofibers containing 0.5 % w/w f-

SWCNTs (a) & (b) at low magnification, (c), & (d) at high magnification.

69

The electrostatic fields and the shear forces present during

electrospinning, and the resultant movement of the PAA chains, serve further to

strengthen the alignment of the polymer with the nanotubes. The net result is

seen as nanotubes dispersed within the polymer fibers. The similar TEM image

results of carbon nanotubes in polymer nanofibers were observed in literature

[74]. Therefore, the acid-functionalization of carbon nanotubes, and the

subsequent hydrogen bonding interactions with the polymer greatly improve the

dispersion and orientation of the nanotubes within the PAA nanofibers at low

levels of loading.


The thermal stability of PI/f-SWCNT nanofiber composites was found to

have improved as compared to the neat PI nanofiber mat (Figure. 5.12), even at

low levels of f-SWCNT.

Figure. 5.12: TGA curve of PI nanofiber webs with and without f-SWCNT at

nitrogen atm.

70

The temperatures for 10 % weight loss in PI/f-SWCNT samples containing

0 %, 0.5 %, 1 % and 2 % SWCNT (by weight) were found to be 482.7 °C, 513.9

°C, 545.2 °C and 585.8 °C respectively. There is a tremendous increase in

decomposition temperature of polyimide in the presence of even very small

quantity of SWCNT. This increase in thermal stability with increase in loading

level of f-SWCNT is due to improvement in interaction between PI and f-

SWCNTs.


The DMA results of PI nanofiber mats with f-SWCNTs (0, 0.5, 1.0, and 2

% w/w) are shown in Figure. 5.13.

Figure. 5.13: DMA curve of PI nanofiber webs with and without f-SWCNT in air

atm.

The tan δ values measured from the peaks in the tan δ vs temperature

curves were 0.3646, 0.3917, 0.4095, and 0.4289 with f-SWCNT content of 0, 0.5,

1, and 2 % w/w. There is a significant increase in the storage modulus values

71

with the increase in SWCNT content. The storage modulus (E’) values were

obtained 1.24 x 108, 1.69 x 108, 1.94 x 108 and 2.29 x 108 Pa at 30 °C for the

samples of PI with 0, 0.5, 1, and 2 % w/w of f-SWCNTs respectively. This

indicates the good interaction between the PI matrix and SWCNTs [74].

The effect of acid-functionalized SWCNT on the tensile properties is

shown in Table 5.2. The ultimate tensile strength (or the tensile strength at

break), elongation at break and tensile modulus increased significantly with

increasing f-SWCNT content from 0 to 2 % w/w. Nearly 80% increase in tensile

strength was observed with just 2 % w/w loading of f-SWCNT to polyimide and

also 30 % – 40 % increase in modulus and elongation with increase in SWCNT

content. The formation of hydrogen bonds between f-SWCNTs and the PAA

matrix, and subsequent alignment of the CNTs in the fibers facilitate more

efficient load transfer between the matrix and the filler.

Table 5.2 Tensile properties of PI/f-SWCNTs nanofiber composites.

S.No. Sample Tensile

strength (Mpa)

Tensile

Modulus (GPa)

Elongation

at break

1. Neat-PI 4.88±0.8 1.45±0.03 76.27±1.8 %

2. PI/f-SWCNT 0.5% 5.90±0.4 1.57±0.02 98.71±1.7 %

3. PI/f-SWCNT 1.0% 6.67±0.5 1.66±0.03 102.23±1.5 %

4. PI/f-SWCNT2.0% 8.84±0.9 1.79±0.04 109.51±1.9 %

5.2.4 Electrical properties

The electrical conductivity of PI/f-SWCNT nanofiber webs was measured

using four-point probe method. The electrical conductivity of neat PI and PI/f-

SWCNT nanofiber webs is shown in Table 5.3. The measurements were carried

out at least four times for each sample and the average values along with

standard deviation are reported. The conductivity of PI/f-SWCNT up to 2 wt%

increased by up to four orders of magnitude compared to neat PI nanofiber. This

72

increase in conductivity is due to the presence of f-SWCNTs in nanofibers and

also the interaction between the f-SWCNTs with PI matrix.

Table 5.3 Electrical conductivity of PI/f-SWCNTs nanofiber composites.

S.No. Sample Electrical conductivity

(S/cm)

1. Neat PI 1.40±0.03 x 10-18

2. PI/f-SWCNT 0.5 % w/w 1.58±0.08 x 10-14

3. PI/f-SWCNT 1 % w/w 3.32±0.07 x 10-14

4. PI/f-SWCNT 2 % w/w 6.17±0.12 x 10-14

5.3 CONCLUSION

Acid-functionalized SWCNTs were incorporated in PAA matrix by in situ

polymerization of monomers PMDA, ODA and IDDA. Bead-free PAA nanofiber

composites with f-SWCNT content 0 - 2 % (by weight) were prepared by

electrospinning method and thermally imidized into PI/f-SWCNT nanofiber webs.

HR-TEM micrographs indicated homogeneous dispersion of functionalized

SWCNTs in the poly (amic acid) matrix, and alignment of the CNTs within the

nanofibers. FTIR and XPS spectra indicated possible hydrogen bonding

interactions between the functionalized CNTs and the polymer matrix, facilitating

alignment of the CNTs in the polymer fibers. The thermal and mechanical

properties of the fibers improved significantly with the incorporation of f-SWCNTs

in polyimide. Storage modulus, ultimate tensile strength and elongation at break

of nanofiber mats reached 2.29 x 108 Pa, 8.84 MPa and 109 %, respectively, for

f-SWCNTs loading level of 2 % w/w in the polyimide matrix. The thermal

decomposition temperature is also increased by ~100 oC at 10 wt % loss.

73

6. PREPARATION AND CHARACTERIZATION OF POLYIMIDE -

GRAPHENE OXIDE NANOFIBER COMPOSITES

6.1 INTRODUCTION

Aromatic polyimides (PI) are one of the most important high performance

materials in microelectronics and aircraft industries because of their superior

mechanical properties, excellent thermal stability, high glass transition

temperature, and good resistance to solvents. Electrospun nanofibers possess

many unique properties including a large specific surface area and superior

mechanical properties, and they have potential use as nanoscale building blocks.

Nanofibers have been successfully prepared for many applications such as

filters, nano electronics, optical and chemical sensors, catalyst systems,

scaffolds for tissue regeneration and immobilized enzymes. The main advantage

of electrostatic spinning is the ability to produce ultra-fine fibers ranging from

nanometer to sub-micron diameters [124].

Wu et al. reported polyimide-based composite films containing carbon

black (CB), carbon nanotube (CNT) and carbon nanofiber (CNF), respectively

[125]. They were prepared using low-molecular-weight poly (amic acid) (PAA), a

precursor of PI, as an impurity-free dispersant. A significant improvement in the

Young’s modulus of the composites was observed compared to neat PI and the

CNT-loaded composites showed a higher Young’s modulus than the CB or CNF-

loaded composite. Si et al. synthesized non-woven polyimide and silica nanofiber

composites combining electrospinning and controlled in-situ sol-gel techniques

[32]. This composite with a 6.58 % w/w of SiO2 content showed an increase of

133 °C in the decomposition temperature and a four-fold increase in the ultimate

tensile strength, compared to a neat PI fabric.

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Graphene and related materials, make attractive materials for electric,

optoelectronic, and photonic devices, due to their fascinating electrical and

mechanical properties such as superior Hall mobilities, thermal conductivity,

current-carrying capabilities, and room temperature ballistic transport. However,

pristine graphene is unsuitable for intercalation in polymer chains, because

graphene tends to agglomerate in the polymer matrix during processing.

Graphene converted to graphene oxide shows improved dispersability in polymer

matrices and organic systems.

Wang et al. investigated the synthesis and properties of graphene oxide

reinforced polyimide nano composite films with low dielectric constant and high

strength [126]. Longun, et al. reported composites of polyimide with nanoscale

graphene synthesized via in situ condensation polymerization [127]. They

reported that the presence of graphene significantly influenced the storage

modulus, the damping ability and the glass transition temperature of the

composite. Graphene contents (≥ 18 vol. %), were found to hinder polymer chain

flexibility and chain motion due to the 2D geometry of graphene.

The present study deals with the synthesis and characterization of GO

and PI nanofiber composites for potential electronic applications. GO was

synthesized from graphite (<20 µm; Sigma Aldrich) by Hummer’s method and

dispersed in N, N dimethyl formamide (DMF) [91]. The polymerization of poly

(amic acid) (PAA) was carried out in the GO-DMF dispersion. The GO content

tried for PI composites are 0.1 %, 1 % and 2 % by weight. The experimental

details are described in chapter 3 (section 3.4 and 3.5). The resulting nanofiber

composites were characterized by Fourier transform infrared spectroscopy

(FTIR), high resolution scanning electron microscopy (HRSEM), x-ray diffraction

(XRD) Raman analysis and Dynamic mechanical analysis (DMA).

75



The FTIR spectra of graphite and GO are shown in Figure. 6.1 (a) & (b).

Among the salient features observed are the bands at 1617 cm-1 and 3171 cm-1

corresponding to the stretching modes of –C=C and -OH groups. The bands at

1060 cm-1 and 1378 cm-1 are attributed to bending modes of -C-O and -C-H

groups in GO respectively. Figure. 6.2 depicts the FTIR spectra of GO-PAA and

Figure. 6.3 shows the GO-PI nanofiber composites. The GO-PAA composites

electrospun on Al foil, showed the carboxylic –OH stretching (3500 cm-1), the -

C=O stretching (1719 cm-1) and amide group vibration modes (1660 cm-1 and

1544 cm-1), consistent with the presence of PAA. After heat treatment, which led

to amidation of PAA between 50 – 125 °C and imidization at around 350°C, the

carboxylic –OH stretching band was found to be absent, indicating the

conversion of PAA to PI. The other characteristic peaks of polyimide such as

C=O asymmetric stretch (1775 cm-1 and 725 cm-1), C–N stretch (1377 cm-1) and

C=O bending (725 cm-1) were also observed. The absence of the C-N-H stretch

(1544 cm-1) indicated complete imidization in the sample.

Figure. 6.1: FTIR spectra of (a) Graphite, and (b) Graphene oxide.

76

Figure. 6.2: FTIR spectra of PAA/GO with GO contents of 0 % (neat PAA), 0.1 %,

1.0 %, and 2.0 % (by weight).

Figure. 6.3: FTIR spectra of PI/GO with GO contents of 0 % (neat PI), 0.1 %, 1.0

%, and 2.0 % (by weight).

77

The Raman spectra of graphite and graphene oxide are shown in Figure.

6.4 (a) & (b). The samples were characterized by two prominent features: a G-

band at 1578 cm-1 arising due to the first order Raman scattering of the E2G

phonon at the Brillouin zone center of sp2 carbon atoms and a D-band arising

from the breathing mode of the k-point phonons of A1G symmetry at 1321 cm-1.

The D-band requires a defect for its activation and hence, it is an indicator of

defects. The Raman spectrum of GO shows an intense D-band at 1321 cm-1 as

expected, and a broad G-band at 1578 cm-1. The ratio of the intensities of the D-

band and G-band is 0.85 and it indicates the presence of defects introduced in

graphene layers during the oxidation process of graphite.

Figure. 6.4: Raman spectra of (a) Graphite and (b) Graphene oxide.

Raman spectroscopy has been widely used for characterizing polymers

and graphene. Figure 6.5 shows the Raman spectra of neat PI and its nanofiber

composites with different GO content. The peak at 1390 cm-1 in neat PI indicates

the C–N stretching of the imide ring. The characteristic Raman features of

graphene namely the D band (~1321 cm-1) and the G band (~1578 cm-1) can

also be observed in the composite samples. However, in the composite samples,

the band of PI at 1390 cm-1 and the D band of GO at 1321 cm-1 were found to

overlap and could not be distinguished as they were broad and close to each

78

other. With increasing GO content in the nanofiber composites, the peaks in the

vicinity of 1320 cm-1 (predominantly due to the graphene D-band) and 1575 cm-1

(graphene G-band) were both found to increase as expected.

Figure. 6.5: Raman spectra of PI/GO with GO contents of 0% (neat PI), 0.1 %,

1.0 % and 2.0 % (by weight).

Figure. 6.6: X-ray diffraction pattern of (a) Graphite and (b) Graphene oxide.

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The distinctive powder X-ray diffractogram features of graphite and the

synthesized GO are presented in Figure. 6.6 (a) & (b). For the graphite sample,

the characteristic peak of hexagonal graphite corresponding to a d-spacing of

0.334 nm was found at 26.35°. Upon conversion of the graphite into GO, the

peak position is shifted to 10.26° corresponding to an interlayer spacing to 0.878

nm. This increase in d-spacing is due to the intercalation of –OH and other

functional groups in between the graphene layer.

Figure. 6.7: AFM micrograph of graphene oxide.

Figure. 6.8: The height profile of the graphene oxide in AFM micrograph.

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Figure 6.7 shows the AFM image of GO deposited on a freshly cleaved

mica slide, which confirms the HRSEM observations. The GO sheets appear to

be folded, wrinkled and stacked above one another in places. A line scan was

conducted on this sample as shown in Figure 6.7. The height profile of the line

scan is shown in Figure 6.8. The thickness of this layer from the mica surface is

roughly 7 nm. Assuming the thickness of each layer of GO to be about 0.878 nm,

it can be inferred that this is 8-layer sheet of GO.

Figure. 6.9: SEM micrographs of (a) and (b) graphite, (c), and (d) graphene

oxide.

The morphology of graphite and graphene oxide was studied by scanning

electron microscope. Figure. 6.9 (a) & (b) show the SEM micrographs of graphite

81

and Figure. 6.9 (c) & (d) attribute to graphene oxide. It is observed that the

exfoliation of graphene layers due to oxidation process which involves the

intercalation of functional groups such as –OH, –COOH, and –O– between the

layers. Figure. 6.10 (a) shows HRSEM image of neat PAA and Figure. 6.8 (b–d)

show the HRSEM images of PAA nanofibers with GO content of 0.1, 1, and 2 %

w/w. Bead or spindle-like structures were observed in the nanofibers in the GO-

PAA nanofiber composites.

Figure. 6.10: Scanning electron micrographs of PAA/GO with GO content of

(a) 0 % (neat PAA), (b) 0.1 % (GO), (c) 1 % (GO), and (d) 2 % (GO) by weight.

The size of the nanofibers in the PAA/GO composites was calculated in

Figure. 6.10 (a-d) using ImageJ software. The diameter of the nanofibers was in

the range of 94–143 nm and the size of the beads were in the range of 0.65–0.77

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µm (Table 6.1). Figure. 6.11 (a-d) shows the HRSEM image of PI nanofibers with

GO content of 0, 0.1, 1, and 2 % w/w. After thermal imidization, the PI/GO

nanofiber composites (Figure. 6.11 (a–d)) did not show a significant change in

either the nanofiber diameters (84–143 nm) or the bead sizes (0.79–0.91 µm).

This is due to the large variability in the nanofiber diameters and bead sizes

relative to the change in mass (volume) of PAA upon conversion to PI. The

density of these beads (numbers over an area of 11×11 m) as observed in the

SEM images was found to increase with increasing content of GO (35, 100 and

110 beads respectively for PI/GO (0.1 %), PI/GO (1 %) and PI/GO (2 %) by

weight).

Figure. 6.11: Scanning electron micrographs of PI/GO with GO content of (a) 0%

(neat PI), (b) 0.1 % (GO), (c) 1 % (GO), and (d) 2 % (GO) by weight.

83

Figure. 6.10 (a) shows PAA nanofibers with few tens of micrometer

lengths and an average diameter of 94 ±18 nm. Figure. 6.11 (a) shows the

presence of PI nanofiber with few tens micrometer length and an average

diameter of 84 ± 16. Thus, shrinkage in the average fiber diameter from 94 nm in

PAA to 84 nm in PI was observed after thermal imidization. The similar type of

shrinkage was also observed in diameter of PAA and PI nanofibers with f-

SWCNTs. This can be attributed to the elimination of water molecules during

imidization. Table. 6.1 indicates the diameters of the nanofibers in composites

containing 0 %, 0.1 %, 1 % and 2 % of GO (by weight).

Table 6.1: Diameter of the PAA and PI nanofiber composites with GO contents of

0 % (neat PAA/PI), 0.1 %, 1.0 % and 2.0 % (by weight).

Sample GO-PAA GO-PI diameter

GO content Fiber (nm) Bead (µm) Fiber (nm) Bead (µm)

Neat (0%) 94 ±18 - 84 ± 16 -

(0.1%) 120 ± 25 0.67±0.14 134 ± 27 0.91±0.23

(1.0%) 125 ± 23 0.77±0.22 105±20 0.83±0.12

(2.0%) 143± 25 0.65±0.11 141±27 0.79±0.13

The TEM images of PAA/GO (1 % w/w) are shown in Figure 6.12. It is

clearly observed from these images that the bead-like structures (0.50–1.20 µm)

are nanofibers that have bulged due to the GO present inside them. Rolled GO

sheets can also be seen extending from the bulged areas into the nanofibers.

We propose that during the electrospinning process as the GO-polymer

solution is squeezed through the syringe needle, the GO gets deformed under

shear. As the jet of the polymer solution exits the needle in its flight towards the

collecting drum, the GO in the jet relaxes and gets bunched up before the

solidification of the jet into a nanofiber. H. Fong et al. have reported similar bead

and spindle-like structures in electrospun nanofibers of poly (ethylene oxide) with

NaCl [128].

84

Figure. 6.12: High resolution transmission electron micrographs of PAA/GO

1 % w/w nanofiber.

Since the sizes of the GO sheets are in the order of a few microns and the

nanofiber diameters are in the order of a 100nm, the bulging of the nanofibers

due to the inclusion of the GO sheets is highly probable. The formation of the

beads are indicative of low interfacial interaction between GO and the polymer

matrix, which encourages GO to bunch up rather than conform to the shape of

the matrix. It is possible to avoid bead formation only at very low content of GO

(~0.07 % w/w) and by specific functionalization of the GO so as to improve its

compatibility with the polymer matrix. However, since the GO used for this study

was not specifically functionalized, an increasing GO content is likely to lead to a

higher number of beads per unit area.

85


Thermal stability is an important property for PI-based nanocomposites

since they are often used as high-performance engineering plastics. The

presence of inert nanofillers in PI can improve the thermal and mechanical

properties. Figure 6.13 shows the TGA curves of neat PAA, PI, GO and PI/GO

nanofiber composites. The TGA curve of GO indicates weight losses of 17 %, 33

% and 50 % at 150 °C, 200 °C and 350 °C respectively. These have been

respectively attributed to evaporation of absorbed water, the decomposition of

labile oxygen functional groups such as hydroxyl, epoxy and carbonyl and the

removal of the more stable oxygen functionalities (such as phenol, carbonyl,

quinone).

Figure. 6.13: TGA curves of PI/GO nanofier webs with GO contents of 0 % (neat

PI), 0.1 %, 1.0 %, and 2.0 % (by weight) at nitrogen atm.

In TGA curve of neat PAA nanofiber web, there was a significant weight

loss (10.7 %) at 250 °C. This is due to the loss of H2O molecules during the

formation of the imide linkage from amic acid. In the case of the neat PI nanofiber

86

mat, there was a weight loss of only about 3% up to 300°C. These results are

indicative of the conversion PAA to PI. Further, all the PI composite samples

showed a significant increase in thermal stability with increasing GO content,

compared to neat PI. The temperatures for a 10 % weight loss in GO-PI samples

containing 0 %, 0.1 %, 1 % and 2 % GO (by weight) were found to be 503 °C,

527 °C, 543 °C, and 603 °C respectively.

6.2.3 Dynamic mechanical properties

The DMA studies of the electrospun PI nanofiber webs with different GO

contents (0 – 2 % w/w) are shown in Figure. 6.14. The glass transition

temperature (Tg) of the samples as measured from the peaks in the tan δ vs.

temperature curves, increased from 317 °C to 323 °C as the GO content

increased from 0 to 2 % w/w.

Figure. 6.14: DMA curves of PI/GO nanofiber webs with GO contents of 0 %

(neat PI), 0.1 %, 1.0 % and 2.0 % (by weight).

87

The storage modulus (E') values also increased from 1.4 x108 to 3.8x108

Pa for the same samples. This is indicative of some interaction between the PI

matrix and the GO filler. This interaction might be the result of carrying out the

polymerization of the samples in the presence of GO leading to possible

hydrogen bonding between the PI matrix and the functional groups on GO.

Moreover, the beads in the nanofiber mat are likely to provide better interlocking

between the fibers, leading to an overall stiffer material (higher E').

The tan δ values of the composites with different contents of GO are

shown in Table. 6.2. The tan δ values of the composites showed a decrease at

0.1% GO content followed by an increase at 1% and 2% GO content. Similar

trends have been found in the literature as well. The decrease in tan δ at 0.1%

GO can be explained by the increased value of E' upon addition of the GO. The

increase in tan δ at higher GO contents could be the consequence of GO

interrupting the connectivity of the polymer chains and facilitating chain slippage,

leading to a higher loss modulus (E").

Table 6.2 DMA analysis of PI/GO with GO contents of 0 % (neat PI), 0.1 %, 1.0

% and 2.0 % (by weight).

Sample Tg (°C)

Storage Modulus x

108 Pa

Tan δ

Neat PI 317±3 1.4 ±0.23 0.428±0.05

PI-GO (0.1%) 319±2 2.5±0.21 0.395±0.03

PI-GO (1.0%) 320±2 2.8±0.18 0.533±0.06

PI-GO (2.0%) 323±3 3.8±0.24 0.510±0.07


Electrical resistance was measured with a two point probe setup which

consists of two silver probes in voltage range of 0–30 V DC. The electrical

resistance, volume resistivity, and conductivity of PI nanofibers with GO up to 2

% w/w are given in Table. 6.3. In spite of the higher conductivity of graphene

88

oxide as compared to PI, the composites with GO content of up to 2 % w/w did

not show a drastic increase in the electrical conductivity as compared to neat PI

since, the GO remains bunched up in the form of beads and isolated from other

beads in the insulating PI nanofibers.

Table 6.3 Electrical properties of PI/GO nanofiber webs with GO contents of 0 %

(neat PI), 0.1 %, 1.0 % and 2.0 % (by weight).

Sample Resistance (Ω) Volume resistivity

(Ω-m)

Conductivity

(S/m)

Neat PI 1.96±0.023×109 1.29±0.01×1012 7.74±0.03×10-18

PI-GO (0.1%) 1.91±0.15×109 9.75±0.80×1011 1.23±0.01×10-12

PI-GO (1.0%) 1.90±0.13×109 9.72±0.91×1011 1.021±0.15×10-12

PI-GO (2.0%) 1.86±0.37×109 1.90±0.31×1012 6.26±0.95×10-13

6.3 CONCLUSION

Polyimide-Graphene oxide (PI-GO) nanofiber composite mats with up to 2

wt% of GO were made by electrospinning of polyamic acid followed by thermal

imidization. The presence of graphene oxide in the composites and the complete

conversion of poly(amic acid) to polyimide were confirmed by Raman

spectroscopy, XRD and FTIR. The graphene oxide formed bead or spindle-

shaped structures in the nanofibers as observed by HRSEM and TEM. The

composites showed a significant improvement in the storage modulus, glass

transition temperatures and thermal stability with increasing graphene oxide

content. The electrical resistance of the composites was found to be largely

unaltered by the addition of GO up to 2 % w/w in PI.

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7. CARBONIZATION OF POLYIMIDE/f-MWCNTs NANOFIBER

COMPOSITES AND THEIR PROPERTIES

7.1 INTRODUCTION

In recent years, polymer nanofibers are widely produced by

electrospinning technique, with the fibers having diameters in the range of a few

nanometers to several micrometers. The nanofibers are prepared from polymer

solutions or melts, and have large surface area and small pores. Because of

these advantages, polymer nanofiber webs are used in many fields such as

defence, biomedical, textiles, agriculture, and membrane applications. Even

though many methods are used to prepare polymer nanofibers such as melt blow

spinning, solution blow spinning, wet spinning etc., the electrospinning is an

easier method and is cost effective. Loading of carbon nanotube in polyimide

matrix composites has been motivated largely by the increase in electrical

conductivity that occurs due to CNT network formation. We have developed the

polyimide nanofibers with f-SWCNT and graphene oxide using electrospinning

method as shown in previous chapters [122, 123].

In the past decade, many researchers have prepared carbon nanofibers

(CNF) from polymer electrospun nanofibers. The majority of the reports are from

poly acrylo nitrile (PAN). However, the length of the fibers is limited and they are

not useful in bulk material applications except as composites with polymers.

Carbon nanofibers prepared by electrospinning and high temperature treatment

of organic precursors is a simple process and a free standing web is obtained. A

few researchers have developed carbon nanofiber webs from PMDA-ODA

polyimide nanofiber web and studied their morphological and electrical properties

[41]. Some research studies have been reported on flexible polyimide nanofibers

by selecting monomers having hexa-flouro group and iso-propylidene group. In

the previous chapters, we reported the preparation of polyimide film and

90

nanofiber web with improved properties such as tensile strength and flexibility by

the addition of IDDA monomer with PMDA and ODA monomers.

Furthermore, heat treatment is an essential step in the preparation of

carbon nanofibers. Radio-frequency (RF) based induction heating is a highly

energy efficient process that produces high local temperatures. Ramakrishnan et

al. have synthesized multi-walled carbon nanotubes from ethanol by radio-

frequency catalytic chemical vapour deposition method [129]. Alexandru et al

have explained that the utilization of inductive heating in the synthesis of carbon

nanotubes and graphene by catalytic chemical vapour deposition, which can

significantly reduce the energy consumption and the overall reaction time [130].

Richard et al have demonstrated the growth of high quality graphene films using

an inductive heating cold wall reactor, where the RF field directly heats the

catalytic copper substrate [131].

In this study, we have carried out the synthesis of carbon nanofibers

prepared by electrospinning method and thermal treatment in the temperature

range from 300 to 1000 ºC by RF induction heating method under nitrogen

atmosphere. Also, the effect of incorporating f-MWCNT in the PI nanofiber web

and the resultant CNF has been studied. Many studies reported the effect of

addition of nanofillers such as carbon nanotubes, graphene, metal nanoparticles,

etc. on the properties and applications of polyimide nanofibers. Though there are

some investigations on synthesis of CNF from polyimide nanofibers, previous

studies have not focused on reinforcing CNT with CNF. Particularly, the present

study focuses on the effect of f-MWCNTs incorporated in the PI on the electrical

conductivity of CNF. The preparation carbonized PI nanofiber with f-MWCNTs

and the characterization and testing methods are given in chapter 3 (section 3.6

and 3.7).

91


7.2.1 Carbon nanofiber yield

The carbonization yield was 54 % and 51 % for the neat PI nanofiber

webs carbonized at 900 and 1000 °C respectively. The yield for carbonized PI/f-

MWCNTs (1 % w/w) nanofiber web was 55 % and 52 % at 900 and 1000 °C

respectively. The carbon nanofiber yield obtained from PI/f-MWCNTs (2 % w/w)

nanofiber web was 57 % and 55 % at 900 °C and 1000 °C respectively. This

indicates that the degradation of polyimide structure leads to evolution of

hydrogen, oxygen and nitrogen, and their products. The yield difference of 1 to

2 % w/w for PI/f-MWCNT with neat PI carbon nanofiber was due to the presence

of f-MWCNTs in PI nanofiber web.


The FT-IR spectra of neat PAA, PI, PAA/f-MWCNT 1 % w/w, and PI/f-

MWCNT 1 % w/w nanofiber webs are shown in Figure. 7.1. The neat PAA shows

a broad peak in the region of 2500 cm-1 - 3500 cm-1 corresponding to –OH and –

NH2 stretching bands, while C=O in –CONH and –C-NH stretch bands are

observed at 1653 cm-1 and 1541 cm-1 respectively. After thermal treatment of

PAA into PI nanofiber web, new bands were seen at 1784 cm-1 for the C=O

symmetric stretch, at 1723 cm-1 for the C=O asymmetric stretch, at 1376 cm-1

for the C-N stretch and 725 cm-1 for C=O bending and these bands confirm the

formation of imide linkage. Figure. 7.2 shows the FT-IR spectra of neat CNF-PI at

900 °C, neat CNF-PI at 1000 °C, CNF-PI-f-MWCNT 1 % w/w at 900 °C, and

CNF-PI-f-MWCNT 1 % w/w at 1000 °C carbonized nanofiber webs. There were

no characteristic peaks observed, which confirms that predominantly only carbon

atoms are present in the nanofiber web after carbonization.

92

Figure. 7.1: FT-IR Spectra of PAA and PI with f-MWCNT (0 & 1.0 % w/w)

nanofibers.

Figure. 7.2: FT-IR Spectra of CNF/PI-f-MWCNT (0, 1 % w/w) at 900 °C & 1000

°C.

93

Raman spectra of (a) Neat PI, (b) PI/f-MWCNT 1 % w/w, (c) neat CNF/PI

at 900 °C, (d) neat CNF-PI at 1000 °C, (e) CNF/PI-f-MWCNT-1 % w/w at 1000

°C, and (f) CNF/PI-f-MWCNT 2 % w/w at 1000 °C nanofiber webs are shown in

Figure. 7.3 (a-f). The Raman spectrum of carbonized PI had a strong peak

around 1600 cm-1 was assigned to G-band due to sp2 hybridization of ordered

carbon atoms having C-C bond stretching motions. A peak around 1350 cm-1 (D-

band) is due to amorphous carbon atoms arising from defect induced region due

to lack of hexagonal symmetry in carbon structure of the carbon nanofiber web.

Figure. 7.3: Raman spectra of (a) Neat PI, (b) PI/f-MWCNT-1 % w/w, (c) CNF/PI

at 900 °C, (d) CNF/PI at 1000 °C, (e) CNF/PI-f-MWCNT 1 % w/w at 1000 °C, and

(f) CNF/PI-f-MWCNT 2 % w/w at 1000 °C nanofiber webs.

94

The integrated intensity ratio (ID/IG) is an important parameter

characterizing the extent of order of the carbon structure in the material. The

intensity ratios of D-band to the G-band (ID/IG) for carbonized nanofibers were

found to be 0.897, 0.887, 0.884 and 0.867, respectively corresponding to cases

(c) to (f). With increase in carbonization temperature and the addition of f-

MWCNTs to PI the (ID/IG) values decreased due to increase in the ordered

arrangement of carbon atoms. It is of interest to note that though higher

temperature can cause the introduction of more defects, it also appears to

facilitate dehydrogenation, de-oxygenation, and de-nitrogenation, and improves

sp2 hybridization, and increases crystallinity. Overall, in combination with the

FTIR results, it may be inferred that all carbonized nanofibers are seen to be

crystalline with defects arising primarily from thermal treatment.

X-ray diffraction patterns of (a) Neat PI (b) PI/f-MWCNT 1 % w/w at 1000

°C, (c) CNF/PI at 1000 °C, and (d) CNF/PI-f-MWCNT 1 % w/w nanofiber webs

are shown in Figure. 7.4 (a-d).

Figure. 7.4: X-ray diffraction pattern of (a) Neat PI, (b) PI/f-MWCNT-1 % w/w, (c)

neat CNF/PI at 1000 °C, and (d) CNF/PI-f-MWCNT 1 % w/w at 1000 °C.

95

XRD patterns of electrospun PI (Figure 7.3 (a) & (b)) show an intense

peak appeared at 16.4° and 16.1° (2θ) which reflect the (1 0 0) of hexagonal

carbon structure with d-spacing of 5.370 and 5.499 Å. There was a relatively

weak peak observed at 25.8° attributing to the d-spacing of 3.443 Å from (1 1 0)

reflections. A broad peak at 2θ = 24° remained for both CNF/PI and CNF/PI-f-

MWCNT 1 % w/w at 1000 °C and also, a small peak at 2θ = 44° for PI nanofiber

carbonized at 1000 °C indicates the initial development of a graphite-like

structure. This, along with the Raman spectroscopy results, is an indication of

possible graphitization-like process at higher temperatures. Whether the

incorporation of CNT will facilitate graphitization at a lower temperature is a

fascinating question that deserves a separate study.

Figure. 7.5: Scanning electron micrographs of (a) PAA, (b) PI, (c) PAA/1 % w/w

f-MWCNTs, and (d) PI/1 % w/w f-MWCNTs nanofiber web.

96

Figure. 7.5 (a-d) shows the HRSEM micrograph of electrospun PAA and

PI nanofiber webs with f-MWCNT content of 0, 1 % w/w. It can be seen that non-

aligned and uniform nanofibers have been obtained. The diameter of nanofibers

was measured using ImageJ software and was found to be in the range of 50-

100 nm.

Figure. 7.6: Scanning electron micrographs of (a) and (b) Neat CNF, (c) and (d).

CNF PI/1 % w/w f-MWCNT webs at different magnification.

It is seen that there are no beads in the nanofibers and this was achieved

by optimizing electrospinning parameters. The weight loss was higher and the

fibers had more shrinkage in plane and the thickness directions, when the

nanofiber webs were subjected to high temperature for carbonization (up to 1000

97

ºC); this led to the fusion of nanofibers and connections between the fibers are

identified (Figure. 7.6 (a-d)). The fusion of fibers creates percolation path for

electrical conduction through the nanofibers. The CNFs with f-MWCNTs have

more percolation threshold and, therefore, are expected to lead to more

electronic conduction when placed in an electric field.

Figure. 7.7: Transmission electron micrographs of PAA-1 % w/w f-MWCNT

nanofibers at various magnifications (a-d).

HRTEM micrographs of PAA nanofibers with f-MWCNT 1 % w/w at

various magnifications are shown in Figure. 7.7 (a-d). It is clearly observed that

the fiber surface is smooth and no beads are present in the nanofibers. It shows

that the multiwalled carbon nanotubes are dispersed inside the fibers and aligned

98

with the fiber direction. It is likely that acid-functionalized MWCNTs interact with

the polymerizing PAA chains through hydrogen-bonding interactions, which,

along with the shear forces and electrostatic forces during electrospinning,

strengthen the alignment of polymer chains with the nanotubes. This is observed

even at low levels of loading.


The tensile properties of neat PI and PI with f-MWCNT (1 & 2 % w/w)

nanofiber webs are shown in Table. 7.1. It can be observed that there is twice the

value increase in tensile strength and significant improvement in elongation at

break % for f-MWCNTs (2 % w/w) reinforced PI nanofiber compared to neat PI.

This is due to the strong interaction between the –COOH, –OH functional groups

present in MWCNT and polyimide backbone chain. Also, we reported similar

interactions between f-SWCNT and polyimide chain in the previous chapters.

The tensile property of CNF samples are not reported due to higher brittleness of

CNF after carbonization.

Table 7.1 Tensile properties of polyimide/f-MWCNT nanofiber webs

S.No. Sample Tensile strength

(MPa)

Elongation at

break

1. Neat PI 4.88±0.5 76.27±0.4 %

2. PI/f-MWCNT 1 % 6.82±0.4 100.54±0.8 %

3. PI/f-MWCNT 2 % 8.49±0.6 108.60±0.5 %


The electrical conductivity of carbon nanofiber webs were measured using four-

point probe method. First, the resistivity was calculated by the formula 7.1,

ρo = 2πS(V/I) (7.1)

99

where ρo is resistivity, S is the distance between the probes in cm and (V/I) is the

resistance in Ω taken as the average value. The resistivity ρ is calculated using

the formula 7.2, because the prepared carbon nanofibers have the thickness of ~

0.001 cm (valid for sample thicknesses < 0.2 cm).

ρ = ρo / (2S/W loge 2) (7.2)

where (2S/W loge 2) is the correction factor, which is applied for infinitely thin

slice samples, and W is the sample thickness.

Then, the conductivity of carbon nanofiber is calculated by formula 7.3,

σ = 1/ ρ (7.3)

The four-probe measurements carried out at least four times of each sample and

the standard deviation values are also determined. Table 7.2 shows the electrical

conductivity of PI nanofiber and carbonized PI nanofiber web. The conductivity of

PI/f-MWCNT up to 2 % w/w increased by four orders of magnitude compared to

neat PI nanofiber.

Table. 7.2 Electrical conductivity of PI nanofiber and CNF with f-MWCNT

(0, 1 and 2 % w/w).

S.No. Sample name Electrical Conductivity

(σ in S/cm)

1. Neat PI 1.40±0.03 x 10-18

2. Neat PI-1 % MWCNT 2.40±0.05 x 10-14

3. Neat PI-2 % MWCNT 1.68±0.06 x 10-13

4. CNF (Neat PI) 900 ºC 2.21±0.03

5. CNF (Neat PI) 1000 ºC 2.28±0.01

6. CNF (PI-1 %MWCNT) 900 ºC 5.62±0.04

7.

8.

9.

CNF (PI-1 %MWCNT) 1000 ºC



5.73±0.06

16.10±0.05

16.90±0.04

100

The electrical conductivity of CNF PI do not change much when

carbonized at 900°C to 1000°C , i.e 2.21 and 2.28 Scm-1 respectively. It was

also found that the conductivity of CNF PI-1 % f-MWCNT nanofiber webs were

5.62 and 5.73 Scm-1 at 900 °C and 1000 °C respectively. A dramatic increase in

conductivity was observed for CNF PI- 2 % w/w f-MWCNT, nearly eight times

higher value than CNF PI. It is interesting to find that the in-situ polyimide with f-

MWCNT nanofiber web after carbonization shows higher electrical conductivity

(>120 % increase) than seen with neat CNFs. This is due to the alignment of

CNTs in nanofibers, which further facilitate the conduction path for electrons after

the fusion of fibers at high temperature.

7.3 CONCLUSION

Polyimide/f-MWCNT nanofiber composite webs were prepared using

electrospinning method and carbonized the nanofiber webs using radio-

frequency inductive heating, where radio frequency field directly heats the

graphite plates consisting PI nanofiber web inside. The conversion of polyimide

nanofiber to carbon nanofiber was confirmed by FT-IR, Raman spectra and XRD

studies. SEM micrographs of CNF show that fusion of fibers occurred during the

carbonization at 1000 ºC. The tensile strength and elongation at break of PI/f-

MWCNT nanofiber web increased twice that of neat PI nanofiber web and this is

because of the strong interaction between PI and f-MWCNT. The electrical

properties were reported and the dramatic increase in conductivity is due to the

heat treatment temperature and presence of f-MWCNT. In the PI/f-MWCNT (1

and 2 % w/w) carbon nanofiber, the CNT may facilitate the electrical conductivity

by providing a path to transfer of electrons through the fused fibers after the

carbonization and results in significantly higher conductivity than neat polyimide

CNF web.

101

8. CONCLUSIONS

The thesis presents the preparation of polyimide films and nanofiber

composites. The PMDA-ODA polyimide and modified PMDA-ODA-IDDA

polyimide were synthesized. The polyimide films were prepared by solution

casting method and characterized. The thermal stability and tensile strength

decreased for PMDA-ODA- IDDA PI system. However, elongation at break %s

increased, this indicates the flexibility of iso-propylidine bridge in IDDA which

lead to more elongation and less rigidity. This flexible PI can be utilized in high

temperature flexible electronic circuits.

Functionalization of SWCNTs was carried out using acid treatment. The

acid-functionalized SWCNTs were incorporated in PAA nanofibers and were

thermally imidized into PI/f-SWCNT nanofiber webs. The presence of oxygen in f-

SWCNTs and nitrogen in poly(amic acid) were confirmed by XPS studies. The

thermal stability increased from 482 ºC (neat PI) to 585 ºC (PI/f-SWCNT 2 %

w/w) at 10 % w/w decomposition. Storage modulus, tensile strength and

elongation at break of nanofiber webs reached 2.29×108 Pa, 8.84 MPa and 109

%, respectively, for PI/f-SWCNTs (2 % w/w). The electrical conductivity of the

fibers improved effectively from 1.40 ± 0.03 x 10-18 (neat PI) up to 6.17 ± 0.12 x

10-14 (PI/f-SWCNT 2 % w/w) with the incorporation of f-SWCNTs in polyimide.

Graphene oxide was prepared from graphite by modified Hummer’s

method and characterized. PMDA-ODA polyimide nanofiber composites with GO

(0, 0.1, 1, and 2 % w/w) were prepared. The presence of graphene oxide in the

composites and the complete conversion of poly amic acid to polyimide were

confirmed by Raman spectroscopy and FTIR. The graphene oxide cluster is

formed as bead or spindle-shaped structures in the nanofibers as observed by

HRSEM and HRTEM. The composites showed a significant improvement in the

storage modulus, glass transition temperatures, thermal stability, and electrical

conductivity of the nanofiber composites with increasing of GO loading in

polyimide nanofiber.

102

Acid functionalized MWCNTs reinforced polyimide nanofiber webs were

prepared. The polyimide nanofiber webs were carbonized using RF-induction

heating method. The SEM micrographs of CNFs confirm that the fusion of fibers

occurred during the carbonization at 1000 ºC. There was an increase in electrical

conductivity with increase in carbonization temperature and also the loading level

of f-MWCNTs in polyimide. The f-MWCNT may facilitate the electrical

conductivity by providing a path to transfer of electrons through the fused fibers

after the carbonization to result in significantly higher conductivity than neat

PI/CNF web.

To conclude the outcomes of this research work will be highly suitable for

development of polymer nanofiber membrane, polymer electrolyte, flexible

polymer electrode, super capacitor, fuel cell and other potential applications.

103

9. SCOPE FOR FUTURE WORK

Ø Utilization of polyimide films in high temperature flexible electronic

applications

Ø Use of carbon nanotube and graphene oxide reinforced polyimide

nanofiber web in membrane applications

Ø The further research to avoid the agglomeration of graphene oxide during

the electrospinning

Ø Preparation of polyimide composite with reduced graphene oxide

Ø Polyimide nanofiber composite webs as electrolyte membrane in Li-ion

rechargeable batteries

Ø Polyimide nanofiber composite webs as reinforcement in polyimide matrix

to prepare polyimide composite film and also as support material in solar

cells

Ø Use of polyimide nanofiber materials in sensor applications

Ø Carbonized polyimide nanofiber webs as reinforcement in polyimide matrix

to prepare polyimide composites for use in flexible electrode applications

104

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TECHNICAL BIOGRAPHY

Mr. DHAKSHNAMOORTHY M. (RRN. 0982205) was born on 1 3 th June 1982,

in Arakkonam, Tamil Nadu. He did his schooling in Govt. Hr. Sec. School, Minnal

and he secured 71% in the Higher Secondary Examination. He received B.Sc.

degree in Chemistry in the year 2003 from RKM Vivekanandha College, Chennai,

Madras University, Chennai. In the year 2005, he completed M.Sc. in Chemistry

from Sacred Heart College, Thiruvalluvar University, Vellore. Further, he received

M.Tech. in Polymer Science and Engineering from College of Engineering (main

campus), Anna University, Chennai in the year 2008. He worked as Junior

Research Fellow in Indian Plywood Industries Research and Training Institute

(IPIRTI), Bangalore for one year (July 2008 – July 2009). He worked in B. S.

Abdur Rahman University as Research Fellow in a DST nano-mission funded

project (July 2009 – December 2012) and he registered Ph.D in B. S. Abdur

Rahman University in August 2009. He was employed with CARE Group of

Institutions, Tiruchirapalli, as Assistant Professor in Department of Materials

Science and Engineering (January 2013 – February 2015) and converted his

Ph.D Full-Time into Part-Time in January 2013. He is currently working as

Research Scientist in M/s. Z-Strand Pvt. Ltd., Bangalore. His area of research

interests includes (i) preparation of polymer nanofibers by electrospinning

method and characterization, (ii) polymer nano composites, (iii) synthesis of high

performance polymer and thermosetting resins, etc. He has published four

papers in reputed journals and presented five papers in the International

Conferences. The e-mail ID is: [email protected] and the contact

number is: 9962633181.

124

List of publications based on the research work

[1] Development of Flexible Low Dielectric Constant Polyimide Films Based

on Iso-Propylidene, Aryl-Ether Linked Dianhydride/Diamine, Inter. J.

Scien. Eng. Res., Vol. 3, pp. 1-8, 2012. (Impact Factor – 3.2)

[2] In-Situ preparation and Characterization of Acid functionalized Single

walled Carbon nanotubes with Polyimide nanofibers, J. Nanosci.

Nanotechnol., Vol. 14, pp. 5011-5018, 2014. (Impact Factor – 1.556)

[3] Synthesis and Characterization of Graphene Oxide-Polyimide Nanofiber

Composites, RSC Adv., Vol. 4, pp. 9743-9749, 2014. (Impact Factor –

3.84)

[4] Carbonization of Electrospun Polyimide/f-Multiwalled Carbon Nanotubes

Nanofiber Webs by RF-Induction Heating, Nanosci. Nanotechnol. Lett.,

Vol. 7, pp. 521-528, 2015. (Impact Factor – 1.431)

Paper presentation in International conferences

[1] Preparation and properties of electrospun Polyimide / Single walled

carbon nanotubes (SWNTs) composite nanofibres, International

conference (ICOANN-2010) - Alagappa University, Karaikudi, Tamilnadu,

India (1-3 Mar 2010).

[2] Synthesis, Chracterization and properties of soluble and insoluble

polyimides, International conference on Advancements in polymeric

materials (APM-2011) - CIPET, Chennai, Tamilnadu, India (25-27 Mar

2011).

[3] Synthesis and Characterization of Graphene Oxide/Polyimide Nanofiber

Composite, International Conference on Advanced Nanomaterials and

Nanotechnology (ICANN-2011), IIT, Guwahati, Assam, India (08-10 Dec

2011).

125

[4] Preparation and Characterization of Electrospun f-SWCNT/Co-Polyimide

Nanofiber Composites, International conference (MAM-12), KSR group of

institutions, Tamilnadu, India (21-25 Nov 2012).

studies on carbon nanotube and graphene oxide …...abstract the latest quest of polymer...

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