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Pre-coating Single-walled Carbon Nanotubes (SWNTs) for Enhanced Dispersion in Nylon Matrices

Permanent Link: http://ufdc.ufl.edu/UFE0042179/00001

Material Information

Title: Pre-coating Single-walled Carbon Nanotubes (SWNTs) for Enhanced Dispersion in Nylon Matrices
Physical Description: 1 online resource (79 p.)
Language: english
Creator: Lin, Cheng-Ying
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: composite, emulsion, interfacial, nylon, single
Chemical Engineering -- Dissertations, Academic -- UF
Genre: Chemical Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: The main objective of this thesis is to effectively enhance the dispersion of single-walled carbon nanotubes (SWNTs) in Nylon 6, 10 matrices, and improve the properties of Nylon 6, 10 by pre-coating SWNTs before bulk-scale polymerization. A pre-coated Nylon 6, 10 sheath on the nanotube sidewall was created by using emulsion polymerization which performed good dispersion quality and surprisingly maintained the inherent properties of SWNTs. Nylon 6, 10 nanocomposites containing pre-coated SWNTs were prepared by in-situ interfacial polymerization. Raman spectroscopy confirmed the limited aggregation of SWNTs during bulk-scale polymerization and the uniformity of appeared bundles along the as-spun fibers. Furthermore, the mechanical properties of Nylon 6, 10 were 1.5 times improved by incorporating low loading of SWNTs (~0.0025 wt.% in initial suspension). The fracture surface of composite was imaging by scanning electron microscopy (SEM) which shows the dispersion of pre-coated SWNTs in the Nylon 6, 10 matrices. The thermal properties of the composite were also evaluated by differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA) which showing an enhanced thermal stability and higher glass transition temperature with the incorporation of pre-coated SWNTs. Overall, the results suggest that pre-coating process before bulk-scale polymerization can effectively improve the performance properties of Nylon 6,10.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Cheng-Ying Lin.
Thesis: Thesis (M.S.)--University of Florida, 2010.
Local: Adviser: Ziegler, Kirk.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2011-08-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0042179:00001

Permanent Link: http://ufdc.ufl.edu/UFE0042179/00001

Material Information

Title: Pre-coating Single-walled Carbon Nanotubes (SWNTs) for Enhanced Dispersion in Nylon Matrices
Physical Description: 1 online resource (79 p.)
Language: english
Creator: Lin, Cheng-Ying
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: composite, emulsion, interfacial, nylon, single
Chemical Engineering -- Dissertations, Academic -- UF
Genre: Chemical Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: The main objective of this thesis is to effectively enhance the dispersion of single-walled carbon nanotubes (SWNTs) in Nylon 6, 10 matrices, and improve the properties of Nylon 6, 10 by pre-coating SWNTs before bulk-scale polymerization. A pre-coated Nylon 6, 10 sheath on the nanotube sidewall was created by using emulsion polymerization which performed good dispersion quality and surprisingly maintained the inherent properties of SWNTs. Nylon 6, 10 nanocomposites containing pre-coated SWNTs were prepared by in-situ interfacial polymerization. Raman spectroscopy confirmed the limited aggregation of SWNTs during bulk-scale polymerization and the uniformity of appeared bundles along the as-spun fibers. Furthermore, the mechanical properties of Nylon 6, 10 were 1.5 times improved by incorporating low loading of SWNTs (~0.0025 wt.% in initial suspension). The fracture surface of composite was imaging by scanning electron microscopy (SEM) which shows the dispersion of pre-coated SWNTs in the Nylon 6, 10 matrices. The thermal properties of the composite were also evaluated by differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA) which showing an enhanced thermal stability and higher glass transition temperature with the incorporation of pre-coated SWNTs. Overall, the results suggest that pre-coating process before bulk-scale polymerization can effectively improve the performance properties of Nylon 6,10.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Cheng-Ying Lin.
Thesis: Thesis (M.S.)--University of Florida, 2010.
Local: Adviser: Ziegler, Kirk.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2011-08-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0042179:00001


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PRE-COATING SINGLE-WALLED CARBON NANOTUBES (SWNTS) FOR
ENHANCED DISPERSION IN NYLON MATRICES


















By

CHENG-YING LIN


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA

2010
































2010 Cheng-Ying Lin































To my family in Taiwan, who, always support me with love and strength









ACKNOWLEDGMENTS

I would like to sincerely thank my advisor, Professor Kirk J. Ziegler, for his patient

guidance throughout this project and my graduate work. I would also like to express my

appreciation to Dr. Peng Jiang who was supportive as one of my committee members.

Moreover, I thank our group members, Justin Hill, Carlos Silvera-Batista, Fahd

Rajab, and Prakash Sugumaran for giving lots of precious suggestions and assistance

to my research. I am especially indebted to Prakash for the X-ray diffraction (XRD)

measurements. I also want to thank Dr. Brennan and his graduate students, Dave

Jackson, Angle Ejiasi, for assisting me with a lot of characterizations and analyses

during this project. I am very grateful for their generous help and permission of using

their equipment.

Finally, I want to express my gratitude to my family in Taiwan, who have supported

me everything with their love and care. Especially the mentally supports from my sister

Cheng-Chi Lin. Special thanks to my dear boyfriend Tzung-Hua Lin, who has always

given me suggestions on my works, encouraged me when I got depressed, and helped

me on everything I need. I will always be grateful for having them on my side.









TABLE OF CONTENTS

page

A C KNOW LEDG M ENTS ............................ ................... ......................................... 4

LIST O F TA BLES .......... ..... ..... ............................................................. ........ 7

LIS T O F F IG U R E S .................................................................. 8

LIST OF ABBREVIATIONS ......................... ..................... 10

A BST RA C T ............... ... ..... ......................................................... ...... 12

CHAPTER

1 INTR O D U CT IO N ............................................................................................. 14

B a ckg ro u nd ......................... ................ .................. ... ... ............... 15
Single-Walled Carbon Nanotubes (SWNTs) Structure and Properties ............... 16
N a notu be s S tructu re ........................................... ......................... ........... 16
Properties .......... ................................................................ 17
Electrical properties ..................... ............... ....... ................. 17
M echanical properties ......................................................... ................. 18
Surface properties..................... .. ........................ .. .. ............. 18
Processes for Preparing Carbon Nanotubes (CNTs)/Polymer Composites ........... 19
Chemically Modified Carbon Nanotubes ........ ...................... .... ............... 19
Composite Processing .............. .................................... 21
M elt m ixing ....................... ...................... ..... .. .. ...... .......... 21
Solution processing of carbon nanotubes and polymer ........................... 22
In-situ Polymerization............................................. ........... 23
Characterizing Single-W called Carbon Nanotubes............................................... 24
Objectives and Background of Making Nylon 610/SWNTs Composite................... 25
O objective ............... ......... ..... ......... .................... .......... 25
Background ............. ......... .... ...... ...... ............................ 26

2 EXPERIMENTAL METHOD AND CHARACTERIZATION................ ............... 31

Experimental Section .......... ............ ......... ................. ............... 31
SW NTs Suspension Dispersion ........................................ ......... ... ............... 31
Emulsion Polymerization by Swelling Surfactant Micelle................................ 31
Synthesis of the Nylon-6, 10 and Nylon-6, 10/SWNTs Composites ................ 32
Characterization........................ ............ 33
Pre-coating of Nylon 6, 10 ................. .. ... ........ .. .............. ... ...... ..... 33
Nylon-610 and Pre-coated SWNT/Nylon composite................ ............... 34









3 RESULTS AND DISCUSSION ............ .......... .... ................... ................. 37

Pre-coating SWNTs with Nylon 6, 10 by Emulsion Polymerization...................... 37
Bulk in-situ Interfacial Polymerization with Pre-coated SWNTs .......................... 41
Material Properties of Pre-coated SWNT/Nylon 6, 10 Composites...................... 45

4 CONCLUSION AND RECOMMENDATIONS ................ ..... ................. 71

C o nclusio n ........................... .............. ..... ................................. 7 1
R ecom m endation for Future W ork................................................. ... .. ............... 72

LIST O F REFERENCES ........... ................... ........................................... 75

B IO G RA PH IC A L S K ETC H ............. ...................................................... ............... 79









LIST OF TABLES


Table page

3-1 Monitored pH values correspond to each experiment procedures.................. 50

3-2 The monitored pH value with each change to the pre-coating procedure.......... 50

3-3 Aggregation ratio (intensity of aggregation peak to the highest peak in radial
breathing modes (RBMs)) of pre-coated single-walled carbon nanotubes
(SWNTs) suspension changes after mixing with sodium hydroxide and
hexamethylene diamine (HMDA) ............... ......... ................. ........... .... 50

3-4 Mechanical properties of pure Nylon 6, 10 and pre-coated SWNT/Nylon
com posite. .......................... .................................................. 50









LIST OF FIGURES


Figure page

1-1 Scanning Tunneling Microscopic (STM) image of a single-walled carbon
nanotubes (SW NTs).................................................................. .. ........... 28

1-2 The schematically honeycomb lattice structure of graphene sheet ................... 28

1-3 Schemetically showing indexed lattice points on a segment of graphene
sheet.............................................. .......... 29

1-4 The density of states (DOS) of SW NT........... .......... .... ................ 29

1-5 Schematically represent the possible surfactant structures...... ........................ 30

1-6 Mechanism of emulsion polymerization to form a sheath of nylon-6,10 around
the sidewall of SWNTs by incorporate two immiscible solvents...................... 30

2-1 Digital photograph shows the fiber pulling motor with constant pulling speed
(-20 rpm)........................................ .......... 36

2-2 The image of the interfacial polymerization processing of continuously pre-
coated SW NT/Nylon composite fibers......... ................. ................ 36

3-1 The condensation reaction of Nylon 6, 10, which involves sebacoyl chloride
and hexamethylene diamine.................. .......... ..................... 51

3-2 Illustration of the new microenvironment formed around the SWNTs................. 51

3-3 Fluorescence spectra (Ex. = 662 nm ) of sodium dodecylbenzene sulfonate
(SDBS)-coated SWNTs suspension; carbon tetrachloride-swelled
suspension; and sebacoyl chloride dissolved in carbon tetrachloride-swelled
suspension. ............. ..... ............. ........................ ......... ..... 52

3-4 Water decomposes the monomer, sebacoyl chloride, which makes the
solution acidic after mixing with the monomer. ........ ................................. 52

3-5 Fluorescence spectra (Ex. = 662 nm) of the initial SDBS-SWNTs suspension;
sebacoyl chloride dissolved in carbon tetrachloride-swelled suspension; and
SWNT suspension after emulsion polymerization. ............................. 53

3-6 Fourier transform-infrared (FT-IR) spectra of pristine high-pressure carbon
monoxide process (HiPco) SWNTs and Nylon-coated SWNTs....................... 53

3-7 Atomic force microscopy (AFM) images and histograms of the distribution of
SWNT diameters in SDBS-coated; pre-coated SWNT suspensions. ............... 54









3-8 Digital photograph showing the formation of the pre-coated SWNTs/Nylon
thin film at the interface of two immiscible solutions.............. ................ ... 55

3-9 Digital photographs of as-spun pre-coated SWNT/Nylon composite fibers
and as-spun pure Nylon 6, 10 fibers........................... ......... ............ 55

3-10 The effect of reducing surface tension with the surfactant............................... 56

3-11 The effect of hexamethylene diamine (HMDA) and sodium hydroxide on the
fluorescence spectra (Ex. = 662 nm) of pre-coated SWNTs suspensions......... 57

3-12 Normalized Raman spectra (Ex. = 785 nm) shows the effect of the presence
of base on the pre-coated SW NT suspension. ........................................... 58

3-13 Raman spectra (Ex. = 785 nm) of the SWNTs radial breathing modes
(RBMs) along the length of the raw fiber. .............. ..................................... 59

3-14 Raman spectra (Ex. = 785 nm) of the SWNTs along the length of the raw
fiber shows the fluorescence region in high Raman shift region .................. 60

3-15 Normalized Rasonence Raman spectra (Ex. = 785 nm) un-coated and pre-
coated SW NT/Nylon fibers at 30 inches long positions. ................ ............... 61

3-16 Near-infrared (NIR) fluorescence emission spectra (Ex. = 662 nm) of SWNTs
after dissolving the nylon from the pre-coated SWNTs/Nylon composite. ......... 62

3-17 Differential scanning calorimeter (DSC) thermograms of composites and pure
Nylon with a heating rate of 10C/min in N2 during the first, second, and
third heating scan. ................................................ .......... ........... 63

3-18 The X-ray diffraction (XRD) pattern of pure Nylon 6, 10 and pre-coated
SW NT/Nylon com posite film ........... ........................................... ........ .... .. 65

3-19 DSC thermograms of composites and pure Nylon with a cooling rate of
20C/min in N2 during the first, second, and third cooling scan. .................. 67

3-20 Thermogravimetric analysis (TGA) curves of pure nylon and pre-coated
SW NTs/Nylon with a heating rate 10C/m in in N2. ..................... ..................... 67

3-21 Digital photographs of the hot-pressed pre-coated SWNT/Nylon composite
sheet and pure Nylon 6, 10 sheet........................ .......... ................ 68

3-22 Digital photographs of the hot-pressed pre-coated SWNT/Nylon composite
specimen bar (left) and pure Nylon 6, 10 specimen bar (right)............. ........... 68

3-23 Stress-strain curves of samples obtained with 2 mm/min crosshead speed....... 69

3-24 Scanning electron microscopy (SEM) images obtained from the vertical
fra ctu re su rfa ce s : ......... ................................................................................... 7 0











AFM

CNTs

DOS

DSC

Ex.

FT-IR

GPC

HiPco

HPR

HMDA

MWNTs

NIR

Nylon 6, 10

PHAE

PVA

RBMs

S.C.

SDBS

SDS

SEM

STM

SWNTs

TEM


LIST OF ABBREVIATIONS

Atomic force Microscopy

Carbon nanotubes

Density of states

Differential scanning calorimeter

Excitation source

Fourier transform-infrared spectroscopy

Gel permeation chromatography

High-pressure carbon monoxide process

HiPco SWNTs from Rice University

Hexamethylene diamine

Multi-walled carbon nanotubes

Near-infrared

Poly(hexamethylenesebacamide)

Polyhydroxy amino ether

Poly(vinyl alcohol)

Radial breathing modes

Sebacoyl chloride

Sodium dodecylbenzene sulfonate

Sodium dodecyl sulfate

Scanning electron spectroscopy

Scanning Tunneling Microscopy

Single-walled carbon nanotubes

Transmission electron microscopy









TGA Thermogravimetric analysis

XRD X-ray diffraction









Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

PRE-COATING SINGLE-WALLED CARBON NANOTUBES (SWNTS) FOR
EXHANCED DISPERSION IN NYLON MATRICES

By

Cheng-Ying Lin

August 2010

Chair: Kirk J. Ziegler
Major: Chemical Engineering

The main objective of this thesis is to effectively enhance the dispersion of single-

walled carbon nanotubes (SWNTs) in Nylon 6, 10 matrices, and improve the properties

of Nylon 6, 10 by pre-coating SWNTs before bulk-scale polymerization. A pre-coated

Nylon 6, 10 sheath on the nanotube sidewall was created by using emulsion

polymerization which performed good dispersion quality and surprisingly maintained the

inherent properties of SWNTs. Nylon 6, 10 nanocomposites containing pre-coated

SWNTs were prepared by in-situ interfacial polymerization. Raman spectroscopy

confirmed the limited aggregation of SWNTs during bulk-scale polymerization and the

uniformity of appeared bundles along the as-spun fibers.

Furthermore, the mechanical properties of Nylon 6, 10 were 1.5 times improved by

incorporating low loading of SWNTs (~0.0025 wt.% in initial suspension). The fracture

surface of composite was imaging by scanning electron microscopy (SEM) which shows

the dispersion of pre-coated SWNTs in the Nylon 6, 10 matrices. The thermal properties

of the composite were also evaluated by differential scanning calorimeter (DSC) and

thermogravimetric analysis (TGA) which showing an enhanced thermal stability and

higher glass transition temperature with the incorporation of pre-coated SWNTs.









Overall, the results suggest that pre-coating process before bulk-scale polymerization

can effectively improve the performance properties of Nylon 6,10.









CHAPTER 1
INTRODUCTION

Single-walled carbon nanotubes (SWNTs) have attracted great interest because of

their remarkable mechanical, electrical, and thermal properties since their discovery in

1993 by lijima (1). Various applications have been demonstrated based on these

extraordinary properties, ranging from nanocomposite materials, sensors, biomedical

application, and electronic devices to energy storage and generation.

In the field of polymer nanocomposites, carbon black, graphite, clay, and silica are

the most common nanofillers used to improve the properties of the polymer. Carbon

nanotubes (CNTs) are one of the newest nanofillers considered for reinforcing the

properties because of their spectacular characteristics. CNTs can be categorized as

SWNTs or multi-walled carbon nanotubes (MWNTs). CNTs have a very high aspect

ratio (typically ca. 300-1000) which allows an anisotropic alignment (assemble in a

certain direction) in the matrices. Unfortunately, CNTs are relatively insoluble in

common solvents (2). Therefore, SWNTs have high a tendency to agglomerate in

bundles due to the strong van der Waals forces of attraction, which cause dispersion

issues that diminish the performance of the composite (3). The lack of dispersion

prevents adequate interfacial bonding between the CNTs and polymer matrix. These

issues are highly relevant to the aligned quality of nanotubes, which is a crucial

challenge when developing a nanotube/polymer nanocomposite. Therefore, optimizing

the SWNT-polymer interfacial interaction has the potential to enhance the dispersion

quality. The ultimate objective is to maintain the inherent properties of SWNTs (optical,

electrical, and mechanical properties) at the same time.









Background

To improve the native performance properties (strength, toughness, thermal

stability etc.) of polymer materials, fiber-reinforced polymer composites have been

widely used in many structural materials. Typically, the volume ratio, shape, and size of

the filler particles highly influence the properties of polymer nanocomposites. In

conventional polymer composites, fillers with micrometer scale (e.g. glass beads,

calcium carbonate) have been used to enhance the polymer mechanical properties.

Nowadays, nanometer-scale fillers with large aspect ratio have been used to further

improve the mechanical properties due to the tremendously large surface area and

volume ratio.

Over the last decade, carbon nanotubes (CNTs) have drawn significant attention

from researchers due to their extraordinary properties. Their excellent electrical

conductivity and high surface area makes them perfect nanoscale electrodes for

devices and sensors. Other applications include field emission electron source for flat

panel displays, where they have advantages over liquid crystal displays, such as low

power consumption, higher brightness, faster response speed, wider visible angle, and

larger operating temperature range (4). However, due to the complexity in technique

and high costs of the material, this application is still under development.

One of the most exciting applications of SWNTs is in developing novel polymer

nanocomposites. Ajayan et at. were the first to use CNTs as the reinforced nanofiller in

a polymer nanocomposite in 1994 (5). Using nanotubes improved the mechanical

properties as well as the electrical and thermal properties of the nanocomposite

structure. Typically, the properties of CNTs/polymer nanocomposites vary with several

factors, such as the synthetic processing and purification of nanotubes, impurities in the









nanotubes, differences in the distribution of nanotubes (i.e., different (n,m) types,

lengths, diameters), the aggregation state in the polymer matrix (i.e. individual or

bundled), and the orientation of nanotubes in the matrix. In terms of tensile modulus,

there are several examples of CNTs improving the mechanical properties of polymer

matrices, such as Poly(vinyl alcohol) (PVA) films (6), epoxy (7), and polyimide (8).

Despite the significant improvement to performance, CNTs have yet to reach the

theoretical maximum performance. This inability to achieve the desired performance is

often attributed to the lack of adequate dispersion throughout the matrix, as discussed

further below.

Single-Walled Carbon Nanotubes (SWNTs) Structure and Properties

Nanotubes Structure

CNTs are a novel form of carbon, which have been classified to the fullerene

structural family consisting of a wall constructed by hexagonal carbon and

hemispherical buckyball caps at both ends. The atomic structure of SWNTs can be

identified by high-resolution scanning tunneling microscopy (STM), as shown in Figure

1-1 (9). The structure of SWNTs can be visualized as a graphene sheet, which has

been rolled into a tube. The atomic structure of SWNTs is dependent on the roll-up

vector (Ch), which yields different chiral angles (9) or chiralities described by the

following equation:

Ch = nal + ma2

where the index (n,m) represent the number of unit vectors (al,a2) along the crystal

lattice of a graphene sheet, shown in Figure 1-2 (10). The roll-up vector not only

determines the index (n,m), but also the diameter of the nanotubes since the interatomic

spacing is known.









Properties

Electrical properties

The chirality of the SWNTs determines their material properties, especially the

electrical properties. It has been shown that the SWNTs can be either metallic or

semiconducting in nature, which is determined by the n and m difference: I n m | = 3q.

The nanotube is classified as a metallic or semimetallic group if q is an integer, resulting

in approximately 1/3 metallic and 2/3 semiconducting SWNTs in typical samples. The

distribution of types of nanotubes based on different (n,m) indices and chiral angle is

shown in Figure 1-3 (11).

Figure 1-4 (11) shows the density of states (DOS) of a semiconducting SWNT.

The DOS describes the number of states occupied at each energy level, which can be

used to illustrate the optical properties of SWNTs. SWNTs exhibit a quasi-one-

dimensional (1 D) system, which yields sharp van Hove peaks in the DOS. The van

Hove singularities depend on the diameter and chirality of the SWNTs and are similar to

molecular energy levels. As shown in Figure 1-4, when SWNTs absorb energy equal to

or greater than E22, the electron can be excited to the conduction band. The excited

electron will go through nonradiative relaxation to the lowest energy state in the

conduction band before recombining with the hole generated in the valence band. This

causes a radiative fluorescence emission (Ell), which also denotes the band gap of

each semiconducting SWNT. Therefore, semiconducting nanotubes with different (n,m)

indices generate near-infrared (NIR) fluorescence in the 800 to 1600 nm wavelength

range at certain wavelength.









Mechanical properties

The mechanical properties of SWNTs have been studied extensively both

theoretically and experimentally for decades. Since the SWNTs have even better tensile

strength and Young's modulus than other nanofillers, they have become an ideal

candidate for structural applications. Theoretically, Krishnan et al. had predicted a wide

range of the elastic modulus from 0.5 to 5.5 TPa and the tensile strength in the range of

20-200 GPa (12). Experimentally, atomic force microscopy (AFM) has been used for

measuring the stiffness of arc-MWNTs, which had an average Young's modulus of 1.28

TPa (13). This measurement is much harder on SWNTs due to handling difficulties.

Salvetat et al. were the first to show a tensile modulus of ~1 TPa for small diameter

SWNTs ropes (14). Walters et al. further investigated the properties of nanotube

bundles; they obtained a yield strength of 45 7 GPa based on an assumed elastic

modulus of 1.25 TPa (15). In addition to the excellent mechanical properties, the density

of SWNTs is approximately 1.33 1.4 g/cm3 (16). Therefore, SWNTs are a strong,

lightweight material with significant promise in ultra-high performance multifunctional

applications.

Surface properties

Although SWNTs have exceptional properties that make them ideal for composite

structures, it remains difficult to disperse and integrate them into polymer matrices. The

dispersion problem exists because of the strong side-by-side van der Waals attractions

between every single nanotube. These interactions lead SWNTs to aggregate together

and form bundles with diameters that range from 10 100 nm. These ropes have

serious implications to composite structures since SWNTs in the ropes may slide by

shearing along the axis, which diminishes their performance in the composite.









Therefore, enhancing the dispersion of SWNTs or preventing the aggregation during

composite formation is very important to their properties and performance.

Processes for Preparing Carbon Nanotubes (CNTs)/Polymer Composites

SWNTs exhibit extraordinary electronic, thermal, mechanical, and optical

properties. Therefore, nanotubes have provoked interest from fundamental to applied

researchers. Despite the significant effort, it remains difficult to integrate CNTs into

applications because of the substantial van der Waals attraction between nanotubes.

The tendency of CNTs to aggregate presents a serious impediment to the mechanical

properties that can be achieved in composites. In order to get an effective reinforcement

using CNTs, the interaction between the CNT and thermoplastic or thermoset polymer

matrix must be enhanced. Modifying the surface of SWNTs to establish a stronger

chemical affinity (convalently or noncovalently) is required to disperse CNTs individually

and uniformly throughout the polymer. The preferred approach will achieve high

dispersion of CNTs without destroying their integrity to utilize the nanotubes effectively

in composites.

In general, fabricating a nanocomposite with CNTs requires a few preliminary

steps: a) eliminating impurities in CNTs; b) removing bundles to maximize the amount of

individual CNTs; c) chemically modifying the surface of CNTs to maximize dispersion.

There are several developed processes, such as solution processing, melt mixing, bulk

mixing, and in-situ polymerization, for making CNT/polymer composites.

Chemically Modified Carbon Nanotubes

CNTs readily form ropes and bundles because of the strong intrinsic van der

Waals force of attraction and their high aspect ratio. The attractive force is ~ 0.5 eV/nm

of nanotube-to-nanotube contact (17). Most common solvents cannot supply enough









solvation forces to suspend CNTs (2). Therefore, the surface of CNTs is typically

modified to aid dispersion. This procedure can be divided into to two main categories:

covalent and non-covalent modifications. Covalent modification requires a strong

chemical bond or graft between the polymer and CNTs. Depending on the way the

polymer chains are formed, covalent modification can be further divided into "grafting to"

and "grafting from" nanotube methodologies. The "grafting to" approach is when

polymers of a specific molecular weight are reacted onto the sidewall of the nanotube

and the polymer is terminated with a radical precursor or reactive group. After this

functionalization step, the nanotubes are integrated into the matrix by initiating another

polymer reaction. However, the pre-formed polymer chains binding on the CNTs

surfaces sterically hinder diffusion of additional macromolecules on the sidewall. On the

"grafting from" approach, polymers are grown around the surface of the nanotube by

using in-situ polymerization. It also has been called surface-initiated polymerization.

This process is efficient, controllable, and prevents the steric hindrance of

macromolecules; however, this method requires careful control of the initiator

concentration and substrate conditions.

Covalent sidewall functionalization destroys the intrinsic properties of the SWNTs,

such as electrical and optical properties, since the conjugated system is disrupted (18).

Therefore, non-covalent modification may be preferred to keep the structural integrity.

Non-covalent surface modification of SWNTs involves the physical adsorption or direct

coating of polymer to the surface of nanotubes. In some cases, surfactants may assist

the dispersion of SWNTs prior to the adsorption of polymer. Surfactants can disperse

either organic or inorganic particles by non-covalent physical adsorption onto the









surface. Anionic surfactants, such as sodium dodecyl sulfate (SDS) and sodium

dodecylbenzene sulfonate (SDBS) are widely used to get SWNTs suspensions with

high dispersion quality. Nonionic surfactants, such as natural (e.g. Gum Arabic) and

artificial polymers (19), have also been used; the bulky hydrophilic groups provide steric

repulsion but it is not as effective as the electrostatic repulsion mechanism of ionic

surfactants.

The adsorption of surfactants around nanotubes forms different types of

structures, which may affect the luminescent behavior of suspended SWNTs. Figure 1-5

(20) shows the possible schematic representation of SDS surfactant adsorption

structures. Generally, it is preferable to form cylindrical micelles of surfactant.

Hemimicellar adsorption, one of the other possible surfactant structures, was shown by

by Resasco et al. (21) to be sterically and energetically unfavorable. The concentration

of surfactant can also cause different surfactant coating structures (i.e. vary the shape

of micelles).

Typical preparation procedures for producing individually suspended SWNTs in

surfactant solutions involve high-shear homogenization, ultrasonication, and

ultracentrifugation (22). Ultracentrifugation is used to remove bundles from the

suspension but it is a time-consuming, low throughput, and high cost bundle-removal

process. Wang et al. have recently demonstrated a high-efficiency separation approach

called interfacial trapping, which utilizes liquid-liquid interfaces (23).

Composite Processing

Melt mixing

Because of the nature of thermoplastic polymers, which can be softened when

heated at certain temperature, melt mixing is desirable technique for blending









nanofillers into polymers that cannot be processed with solution processing.

Furthermore, some traditional processing techniques, such as injection molding, blow

extrusion, and blow molding can be used when melt-mixing CNTs into the thermoplastic

polymers. These processes are simple, fast, and widely used in the plastic industry (24,

25).

In general, melt mixing involves melting polymers into viscous melts. Additives,

such as CNT materials, can be blended with the viscous liquid by the application of high

shear mixing. The sample can be fabricated by developed techniques, such as

extrusion, compression molding, or injection molding, based on the desired morphology

or shape of the composite.

By using high-shear mixing or increasing the processing period, CNT dispersion

can be enhanced. However, the melts' viscosity is much higher for nanofibers than

processing large-scale fibers; therefore, the viscosity of nanotubes/polymer should be

determined before using this processing technique. The melting properties can be

highly affected by adding nanotubes, resulting in potential polymer degradation under

high shear rates. Therefore, the processing conditions should be carefully monitored.

Solution processing of carbon nanotubes and polymer

Solution-based processing is the most common method for preparing carbon

nanotube-polymer composites. It has the advantage of low viscosity since the

nanotubes are mixed with the polymer in a chosen solvent. Typically, solution-

processing procedures are different but still follow a general protocol. First, the

nanotube powder is dispersed in a liquid medium (either solvent or polymer solution) by

vigorous agitation and sonication to deagglomerate the nanotubes. The nanotubes are

then mixed with the targeted polymer by energetic agitation. In general, high shear









mixing, reflux, and magnetic stirring are the most commonly used agitation methods.

High power tip, cup-horn, or mild bath sonication are usually used to provide sonication.

The solvent is then evaporated with or without vacuum to form a composite film.

In contrast to melt mixing, solution processing can either work on thermosetting or

thermoplastic polymer matrices. An early example of a solution based approach to CNT

composites used thermosetting epoxy matrices, as described by Ajayan et al. (5).

Concerning the formation of carbon nanotube-thermoplastic matrices using solution

processing method, Jin et al. (26) have reported an approach using polyhydroxy amino

ether (PHAE) as the matrix.

The dispersion quality of nanotubes in the chosen solvent is very important in this

process. However, the low solubility of nanotubes in most common solvents limits the

application of this approach. Surfactants, commonly SDS, have been introduced before

mixing the CNTs with the polymer to solve this problem (27-29). Despite the added

dispersant, reagglomeration of the nanotubes within the film can occur during the

casting/evaporation process. Du et al. proposed an approach that utilized the

coagulation mechanism to prevent forming bundles (30).

In-situ Polymerization

In-situ polymerization is one of the most effective processes for polymer

nanocomposite fabrication. This is a method for enhancing integration and dispersion of

nanoparticles between phases (31). Similar to solution-based process, improved

dispersion of nanotubes in the initial solution (i.e. monomer and solvent) and in the

composite would aid performance. However, in-situ polymerization and solution

processing may have contamination problems due to the use of solvents.









The fabrication involves dispersing nanotubes in the monomer followed by

polymerizing the monomer. Generally, the main advantage is the capability to obtain

molecular-scale reinforcement due to the small size of monomeric molecules.

Combined with the preliminary production of polymer-grafted nanotubes that can have

better affinity with polymer chains, the uniformity of final composite adducts will be

higher than directly mixing nanotubes and polymers in solution. These processes also

allow higher loading of CNTs. This method has been used with conductive polymers,

such as polypyrrole (32), and also can be used on aligned CNT arrays to grow

nanotube/ polymer coaxial wires (33). Typically, epoxy nanocomposite is one of the

major materials being studied by in-situ polymerization.

Furthermore, in-situ polymerization process allows covalent bonds to be formed

between functionalized nanotubes and the polymer matrix using condensation

reactions. A non-covalent bonding between the nanotubes and the polymer matrix in a

nanocomposite is also possible through in-situ polymerization. Fan et al. (34) reported a

study of in-situ radical polymerization of pyrrole. They used spectroscopic

characterization to conclude non-covalent bonding between the CNTs and polymer

chains. These non-covalent interactions can maintain the inherent properties of CNTs,

such as the fluorescence emission and mechanical properties. Maintaining the inherent

properties of the CNTs is also used in this thesis.

Characterizing Single-Walled Carbon Nanotubes

NIR fluorescence (11, 22, 35), Raman (36), and optical absorbance spectroscopy

(37) are the most common measurement techniques to analyze the dispersion quality of

SWNTs suspension. NIR fluorescence can identify the individually suspended SWNTs

based on the uniqueness of fluorescent emission from individual semiconducting









SWNTs (22). The concentration of SWNTs suspension can be obtained from the

absorbance spectra (38, 39). Raman spectroscopy of CNTs has two major peaks, which

are the radial breathing modes (RBMs) at low frequency and the tangential multi-feature

(G-band) at high frequency. The energy of the vibration mode in RBMs depends on the

diameter of SWNTs, so a general diameter distribution of SWNTs can be obtained

during the analysis of RBMs. There is also a peak in the RBMs that represents the

quantity of aggregation (40) since the phonon modes of nanotubes are easily disturbed

by the environment.

As mentioned earlier, NIR fluorescence is capable of determining the Ell

transitions of semiconducting SWNTs. It is important to note that SWNT bundles suffer

a disappearance of the NIR fluorescence since the presence of metallic SWNTs in the

bundles quench the luminescence from adjacent semiconducting nanotubes (22).

Likewise, different solvents and surfactants (structure too) affect the optical spectra of

SWNTs. Each solvent generates a specific adsorption and emission spectra, and such

behavior is called solvatochromic effects typically characterized by the solvatochromic

shift of the spectra. Increasing the dielectric constant (K) of the surrounding media,

tends to shift the optical transition energy of SWNTs to lower energies (19, 41). The

solvatochromic shift helps us understand changes to the environment surrounding

SWNTs during processing.

Objectives and Background of Making Nylon 610/SWNTs Composite

Objective

While SWNTs have been widely applied in making innovative nanocomposites,

dispersing individual SWNTs in the matrices remains a challenge. Many researchers

have applied functionalized CNTs to get better dispersion by enhancing the interaction









with the polymer matrix (42-44). However, the generation of covalent bonds diminishes

the advantageous electrical, optical, and thermal properties of the SWNTs and may

even reduce the mechanical properties of SWNTs (3, 45). Therefore, surfactant-aided

dispersion is preferred in this research.

Polyamide is an important thermoplastic polymer in industry and has been

investigated in making nanotube/polyamide nanocomposites (MWNTs are typically

used) (46-48). This study applies a simple and fast processing route to incorporate

SWNTs into polymer matrix instead of MWNTs due to the better mechanical properties

of SWNTs. Poly(hexamethylenesebacamide) (Nylon 6,10) is the chosen polymer matrix

since it is easily synthesized via in-situ interfacial polymerization.

Background

This thesis aims to enhance the dispersion of SWNTs by controlling the interfacial

properties of SWNTs. In order to effectively incorporate SWNTs into the polymer matrix,

a chemical procedure is necessary to modify the nanotubes so that they have higher

affinity with the polymer matrix. A non-covalent bonding modification has been chosen

to increase the dispersion and binding with the matrix so that the inherent electrical,

optical and even mechanical properties of SWNTs can be maintained. Wang et al. has

shown that mixing some water-immiscible organic solvents with surfactant-SWNT

suspensions forms an emulsion-like environment that encapsulates SWNTs. The

hydrophobic region of the surfactant, which surrounds the SWNTs, is swelled by the

organic solvent (49). To take advantage of this microenvironment, the organic solvent

can be the media to build non-covalently modified SWNTs. Chen et al. has described

the in-situ emulsion polymerization of nylon-6, 10 by using the swelling mechanism of

surfactant structure of SWNTs. On the basis of the interfacial polymerization and









swelling behavior, two immiscible solvents should be used to generate a thin layer of

polymer around the sidewall of SWNTs. Figure 1-6 shows the reaction scheme for this

emulsion polymerization (50). In this thesis, the non-covalently pre-coated SWNTs are

integrated into a bulk Nylon-6, 10 composite. This approach provides steric hindrance to

aggregation of SWNTs during the bulk polymerization. Continuous fibers can be spun

from the interface of two immiscible phases, which is described further in Chapter 3.





























Figure 1-1. Scanning Tunneling Microscopic (STM) image of a SWNT (9).


\Armchair
\^s


ZigeZag


a,

/2,


ma ,
le


na,


Figure 1-2. The schematically honeycomb lattice structure of graphene sheet. Showing
the chiral angle (9) and rolled-up vectors (Ch) (10).






























Figure 1-3. Schemetically showing indexed lattice points on a segment of graphene
sheet. Nanotubes designated (n,m) are obtained by rolling the sheet along a
roll-up vector, which omit the metallic nanotubes. The chiral angle a is ranged
from 0 to 30 (11).


t J E. E22,,
S0 fluorescence absorption
1 VV2

-2 V2

-3
valence
-4

-5
0 2 4 6 8 10
Density of Electronic States


Figure 1-4. The density of states (DOS) of SWNT. Solid lines represent the electron
transitions between energy levels; dashed lines are nonradiative relaxations
(11).



















Figure 1-5. Schematically represent the possible surfactant structures a) cylindrical
surfactant micelles encapsulate SWNTs; b) Hemimicellar adsorption
surfactant structure on a SWNT; c) Random arrangement of surfactant
molecules on a SWNT (20).


Figure 1-6. Mechanism of emulsion polymerization to form a sheath of nylon-6,10
around the sidewall of SWNTs by incorporate two immiscible solvents (50).


Iv,


4MO;~









CHAPTER 2
EXPERIMENTAL METHOD AND CHARACTERIZATION

Experimental Section

Single-walled Carbon Nanotubes (SWNTs) Suspension Dispersion

Aqueous single-walled carbon nanotubes (SWNTs) suspensions were prepared

with a given initial mass (~40 mg) of raw SWNTs (Rice HiPco SWNTs from Rice

University (HPR) 177.1) and suspended in 200 mL of aqueous sodium dodecylbenzene

sulfonate (SDBS) (Sigma-Aldrich) surfactant solution (1 wt.%). High-shear

homogenization (IKA T-25 Ultra-Turrax) at 10,000 rpm for ~1.5 hr and ultrasonication

(Misonix S3000) with 90% amplitude for 10 min were used for aiding dispersion. After

ultrasonication, the SWNTs suspension was ultracentrifuged at 20,000 rpm (Beckman

Coulter Optima L-90 K) for 3 hr to remove the SWNTs bundles.

Emulsion Polymerization by Swelling Surfactant Micelle

5 pL sebacoyl chloride (Fluka, >95.0%) was added to 5 mL carbon tetrachloride

(Sigma-Aldrich, 99%) to form a 0.005 M sebacoyl chloride solution. To swell the SWNTs

with surfactant structure by sebacoyl chloride solution, 5 mL of the aqueous SDBS-

SWNTs suspension was added gently to 5 mL of sebacoyl chloride solution. Such two-

phase mixture was shaken by Vortex mixer at 1,500 rpm for 30 sec to form the solvent-

swelled microenvironments around SWNTs. The aqueous solvent-coated SWNTs

suspension was removed carefully from the bulk carbon tetrachloride after 1-1.5 hr

settling for stable phase separation to prevent further emulsification. Hexamethylene

diamine (HMDA) was liquefied by heating to about 600C. Liquid monomer

hexamethylene diamine (~0.002 mL) was then directly added to the solvent-coated

aqueous SWNTs suspension. An obvious color change of polymerized SWNTs









suspension to bluish-grey can be observed right after adding the second monomer

which implies the formation of nylon-6, 10.

Synthesis of the Nylon-6, 10 and Nylon-6, 10/SWNTs Composites

The nylon 610/SWNTs composite was synthesized by in-situ polymerization with

two immiscible solutions. The top organic solution of the interfacial polymerization

reaction contained the diacid chloride (sebacoyl chloride (>95%, Fluka)). 0.4 mL of

sebacoyl chloride was dissolved in 5 mL hexane formed a ~ 0.4M sebacoyl chloride

solution. The aqueous phase contained the diamine (hexamethylene diamine (98%,

Aldrich)). 0.4 mL of 1, 6-hexamethylene diamine was again liquefied by heating to ~

60C before adding to 5 mL of the nylon-6,10 pre-coating SDBS-SWNTs suspension in

a 20 mL beaker. The aqueous to organic phase volume ratio was optimized by

observing a visually low carbon nanotube aggregation and a relative high yield of fibers.

The organic sebacoyl chloride solution then was gently poured on the top of the

monomer added SWNTs suspension. A thin film was formed at the interface of two

immiscible solutions immediately due to the condensation reaction. The thin film was

carefully took out with tweezers, and mounted on a motored glass rod with a ~ 20 rpm

rolling speed, as shown in Figure 2-1. The nylon-6, 10/SWNTs composite products was

continuously formed a fine thread like fiber due to the high reaction rate, as shown in

Figure 2-2. The collected nylon-6,10/SWNTs fiber was washed with acetone (Fisher)

and followed with distilled water several times to remove the residual unreacted

monomers and solvents, then such rinsed fiber was then soaked in 200 mL distilled

water for at least 4 hours to remove the surfactant and other impurities. The sample was

desiccated in a vacuum oven under reduced pressure (~50 inHg) at 70-80C for 12-24

hours.









Characterization


Pre-coating of Nylon 6, 10

The Near infrared (NIR)-fluorescence from the individually suspended

semiconducting SWNTs of all the aqueous SWNTs suspensions were characterized

with the Applied NanoFluorescence Nanospectrolyzer (Huston, TX) with excitation from

662 and 784 nm. The different degree of aggregation in SDBS-coated SWNTs

suspensions after emulsion polymerization had been characterized with a liquid probe

by a Renishaw Invia Bio Raman with a 785 nm diode laser source. All the Raman

spectra were normalized to the G-band (1590 cm-1) after the baseline correction.

The distribution of nanotubes diameters was collected from tapping mode atomic

force microscopy (AFM) images on a Digital Instruments Dimension 3100. The SDBS-

coated SWNTs and Nylon-coated SWNTs suspensions were spin-coated onto fresh

mica and gently rinsed with de-ionized water to removed excess surfactant to get the

images. The average diameters of SDBS-coated and Nylon-coated SWNTs were

determined from 10 AFM images of each sample with the NanoScope v5.30r1 software.

The histogram recorded at least 125 SWNTs for each sample showing the distributions

of the nanotubes diameters.

The presence of Nylon by emulsion polymerization was characterized by Fourier

transform-infrared (FT-IR) spectroscopy (Nicolet MAGNA 760 FTIR). The surfactant

was removed prior the FT-IR analysis by adding 5 mL ethyl acetate to the Nylon-coated

SWNTs suspension. The mixture was then shaken with a Vortex mixer at 2,000 rpm for

30 sec, and settled for phase separation. The Nylon-coated SWNTs suspension was

then taken out and freeze dried (LABCONCO Freeze Dryer 8) overnight, and a gray

powder of the Nylon-coated SWNTs was ready to analyze.









Nylon-610 and Pre-coated SWNT/Nylon composite

The thermal properties of Nylon-610 and pre-coated SWNT/Nylon composite were

characterized by differential scanning calorimeter (DSC) (Seiko DSC6200 instrument).

Each sample was heated from 30 to 260C with 10C/min heating rate, held at 260C

for 1 min, and cooled at the specified cooling rate (20C/min) to room temperature. The

thermal properties of samples were monitored under this heat treatment for 3 cycles.

The glass melting temperature and glass transition temperature were calculated from

the endothermic peaks and steps in the DSC baseline, respectively. The thermal

decomposed temperature was determined with the TGA. The samples were heated

from 30 to 800C withlO0C/min heating rate in nitrogen atmospheres. The weight loss

was recorded and normalized to the initial weight of each sample. Analysis was

performed on the resulting weight (%) vs. temperature curves.

The crystallinity of fibers was monitored by both DSC and X-ray diffraction (XRD).

The XRD patterns were recorded by using APD 3720 diffractometer operating at a

voltage of 40kV and a current of 25A using Cu Ka radiation. The scanning step size is

0.020 in 2 sec per step.

The degree of aggregation of pre-coated SWNTs in the composites was monitored

by Raman spectra with a 785 nm laser diode source. The samples were taken along in

different positions and pressed between two microscope slides. The pre-coated

SWNT/Nylon composite was dissolved in formic acid with the aid of bath-sonicated for

measuring the photoluminescence of nanotubes by Applied NanoFluorescence

Nanospectrolyzer.

The tensile properties of the pre-coated SWNT/Nylon composite and Nylon-610

were determined by a universal test machine (Model no. 1122, Instron, USA) at a cross-









head speed of 2 mm/min. Sheets of the composite for the mechanical testing were

fabricated by hot pressing (Lab Press, CARVER) at above melting temperature (~230

C) in atmosphere and followed by heating samples in tube furnace at 220C in vacuum

condition to get rid of bubbles. A second-step hot press was performed at ~230C under

a pressure of 250 psi to mold the samples to sheet type specimen before testing. The

cooling process during above heat treatments was slow cooling in atmosphere.

The surface morphology was observed using a scanning electron microscopy

(JEOL6335F FEG-SEM) at an accelerating voltage of 15 kV. The samples were

collected after the tensile test, and then coated with platinum.



























Figure 2-1. Digital photograph shows the fiber pulling motor with constant pulling speed
(-20 rpm).


Figure 2-2. The image of the interfacial polymerization processing of continuously pre-
coated SWNT/Nylon composite fibers









CHAPTER 3
RESULTS AND DISCUSSION

The condensation reaction in synthesizing Nylon 6, 10, which involves the

monomers sebacoyl chloride and hexamethylene diamine (HMDA), is rapid with

hydrochloric acid being a by-product, as shown in Figure 3-1. Sebacoyl chloride and

HMDA selectively dissolve in organic and aqueous solvents, respectively. Therefore,

the polymerization reaction only occurs at the interface of the liquid phases, which is

known as interfacial polymerization. This thesis focuses on spinning Nylon 6, 10

composite fibers through interfacial polymerization. However, the process is split into

two steps to improve the dispersion and interaction of the single-walled carbon

nanotubes (SWNTs) with the nylon matrix. Recently, it was demonstrated that mixing

aqueous SWNTs suspensions with immiscible organic solvents can change the

environment around the nanotubes into an emulsion-like microenvironment around

SWNTs, as shown in Figure 3-2 (49, 50). These new solvent-swelled structures enable

new functionalization schemes on the sidewalls of nanotubes. Taking advantage of

interfacial polymerization and swelling the surfactant structure around the sidewall of

nanotubes, a sheath of Nylon 6, 10 around SWNTs can be formed as the first step (50).

This step was developed previously but will be reviewed in Section 3.1 because of its

importance in integrating the pre-coated SWNTs into the composite fibers in the second

step of the process.

Pre-coating SWNTs with Nylon 6, 10 by Emulsion Polymerization

The Near infrared (NIR) fluorescence from individually-suspended semiconducting

nanotubes is sensitive to the environment around the nanotubes. The solvent-swelled

SWNTs maintain the fluorescent properties, although the peak positions will be shifted









due to solvatochromic effects (49, 51). Carbon tetrachloride forms these solvent

microenvironments in sodium dodecylbenzene sulfonate (SDBS)-SWNT suspensions

and was chosen to be the media for this reaction since sebacoyl chloride has high

solubility in it. Figure 3-3 shows the well-resolved fluorescence spectra characteristic of

the SDBS-SWNT suspension in different environments. The fluorescence emission of

the suspension shows slight blue-shifts and an increase in intensity for nanotubes with

larger diameters after mixing with carbon tetrachloride.

These spectral changes are associated with the different microenvironment

around SWNTs (49, 50). The blue shifts of the fluorescence peaks imply that the

nanotubes are in a less polar and homogeneous environment, as shown in Figure 3-2

(51). The increase of the intensity may be because of the polarity change or

reorganization of the surfactant structure that minimizes the quenching effect (49, 50,

52). As expected, the peak positions of the suspension after mixing with sebacoyl

chloride solutions match the peak positions of the solvent-swelled suspension. Such

identical fluorescence peak positions indicate similar polarity around the nanotubes due

to the same microenvironment (i.e., CC14). However, mixing a 0.005M sebacoyl chloride

in carbon tetrachloride with the SDBS-SWNTs suspension yields a drastic decrease in

fluorescence intensity for every SWNT (n, m) types when compared with either the

initial suspension or the solvent-swelled suspension. Nanotubes with larger diameters

show more noticeable decreases in intensity, namely more sensitive response with the

change of the microenvironment. Some researchers have also found the higher

sensitivity of large diameter SWNTs in quenching mechanism (52-54).









The decrease in the fluorescence intensity could be the presence of the sebacoyl

chloride within the emulsion-like phase surround SWNTs. Table 3-1 shows the

monitored pH value at every step in the coating process. The pH value after mixing with

the 0.005M sebacoyl chloride solutions changed to 2.14, which is considerably more

acidic than both the initial and solvent-swelled suspensions. Sebacoyl chloride can

slightly decompose with the absorption of water and forms strong acids, as the water-

decomposition reaction in Figure 3-4 shows. It has been demonstrated that acidic

environments can quench the SWNTs fluorescence intensity because of the protons

(55-58). Therefore, the acidic environment and the doping effect (50, 59-61) may be the

reasons for the drastic decrease of the intensity. Regardless, these results indirectly

confirm the presence of sebacoyl chloride surrounding the SWNTs.

The decreased fluorescence intensity with the presence of sebacoyl chloride

reversed once adding the water-soluble monomer HMDA. Figure 3-5 shows a

noticeable increase in intensity after mixing with the HMDA no significant increase in pH

(~2.89). The spectra also show slight red shifts in peak position, which indicates a

change to the microenvironment around the nanotubes. Once again, the SWNTs with

larger diameters have higher sensitivity to the new environment. These changes to the

spectral properties imply the consumption of sebacoyl chloride during the condensation

reaction with HMDA (see Figure 3-1) at the SWNT interface rather than fewer protons

present in the suspension. This might be expected since the by-product from the water-

decomposed reaction of sebacoyl chloride would not be involved in the emulsion

polymerization reaction. Similar fluorescence spectral changes are also observed after

mixing the solvent and the polymerization with the excitation source at 784 nm.









Pure Nylon 6, 10 synthesized via interfacial polymerization forms a white powder

with characteristic Fourier transform-infrared spectroscopy (FT-IR) stretches of the

amide-l peak (C=O stretching) at 1640 cm-1, the amide-ll peak (N-H bending) at 1545

cm-1, the C-H stretching at 2860 and 2940 cm-1, and the N-H stretching at 3330 cm-

(50, 62). To minimize the signal from the excess surfactant, ethyl acetate was used to

remove the surfactant in the Nylon-coated SWNTs suspension. The suspension was

then solidified with freeze-drying, forming a homogenous gray powder. Figure 3-6

shows the FT-IR spectra of pristine SWNTs and nylon-coated SWNTs. The FT-IR

spectrum of the Nylon-coated SWNTs shows amide-I, amide-ll, and N-H stretching

groups at 1637, 1569, and 3338 cm-1, respectively. These indicate the existence of

Nylon 6, 10 via the emulsion polymerization in the Nylon-coated SWNTs when

compared with the pristine SWNTs (50).

A tapping mode atomic force microscopy (AFM) image in Figure 3-7a illustrates

the morphology of suspended SDBS-coated SWNTs. Individually suspended SWNTs

are clearly observed throughout the mica substrate. The histogram in Figure 3-8a

shows an average diameter of 1.2 nm, which is reasonable for high-pressure carbon

monoxide process (HiPco) SWNTs coated with a surfactant. Figure 3-7b shows the

Nylon 6, 10-coated SWNTs suspension after the emulsion polymerization as-deposited

mica substrate. Some individual SWNTs can still be observed after the emulsion

polymerization; however, the morphology around SWNTs has noticeable changes when

compared with the initial suspension. The diameter distribution of the Nylon 6, 10-

coated SWNTs in the histogram in Figure 3-7b becomes broader and has a larger

deviation from the average diameter (7.3 nm). Moreover, the significant increase in the









average diameter after the polymerization implies the coating of Nylon 6, 10 around

individual SWNTs.

Bulk in-situ Interfacial Polymerization with Pre-coated SWNTs

The in-situ interfacial polymerization reaction to form Nylon 6, 10 involves two

immiscible solutions, namely, an aqueous phase containing HMDA and an organic

solution containing sebacoyl chloride. The composite is formed by introducing SWNTs

pre-coated with Nylon to the aqueous suspension. A thin film of the polymer composite

forms at the water-oil interface immediately, as shown in Figure 3-8. A clear distinction

can be seen between the neat Nylon 6, 10 and pre-coated SWNT/Nylon composite with

the naked eye based on the different color, as shown in Figure 3-9, even with a dilute

pre-coated SWNTs suspension (~20 mg/L). The concentration ratio ([sebacoyl cholride]

: [HMDA] ~ 1.5) of monomers was determined by observing a visually low carbon

nanotube aggregations and a relative high yield of fibers.

One of the major roles of surfactants is to reduce the surface tension of solutions;

however, the reduction in surface tension can be problematic to the spinning of fiber

resulting in discontinuous fibers ,as shown in Figure 3-10a. Therefore, the pre-coated

SWNTs suspension should be below the organic phase, which will help to strip the

surfactant from the fiber as it forms. Hexane was chosen as the organic solvent for the

bulk polymerization reaction rather than carbon tetrachloride because of the lower

density than the pre-coated SDBS-SWNTs suspension and high solubility of sebacoyl

chloride. This system was able to stabilize the surface of as-spun pre-coated

SWNTs/Nylon 6, 10 composite fibers. (Figure 3-10b)

When synthesizing Nylon 6, 10 by in-situ interfacial polymerization, sodium

hydroxide (less than 1M) is typically added to neutralize the byproduct (hydrochloric









acid) of the condensation reaction. However, it is well known that the pH of the solution

has a significant impact on the absorption and fluorescence of SWNTs suspended with

ionic surfactants. SDBS has been demonstrated to be an effective surfactant to

suspend HiPco carbon nanotubes (63) with good resolution of the spectral features and

high concentrations of SWNTs (19). Tan et al. (64) reported that the SWNTs have good

dispersion in SDBS solution in basic conditions up to pH=13 (64). However, the

SDBS/nylon-coated SWNT suspension in this study became milky after adding ~70 mg

sodium hydroxide, which caused visible aggregation of the nanotubes. The mixture

became clear again after a small amount of HMDA was added, although the

aggregation of the SWNTs was irreversible. A 1 wt.% solution of only SDBS was taken

as a reference and similar behavior was observed, which implies the effect could be

associated with the interaction of SDBS and sodium hydroxide. Interestingly, this does

not have negative effects on pure SDBS-SWNT suspensions. Figure 3-11 shows the

fluorescence emission of SWNTs excited at 662 nm after adding the sodium hydroxide

and HMDA. The spectra show a decrease in intensity and a slight blue shift in peak

positions (see (7,5) and (8,3) located at 1026 and 956 nm, respectively) after adding

only HMDA or sodium hydroxide, especially for the larger diameter SWNTs. The

decrease in intensity is more significant for the addition of base to the pre-coated SWNT

suspension. Interestingly, the fluorescence intensity for smaller diameter SWNTs

increases and the peak positions shift back after adding both to the suspension. Table

3-2 lists the monitored pH value in these different environments. The pH gets slightly

higher (pH=13.07) with the presence of HMDA, which returned the milky suspension

back to a clear suspension. The drastic decrease in fluorescence intensity suggests that









both HMDA and high pH may destabilize the coating that enables dispersion of the pre-

coated SWNTs in the aqueous phase.

Figure 3-12 shows the resonance Raman spectra for these suspensions. The

broad and intense peaks in the spectra are fluorescence from the SWNTs. Once again,

the fluorescence nearly disappears after adding the sodium hydroxide. The radial

breathing modes (RBMs) peaks after adding NaOH show a slight broadening of the

spectra, indicating that the resonant frequency associated with the radial expansion of

the nanotubes has changed. This could indicate that high pH disrupts the coating layer

around the pre-coated SWNTs. After adding the HMDA, the intensity of the RBMs

decreases but the aggregation ratio, which is the ratio of the intensity of the so called

aggregation peak (-270 cm-1) (40) and the highest intensity in RBM region, as shown in

Table 3-3. The aggregation ratios between the initial suspension and the pre-coated

suspension are similar which implies the sustained dispersion quality after emulsion

polymerization. However, the aggregation ratio is nearly two times higher than the initial

suspension after adding NaOH.

Solid state resonance Raman spectroscopy monitored the dispersion of SWNTs

along the as-spun composite fibers. The dimension of the as-spun fiber is about 1.5 mm

in diameter and 4 m in length. The raw fiber was cut into pieces to characterize the

surface by Raman spectroscopy. The aggregation peak at ~270 cm-1 in the RBMs of the

Raman spectra was used to characterize the aggregations status in the fiber, as shown

in Figure 3-13. Interestingly, the intensity of the aggregation peaks decrease with the

length of the fibers. This indicates that the uniformity of pre-coated SWNTs in the Nylon

6, 10 matrix is maintained for approximately 100 inches. Furthermore, the fluorescence









was also observed in the Raman spectra, which indicates the SWNTs were not likely

aggregated but dispersed individually in the Nylon matrix, as shown in Figure 3-14. As

discussed above, the aggregation that occurs after prolonged spinning may be due to

the formation of hydrochloric acid during the condensation reaction.

An SDBS-suspension without pre-coating process was used as the aqueous

solution directly in interfacial polymerization to compare to the pre-coated SWNT fibers.

Figure 3-15 shows the Raman spectra of both un-coated and pre-coated SWNT/Nylon

fibers at identical positions along the fiber (~30 inches). The inset graph displays the

RBMs region, which clearly shows the discernable aggregation peak in the fiber with un-

coated SWNTs. Although the pre-coated fiber shows higher intensity of the aggregation

peak which may due to the differentiate nanotubes concentrations, the broader and

indistinct peak may imply less aggregation in pre-coated SWNT/Nylon 610 composite.

The presence of the D-band usually means the formation of defects by functionalizing

the carbon nanotubes; however, the spectra show no indication of covalently

functionalization of SWNTs during the process for either the un-coated or pre-coated

composites. The Nylon 6, 10 pre-coated around the nanotubes may form a protective

layer for any further defect on the sidewall of nanotubes; on the other hand, the

exposure of nanotubes may cause formation of defects during the bulk polymerization

for the SDBS-coated SWNTs. Furthermore, the pre-coated fiber also showed higher

fluorescence intensity from the (8,3) and (6,5) SWNT types in the high Raman shift

region. This also indicates the presence of individually-suspended SWNTs in the Nylon

6, 10 matrix.









The high dispersion of SWNTs throughout the pre-coated SWNT fiber can also be

confirmed by the fluorescence spectra of SWNTs after dissolving the polymer with

formic acid. As shown in Figure 3-16, distinct fluorescence characteristic of SWNTs can

still be observed, indicating the presence of individually-suspended SWNTs. This

provides further indication that SWNTs remain dispersed throughout the matrix during

processing since no additional energy is supplied to aid dispersion.

Material Properties of Pre-coated SWNT/Nylon 6, 10 Composites

The thermal properties of both Nylon 6, 10 and pre-coated SWNT/Nylon 6, 10

composites should be monitored before heat treatment. Differential scanning

calorimetry (DSC) and thermogravimetric analysis (TGA) were used to characterize the

melting temperature, glass transition temperature (Tg), and decomposition temperature.

DSC as well as X-ray diffraction (XRD) are widely used to determine the crystallinity of

polymers.

Neat Nylon 6, 10, pre-coated SWNT/Nylon 6, 10 composites, hot pressed Nylon 6,

10, and hot pressed composites were taken as samples. The samples were subjected

to three cycles of heating and cooling between room temperature and 260C with a

ramp rate 10C/min and cooling rate 20C/min in nitrogen. Figure 3-17 shows the mass-

normalized DSC data in each of the three heating scans. The endothermic peaks show

the melting temperature (Tm) of the samples, and the enthalpy of fusion (AHf) was

determined from the area under the endotherm. In the first heat scan (Figure 3-17a),

there are some broad endothermic peaks at around 60C, which may be due to

impurities in the samples from the polymerization process. To eliminate the effect of the

impurities from the DSC measurements, a second and third scan was used to get

reliable thermal properties. The melting temperatures and the DSC curves are almost









identical in the second and third heating scan, which indicates the impurities were

eliminated. The pre-coated SWNT/Nylon 6, 10 has ~3C higher melting temperature

than the neat Nylon 6, 10 in these three heating scans. However, the composite shows

5-10% lower crystallinity in comparison to the neat Nylon 6, 10 for either the raw fibers

or hot-pressed fiber in the first heating scan. This reduction in crystallinity could be

associated with introducing SWNTs into the matrix since the alignment or orientation of

SWNTs may influence the crystalline structure of pure nylon during interfacial

polymerization. Interestingly, the melting temperatures for raw fibers slightly increased

with the heating scan, whereas, the Tm for the hot-pressed samples decreased after the

DSC cycles. Also, the raw fibers have smaller decreases in crystallinity within these

three cycles than the hot-pressed samples, which indicate that the polymer degrades

after heat treatment. Moreover, the pressed Nylon 6, 10 had a more significant

decrease in crystallinity in the cycles than the hot-pressed pre-coated SWNT/Nylon 6,

10 composite. This indicates that the composite has higher thermal stability. The

crystallinity can be further evaluated by X-ray diffraction (XRD). As shown in Figure 3-

18, XRD patterns are similar between the pre-coated SWNT/Nylon composite and pure

Nylon 6, 10. It shows major peaks at 29 = 170, 200 and 240 for both of the films, and

only shows slightly differentiated in the intensity. The identical peak positions imply the

same crystal structures in composite and pure Nylon 6, 10. However, the change of

intensities needs more studies to interpret depending thickness and heat treatment.

Figure 3-19 shows the cooling scan of the samples, which shows the recrystallization

temperatures and the heat of fusion of the exothermic peaks. The cooling curves also

show similar thermal behavior to the heating curves. In order to get the glass transition









temperatures of the in-situ polymerized Nylon 6, 10 and pre-coated SWNT/Nylon

composite, a slower ramping rate (5C/min) and lower onset temperature (0C) should

be used on the samples. The observed glass transition temperature from the DSC curve

(not shown) was calculated by the software, and the Tg for the Nylon 6, 10 and the pre-

coated SWNT/Nylon composite are ~45C and ~58C, respectively.

The thermal stability of the samples was also confirmed by TGA in a nitrogen

atmosphere with 10C/min heating rate. Figure 3-20 shows the TGA data in nitrogen.

The thermogram displays that the degradation temperature at a 5% weight loss (Td5%) is

355C and 394C for Nylon 610 and pre-coated SWNT/Nylon composite, respectively.

The higher decomposition temperature of the composite confirmed the higher thermal

stability with the incorporation of pre-coated SWNTs. It has been found that the

incorporation of high thermal conducting materials (e.g. nanotubes) can enhance the

thermal conductivity of composites and increase the thermal stability (65-67).

One of the main purposes of this thesis is to improve the mechanical strength of

Nylon composites by incorporating pre-coated SWNTs into the matrix. Fibers of pre-

coated SWNT/Nylon composites and neat Nylon 6, 10 were transformed to film samples

by compression mold using a hot press at 230C for mechanical testing. The sheet-like

specimens were made with thicknesses of 0.02 inches and cut into strips. Digital

pictures of the sheet-like films and specimen bars are showing in Figure 3-21 and

Figure 3-22, respectively. Mechanical property values reported here are the averages of

the results for tests run on more than five specimens.

Six specimens of each sample (pre-coated SWNT Nylon and neat Nylon 6, 10)

were prepared to get an average of the tensile properties using a Instron test machine









at room temperature. Sample stress-strain curves for neat Nylon and pre-coated SWNT

composite are presented in Figure 3-23, which shows one of the best results for the

composite. As strain is applied, the materials exhibit an elastic region up to A and A'

with the composite having a larger slope. Beyond these points, permanent deformation

occurs with a constant load until passing their natural stretch ratio (B and B'). The

curves then show a typical polymer characteristic with the presence of strain hardening

between B-C and B'-C'. In this case, the permanent deformation of composite is larger

than the pure Nylon 6, 10, which denotes more flexibility of the hot pressed composite.

The average mechanical properties of each sample are shown in Table 3-4. The

average value of the ultimate tensile strength and Young's modulus for the neat Nylon

and pre-coated SWNT/Nylon composite were 23.77 3.92 MPa and 273.7088 80.18

MPa, respectively. Compared with the pure Nylon 6, 10, the pre-coated SWNT/Nylon

composite shows 1.5 times higher strength and 1.25 times higher modulus. However,

the elongation at break of both pure Nylon 6, 10 and pre-coated SWNT/Nylon

composite have similar values only with higher deviation with the composite. It is clear

that the deviation in parameters is always higher with the composite specimens, which

may be associated with the incorporation of pre-coated SWNTs. This could be due to

differences in concentration or dispersion; however, the alignment of the nanoparticles

in polymer matrices also highly influences the mechanical properties of the materials.

This latter effect could explain the observed behavior since the raw fibers had to be

pressed into sheets to remove voids for evaluating the mechanical properties, resulting

in randomly assembled SWNTs in the molten phase. Therefore, the mechanical









properties along the whole sheet of film might be different. In addition, the orientation

when cutting the specimen will be critical to the measured results.

There are usually two factors that explain the improvement of mechanical

properties with the incorporation of carbon nanotubes: a) good dispersion of SWNTs in

the polymer matrix and b) strong van der Waals interactions between the carbon

nanotubes and polymer chains. The use of the pre-coated SWNTs in the preparation of

composites can benefit both of these processes. First, the pre-coating provides a barrier

to the aggregation of SWNTs in the polymer matrix. Second, the polymer chains may

cross-link with un-reacted ends of the polymer chains on the Nylon sheath, enhancing

the interaction between SWNTs and Nylon.

The fracture surface morphology of hot-pressed pure Nylon 6, 10 and pre-coated

SWNT/Nylon composite films were obtained by scanning electron microscopy (SEM),

as shown in Figure 3-24. Apparently, the pure Nylon 6, 10 film shows homogeneous

structures on the fracture surface. Compared with the pure Nylon 6, 10, the pre-coated

SWNT/Nylon composite film shows that the pre-coated SWNTs were uniformly

dispersed in the Nylon matrices (arrows pointed). The discernable nanotubes in Figure

3-24d have approximate 50 nm in diameter, which implies the presence of Nylon-coated

nanotubes bundles rather than the actual SWNTs bundle. Moreover, the fracture

surface of the composite shows good adhesion between SWNTs and Nylon 6, 10 in the

polymer composite.









Table 3-1. Monitored pH values correspond to each experiment procedures. Red color denotes an acidic environment in
the suspension, which can affect the fluorescence and dispersion properties.
CCl4-coated Nylon 6, 10-coated
SDBS suspension CCu-coated CC4(S.C.)-coated suspension Nyon ,10-
suspension suspension
pH value 7.28 8.08 2.14 2.89


Table 3-2. The monitored pH value with each change to the pre-coating procedure. Red and blue colors denote an acidic
and basic environment in the suspension, respectively.
SDBS Cd CCI4(S.C.)- Nylon 6, 10- Pre-coated Pre-coated (6) +
supenion spenion coated coated suspension + suspension + 100pL
suspension suspension HMDA NaOH HMDA
pH 7.28 8.08 2.14 2.89 12.26 13 13.07
value


Table 3-3. Aggregation ratio (intensity of aggregation peak to the highest peak in RBMs) of pre-coated SWNT suspension
changes after mixing with sodium hydroxide and HMDA.
Highest peak intensity in RBMs Aggregation peak Aggregation ratio
(1)SDBS suspension 0.81689 0.08859 0.108448
(2)Pre-coated suspension 0.75171 0.0958 0.127443
(3)NaOH in pre-coated 0.6957 0.15365 0.220857
suspension
(4)(3)+100pL HMDA 0.31767 0.07718 0.242957


Table 3-4. Mechanical properties of pure Nylon 6, 10 and pre-coated SWNT/Nylon composite.
Young's Modulus E (MPa) Ultimate Tensile strength Elongation (%)
ay (MPa)
Nylon 6, 10 217.256 38.85 15.34 1.67 11.35475 3.53
Nylon 6, 10/pre-coated 273.7088 80.18 23.77 3.92 11.06 4.09
SWNTs









S0 H H
-(CH2)8 + H2N-(CH2)6-NH2 ( H2)8 N N + HCI
C C (CH2)6
o 0 0 n


Sebacoyl Chloride


Hexamethylene Diamine


Figure 3-1. The condensation reaction of Nylon 6, 10, which involves sebacoyl chloride
and hexamethylene diamine.


Figure 3-2. Illustration of the new microenvironment formed around the SWNTs. The
solvent swells the hydrophobic core of the micelle surrounding SWNTs,
providing a new approach to prepare coatings around SWNTs.


Nylon 6, 10














(2) (1) SDBS suspension
1.0 --- (3)CCl4(S.C) coated
(1) -- (2)CC14 coated
0.8

-- 0.6


S0.4
(3)

0.2

0.0

900 1000 1100 1200 1300 1400
Wavelength (nm)


Figure 3-3. Fluorescence spectra (Ex. = 662 nm ) of (1) SDBS-coated SWNTs
suspension; (2) carbon tetrachloride-swelled suspension; and (3) sebacoyl
chloride dissolved in carbon tetrachloride-swelled suspension.



0

a + H OH a
+ +HO-----

0 o

Figure 3-4. Water decomposes the monomer, sebacoyl chloride, which makes the
solution acidic after mixing with the monomer.

















0.8 -4 III -- iun -cOateu


? 0.6 (4)


S0.4


0.2
(3)

0.0 -

900 1000 1100 1200 1300 1400
Wavelength (nm)


Figure 3-5. Fluorescence spectra (Ex. = 662 nm) of the (1) initial SDBS-SWNTs
suspension; (2) sebacoyl chloride dissolved in carbon tetrachloride-swelled
suspension; and (3) SWNT suspension after emulsion polymerization.


3500 3000 2500 2000 1500


1000


Wavenumber (cm-')


Figure 3-6. FT-IR spectra of pristine HiPco SWNTs and Nylon-coated SWNTs.


















































Figure 3-7. AFM images and histograms of the distribution of SWNT diameters in (a)
SDBS-coated; (b) pre-coated SWNT suspensions.








54








'I


U
.4
.1


Figure 3-8. Digital photograph showing the formation of the pre-coated SWNTs/Nylon
thin film at the interface of two immiscible solutions in a reaction beaker
(50mL). Top layer: sebacoyl chloride dissolved in hexane; bottom layer: pre-
coated SWNTs and HMDA aqueous suspension.


W 1 (b)


Figure 3-9. Digital photographs of (a) as-spun pre-coated SWNT/Nylon composite
fibers and (b) as-spun pure Nylon 6, 10 fibers.









(a)


(b)


Figure 3-10. The effect of reducing surface tension with the surfactant. (a) A low
density solvent (p<1 g/cm3 e.g. hexane) sits on the top layer above the Nylon-
coated SDBS-SWNTs suspension. (b) A high density solvent (p>1 g/cm3) sits
on the bottom of the Nylon-coated SDBS-SWNTs suspension. The digital
photographs show the structure of the as-spun fibers.

















-- I~Ik4#) pr'c-icOicu suspcllSiuOIl-Uaisc-iivl~t
0.8


0.6


., 0.4



Z 0.2



0.0

900 1000 1100 1200 1300 1400
Wavelength(nm)



Figure 3-11. The effect of HMDA and sodium hydroxide on the fluorescence spectra
(Ex. = 662 nm) of pre-coated SWNT suspensions. The spectra were
normalized to the highest peak intensity of the pre-coated SDBS-SWNTs
suspension.



















2






S100 200 300 400



0
500 1000 1500 2000 2500 3000

Raman shift (cm-1 )

Figure 3-12. Normalized Raman spectra (Ex. = 785 nm) shows the effect of the
presence of base on the pre-coated SWNT suspension. (1) SDBS-SWNT
suspension; (2) pre-coated suspension; (3) pre-coated suspension with high
loading of NaOH; and (4) addition of 100pL of HMDA to (3). The inset shows
the SWNT RBMs of each sample.













0.6

0.5"-2.4"
22.4"-24"
50.5"-52.3"
(C 75.4"-77"
S0.4 102"-104"
138"-139.75"




N
0.2


O
Z


0.0

150 200 250 300 350

Raman Shift (cm1)



Figure 3-13. Raman spectra (Ex. = 785 nm) of the SWNTs RBMs along the length of
the raw fiber. The spectra were normalized to the G-band of each spectrum.












1.0


0.5"-2.4"
0.8 22.4"-24"
50.5"-52.3" (6,5)
"- 75.4"-77"
0.6 102"-104"
C 138"-139.75"


0.4 (8,3)



E 0.2
L-
0
z

0.0 -


1500 2000 2500 3000

Raman Shift (cm1)


Figure 3-14. Raman spectra (Ex. = 785 nm) of the SWNTs along the length of the raw
fiber shows the fluorescence region in high Raman shift region. The spectra
were normalized to the G-band of each spectrum.
















1.0 ,
04 (b)

o 02
S(6,5)
1 o00o
C150 200 250 300 350
0.5 (a)

N





0.0

0 500 1000 1500 2000 2500 3000

Raman Shift (cm1)



Figure 3-15. Normalized Rasonence Raman spectra (Ex. = 785 nm) (a) un-coated and
(b) pre-coated SWNT/Nylon fibers at 30 inches long positions. The inset
shows the SWNT RBMs of each sample.

















o -- pre-coated SWNT/Nylon
S- (7,5) in formic acid
1.5
C (8,3)

Co
> 1.0
A (9,5)


0.5
a-


0.0
900 1000 1100 1200 1300 1400
Wavelength (nm)


Figure 3-16. NIR fluorescence emission spectra (Ex. = 662 nm) of SWNTs after
dissolving the nylon from the pre-coated SWNTs/Nylon composite.

































100


150


Temperature (C)


50 100 150 200 250


Temperature (C)


Figure 3-17. DSC thermograms of composites and pure Nylon with a heating rate of
10C/min in N2 during the (a) first, (b) second, and (c) third heating scan. The
marked number indicates the melting temperature (enthalpy of fusion).


(a)
220_540C (76 83)



218-09C (79-48)\J


221-63C (7456)



2160C (81 2)
p-- pre-coated SWTEIJylon610 -
Nylon-610
- pressed pre-coated SVITINylon610
-- pressed Nylon-610















x -I 221 15C (69_06)







0---
L. 220_67C (65_17)



Spre-coated SVMTTNylon 610
-- Nylon-610 214 57C (60 51)
pressed pre-coated SVI TINylon 610
-- pressed Nylon610
50 100 150 200

Temperature (C)


Figure 3-17. Continued





































64






















(b)










-I I I I
10 15 20 25 30 35 40
2e



Figure 3-18. The XRD pattern of (a) pure Nylon 6, 10 and (b) pre-coated SWNT/Nylon
composite film.














S- pre-coated SWN TfNylon 610
S- Nylon-610
pressepre re-coated SVN TN.ylon 610
188-37.C (45.69)
-- pressed Nylon 610C


/ 188-230C (58.65)
E


187-91 C (42-2)
ir


183 19oC (5903)



50 100 150 200 250
Temperature (C)




-- pre-coatd SWiNMNylon 610
o -- lon-,10 190616C (38.9)
S -- pressed pre-coated SWINNylon 610
pressed Nylon-SI 0



5 1188 290C (59_85)



L. 1871930C (4433)



18280C (51.67)

(b)
50 100 150 200 250
Temperature (C)



Figure 3-19. DSC thermograms of composites and pure Nylon with a cooling rate of
20C/min in N2 during the (a) first, (b) second, and (c) third cooling scan. The
marked number indicates the recrystallization temperature (enthalpy of
recrystallization).













- pre-coated SWlN TENylon 610
-- Nyon-610 194 36C (37 31)
-- pressed pre-coated SVWlNTINylon 610
-- pressed Nylon-610



188 790C (58 47)


187_93C (44_33)



182_50C (56_11)

(c) I
50 100 n50 200 250

Temperature (C)


Figure 3-19. Continued


60


40


I I II I I I I I
0 100 200 300 400 500 600 700 800

Temperature (oC)


Figure 3-20. TGA curves of pure nylon and pre-coated SWNTs/Nylon with a heating
rate 10C/min in N2. The data were normalized to their initial weight




67


- pre-coated SWNTINylon
- Nylon-6,10

























Figure 3-21. Digital photographs of the hot-pressed (a) pre-coated SWNT/Nylon
composite sheet and (b) pure Nylon 6, 10 sheet.


Figure 3-22. Digital photographs of the hot-pressed pre-coated SWNT/Nylon composite
specimen bar (left) and pure Nylon 6, 10 specimen bar (right)















25 Hre-coated bVVN I /Nylon


20

a-
15 C'

.. A
U) 10 -A' B'


5



0 5 10 15
Strain (%)


Figure 3-23. Stress-strain curves of samples obtained with 2 mm/min crosshead speed.
A/A': yield point; B/B': onset of strain hardening; C/C' rupture point.



































Figure 3-24. SEM images obtained from the vertical fracture surfaces: (a)(b) pure Nylon
6, 10 film after tensile test (x10,000; x30,000); (c)(d) pre-coated SWNT/Nylon
composite film after tensile test.









CHAPTER 4
CONCLUSION AND RECOMMENDATIONS

Conclusion

This research project is mainly focused on enhancing the dispersion of single-

walled carbon nanotubes (SWNTs) in the polymer matrices by using non-covalent

SWNTs surface modification. Besides the dispersion of SWNTs, the alignment of

SWNTs was also one of the critical factors affect the performance of the composite. The

in-situ interfacial polymerization can potentially align the SWNTs in the as-spun fiber

because of the constant pulling force. By doing so, it is expected to improve the

properties of polymer by introducing carbon nanotubes. Nylon 6, 10 was chosen as the

target polymer which is one of the polyamides due to their good mechanical properties

and widely used in industrial or commercial purposes.

Surfactant was used to assist the pristine nanotubes dispersed stable in aqueous

phase. Sodium dodecylbenzene sulfonate (SDBS) was used to disperse SWNTs in

water since the effectively suspend SWNTs and being yield the best resolved spectral

features. The pre-coating process involves the solvent-swelled mechanism and

emulsion interfacial polymerization, which modified the surface properties of SWNTs

without diminish the inherent fluorescence. The coating of Nylon around nanotubes was

analyzed by the Fourier transform-infrared spectroscopy (FT-IR) and atomic force

microscopy (AFM) which confirmed the presence of Nylon and the coating thickness

(2.5 4.5 nm). This Nylon-coated SWNTs suspension was directly used as the aqueous

phase in the in-situ interfacial polymerization. The as-spun fiber was evaluated by

Raman spectroscopy, differential scanning calorimeter (DSC), thermogravimetric

analysis (TGA), and spectrometer. Scanning electron microscopy (SEM) images show









the fracture surface morphology of hot-pressed samples which tells the dispersion

quality of nanotubes in the matrices.

The pre-coated SWNT/Nylon composites were prepared by in-situ interfacial

polymerization of two immiscible solutions. A non-covalently coating of Nylon 6, 10

around the SWNTs by emulsion polymerization are well-dispersed in SDBS-assisted

suspension prior to bulk-scale in-situ interfacial polymerization. Nylon 6, 10 sheath

performed as a protective layer to an acidic environment which prevented the

aggregation and fluorescence quenching effect during the bulk-scale polymerization.

The pre-coated SWNTs showed good dispersion and affinity in the Nylon 6, 10

matrices. It was found that the young's modulus, tensile strength, and the thermal

stability of Nylon 6, 10 fibers are greatly improved almost without damaging the pristine

Nylon crystallinity even with a small loading (~0.0025 wt.%) of incorporated pre-coated

SWNTs. Moreover, the inherent fluorescence emission of SWNTs was retained after the

in-situ interfacial polymerization. This composite synthetic approach was simple and fast

which may be easily applied to different polyamides.

Recommendation for Future Work

This research has confirmed that the pre-coating process can effectively aid the

dispersion of SWNTs in Nylon 6, 10, and form a protective layer to acid environments. It

is successful to improve the mechanical and thermal properties with the low loading of

SWNTs. We have used SEM to monitor the morphology of the composite fracture

surface, and DSC and TGA to analyze the thermal properties of the polymer. Utilize

other techniques such as transmission electron microscopy (TEM) can further observe

the Nylon coating structure of SWNTs closely in the matrices.









According to the synthetic process, it is necessary to purify the polymer by soaking

them in DI water because the presence of the impurities may damage the polymer

structures or degrade the thermal properties. However, there are some researches

indicates Nylon may decompose with water or acid. Due to the problems, it could be

beneficial to extract the impurities by other methods, such as Soxhlet extraction. It was

also found that both pure Nylon 6, 10 or the composite have burning problems when

heating to certain temperature under the atmosphere. It could be damage or degrade

the mechanical properties of the material. Therefore, it will be ideal to heat the polymer

or it's composites under either vacuum or some inert gas (e.g. nitrogen, argon) to

maintain the polymer structures and the properties.

Based on the results showing in this thesis, it is highly possible to enhance the

performance properties of the polymer by increasing initial SWNTs concentration which

gives higher loading of SWNTs in the composites. The pulling speed could be also one

of the factors to change the degree of polymerization or the interaction between the

SWNTs and polymers. Synthetic polymers may have different molecular weight due to

the varieties of methodology on processing. By measuring the molecular weight, the

degree of polymerization can be easily known. Typically, gel permeation

chromatography (GPC) is the way to determine the molecular weight of polymers. The

Nylon coating thickness or the uniformity around carbon nanotubes can be estimated by

such technique, which affects the dispersion and the amount of SWNTs bundles.

Presently, we used the traditional way (DSC) to evaluate the Tg of the composite

and the Nylon. However, the lower sensitivity of the endotherms appearance is highly

related to the sample weight and the heating rate. Dynamical mechanical analysis









(DMA) is the other technique to get both thermal and mechanical properties. It has

higher sensitivities to get the Tg of polymers. The precise Tg can be obtained by using

this analytical tool.

With the success of introducing SWNTs in Nylon matrices by using the fast and

simple in-situ interfacial polymerization, it is potentially easy to apply to different

polymers with this technique. It is also worth to try other surfactant-assisted SWNTs

suspension or different solvent-swelled emulsion polymerization which may further

optimize properties of the composites.









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BIOGRAPHICAL SKETCH

Cheng-Ying Lin received her Bachelor of Science degree in chemical engineering

from National Central University in June 2008. She began her graduate studies at the

University of Florida in August 2008 with a Master of Science (Non-Thesis) program.

She joined Professor Kirk J. Ziegler's research group in February 2009 for research

experiences and transferred to Master of Science program in August 2009 for advanced

study.





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1 PRE COATING SINGLE WALLED CARBON NANOTUBES (SWNTS) FOR ENHANCED DISPERSION IN NYLON MATRICES By CHENG YING LIN A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREM ENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2010

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2 2010 Cheng Ying Lin

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3 To my family in Taiwan, who, always support me with love and strength

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4 ACKNOWLEDGMENTS I would like to sincerely thank my advisor, Professor Kirk J. Ziegler, for his patient guidance throughout this project and my graduate work. I would also like to express my appreciation to Dr. Peng Jiang who was supportive as one of my committee members. Moreover, I thank our gro up members, Justin Hill, Carlos Silvera Batista, Fahd Rajab, and Prakash Sugumaran for giving lots of precious suggestions a nd assistances to my research. I am especially indebted to Prakash for the X ray diffraction ( XRD ) measurements. I also want to than k Dr. Brennan and his graduate students, Dave Jackson, Angle Ejiasi, for assisting me with a lot of characterizations and analyses during this project. I am very grateful for their generous help and permission of using their equipments. Finally, I want to express my gratitude to my family in Taiwan, who have supported me everything with their love and care. Especially the mentally supports from my sister Cheng Chi Lin. Special thanks to my dear boyfriend Tzung Hua Lin, who has always given me suggestions o n my works, encouraged me when I got depressed, and helped me on everything I need. I will always be grateful for having them on my side.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 7 LIST OF FIGURES ................................ ................................ ................................ .......... 8 LIST OF ABBREVIATIONS ................................ ................................ ........................... 10 ABSTRACT ................................ ................................ ................................ ................... 12 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 14 Background ................................ ................................ ................................ ............. 15 Single Walled Carbon Nanotubes (SWNTs) Structure and Properties ................... 16 Nanotubes Structure ................................ ................................ ........................ 16 Properties ................................ ................................ ................................ ......... 17 Electrical properties ................................ ................................ ................... 17 Mechanical properties ................................ ................................ ................ 18 Surface properties ................................ ................................ ...................... 18 Processes for Preparing Carbon N anotubes ( CNT s) /Polymer Composites ............ 19 Chemically Modified Carbon Nanotubes ................................ .......................... 19 Composite Processing ................................ ................................ ..................... 21 Melt mixing ................................ ................................ ................................ 21 Solution processing of carbon nanotubes and polymer ............................. 22 In situ Polymerization ................................ ................................ ................. 23 Characterizing Single Walled Carbon Nanotubes ................................ ................... 24 Objectives and Background of Making Nylon 610/SWNTs Composite ................... 25 Objective ................................ ................................ ................................ .......... 25 Background ................................ ................................ ................................ ...... 26 2 EXPERIMENTAL METHOD AND CHARACTERIZATION ................................ ...... 31 Experimental Section ................................ ................................ .............................. 31 SWNTs Suspension Dispersion ................................ ................................ ....... 31 Emulsion Polymerization by Swelling Surfactant Micelle ................................ .. 31 Synthesis of the Nylon 6, 10 and Nylon 6, 10/SWNTs Comp osites ................. 32 Characterization ................................ ................................ ................................ ...... 33 Pre coating of Nylon 6, 10 ................................ ................................ ................ 33 Nylo n 610 and Pre coated SWNT/Nylon composite ................................ ......... 34

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6 3 RESULTS AND DISCUSSION ................................ ................................ ............... 37 Pre coating SWNTs with Nylon 6, 10 by Emulsion Polyme rization ......................... 37 Bulk in situ Interfacial Polymerization with Pre coated SWNTs .............................. 41 Material Properties of Pre coated SWNT/Nylon 6, 10 Composites ......................... 45 4 CONCLUSION AND RECOMMENDATIONS ................................ ......................... 71 Conclusion ................................ ................................ ................................ .............. 71 Recommendation for Future Work ................................ ................................ .......... 72 LIST OF REFERENCES ................................ ................................ ............................... 75 BIOGRAPHICAL SKETCH ................................ ................................ ............................ 79

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7 LIST OF TABLES Table page 3 1 Monitored pH values correspond to each experiment procedures ...................... 50 3 2 The monitored pH value with each ch ange to the pre coating procedure. .......... 50 3 3 Aggregation ratio (intensity of aggregation peak to the highest peak in radial breathing modes ( RBMs )) of pre coated single walled carbon nanotubes ( S WNT s) suspension changes after mixing with sodium hydroxide and h examethylene diamine ( HMDA ) ................................ ................................ ....... 50 3 4 Mechanical properties of pure Nylon 6, 10 and pre coated SWNT/Nylon composite. ................................ ................................ ................................ .......... 50

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8 LIST OF FIGURES Figure page 1 1 Scanning Tunneling Microscopic (STM) image of a single walled carbon nanotubes ( SWNT s ) ................................ ................................ .......................... 28 1 2 The schematically honeycomb lattice structure of graphene sheet.. .................. 28 1 3 Schemetically showing indexed lattice points on a segment of graphene shee t ................................ ................................ ................................ ................... 29 1 4 The density of states (DOS) of SWNT.. ................................ .............................. 29 1 5 Schematically represent the possible surfactant structures ................................ 30 1 6 Mechanism of emulsion polymerization to form a sheath of nylon 6,10 around the sidewall of SWNTs by incorporate two immiscible solvents. ......................... 30 2 1 Di gital photograph shows the fiber pulling motor with constant pulling speed (~20 rpm). ................................ ................................ ................................ ........... 36 2 2 The image of the interfacial polymerization processing of continuously pre coated SWNT/Nylon compos ite fibers ................................ ................................ 36 3 1 The condensation reaction of Nylon 6, 10, which involves sebacoyl chloride and hexamethylene diamine. ................................ ................................ .............. 51 3 2 I llustration of the new microenvironment formed around the SWNTs. ................ 51 3 3 Fluorescence spectra (Ex. = 662 nm ) of sodium dodecylbenzene sulfonate ( SDBS ) coated SWNTs suspension; carbon tetrachl oride swelled suspension; and sebacoyl chloride dissolved in carbon tetrachloride swelled suspension. ................................ ................................ ................................ ........ 52 3 4 Water decomposes the monomer, sebacoyl chloride, which makes the solution acidic a fter mixing with the monomer. ................................ ................... 52 3 5 Fluorescence spectra (Ex. = 662 nm) of the initial SDBS SWNTs suspension; sebacoyl chloride dissolved in carbon tetrachloride swelled suspension; and SWNT suspension after emulsion polymerization. ................................ ............. 53 3 6 Fourier transform infrared ( FT IR ) spectra of pristine high pressure carbon monoxide process ( HiPco ) SWNTs and Nylon coated SWNTs. ......................... 53 3 7 Atomic force microscopy ( AFM ) images and histograms of the distribution of SWNT diameters in SDBS coated; pre coated SWNT suspensions. ............... 54

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9 3 8 Digital photograph showing the formation of the pre coated SWNTs/Nylon thin film at the interface of two immiscible solut ions .. ................................ ......... 55 3 9 Digital photographs of as spun pre coated SW NT/Nylon composite fibers and as spun pure Nylon 6, 10 fibers. ................................ ................................ 55 3 10 The effect of reducing surface tension with the surfactant.. ................................ 56 3 11 The effect of h examethylene diamine ( HMDA ) and sodium hydroxide on the fluorescence spectra (Ex. = 662 nm) of pre coated SWNT s suspensions.. ........ 57 3 12 Normalized Raman spectra (Ex. = 785 nm) shows the effect of the presence of base on the pre coated SWNT suspension. ................................ ................... 58 3 13 Raman spectra (Ex. = 785 nm) of the SWNTs radial breathing modes ( RBMs ) along the length of the ra w fiber. ................................ ........................... 59 3 14 Raman spectra (Ex. = 785 nm) of the SWNTs along the length of the raw fiber shows the fluorescence region in high Raman shift region. ........................ 60 3 15 Normalized Rasonence Raman spectra (Ex. = 785 nm) un coated and pre coated SWNT/Nylon fibers at 30 inches long positions. ................................ ..... 61 3 16 Near infrared ( NIR ) fluoresce nce emission spectra (Ex. = 662 nm) of SWNTs after dissolving the nylon from the pre coated SWNTs/Nylon composite. .......... 62 3 17 Differential scanning calorimeter ( DSC ) thermograms of composites and pure Nylon with a heating rate of 10 C /min in N 2 during the first, second, and third heating scan. ................................ ................................ .............................. 63 3 18 The X ray diffraction ( XRD ) pattern of pure Nylon 6, 10 and pre coated SWNT/Nyl on composite film. ................................ ................................ .............. 65 3 19 DSC thermograms of composites and pure Nylon with a cooling rate of 20 C /min in N 2 during the first, second, and third cooling scan. ...................... 67 3 20 Thermogravimetric analysis ( TGA ) curves of pure nylon and pre coated SWNTs/Nylon with a heating rate 10 C /min in N 2 ................................ ............. 67 3 21 Digital photographs of t he hot pressed pre coated SWNT/Nylon composite sheet and pure Nylon 6, 10 sheet. ................................ ................................ ..... 68 3 22 Digital photographs of the hot pressed pre coated SWNT/Nylon composite specimen bar (left) and pure N ylon 6, 10 specimen bar (right) ........................... 68 3 23 Stress strain curves of samples obtained with 2 mm/min crosshead speed.. ..... 69 3 24 Scannin g electron microscopy ( SEM ) images obtained from the vertical fracture surfaces: ................................ ................................ ................................ 70

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10 LIST OF ABBREVIATION S AFM Atomic force Microscopy CNTs Carbon nanotubes DOS Density of states DSC Differential scanning calorimeter Ex. Exc itation source FT IR Fourier transform infrared spectroscopy GPC Gel permeation chromatography HiPco High pressure carbon monoxide process HPR HiPco SWNTs from Rice University HMDA Hexamethylene diamine MWNTs Multi walled carbon nanotubes NIR Near infrared Nylon 6, 10 Poly(hexamethylenesebacamide) PHAE Polyhydroxy amino ether PVA Poly(vinyl alcohol) RBMs Radial breathing modes S.C. Sebacoyl chloride SDBS Sodium dodecylbenzene sulfonate SDS Sodium dodecyl sulfate SEM Scanning electron spectroscopy STM Scanni ng Tunneling Microscopy SWNTs Single walled carbon nanotubes TEM Transmission electron microscopy

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11 TGA Thermogravimetric analysis XRD X ray diffraction

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12 Abstract of Thesis Presented to the Graduate School of the University of Florida in Par tial Fulfillment of the Requirements for the Degree of Master of Science PRE COATING SINGLE WALLED CARBON NANOTUBES (SWNTS) FOR EXHANCED DISPERSION IN NYLON MATRICES By Cheng Ying Lin August 2010 Chair: Kirk J. Ziegler Major: Chemical Engineering The m ain objective of this thesis is to effectively enhance the dispersion of s ingle walled carbon nanotubes (SWNTs) in Nylon 6, 10 matrices and improve the properties of Nylon 6, 10 by pre coating SWNTs before bulk scale polymerization A pre coated Nylon 6, 10 sheath on the nanotube sidewall was created by using emulsion polymerization which performed good dispersion quality and surprisingly maintained the inherent properties of SWNTs. Nylon 6, 10 nanocomposites containing pre coated SWNTs were prepared by i n situ interfacial polymerization. Raman spectroscopy confirmed the limited aggregation of SWNTs during bulk scale polymerization and the uniformity of appeared bundles along the as spun fibers. Furthermore, the mechanical properties of Nylon 6, 10 were 1.5 times improved by incorporating low loading of SWNTs (~0.0025 wt .% in initial suspension). The fracture surface of composite was imaging by scanning electron microscopy (SEM) which shows the dispersion of pre coated SWNTs in the Nylon 6, 10 matrices. The thermal properties of the composite were also evaluated by differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA) which showing an enhanced thermal stability and higher glass transition temperature with the incorporation of pre coated SWNTs.

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13 Overall, the results suggest that pre coating process before bulk scale polymerization can effectively improve the performance properties of Nylon 6,10.

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14 CHAPTER 1 INTRODUCTION Single walled carbon nanotubes (SWNTs) have attracted great interest bec ause of their remarkable mechanical, electrical, and thermal properties since their discovery in 1993 by Iijima ( 1 ). Various applications have been demonstrated based on these extraordinary properties, ranging from nanocomposite materials, sensors, biomedi cal application, and electronic devices to energy storage and generation. In the field of polymer nanocomposites, carbon black, graphite, clay, and silica are the most common nanofillers used to improve the properties of the polymer. Carbon nanotubes (CNT s) are one of the newest nanofillers considered for reinforcing the properties because of their spectacular characteristics. CNTs can be categorized as SWNTs or multi walled carbon nanotubes (MWNTs). CNTs have a very high aspect ratio (typically ca. 300 10 00) which allows an anisotropic alignment (assemble in a certain direction) in the matrices. Unfortunately, CNTs are relative ly insoluble in common solvents ( 2 ) Therefore, SWNTs have high a tendency to agglomerate in bundles due to the strong van der Waal s forces of attraction, which cause dispersion issues that diminish t he performance of the composite ( 3 ) The lack of dispersion prevents adequate interfacial bonding between the CNTs and polymer matrix. These issues are highly relevant to the aligned qual ity of nanotubes, which is a crucial challenge when developing a nanotube/polymer nanocomposite. Therefore, optimizing the SWNT polymer interfacial interaction has the potential to enhance the dispersion quality. The ultimate objective is to maintain the i nherent properties of SWNTs (optical, electrical, and mechanical properties) at the same time.

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15 Background To improve the native performance properties (strength, toughness, thermal stability etc.) of polymer materials, fiber reinforced polymer composites h ave been widely used in many structural materials. Typically, the volume ratio, shape, and size of the filler particles highly influence the properties of polymer nanocomposites. In conventional polymer composites, fillers with micrometer scale (e.g. glass beads, calcium carbonate) have been used to enhance the polymer mechanical properties. Nowadays, nanometer scale fillers with large aspect ratio have been used to further improve the mechanical properties due to the tremendously large surface area and vol ume ratio. Over the last decade, carbon nanotubes (CNTs) have drawn significant attention from researchers due to their extraordinary properties. Their excellent electrical conductivity and high surface area makes them perfect nanoscale electrodes for dev ices and sensors. Other applications include field emission electron source for flat panel displays, where they have advantages over liquid crystal displays, such as low power consumption, higher brightness, faster response speed, wider visible angle, and lar ger operating temperature range ( 4 ) However, due to the complexity in technique and high costs of the material, this application is still under development. One of the most exciting applications of SWNTs is in developing novel polymer nanocomposites. A jayan et at. were the first to use CNTs as the reinforced nanofiller in a polymer nanocomposite in 1994 ( 5 ). Using nanotubes improved the mechanical properties as well as the electrical and thermal properties of the nanocomposite structure. Typically, the properties of CNTs/polymer nanocomposites vary with several factors, such as the synthetic processing and purification of nanotubes, impurities in the

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16 nanotubes, differences in the distribution of nanotubes (i.e., different (n,m) types, lengths, diameters) the aggregation state in the polymer matrix (i.e. individual or bundled), and the orientation of nanotubes in the matrix. In terms of tensile modulus, there are several examples of CNTs improving the mechanical properties of polymer matrices, such as Pol y(vinyl alcohol) ( PVA ) films ( 6 ), epoxy ( 7 ), and polyimide ( 8 ). Despite the significant improvement to performance, CNTs have yet to reach the theoretical maximum performance. This inability to achieve the desired performance is often attributed to the lac k of adequate dispersion throughout the matrix, as discussed further below. Single Walled Carbon Nanotubes (SWNTs) Structure and Properties Nanotubes Structure CNTs are a novel form of carbon, which have been classified to the fullerene structural family c onsisting of a wall constructed by hexagonal carbon and hemispherical buckyball caps at both ends. The atomic structure of SWNTs can be identified by high resolution scanning tunneling microscopy (STM), as shown in Figure 1 1 ( 9 ) The structure of SWNTs ca n be visualized as a graphene sheet, which has been rolled into a tube. The atomic structure of SWNTs is dependent on the roll up vector ( C h ), which yields different chiral angles ( ) or chiralities described by the following equation: C h = n a 1 + m a 2 where the index (n,m) represent the number of unit vectors ( a 1 ,a 2 ) along the crystal lattice of a graphene sheet, shown in Figure 1 2 ( 10 ) The roll up vector not only determines the index (n,m), but also the diameter of the nanotubes since the interatomic spac ing is known.

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17 Properties Electrical properties The chirality of the SWNTs determines their material properties, especially the electrical properties. It has been shown that the SWNTs can be either metallic or semiconducting in nature, which is determined b y the n and m difference: | n m | = 3q. The nanotube is classified as a metallic or semimetallic group if q is an integer, resulting in approximately 1/3 metallic and 2/3 semiconducting SWNTs in typical samples. The distribution of types of nanotubes bas ed on different (n,m) indices and chiral angle is shown in Figure 1 3 ( 11 ). Figure 1 4 ( 11 ) shows the density of states (DOS) of a semiconducting SWNT. The DOS describes the number of states occupied at each energy level, which can be used to illustrate t he optical properties of SWNTs. SWNTs exhibit a quasi one dimensional (1D) system, which yields sharp van Hove peaks in the DOS. The van Hove singularities depend on the diameter and chirality of the SWNTs and are similar to molecular energy levels. As sho wn in Figure 1 4, when SWNTs absorb energy equal to or greater than E 22 the electron can be excited to the conduction band. The excited electron will go through nonradiative relaxation to the lowest energy state in the conduction band before recombining w ith the hole generated in the valence band. This causes a radiative fluorescence emission (E 11 ), which also denotes the band gap of each semiconducting SWNT. Therefore, semiconducting nanotubes with different (n,m) indices generate near infrared (NIR) fluo rescence in the 800 to 1600 nm wavelength range at certain wavelength.

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18 Mechanical properties The mechanical properties of SWNTs have been studied extensively both theoretically and experimentally for decades. Since the SWNTs have even better tensile streng candidate for structural applications. Theoretically, Krishnan et al. had predicted a wide range of the elastic modulus from 0.5 to 5.5 TPa and the tensile stre ngth in the range of 20 200 GPa ( 12 ) Experimentally, atomic force microscopy (AFM) has been used for measuring the stiffness of arc TPa ( 13 ) This measurement is much harder on SWNTs due to handling difficulties. Salvetat et a l. were the first to show a tensile modulus of ~1 TPa for small diameter SWNTs ropes ( 14 ) Walters et al. further investigated the properties of nanotube bundles; they obtained a yield strength of 45 7 GPa based on an assumed elastic modulus of 1.25 TPa ( 15 ) In addition to the excellent mechanical properties, the density of SWNTs is approximately 1.33 1.4 g/cm 3 ( 16 ). Therefore, SWNTs are a strong, lightweight material with significant promise in ultra high performance multifunctional applications. Surf ace properties Although SWNTs have exceptional properties that make them ideal for composite structures, it remains difficult to disperse and integrate them into polymer matrices. The dispersion problem exists because of the strong side by side van der Waa ls attractions between every single nanotube. These interactions lead SWNTs to aggregate together and form bundles with diameters that range from 10 100 nm. These ropes have serious implications to composite structures since SWNTs in the ropes may slide by shearing along the axis, which diminishes their performance in the composite.

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19 Therefore, enhancing the dispersion of SWNTs or preventing the aggregation during composite formation is very important to their properties and performance. Processes for Prep aring Carbon Nanotubes ( CNT s) /Polymer Composites SWNTs exhibit extraordinary electronic, thermal, mechanical, and optical properties. Therefore, nanotubes have provoked interest from fundamental to applied researchers. Despite the significant effort, it re mains difficult to integrate CNTs into applications because of the substantial van der Waals attraction between nanotubes. The tendency of CNTs to aggregate presents a serious impediment to the mechanical properties that can be achieved in composites. In o rder to get an effective reinforcement using CNTs, the interaction between the CNT and thermoplastic or thermoset polymer matrix must be enhanced. Modifying the surface of SWNTs to establish a stronger chemical affinity (convalently or noncovalently) is re quired to disperse CNTs individually and uniformly throughout the polymer. The preferred approach will achieve high dispersion of CNTs without destroying their integrity to utilize the nanotubes effectively in composites. In general, fabricating a nanocom posite with CNTs requires a few preliminary steps: a) eliminating impurities in CNTs; b) removing bundles to maximize the amount of individual CNTs; c) chemically modifying the surface of CNTs to maximize dispersion. There are several developed processes, such as solution processing, melt mixing, bulk mixing, and in situ polymerization, for making CNT/polymer composites. Chemically Modified Carbon Nanotubes CNTs readily form ropes and bundles because of the strong intrinsic van der Waals force of attracti on and their high aspect ratio. The attractive force is ~ 0.5 eV/nm of nanotube to nanotube contact ( 17 ) Most common solvents cannot supply enough

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20 solvation forces to suspend CNTs ( 2 ) Therefore, the surface of CNTs is typically modified to aid dispersion This procedure can be divided into to two main categories: covalent and non covalent modifications. Covalent modification requires a strong chemical bond or graft between the polymer and CNTs. Depending on the way the polymer chains are formed, covalent polymers of a specific molecular weight are reacted onto the sidewall of the nanotube and the polymer is terminated with a radical precursor or reactive group. After this functionalization step, the nanotubes are integrated into the matrix by initiating another polymer reaction. However, the pre formed polymer chains binding on the CNTs surfaces sterically hinder diffusion of additional macromolecules on the sidewall. On the using in situ polymerization. It also has been called surface initiated polymerization. This process is efficient, control lable, and prevents the steric hindrance of macromolecules; however, this method requires careful control of the initiator concentration and substrate conditions. Covalent sidewall functionalization destroys the intrinsic properties of the SWNTs, such as electrical and optical properties, since the conjugated system is disrupted ( 18 ) Therefore, non covalent modification may be preferred to keep the structural integrity. Non covalent surface modification of SWNTs involves the physical adsorption or direct coating of polymer to the surface of nanotubes. In some cases, surfactants may assist the dispersion of SWNTs prior to the adsorption of polymer. Surfactants can disperse either organic or inorganic particles by non covalent physical adsorption onto the

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21 su rface. Anionic surfactants, such as sodium dodecyl sulfate (SDS) and sodium dodecylbenzene sulfonate (SDBS) are widely used to get SWNTs suspensions with high dispersion quality. Nonionic surfactants, such as natural (e.g. Gum Arabic) and artificial polyme rs ( 19 ), have also been used; the bulky hydrophilic groups provide steric repulsion but it is not as effective as the electrostatic repulsion mechanism of ionic surfactants. The adsorption of surfactants around nanotubes forms different types of structures which may affect the luminescent behavior of suspended SWNTs. Figure 1 5 ( 20 ) shows the possible schematic representation of SDS surfactant adsorption structures. Generally, it is preferable to form cylindrical micelles of surfactant. Hemimicellar adsorp tion, one of the other possible surfactant structures, was shown by by Resasco et al. ( 21 ) to be sterically and energetically unfavorable. The concentration of surfactant can also cause different surfactant coating structures (i.e. vary the shape of micell es). Typical preparation procedures for producing individually suspended SWNTs in surfactant solutions involve high shear homogenization, ultrasonication, and ultracentrifugation ( 22 ). Ultracentrifugation is used to remove bundles from the suspension but it is a time consuming, low throughput, and high cost bundle removal process. Wang et al. have recently demonstrated a high efficiency separation approach called interfacial trapping, which utilizes liquid liquid interfaces ( 23 ). Composite Processing Melt mixing Because of the nature of thermoplastic polymers, which can be softened when heated at certain temperature, melt mixing is desirable technique for blending

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22 nanofillers into polymers that cannot be processed with solution processing. Furthermore, some traditional processing techniques, such as injection molding, blow extrusion, and blow molding can be used when melt mixing CNTs into the thermoplastic polymers. These processes are simple, fast, and widely used in the plastic industry ( 24 25 ). In genera l, melt mixing involves melting polymers into viscous melts. Additives, such as CNT materials, can be blended with the viscous liquid by the application of high shear mixing. The sample can be fabricated by developed techniques, such as extrusion, compress ion molding, or injection molding, based on the desired morphology or shape of the composite. By using high shear mixing or increasing the processing period, CNT dispersion p rocessing large scale fibers; therefore, the viscosity of nanotubes/polymer should be determined before using this processing technique. The melting properties can be highly affected by adding nanotubes, resulting in potential polymer degradation under hig h shear rates. Therefore, the processing conditions should be carefully monitored. Solution processing of carbon nanotubes and polymer Solution based processing is the most common method for preparing carbon nanotube polymer composites. It has the advantag e of low viscosity since the nanotubes are mixed with the polymer in a chosen solvent. Typically, solution processing procedures are different but still follow a general protocol. First, the nanotube powder is dispersed in a liquid medium (either solvent o r polymer solution) by vigorous agitation and sonication to deagglomerate the nanotubes. The nanotubes are then mixed with the targeted polymer by energetic agitation. In general, high shear

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23 mixing, reflux, and magnetic stirring are the most commonly used agitation methods. High power tip, cup horn, or mild bath sonication are usually used to provide sonication. The solvent is then evaporated with or without vacuum to form a composite film. In contrast to melt mixing, solution processing can either work o n thermosetting or thermoplastic polymer matrices. An early example of a solution based approach to CNT composites used thermosetting epoxy matrices, as described by Ajayan et al. ( 5 ) Concerning the formation of carbon nanotube thermoplastic matrices us ing solution processing method, Jin et al. ( 26 ) have reported an approach using polyhydroxy amino ether (PHAE) as the matrix. The dispersion quality of nanotubes in the chosen solvent is very important in this process. However, the low solubility of nanotu bes in most common solvents limits the application of this approach. Surfactants, commonly SDS, have been introduced before mixing the CNTs with the polymer to solve this problem ( 27 29 ). Despite the added dispersant, reagglomeration of the nanotubes withi n the film can occur during the casting/evaporation process. Du et al. proposed an approach that utilized the coagulation mechanism to prevent forming bundles ( 30 ). In situ Polymerization In situ polymerization is one of the most effective processes for po lymer nanocomposite fabrication. This is a method for enhancing integration and dispersion of nanoparticles between phases ( 31 ). Similar to solution based process, improved dispersion of nanotubes in the initial solution (i.e. monomer and solvent) and in t he composite wou ld aid performance. However, in situ polymerization and solution processing may have contamination problems due to the use of solvents.

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24 The fabrication involves dispersing nanotubes in the monomer followed by polymerizing the monomer. Gener ally, the main advantage is the capability to obtain molecular scale reinforcement due to the small size of monomeric molecules. Combined with the preliminary production of polymer grafted nanotubes that can have better affinity with polymer chains, the un iformity of final composite adducts will be higher than directly mixing nanotubes and polymers in solution. These processes also allow higher loading of CNTs. This method has been used with conductive polymers, such as polypyrrole ( 32 ) and also can be use d on aligned CNT arrays to grow nanotube/ polymer coaxial wires ( 33 ). Typically, epoxy nanocomposite is one of the major materials being studied by in situ polymerization. Furthermore, in situ polymerization process allows covalent bonds to be formed betwe en functionalized nanotubes and the polymer matrix using condensation reactions. A non covalent bonding between the nanotubes and the polymer matrix in a nanocompos ite is also possible through in situ polymerization. Fan et al. ( 34 ) reported a study of in situ radical polymerization of pyrrole. They used spectroscopic characterization to conclude non covalent bonding between the CNTs and polymer chains. These non covalent interactions can maintain the inherent properties of CNTs, such as the fluorescence em ission and mechanical properties. Maintaining the inherent properties of the CNTs is also used in this thesis. Characterizing Single Walled Carbon Nanotubes NIR fluorescence ( 11, 22, 35 ) Raman ( 36 ) and optical absorbance spectroscopy ( 37 ) are the most co mmon measurement techniques to analyze the dispersion quality of SWNTs suspension. NIR fluorescence can identify the individually suspended SWNTs based on the uniqueness of fluorescent emission from individual semiconducting

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25 SWNTs ( 22 ). The concentration o f SWNTs suspension can be obtained from the absorbance spectra ( 38, 39 ). Raman spectroscopy of CNTs has two major peaks, which are the radial breathing modes (RBMs) at low frequency and the tangential multi feature (G band) at high frequency. The energy of the vibration mode in RBMs depends on the diameter of SWNTs, so a general diameter distribution of SWNTs can be obtained during the analysis of RBMs. There is also a peak in the RBMs that represents the quantity of aggregation ( 40 ) since the phonon modes of nanotubes are easily disturbed by the environment. As mentioned earlier, NIR fluorescence is capable of determining the E 11 transitions of semiconducting SWNTs. It is important to note that SWNT bundles suffer a disappearance of the NIR fluorescence sin ce the presence of metallic SWNTs in the bundles quench the luminescence from adjacent semiconducting nanotubes ( 22 ). Likewise, different solvents and surfactants (structure too) affect the optical spectra of SWNTs. Each solvent generates a specific adsorp tion and emission spectra, and such behavior is called solvatochromic effects typically characterized by the solvatochromic tends to shift the optical transition en ergy of SWNTs to lower energies ( 19, 41 ). The solv atochromic shift helps us understand changes to the environment surrounding SWNTs during processing. Objectives and Background of Making Nylon 610/SWNTs Composite Objective While SWNTs have been widely applied in making innovative nanocomposites, dispersi ng individual SWNTs in the matrices remains a challenge. Many researchers have applied functionalized CNTs to get better dispersion by enhancing the interaction

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26 with the polymer matrix ( 42 44 ). However, the generation of covalent bonds diminishes the advan tageous electrical, optical, and thermal properties of the SWNTs and may even reduce the mechanical properties of SWNTs ( 3, 45 ). Therefore, surfactant aided dispersion is preferred in this research. Polyamide is an important thermoplastic polymer in indu stry and has been investigated in making nanotube/polyamide nanocomposites (MWNTs are typically used) ( 46 48 ). This study applies a simple and fast processing route to incorporate SWNTs into polymer matrix instead of MWNTs due to the better mechanical prop erties of SWNTs. Poly(hexamethylenesebacamide) (N ylon 6,10) is the chosen polymer matrix since it is easily synthesized via in situ interfacial polymerization. Background This thesis aims to enhance the dispersion of SWNTs by controlling the interfacial p roperties of SWNTs. In order to effectively incorporate SWNTs into the polymer matrix, a chemical procedure is necessary to modify the nanotubes so that they have higher affinity with the polymer matrix. A non covalent bonding modification has been chosen to increase the dispersion and binding with the matrix so that the inherent electrical, optical and even mechanical properties of SWNTs can be maintained. Wang et al. has shown that mixing some water immiscible organic solvents with surfactant SWNT suspens ions forms an emulsion like environment that encapsulates SWNTs. The hydrophobic region of the surfactant, which surrounds the SWNTs, is swelled by the organic solvent ( 49 ). To take advantage of this microenvironment, the organic solvent can be the media t o build non covalently modified SWNTs. Chen et al. has described the in situ emulsion polymerization of nylon 6, 10 by using the swelling mechanism of surfactant structure of SWNTs. On the basis of the interfacial polymerization and

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27 swelling behavior, two immiscible solvents should be used to generate a thin layer of polymer around the sidewall of SWNTs. Figure 1 6 shows the reaction scheme f or this emulsion polymerization ( 50 ). In this thesis, the non covalently pre coated SWNTs are integrated into a bulk Nylon 6, 10 composite. This approach provides steric hindrance to aggregation of SWNTs during the bulk polymerization. Continuous fibers can be spun from the interface of two immiscible phases, which is described further in Chapter 3.

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28 Figure 1 1. Scan ning Tunneling Microscopic (STM) image of a SWNT ( 9 ) Figure 1 2. The schematically honeycomb lattice structure of graphene sheet. Showing the chiral a up vectors (Ch) ( 10 )

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29 Figure 1 3. Schemetically showing indexed lattice points on a segment of graphene sheet Nanotubes designated (n,m) are obtained by rolling the sheet along a roll up vector which omit the metallic nanotubes from 0 to 30 ( 11 ) Figure 1 4. The density of states (DOS) of SWNT. Solid lines represent the electron transitions between energy levels; dashed lines are nonradiative relaxations ( 11 )

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30 Figure 1 5. Schematically repr esent the possible surfactant structures a) cylindrical surfactant micelles encapsulate SWNTs; b) Hemimicellar adsorption surfactant structure on a SWNT; c) Random arrangement of surfactant molecules on a SWNT ( 20 ). Figure 1 6. Mechanism of emulsion polymerization to form a sheath of nylon 6,10 around the sidewall of SWNTs by incorporate two immiscible solvents ( 50 ).

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31 CHAPTER 2 EXPERIMENTAL METHOD AND CHARACTERIZATION Experimental Section Single walled Carbon Nanotubes ( SWN Ts ) S uspension Dispersion Aqueous single walled carbon nanotubes ( SWNTs ) suspensions were prepared with a given initial mass (~40 mg) of raw SWNTs (Rice HiPco SWNTs from Rice University ( HPR ) 177.1) and suspended in 200 mL of aqueous sodium dodecylbenzene sulfonate ( SDBS ) (Sigma Ald rich) surfactant solution (1 wt. %). High shear homogenization ( IKA T 25 Ultra Turrax ) at 10,000 rpm for ~1.5 hr and ultrasonication (Misonix S3000) with 90% amplitude for 10 min were used for aiding dispersion. After ultrasonicat ion, the SWNTs suspension was ultracentrifuged at 20,000 rpm (Beckman Coulter Optima L 90 K) for 3 hr to remove the SWNTs bundles. Emulsion Polymerization by S welling Surfactant M icelle was added to 5 mL carbon tetra chloride (Sigma Aldrich, 99%) to form a 0.005 M sebacoyl chloride solution. To swell the SWNTs with surfactant structure by sebacoyl chloride solution, 5 mL of the aqueous SDBS SWNTs suspension was added gently to 5 mL of sebacoyl chloride solution. Such t wo phase mixture was shaken by Vortex mixer at 1,500 rpm for 30 sec to form the solvent swelled microenvironments around SWNTs. The aqueous solvent coated SWNTs suspension was removed carefully from the bulk carbon tetrachloride after 1 1.5 hr settling for stable phase separation to prevent further emulsification. Hexamethylene diamine (HMDA) was liquefied by heating to about 60C. Liquid monomer hexamethylene diamine (~0.002 mL) was then directly added to the solvent coated aqueous SWNTs suspension. An obv ious color change of polymerized SWNTs

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32 suspension to bluish grey can be observed right after adding the second monomer which implies the formation of nylon 6, 10. Synthesis of the Nylon 6, 10 and Nylon 6, 10/SWNTs C omposites The nylon 610/SWNTs composite w as synthesized by in situ polymerization with two immiscible solutions. The top organic solution of the interfacial polymerization reaction contained the diacid chloride (sebacoyl chloride (>95%, Fluka)). 0.4 mL of sebacoyl chloride was dissolved in 5 mL h exane formed a ~ 0.4M sebacoyl chloride solution. The aqueous phase contained the diamine (hexamethylene diamine (98%, Aldrich)). 0.4 mL of 1, 6 hexamethylene diamine was again liquefied by heating to ~ 60C before adding to 5 mL of the nylon 6,10 pre coat ing SDBS SWNTs suspension in a 20 mL beaker. The aqueous to organic phase volume ratio was optimized by observing a visually low carbon nanotube aggregation and a relative high yield of fibers. The organic sebacoyl chloride solution then was gently poured on the top of the monomer added SWNTs suspension. A thin film was formed at the interface of two immiscible solutions immediately due to the condensation reaction. The thin film was carefully took out with tweezers, and mounted on a motored glass rod with a ~ 20 rpm rolling speed, as shown in Figure 2 1. The nylon 6, 10/SWNTs composite products was continuously formed a fine thread like fiber due to the high reaction rate, as shown in Figure 2 2. The collected nylon 6,10/SWNTs fiber was washed with acetone (Fisher) and followed with distilled water several times to remove the residual unreacted monomers and solvents, then such rinsed fiber was then soaked in 200 mL distilled water for at least 4 hours to remove the surfactant and other impurities. The sample was desiccated in a vacuum oven under reduced pressure (~50 inHg) at 70 80C for 12 24 hours.

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33 Characterization Pre coating of Nylon 6, 10 The Near infrared ( NIR ) fluorescence from the individually suspended semiconducting SWNTs of all the aqueous SWNTs su spensions were characterized with the Applied NanoFluorescence Nanospectrolyzer (Huston, TX) with excitation from 662 and 784 nm. The different degree of aggregation in SDBS coated SWNTs suspensions after emulsion polymerization had been characterized with a liquid probe by a Renishaw Invia Bio Raman with a 785 nm diode laser source. All the Raman spectra were normalized to the G band (1590 cm 1 ) after the baseline correction. The distribution of nanotubes diameters was collected from tapping mode atomic f orce microscopy (AFM) images on a Digital Instruments Dimension 3100. The SDBS coated SWNTs and Nylon coated SWNTs suspensions were spin coated onto fresh mica and gently rinsed with de ionized water to removed excess surfactant to get the images. The aver age diameters of SDBS coated and Nylon coated SWNTs were determined from 10 AFM images of each sample with the NanoScope v5.30r1 software. The histogram recorded at least 125 SWNTs for each sample showing the distributions of the nanotubes diameters. The p resence of Nylon by emulsion polymerization was characterized by Fourier transform infrared ( FT IR ) spectroscopy (Nicolet MAGNA 760 FTIR). The surfactant was removed prior the FT IR analysis by adding 5 mL ethyl acetate to the Nylon coated SWNTs suspension The mixture was then shaken with a Vortex mixer at 2,000 rpm for 30 sec, and settled for phase separation. The Nylon coated SWNTs suspension was then taken out and freeze dried (LABCONCO Freeze Dryer 8) overnight, and a gray powder of the Nylon coated SW NTs was ready to analyze.

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34 Nylon 610 and Pre coated SWNT/Nylon composite The thermal properties of Nylon 610 and pre coated SWNT/Nylon composite were characterized by differential scanning calorimeter (DSC) ( Seiko DSC6200 instrument ). Each sample was hea ted from 30 to 260C with 10C/min heating rate, held at 260C for 1 min, and cooled at the specified cooling rate (20C/min) to room temperature. The thermal properties of samples were monitored under this heat treatment for 3 cycles. The glass melting te mperature and glass transition temperature were calculated from the endothermic peaks and steps in the DSC baseline, respectively. The thermal decomposed temperature was determined with the TGA. The samples were heated from 30 to 800C with10C/min heating rate in nitrogen atmospheres. The weight loss was recorded and normalized to the initial weight of each sample. Analysis was performed on the resulting weight (%) vs. temperature curves. The crystallinity of fibers was monitored by both DSC and X ray diff raction (XRD). The XRD patterns were recorded by using APD 3720 d iffractometer operating at a voltage of 40kV and a current of 25A using Cu K radiation The scanning step size is 0.02 in 2 sec per step. The degree of aggregation of pre coated SWNTs in the composites was monitored by Raman spectra with a 785 nm laser diode source. The samples were taken along in different positions and pressed between two microscope slides. The pre coated SWNT/Nylon composite was dissolved in formic acid with the aid of bath sonicated for measuring the photoluminescence of nanotubes by Applied NanoFluorescence Nanospectrolyzer. The tensile properties of the pre coated SWNT/Nylon composite and Nylon 610 were determined by a universal test machine (Model no. 1122, Instron, USA) at a cross

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35 head speed of 2 mm/min. Sheets of the composite for the mechanical testing were fabricated by hot pressing (Lab Press, CARVER) at above melting temperature (~230 C ) in atmosphere and followed by heating samples in tube furnace at 220 C in vacuum condition to get rid of bubbles. A second step hot press was performed at ~230 C under a pressure of 250 psi to mold the samples to shee t type specimen before testing. The cooling process during above heat treatments was slow cooling in atmosphere. The surface morphology was observed using a scanning electron microscopy (JEOL6335F FEG SEM) at an accelerating voltage of 15 kV. The samples w ere collected after the tensile test, and then coated with platinum.

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36 Figure 2 1 Digital photograph shows t he fiber pulling motor with constant pulling speed (~20 rpm). Figure 2 2. The image of the interfacial polymerization processing of contin uously pre coated SWNT/ Nylon composite fibers

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37 CHAPTER 3 RESULTS AND DISCUSSI ON The condensation reaction in synthesizing Nylon 6, 10, which involves the monomers sebacoyl chloride and hexamethylene diamine (HMDA), is rapid with hydrochloric acid being a b y product, as shown in Figure 3 1 Sebacoyl chloride and HMDA selectively dissolve in organic and aqueous solvents, respectively. Therefore, the polymerization reaction only occurs at the interface of the liquid phases, which is known as interfacial polyme rization. This thesis focuses on spinning Nylon 6, 10 composite fibers through interfacial polymerization. However, the process is split into two steps to improve the dispersion and interaction of the single walled carbon nanotubes ( SWNTs ) with the nylon m atrix. Recently, it was demonstrated that mixing aqueous SWNTs suspensions with immiscible organic solvents can change the environment around the nanotubes into an emulsion like microenvironment around SWNTs, as shown in Figure 3 2 ( 49, 50 ). These new solv ent swelled structures enable new functionalization schemes on the sidewalls of nanotubes. Taking advantage of interfacial polymerization and swelling the surfactant structure around the sidewall of nanotubes, a sheath of Nylon 6, 10 around SWNTs can be fo rmed as the first step ( 50 ). This step was developed previously but will be reviewed in Section 3.1 because of its importance in integrating the pre coated SWNTs into the composite fibers in the second step of the process. Pre coating SWNTs with Nylon 6, 10 by Emulsion Polymerization The Near infrared ( NIR ) fluorescence from individually suspended semiconducting nanotubes is sensitive to the environment around the nanotubes. The solvent swelled SWNTs maintain the fluorescent properties, although the peak p ositions will be shifted

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38 due to solvatochromic effects ( 49, 51 ). Carbon tetrachloride forms these solvent microenvironments in sodium dodecylbenzene sulfonate ( SDBS ) SWNT suspensions and was chosen to be the media for this reaction since sebacoyl chloride has high solubility in it. Figure 3 3 shows the well resolved fluorescence spectra characteristic of the SDBS SWNT suspension in different environments. The fluorescence emission of the suspension shows slight blue shifts and an increase in intensity for n anotubes with larger diameters after mixing with carbon tetrachloride. These spectral changes are associated with the different microenvironment around SWNTs ( 49, 50 ). The blue shifts of the fluorescence peaks imply that the nanotubes are in a less polar and homogeneous environment, as shown in Figure 3 2 ( 51 ). The increase of the intensity may be because of the polarity change or reorganization of the surfactant structure that minimizes the quenching effect ( 49, 50, 52 ). As expected, the peak positions of the suspension after mixing with sebacoyl chloride solutions match the peak positions of the solvent swelled suspension. Such identical fluorescence peak positions indicate similar polarity around the nanotubes due to the same microenvironment (i.e., CCl 4 ). However, mixing a 0.005M sebacoyl chloride in carbon tetrachloride with the SDBS SWNTs suspension yields a drastic decrease in fluorescence intensity for every SWNT (n, m) types when compared with either the initial suspension or the solvent swelled sus pension. Nanotubes with larger diameters show more noticeable decreases in intensity, namely more sensitive response with the change of the microenvironment. Some researchers have also found the higher sensitivity of large diameter SWNTs in quenching mecha nism ( 52 54 ).

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39 The decrease in the fluorescence intensity could be the presence of the sebacoyl chloride within the emulsion like phase surround SWNTs. Table 3 1 shows the monitored pH value at every step in the coating process. The pH value after mixing w ith the 0.005M sebacoyl chloride solutions changed to 2.14, which is considerably more acidic than both the initial and solvent swelled suspensions. Sebacoyl chloride can slightly decompose with the absorption of water and forms strong acids, as the water decomposition reaction in Figure 3 4 shows. It has been demonstrated that acidic environments can quench the SWNTs fluorescence intensity because of the protons ( 55 58 ). Therefore, the acidic environment and the doping effect ( 50, 59 61 ) may be the reasons for the drastic decrease of the intensity. Regardless, these results indirectly confirm the presence of sebacoyl chloride surrounding the SWNTs. The decreased fluorescence intensity with the presence of sebacoyl chloride reversed once adding the water sol uble monomer HMDA. Figure 3 5 shows a noticeable increase in intensity after mixing with the HMDA no significant increase in pH (~2.89). The spectra also show slight red shifts in peak position, which indicates a change to the microenvironment around the n anotubes. Once again, the SWNTs with larger diameters have higher sensitivity to the new environment. These changes to the spectral properties imply the consumption of sebacoyl chloride during the condensation reaction with HMDA (see Figure 3 1 ) at the SWN T interface rather than fewer protons present in the suspension. This might be expected since the by product from the water decomposed reaction of sebacoyl chloride would not be involved in the emulsion polymerization reaction. Similar fluorescence spectra l changes are also observed after mixing the solvent and the polymerization with the excitation source at 784 nm.

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40 Pure Nylon 6, 10 synthesized via interfacial polymerization forms a white powder with characteristic Fourier transform infrared spectroscopy ( FT IR ) stretches of the amide I peak (C=O stretching) at 1640 cm 1 the amide II peak (N H bending) at 1545 cm 1 the C H stretching at 2860 and 2940 cm 1 and the N H stretching at 3330 cm 1 ( 50, 62 ). To minimize the signal from the excess surfactant, et hyl acetate was used to remove the surfactant in the Nylon coated SWNTs suspension. The suspension was then solidified with freeze drying, forming a homogenous gray powder. Figure 3 6 shows the FT IR spectra of pristine SWNTs and nylon coated SWNTs. The FT IR spectrum of the Nylon coated SWNTs shows amide I, amide II, and N H stretching groups at 1637, 1569, and 3338 cm 1 respectively. These indicate the existence of Nylon 6, 10 via the emulsion polymerization in the Nylon coated SWNTs when compared with t he pristine SWNTs ( 50 ). A tapping mode atomic force microscopy ( AFM ) image in Figure 3 7a illustrates the morphology of suspended SDBS coated SWNTs. Individually suspended SWNTs are clearly observed throughout the mica substrate. The histogram in Figure 3 8a shows an average diameter of 1.2 nm, which is reasonable for high pressure carbon monoxide process ( HiPco ) SWNTs coated with a surfactant. Figure 3 7b shows the Nylon 6, 10 coated SWNTs suspension after the emulsion polymerization as deposited mica subs trate. Some individual SWNTs can still be observed after the emulsion polymerization; however, the morphology around SWNTs has noticeable changes when compared with the initial suspension. The diameter distribution of the Nylon 6, 10 coated SWNTs in the hi stogram in Figure 3 7b becomes broader and has a larger deviation from the average diameter (7.3 nm). Moreover, the significant increase in the

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41 average diameter after the polymerization implies the coating of Nylon 6, 10 around individual SWNTs. Bulk in si tu Interfacial Polymerization with Pre coated SWNTs The in situ interfacial polymerization reaction to form Nylon 6, 10 involves two immiscible solutions, namely, an aqueous phase containing HMDA and an organic solution containing sebacoyl chloride. The co mposite is formed by introducing SWNTs pre coated with Nylon to the aqueous suspension. A thin film of the polymer composite forms at the water oil interface immediately, as shown in Figure 3 8 A clear distinction can be seen between the neat Nylon 6, 10 and pre coated SWNT/Nylon composite with the naked eye based on the different color, as shown in Figure 3 9 even with a dilute pre coated SWNTs suspension (~20 mg/L). The concentration ratio ([sebacoyl cholride] : [HMDA] ~ 1.5) of monomers was determined by observing a visually low carbon nanotube aggregations and a relative high yield of fibers. One of the major roles of surfactants is to reduce the surface tension of solutions; however, the reduction in surface tension can be problematic to the spinning of fiber resulting in discontinuous fibers ,as shown in Figure 3 10a. Therefore, the pre coated SWNTs suspension should be below the organic phase, which will help to strip the surfactant from the fiber as it forms. Hexane was chosen as the organic solvent for the bulk polymerization reaction rather than carbon tetrachloride because of the lower density than the pre coated SDBS SWNTs suspension and high solubility of sebacoyl chloride. This system was able to stabilize the surface of as spun pre coated SWNT s/Nylon 6, 10 composite fibers (Figure 3 10b) When synthesizing Nylon 6, 10 by in situ interfacial polymerization, sodium hydroxide (less than 1M) is typically added to neutralize the byproduct (hydrochloric

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42 acid) of the condensation reaction. However, it is well known that the pH of the solution has a significant impact on the absorption and fluorescence of SWNTs suspended with ionic surfactants. SDBS has been demonstrated to be an effective surfactant to suspend HiPco carbon nanotubes ( 63 ) with good reso lution of the spectral features and high concentrations of SWNTs ( 19 ). Tan et al. ( 64 ) reported that the SWNTs have good dispersion in SDBS solution in basic conditions up to pH=13 ( 64 ). However, the SDBS/nylon coated SWNT suspension in this study became m ilky after adding ~70 mg sodium hydroxide, which caused visible aggregation of the nanotubes. The mixture became clear again after a small amount of HMDA was added, although the aggregation of the SWNTs was irreversible. A 1 wt .% solution of only SDBS was taken as a reference and similar behavior was observed, which implies the effect could be associated with the interaction of SDBS and sodium hydroxide. Interestingly, this does not have negative effects on pure SDBS SWNT suspensions Figure 3 11 shows the fluorescence emission of SWNTs excited at 662 nm after adding the sodium hydroxide and HMDA. The spectra show a decrease in intensity and a slight blue shift in peak positions (see (7,5) and (8,3) located at 1026 and 956 nm, respectively) after adding only HMDA or sodium hydroxide, especially for the larger diameter SWNTs. The decrease in intensity is more significant for the addition of base to the pre coated SWNT suspension. Interestingly, the fluorescence intensity for smaller diameter SWNTs increases an d the peak positions shift back after adding both to the suspension. Table 3 2 lists the monitored pH value in these different environments. The pH gets slightly higher (pH=13.07) with the presence of HMDA, which returned the milky suspension back to a cle ar suspension. The drastic decrease in fluorescence intensity suggests that

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43 both HMDA and high pH may destabilize the coating that enables dispersion of the pre coated SWNTs in the aqueous phase. Figure 3 12 shows the resonance Raman spectra for these sus pensions. The broad and intense peaks in the spectra are fluorescence from the SWNTs. Once again, the fluorescence nearly disappears after adding the sodium hydroxide. The radial breathing modes (RBMs) peaks after adding NaOH show a slight broadening of th e spectra, indicating that the resonant frequency associated with the radial expansion of the nanotubes has changed. This could indicate that high pH disrupts the coating layer around the pre coated SWNTs. After adding the HMDA, the intensity of the RBMs d ecreases but the aggregation ratio, which is the ratio of the intensity of the so called aggregation peak (~270 cm 1 ) ( 40 ) and the highest intensity in RBM region, as shown in Table 3 3 The aggregation ratios between the initial suspension and the pre coa ted suspension are similar which implies the sustained dispersion quality after emulsion polymerization. However, the aggregation ratio is nearly two times higher than the initial suspension after adding NaOH. Solid state resonance Raman spectroscopy monit ored the dispersion of SWNTs along the as spun composite fibers. The dimension of the as spun fiber is about 1.5 mm in diameter and 4 m in length. The raw fiber was cut into pieces to characterize the surface by Raman spectroscopy. The aggregation peak at ~270 cm 1 in the RBMs of the Raman spectra was used to characterize the aggregations status in the fiber, as shown in Figure 3 13 Interestingly, the intensity of the aggregation peaks decrease with the length of the fibers. This indicates that the uniform ity of pre coated SWNTs in the Nylon 6, 10 matrix is maintained for approximately 100 inches. Furthermore, the fluorescence

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44 was also observed in the Raman spectra, which indicates the SWNTs were not likely aggregated but dispersed individually in the Nylon matrix, as shown in Figure 3 14. As discussed above, the aggregation that occurs after prolonged spinning may be due to the formation of hydrochloric acid during the condensation reaction. An SDBS suspension without pre coating process was used as the aq ueous solution directly in interfacial polymerization to compare to the pre coated SWNT fibers. Figure 3 15 shows the Raman spectra of both un coated and pre coated SWNT/Nylon fibers at identical positions along the fiber (~30 inches). The inset graph disp lays the RBMs region, which clearly shows the discernable aggregation peak in the fiber with un coated SWNTs. Although the pre coated fiber shows higher intensity of the aggregation peak which may due to the differentiate nanotubes concentrations, the broa der and indistinct peak may imply less aggregation in pre coated SWNT/Nylon 610 composite. The presence of the D band usually means the formation of defects by functionalizing the carbon nanotubes; however, the spectra show no indication of covalently func tionalization of SWNTs during the process for either the un coated or pre coated composites. The Nylon 6, 10 pre coated around the nanotubes may form a protective layer for any further defect on the sidewall of nanotubes; on the other hand, the exposure of nanotubes may cause formation of defects during the bulk polymerization for the SDBS coated SWNTs. Furthermore, the pre coated fiber also showed higher fluorescence intensity from the (8,3) and (6,5) SWNT types in the high Raman shift region. This also in dicates the presence of individually suspended SWNTs in the Nylon 6, 10 matrix.

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45 The high dispersion of SWNTs throughout the pre coated SWNT fiber can also be confirmed by the fluorescence spectra of SWNTs after dissolving the polymer with formic acid. As shown in Figure 3 16 distinct fluorescence characteristic of SWNTs can still be observed, indicating the presence of individually suspended SWNTs. This provides further indication that SWNTs remain dispersed throughout the matrix during processing since n o additional energy is supplied to aid dispersion. Material Properties of Pre coated SWNT/Nylon 6, 10 Composites The thermal properties of both Nylon 6, 10 and pre coated SWNT/Nylon 6, 10 composites should be monitored before heat treatment. Differential scanning calorimet ry (DSC) and thermogravimetric analysis (TGA) were used to characterize the melting temperature, glass transition temperature ( T g ), and decomposition temperature. DSC as well as X ray diffraction (XRD) are widely used to determine the cry stallinity of polymers. Neat Nylon 6, 10, pre coated SWNT/Nylon 6, 10 composites, hot pressed Nylon 6, 10, and hot pressed composites were taken as samples. The samples were subjected to three cycles of heating and cooling between room temperature and 260 C with a ramp rate 10 C /min and cooling rate 20C/min in nitrogen. Figure 3 17 shows the mass normalized DSC data in each of the three heating scans. The endothermic peaks show the melting temperature ( T m ) of the samples, and the enthalpy of fusion ( f ) was determined from the area under the endotherm. In the first heat scan ( Figure 3 17a ), there are some broad endothermic peaks at around 60C, which may be due to impurities in the samples from the polymerization process. To eliminate the effect of t he impurities from the DSC measurements, a second and third scan was used to get reliable thermal properties. The melting temperatures and the DSC curves are almost

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46 identical in the second and third heating scan, which indicates the impurities were elimina ted. The pre coated SWNT/Nylon 6, 10 has ~3C higher melting temperature than the neat Nylon 6, 10 in these three heating scans. However, the composite shows 5 10% lower crystallinity in comparison to the neat Nylon 6, 10 for either the raw fibers or hot p ressed fiber in the first heating scan. This reduction in crystallinity could be associated with introducing SWNTs into the matrix since the alignment or orientation of SWNTs may influence the crystalline structure of pure nylon during interfacial polymeri zation. Interestingly, the melting temperatures for raw fibers slightly increased with the heating scan, whereas, the T m for the hot pressed samples decreased after the DSC cycles. Also, the raw fibers have smaller decreases in crystallinity within these t hree cycles than the hot pressed samples, which indicate that the polymer degrades after heat treatment. Moreover, the pressed Nylon 6, 10 had a more significant decrease in crystallinity in the cycles than the hot pressed pre coated SWNT/Nylon 6, 10 compo site. This indicates that the composite has higher thermal stability. The crystallinity can be further evaluated by X ray diffraction (XRD). As shown in Figure 3 1 8 XRD patterns are similar between the pre coated SWNT/Nylon composite and pure Nylon 6, 10. It shows major peaks at 2 = 17 20 and 24 for both of the films, and only shows slightly differentiated in the intensity. The identical peak positions imply the same crystal structures in composite and pure Nylon 6, 10. However, the change of intensit ies needs more studies to interpret depending thickness and heat treatment. Figure 3 19 shows the cooling scan of the samples, which shows the recrystallization temperatures and the heat of fusion of the exothermic peaks. The cooling curves also show simil ar thermal behavior to the heating curves. In order to get the glass transition

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47 temperatures of the in situ polymerized Nylon 6, 10 and pre coated SWNT/Nylon composite, a slower ramping rate (5C/min) and lower onset temperature (0C) should be used on the samples. The observed glass transition temperature from the DSC curve (not shown) was calculated by the software, and the T g for the Nylon 6, 10 and the pre coated SWNT/Nylon composite are ~45C and ~58C, respectively. The thermal stability of the sampl es was also confirmed by TGA in a nitrogen atmosphere with 10C/min heating rate. Figure 3 20 shows the TGA data in nitrogen. The thermogram displays that the degradation temperature at a 5% weight loss ( T d5% ) is 355C and 394C for Nylon 610 and pre coate d SWNT/Nylon composite, respectively. The higher decomposition temperature of the composite confirmed the higher thermal stability with the incorporation of pre coated SWNTs. It has been found that the incorporation of high thermal conducting materials (e. g. nanotubes) can enhance the thermal conductivity of composites and increase the thermal stability ( 65 67 ). One of the main purposes of this thesis is to improve the mechanical strength of Nylon composites by incorporating pre coated SWNTs into the matrix Fibers of pre coated SWNT/Nylon composites and neat Nylon 6, 10 were transformed to film samples by compression mold using a hot press at 230C for mechanical testing. The sheet like specimens were made with thicknesses of 0.02 inches and cut into strips Digital pictures of the sheet like films and specimen bars are showing in Figure 3 21 and Figure 3 22 respectively. Mechanical property values reported here are the averages of the results for tests run on more than five specimens. Six specimens of eac h sample (pre coated SWNT Nylon and neat Nylon 6, 10) were prepared to get an average of the tensile properties using a Instron test machine

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48 at room temperature. Sample stress strain curves for neat Nylon and pre coated SWNT composite are presented in Figu re 3 23 which shows one of the best results for the with the composite having a larger slope. Beyond these points, permanent deformation occurs with a constant load un curves then show a typical polymer characteristic with the presence of strain hardening between B than the pure Nylon 6, 10, which denotes more flexibility of the hot pressed composite. The average mechanical properties of each sample are shown in Table 3 4 The and pre coated SWNT/Nylon comp osite were 23.77 3.92 MPa and 273.7088 80.18 MPa, respectively. Compared with the pure Nylon 6, 10, the pre coated SWNT/Nylon composite shows 1.5 times higher strength and 1.25 times higher modulus. However, the elongation at break of both pure Nylon 6 10 and pre coated SWNT/Nylon composite have similar values only with higher deviation with the composite. It is clear that the deviation in parameters is always higher with the composite specimens, which may be associated with the incorporation of pre co ated SWNTs. This could be due to differences in concentration or dispersion; however, the alignment of the nanoparticles in polymer matrices also highly influences the mechanical properties of the materials. This latter effect could explain the observed be havior since the raw fibers had to be pressed into sheets to remove voids for evaluating the mechanical properties, resulting in randomly assembled SWNTs in the molten phase. Therefore, the mechanical

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49 properties along the whole sheet of film might be diffe rent. In addition, the orientation when cutting the specimen will be critical to the measured results. There are usually two factors that explain the improvement of mechanical properties with the incorporation of carbon nanotubes: a ) good dispersion of SW NTs in the polymer matrix and b ) strong van der Waals interactions between the carbon nanotubes and polymer chains. The use of the pre coated SWNTs in the preparation of composites can benefit both of these processes. First, the pre coating provides a barr ier to the aggregation of SWNTs in the polymer matrix. Second, the polymer chains may cross link with un reacted ends of the polymer chains on the Nylon sheath, enhancing the interaction between SWNTs and Nylon. The fracture surface morphology of hot press ed pure Nylon 6, 10 and pre coated SWNT/Nylon composite films were obtained by scanning electron microscopy (SEM), as shown in Figure 3 24. Apparently, the pure Nylon 6, 10 film shows homogeneous structures on the fracture surface. Compared with the pure N ylon 6, 10, the pre coated SWNT/Nylon composite film shows that the pre coated SWNTs were uniformly dispersed in the Nylon matrices (arrows pointed). The discernable nanotubes in Figure 3 24d have approximate 50 nm in diameter, which implies the presence o f Nylon coated nanotubes bundles rather than the actual SWNTs bundle. Moreover, the fracture surface of the composite shows good adhesion between SWNTs and Nylon 6, 10 in the polymer composite.

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50 Table 3 1. Monitored pH values correspond to each experimen t procedures. Red color denotes an acidic environment in the suspension, which can affect the fluorescence and dispersion properties SDBS suspension CCl 4 coated suspension CCl4(S.C.) coated suspension Nylon 6, 10 coated suspension pH value 7.28 8. 08 2.14 2.89 Table 3 2. The monitored pH value with each change to the pre coating procedure. Red and blue color s denote an acidic and basic environment in the suspension, respectively. SDBS suspension CCl 4 coated suspension CCl4(S.C.) coated suspen sion Nylon 6, 10 coated suspension Pre coated suspension + HMDA Pre coated suspension + NaOH (6) + HMDA pH value 7.28 8.08 2.14 2.89 12.26 13 13.07 Table 3 3. Aggregation ratio (intensity of aggregation peak to the highest peak in RBMs) of pre coated SWNT suspension changes after mixing with sodium hydroxide and HMDA. Highest peak intensity in RBMs Aggregation peak Aggregation ratio (1)SDBS suspension 0.81689 0.08859 0.108448 (2)Pre coated suspension 0.75171 0.0958 0.127443 (3)NaOH in pre coated suspension 0.6957 0.15365 0.220857 (4)(3)+100 0.31767 0.07718 0.242957 Table 3 4. Mechanical properties of pure Nylon 6, 10 and pre coated SWNT/Nylon composite Ultimate Tensile strength y (MPa) Elongation (%) Nylon 6, 10 217.256 38.85 15.34 1.67 11.35475 3 .53 Nylon 6, 10/pre coated SWNTs 273.7088 80.18 23.77 3.92 11.06 4.09

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51 Figure 3 1. The condensation react ion of Nylon 6, 10 which involves sebacoyl chloride and hexamethylene diamine. Figure 3 2 Illustration of the new microenvironment formed around the SWNTs. The solvent swells the hydrophobic core of the micelle surrounding SWNTs, providing a new approach to prepare coatings around SWNTs

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52 Figure 3 3 Fluorescence spectra (Ex. = 662 nm ) of (1) SDBS coated SWNTs suspension; (2) carbon tetrachloride swelled suspension; and (3) sebacoyl chloride dissolved in carbon tetrachloride swelled suspension. Figure 3 4. W ater decompose s the monomer sebacoyl chloride, which makes the solut ion acidic after mixing with the monomer.

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53 Figure 3 5. Fluorescence spectra (Ex. = 662 nm) of the (1) initial SDBS SWNTs suspension; (2) sebacoyl chloride dissolved in carbon tetrachloride swelled suspension ; and (3) SWNT suspen sion after emulsion polymerization. Figure 3 6 FT IR spectra of pristine HiPco SWNTs and Nylon coated SWNTs

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54 Figure 3 7 AFM images and histograms of the distribution of SWNT diameter s in (a) SDBS coated; (b) pre coated SWNT suspensions.

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55 Figure 3 8 Digital photograph show ing the formation of the pre coated SWNTs/Nylon thin film at the interface of two immiscible solutions in a reaction beaker (50mL). Top layer: sebacoyl chloride dissolved in hexane; bottom layer: pre coated SWNTs and HM DA aqueous suspension. Figure 3 9 Digital photographs of (a) as spun pre coated SWNT/Nylon composite fiber s and (b) as spun pure Nylon 6, 10 fiber s

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56 Figure 3 10 The effect of reducing surface tension with the surfactant. (a) A l ow density solve nt 3 e.g. hexane) sit s on the top layer above the Nylon coated SDBS SWNTs suspension. (b) A h 3 ) sit s on the bottom of the Nylon coated SDBS SWNTs suspension. The digital photographs show the structure of the as spun fibe rs.

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57 Figure 3 1 1 The effect of HMDA and sodium hydroxide on the f luorescence spectra (Ex. = 662 nm) of pre coated SWNT suspensions The spectra were normalized to the highest peak intensity of the pre coated SDBS SWNTs suspensio n.

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58 Figure 3 12 Normalized Raman spectra (Ex. = 785 nm) shows the effect of the presence of base on the pre coated SWNT suspension (1) SDBS SWNT suspension ; (2) pre coated suspension ; (3) pre coated suspension with high load ing of NaOH ; and (4) additi on of of HMDA to (3) The inset shows the SWNT RBMs of each sample

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59 Figure 3 1 3 Raman spectra (E x = 785 nm) of the SWNTs RBMs along the length of the raw fiber. The spectra were normalized to the G band of each spectrum.

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60 Figure 3 14. Raman spectra (Ex. = 785 nm) of the SWNTs along the length of the raw fiber shows the fluorescence region in high Raman shift region. The spectra were normalized to the G band of each spectrum.

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61 Figure 3 15 Normalized Rasonence Raman spectra (Ex. = 785 nm) (a) un coated and (b) pre coated SWNT/Nylon fibers at 30 inches long positions. The inset shows the SWNT RBMs of each sample.

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62 Figure 3 16 NIR fluorescence emission spectra (Ex. = 662 nm) of SWNTs after dissolving the nylon from the pre coated SWNTs/Nylon composite

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63 Figure 3 17 DSC thermograms of composites and pure Nylon with a heating rate of 10 C /min in N 2 during the (a) first (b) second and (c) third heating scan. The marked number indicat es the melting temperature (enthalpy of fusion).

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64 Figure 3 17. Continued

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65 Figure 3 18 The XRD pattern of (a) pure Nylon 6, 10 and (b) pre coated SWNT/Nylon composite film.

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66 Figure 3 1 9 DSC thermograms of composites and pure Nylon with a cooling rate of 20 C /min in N 2 during the (a) first (b) second and (c) third cooling scan. The marked number indicates the recrystallization temperature (enthalpy of recrystallization).

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67 Figure 3 1 9 Continued Figure 3 20 TGA curves of pure nylon and pre coated SWNTs/Nylon with a heating rate 10 C /min in N 2 The data were normalized to their initial weight

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68 Figure 3 21 Digital photographs of the hot pressed (a) pre coated SWNT/Nylon composite sheet and (b) pure Nylon 6 10 sheet. Figure 3 22. Digital photographs of the hot pressed pre coated SWNT/Nylon composite specimen bar (left) and pure Nylon 6, 10 specimen bar (right)

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69 Figure 3 23 Str ess str ain curves of samples obtained with 2 mm/ min crosshead speed.

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70 Figure 3 24. SEM images obtained from the vertical fracture surfaces: (a)(b) pure Nylon 6, 10 film after tensile test (x10,000; x30,000); (c)(d) pre coated S WNT/Nylon composite film after tensile test.

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71 CHAPTER 4 CONCLUSION AND RECOM MENDATIONS Conclusion This research project is mainly focused on enhancing the dispersion of single walled carbon nanotubes ( SWNTs ) in the polymer matrices by using non covalent S WNTs surface modification. Besides the dispersion of SWNTs, the alignment of SWNTs was also one of the critical factors affect the performance of the composite. The in situ interfacial polymerization can potentially align the SWNTs in the as spun fiber bec ause of the constant pulling force. By doing so, it is expected to improve the properties of polymer by introducing carbon nanotubes. Nylon 6, 10 was chosen as the target polymer which is one of the polyamides due to their good mechanical properties and wi dely used in industrial or commercial purposes. Surfactant was used to assist the pristine nanotubes dispersed stable in aqueous phase. S odium dodecylbenzene sulfonate ( SDBS ) was used to disperse SWNTs in water since the effectively suspend SWNTs and bein g yield the best resolved spectral features. The pre coating process involves the solvent swelled mechanism and emulsion interfacial polymerization, which modified the surface properties of SWNTs without diminish the inherent fluorescence. The coating of N ylon around nanotubes was analyzed by the Fourier transform infrared spectroscopy ( FT IR ) and atomic force microscopy ( AFM ) which confirmed the presence of Nylon and the coating thickness (2.5 4.5 nm). This Nylon coated SWNTs suspension was directly used as the aqueous phase in the in situ interfacial polymerization. The as spun fiber was evaluated by Raman spectroscopy, differential scanning calorimeter ( DSC ) thermogravimetric analysis ( TGA ) and spectrometer. Scanning electron microscopy ( SEM ) images s how

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72 the fracture surface morphology of hot pressed samples which tells the dispersion quality of nanotubes in the matrices. The pre coated SWNT/Nylon composites were prepared by in situ interfacial polymerization of two immiscible solutions. A non covalent ly coating of Nylon 6, 10 around the SWNTs by emulsion polymerization are well dispersed in SDBS assisted suspension prior to bulk scale in situ interfacial polymerization. Nylon 6, 10 sheath performed as a protective layer to an acidic environment which p revented the aggregation and fluorescence quenching effect during the bulk scale polymerization. The pre coated SWNTs showed good dispersion and affinity in the Nylon 6, 10 stability of Nylon 6, 10 fibers are greatly improved almost without damaging the pristine Nylon crystallinity even with a small loading (~0.0025 wt .% ) of incorporated pre coated SWNTs. Moreover, the inherent fluorescence emission of SWNTs was retained afte r the in situ interfacial polymerization. This composite synthetic approach was simple and fast which may be easily applied to different polyamides. Recommendation for Future Work This research has confirmed that the pre coating process can effectively aid the dispersion of SWNTs in Nylon 6, 10, and form a protective layer to acid environments. It is successful to improve the mechanical and thermal properties with the low loading of SWNTs. We have used SEM to monitor the morphology of the composite fracture surface, and DSC and TGA to analyze the thermal properties of the polymer. Utilize other techniques such as transmission electron microscopy (TEM) can further observe the Nylon coating structure of SWNTs closely in the matrices.

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73 According to the synthetic process, it is necessary to purify the polymer by soaking them in DI water because the presence of the impurities may damage the polymer structures or degrade the thermal properties. However, there are some researches indicates Nylon may decompose with wa ter or acid. Due to the problems, it could be beneficial to extract the impurities by other methods, such as Soxhlet extraction. It was also found that both pure Nylon 6, 10 or the composite have burning problems when heating to certain temperature under t he atmosphere. It could be damage or degrade the mechanical properties of the material. Therefore, it will be ideal to heat the polymer maintain the polymer structures and t he properties. Based on the results showing in this thesis, it is highly possible to enhance the performance properties of the polymer by increasing initial SWNTs concentration which gives higher loading of SWNTs in the composites. The pulling speed could be also one of the factors to change the degree of polymerization or the interaction between the SWNTs and polymers. Synthetic polymers may have different molecular weight due to the varieties of methodology on processing. By measuring the molecular weight the degree of polymerization can be easily known. Typically, gel permeation chromatography (GPC) is the way to determine the molecular weight of polymers. The Nylon coating thickness or the uniformity around carbon nanotubes can be estimated by such tech nique, which affects the dispersion and the amount of SWNTs bundles. Presently, we used the traditional way (DSC) to evaluate the T g of the composite and the Nylon. However, the lower sensitivity of the endotherms appearance is highly related to the sampl e weight and the heating rate. Dynamical mechanical analysis

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74 (DMA) is the other technique to get both thermal and mechanical properties. It has higher sensitivities to get the T g of polymers. The precise T g can be obtained by using this analytical tool. Wi th the success of introducing SWNTs in Nylon matrices by using the fast and simple in situ interfacial polymerization, it is potentially easy to apply to different polymers with this technique. It is also worth to try other surfactant assisted SWNTs suspen sion or different solvent swelled emulsion polymerization which may further optimize properties of the composites.

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79 BIOGRAPHICAL SKETCH Cheng Ying Lin received her Bachelor of Science degree in chemical e ngineering from National Central University in June 2008. She began her graduate studies at the University of Florida in August 2008 with a Master of Science (Non Thesis) program. experiences and transferred to Master of Science program in August 2009 for advanced study.