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Synthesis and Characterization of "Click Inspired" Thiol-Ene-Synthesis and Characterization of "Click Inspired" Thiol-Ene-

Fullerene Nanocomposites Fullerene Nanocomposites

Amber Danielle Windham University of Southern Mississippi

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The University of Southern Mississippi

SYNTHESIS AND CHARACTERIZATION OF “CLICK-INSPIRED” THIOL-ENE-

FULLERENE NANOCOMPOSITES

by

Amber Danielle Windham

Abstract of a Dissertation

Submitted to the Graduate School

of The University of Southern Mississippi

in Partial Fulfillment of the Requirements

for the Degree of Doctor of Philosophy

May 2016

ii

ABSTRACT

SYNTHESIS AND CHARACTERIZATION OF "CLICK-INSPIRED" THIOL-ENE-

FULLERENE NANOCOMPOSITES

by Amber Danielle Windham

May 2016

This work explores the covalent incorporation of C60 and Sc3N@C80 fullerenes

into thiol-ene networks by a facile, one-pot photochemical reaction with multifunctional

thiol and alkene monomers. Synthesis of disulfide bonds within the tri-functional thiol

monomer served to photochemically initiate the reaction when cleaved. This was

followed by thermal curing of the pre-polymer for preparation of fullerene-containing

thiol-ene films. Films were characterized by standard techniques including infrared

spectroscopy, gel percent, and thermogravimetric analysis. The role of C60 and

Sc3N@C80 as suitable alkenes for free-radical reaction with multifunctional thiols was

investigated by preparing a series of thiol-films possessing a significant stoichiometric

deficiency of alkyl ene monomer. Dynamic mechanical analysis of these films revealed

an enhancement of the storage modulus values, increased glass transition temperatures,

and increased network crosslink density with added C60 and Sc3N@C80. Investigation of

the fullerene dispersion throughout the polymer was performed by transmission electron

microscopy and revealed no significant aggregation of C60 or Sc3N@C80 in 1 wt %

samples prepared by the pre-polymer method.

To extend possible applications of this novel polymer nanocomposite, the

preparation of fullerene-containing microspheres was pursued. An acoustic excitation

coaxial flow (AECF) method was developed in this study to yield microspheres of

iii

narrow size dispersions. Microspheres were analyzed via optical microscopy, scanning

electron microscopy, and dynamic light scattering. Microsphere production was

determined to be dependent on the settings of the applied acoustic energy, as well as the

viscosity of the discrete polymer solution, and flow rate of the non-solvent carrier phase.

This presents the AECF method as a tunable technique for control of the microsphere

diameter, with ability to prepare thiol-ene microspheres as small as 200 nm in diameter.

C60 and Sc3N@C80 thiol-ene microspheres were prepared and characterized by differential

scanning calorimetry, thermogravimetric analysis, and optical microscopy.

COPYRIGHT BY

AMBER DANIELLE WINDHAM

2016

SYNTHESIS AND CHARACTERIZATION OF “CLICK-INSPIRED” THIOL-ENE-

FULLERENE NANOCOMPOSITES

by

Amber Danielle Windham

A Dissertation

Submitted to the Graduate School

and the Department of Chemistry and Biochemistry

at The University of Southern Mississippi

in Partial Fulfillment of the Requirements

for the Degree of Doctor of Philosophy

Approved:

______________________________________________

Dr. J. Paige Buchanan, Committee Chair

Associate Professor, Chemistry and Biochemistry

______________________________________________

Dr. Randy K. Buchanan, Committee Member

Senior Research Analyst, U. S. Army Engineer Research Development Center

______________________________________________

Dr. Song Guo, Committee Member

Assistant Professor, Chemistry and Biochemistry

______________________________________________

Dr. Wujian Miao, Committee Member

Associate Professor, Chemistry and Biochemistry

______________________________________________

Dr. Karl J. Wallace, Committee Member

Associate Professor, Chemistry and Biochemistry

______________________________________________

Dr. Karen S. Coats

Dean of the Graduate School

May 2016

iv

DEDICATION

This work is dedicated to the memory of my ambitious and greatly talented

grandparents who would be “tickled pink” to know I became a scientist: Drs. Charles &

Clare Lindsey, and Mr. Joseph Windham. I have received endless support from my

family and friends through my graduate career. Without them, this work would not be

possible. Thank you all for believing in me every step of the way as I chase my dreams.

v

ACKNOWLEDGMENTS

I would like to begin by expressing my utmost gratitude to my advisor, Dr. J.

Paige Buchanan, for mentoring me through my graduate studies. She has taught me many

valuable skills that I am certain will help me throughout my career. Thanks to all

members of the Buchanan Research Group, past and present, graduate and undergraduate.

It has been a joy to work with so many talented and driven individuals. I would like to

extend gratitude to my committee members for their guidance and encouragement: Dr.

Randy K. Buchanan, Dr. Song Guo, Dr. Wujian Miao, and Dr. Karl J. Wallace.

I would also like to recognize those who aided me in the completion of this work:

Dr. Anton D. Netchaev, Dr. James Wynne, Dr. Kenneth Curry, Dr. Spencer Giles, Jessica

Douglas, David Delatte, Patrick Lowe, Keith Conley, and Daniel Jovinelli. Without the

skills, insight, and collaboration of these scientists, this work would be far from

complete.

Thanks to the University of Southern Mississippi Department of Chemistry and

Biochemistry and the National Science Foundation for financial support. Finally, I would

like to recognize all the individuals in the Department of Chemistry and Biochemistry:

students, professors, faculty, and staff. The kindness and support I have received from the

wonderful people I have encountered during my time at USM has made this challenging

journey a memorable experience for which I am grateful.

vi

TABLE OF CONTENTS

ABSTRACT........................................................................................................................ii

DEDICATION....................................................................................................................iv

ACKNOWLEDGMENTS...................................................................................................v

LIST OF TABLES.............................................................................................................vii

LIST OF ILLUSTIRATIONS..........................................................................................viii

LIST OF SCHEMES..........................................................................................................xi

LIST OF ABBREVIATIONS...........................................................................................xii

CHAPTER

I. INTRODUCTION.......................................................................................1

Fullerenes

Fullerene-Containing Polymers

“Click” Chemistry and the Thiol-Ene Reaction

Polymer Microspheres

Scope of Current Work

II. SYNTHESIS AND CHARACTERIZATION OF FULLERENE-

CONTAINING THIOL-ENE NANOCOMPOSITE THIN FILMS..........20

Introduction

Materials, Method, and Characterization

Results and Discussion

III. ACOUSTIC EXCITATION COAXIAL FLOW METHOD FOR

CONTROLLED PRODUCTION OF FULLERENE-CONTAINING

THIOL-ENE MICROSPHERES...............................................................59

Introduction

Materials, Method, and Characterization

Results and Discussion

IV. CONCLUSION.........................................................................................83

REFERENCES..................................................................................................................87

vii

LIST OF TABLES

Table

1. Sample Composition and Gel % of Thiol-Ene-Fullerene Films............................38

2. TGA Results of C60-Thiol-Ene Films Prepared with S-S Photoinitiator...............42

3. Dynamic Mechanical Analysis of Fullerene-Thiol-Ene Films..............................44

4. Study of Effect of Frequency and Amplitude on Microsphere Diameter Using

APE-TMPMP.........................................................................................................69

5. Dilution Study with Microspheres Composed of TTT-TMPMP...........................73

6. Flow Rate Study of Microspheres Prepared with 1%-HEX-TTT-TMPMP at

77 Hz 4 VPP..........................................................................................................75

viii

LIST OF ILLUSTRATIONS

Figure

1. Empty Cage Fullerenes (A) C60 and (B) C70, (C) Endohedral Metallofullerene,

Sc2@C84, and (D) Metallic Nitride Fullerene, Sc3N@C80.......................................4

2. Examples of Metals and Metal Combinations that can be Bound to Nitrogen in the

Trimetallic Nitride Cluster Within C80 Cage...........................................................4

3. Illustration of the Electrostatic Relationship of the Metallic Nitride Cluster with

the C80 Cage in Sc3N@C80......................................................................................4

4. Examples of Covalently Incorporated Fullerene-Containing Polymers: (A)

Fullerene Side-Chain Polymer, (B) Fullerene Main-Chain Polymer, (C)

Crosslinked Fullerene-Containing Network, (D) Fullerene End-Capped Polymer,

(E) Fullerene Star-shaped Polymer, and (F) All-C60 Polymer.................................8

5. General Structures of Common Commercially Available Thiol Monomers: (A)

Alkyl-3-Mercaptopropionates, (B) Alkylthioglycolates, and (C) Alkyl Thiols....13

6. 1H-NMR Spectrum of TMPMP and S-S, Relative to Benzene Shift at 7.15 ppm.

Asterisk (*) Indicates Impurity..............................................................................24

7. Multifunctional Thiol and Ene Monomers and Irgacure 819 Photoinitiator Used in

the Synthesis of C60-Thiol-Ene Nanocomposites..................................................25

8. Side-by-Side Comparison of 1 wt % C60-Thiol-Ene Films Prepared by (Left)

Blending in Monomers in the Absence of Solvent, and (Right) Covalent

Incorporation by Pre-Polymer Photoreaction........................................................28

9. 20 Wt % C60-Thiol-Ene Films Prepared with S-S and Tri-Functional Alkyl Enes

Resulting in a Buckled Morphology, Where Ene Monomer and Cure Conditions

are as Noted: (A) APE, 40 ⁰C for 1 h, 60 ⁰C for 1 h, and 80 ⁰C for 12 h, (B) APE,

65 ⁰C 14 hr, (C) APE, 100 ⁰C 14 h, (D) APE, Cycled at RT 48 h, 70 ⁰C 1 h, for

Two Cycles (E) APE, Cycled at 100 ⁰C 10 min, RT 14 h, for Two Cycles (F)

TTT, 70 ⁰C 2 h then 85 ⁰C 14 h.............................................................................29

10. 10 Wt % C60-Thiol-Ene Film Prepared with Diene, TMPDE, Cured at 75 ⁰C for

72 Hours.................................................................................................................30

11. Representative IR Spectrum of a 1 Wt % C60-Thiol-Ene Fullerene Film Prepared

with TMPDE..........................................................................................................32

ix

12. Thermogravimetric Analysis of 0.75 C60-Thiol-Ene Film Series in N2 Atmosphere

where Wt % C60 is Noted.......................................................................................39

13. Thermogravimetric Analysis of 1.0 C60-Thiol-Ene Film Series in N2 Atmosphere

where Wt % C60 is Noted.......................................................................................40

14. Dynamic Storage Moduli (E') of 0.75 Series C60-Thiol-Ene Films.......................44

15. Loss Tangent (Tan δ) Plot of 0.75 Series C60-Thiol-Ene Films.............................46

16. Dynamic Storage Moduli (E’) of 1.0 Series C60-Thiol-Ene Films........................47

17. Loss Tangent (Tan δ) Plot of 1.0 Series C60-Thiol-Ene Films...............................48

18. Dynamic Storage Moduli of Fullerene-Thiol-Ene Films Prepared with Irgacure

819 and/or Sc3N@C80............................................................................................49

19. Tan δ Plot for Fullerene-Thiol-Ene Films Prepared with Irgacure 819 and/or

Sc3N@C80..............................................................................................................52

20. Calculated Crosslink Density (XLD) For All Films at 25 ⁰C...............................54

21. TEM Image of 1 Wt % 0.75 C60-Thiol-Ene Film Which Appears Continuous

Throughout and No Aggregation of Nanoparticles is Observed...........................54

22. TEM Image of 1 Wt % 1.0 C60-Thiol-Ene Film Which Appears Continuous

Throughout and No Aggregation of Nanoparticles is Observed...........................55

23. TEM Image of 1 Wt % 1.0 Sc3N@C80-Thiol-Ene Film Where No Aggregation is

Visible....................................................................................................................56

24. Tem Image of 1 Wt % 0.75 Sc3N@C80-Irgacure 819 Thiol-Ene Film with No

Visible Aggregation...............................................................................................56

25. TEM Images of 1 Wt % 0.75 C60-Irgacure 819 Thiol-Ene Film Where No

Aggregation is Observed........................................................................................57

26. TEM Image of 10 Wt % 0.75 C60-Thiol-Ene, Where Aggregation of C60 Particles

Appears Consistently Throughout the Network....................................................58

27. TEM Image of 10 Wt % 1.0 C60-Thiol-Ene Network, Where Aggregation of C60

Particles is Not Continuous Throughout the Sample.............................................58

28. Elements That Were Combined in the Development of the AECF Experimental

Technique...............................................................................................................60

29. Acoustic Excitation Coaxial Flow (AECF) Experimental Setup...........................62

x

30. Results of AECF Characterization: Output Voltage Measured as a Result of the

Applied Frequencies at Different Amplitudes. Inset: Maximum Output Voltage

Measured at Tested Amplitudes.............................................................................66

31. Effect of Amplitude on Microspheres Prepared Using APE-TMPMP at 77 Hz,

Where OM Images are Displayed in Series A, SEM Images Displayed in Series

B, and Number in Upper Left Corner Indicates Amplitude in VPP......................70

32. DSC Thermograms of APE-TMPMP Network, and TTT-TMPMP Network,

Where Glass Transition Temperatures are Noted..................................................71

33. Effect of Viscosity on Microsphere Diameter, Using HEX-TTT-TMPMP at 77 Hz

and 4 VPP. A: SEM Images B: Optical Microscope Images Where Wt % Hexane

in TTT is (1) 1, (2) 2.5, (3) 5, (4) 7.5, (5) 10 Wt %...............................................74

34. OM of C60-Containing Microspheres Prepared with TTT/S-S. Microspheres

Exhibit a Buckled Architecture, as Seen in the Thin Films with the Same

Composition...........................................................................................................77

35. OM of C60-Containing Microspheres Prepared with TMPDE/S-S........................78

36. OM Images. (Left) 1 Wt % Sc3N@C80 –TTT-TMPMP Microspheres with 25

mL/min Continuous Flow Rate. (Right) Sc3N@C80 –TTT-TMPMP Microspheres

with a Greater Continuous Flow Rate of 50 mL/min............................................80

37. DSC of Sc3N@C80-TTT-TMPMP Microspheres Compared to TTT-TMPMP

Control, Where Glass Transition is Noted.............................................................81

38. TGA Analysis of Sc3N@C80-Containing TTT-TMPMP Microspheres Under N2

Environment...........................................................................................................82

xi

LIST OF SCHEMES

Scheme

1. Strategic Functionalization of C60 Fullerene to Yield Crosslinked Fullerene-Thiol-

Ene Network via Fullerene-Urethane Linkage......................................................10

2. The Hüisgen Azide-Alkyne 1,3-Dipolar Cycloaddition........................................11

3. General Radical Mechanism for the Photoinitiation and Propagation of a Thiol-

Ene "Click" Reaction, Where kp is the Rate Constant for Propagation and kct is the

Rate Constant for Chain Transfer..........................................................................12

4. Utilization of Ambient Oxygen as a Reactant in a Free-Radical Thiol-Ene

Reaction.................................................................................................................14

5. Overview of Crosslinked Fullerene-Containing Polymer Networks from Previous

and Current Work..................................................................................................18

6. Oxidation of TMPMP to Disulfide (S-S)...............................................................24

7. Competition Reaction for Addition to C60 Occurring Between Thiyl (RS•) and

Photoinitiator Radicals (PI•) upon Cleavage by UV Light....................................26

8. General Preparation of "Click-Inspired" C60-Containing Thiol-Ene Films with Tri-

Functional Enes......................................................................................................27

9. Preparation of "Click-Inspired" C60-Containing Thiol-Ene Films with Di-

Functional TMPDE................................................................................................31

10. Competition Reaction of Thiyl Radical Between Fullerene and Alkyl Ene, Where

the Rate of Propagation with Fullerene is Expected to be Significantly Slower

Due to Resonance-Stabilized Radical....................................................................34

xii

LIST OF ABBREVIATIONS

AECF Acoustic Excitation Coaxial Flow

AIBN 2,2’-Azobisisobutyronitrile

APE Pentaerythritol Allyl Ether

BDE Bond Dissociation Energy

DABCO [1,4]Diazobicyclo(2.2.2)Octane

DCM Dichloromethane

DLS Dynamic Light Scattering

DMA Dynamic Mechanical Analysis

DMPA 2,2-dimethoxy-2-phenylacetophenone

DSC Differential Scanning Calorimetry

ESR Electron-Spin Resonance

FT-IR Fourier Transform-Infrared

MN Mean Number

MNF Metallic Nitride Fullerene

NMR Nuclear Magnetic Resonance

OM Optical Microscopy

OPV Organic Photovoltaic

PCBM [6,6]Phenyl C61-Butyric Acid Methyl Ester

PMMA Poly(methyl)methacrylate

PS Polystyrene

PTFE Polytetrafluoroethylene

PU Polyurethane

RT Room Temperature

SBS Polystyrene-block-Polybutadiene-block-Polystyrene

xiii

SD Standard Deviation

SEM Scanning Electron Microscopy

SIS Polystyrene-block-Polyisoprene-block-Polystyrene

S-S Trimethylolpropane Tris-3-Mercaptopropionate Disulfide

TEM Transmission Electron Microscopy

TGA Thermogravimetric Analysis

TMPDE Trimethylolpropane Diallyl Ether

TMPMP Trimethylolpropane Tris-3-Mercaptopropionate

TTT Triallyl-1,3,5-Triazine-2,4,6-(1H,3H,5H)-Trione

UV Ultraviolet

VPP Volts Peak-to-Peak

XLD Crosslink Density

1

CHAPTER I

INTRODUCTION

Fullerenes

Since the introduction of Buckminsterfullerene, or C60 fullerene, in 1985, the

scientific community has been avidly researching the properties and applications of this

unique molecule.1 The molecule is comprised of 12 pentagons and 20 hexagons fused

together containing a total of sixty sp2-hybridized carbon atoms connected in a hollow

spherical arrangement (Figure 1A). C60 possesses 30 conjugated carbon-carbon double

bonds that offer great possibilities for exciting chemistry. In addition to the most

abundant C60, less abundant fullerenes include the smaller C20 2 and C36,

3 and higher-

order empty cage fullerenes C70 (Figure 1B),4 C76,5 C78,

6 C82,7 C84, C86,

8 and C90.

C60 exhibits a variety of photophysical, electrochemical, magnetic, optical, and

conducting properties that make it unique.9 [60]Fullerenes possess high electron affinity

of 2.6-2.8 eV.10 This, combined with the conjugation occurring in the molecule, gives

fullerenes outstanding radical scavenging ability. For this reason, C60 is considered to be a

“radical sponge”.11 Another valuable characteristic of fullerenes is their ability to form

singlet oxygen.12 There is also interest in the use of fullerenes for energy applications

such as hydrogen storage13 and supercapacitors.14 Together these properties make

fullerenes valuable for extensive applications in the fields of biology, chemistry, and

physics.

Fullerenes are sparingly soluble in monomers and common solvents, such as

chloroform and acetone, limiting their use in many applications.15 Upon chemical

modification of the fullerene’s surface, solubility can be modified significantly. For

2

example, the solubility of C60(OH)x can range from organic to aqueous depending on the

number of attached hydroxyl groups, where the addition of 6 hydroxyl groups renders the

fullerenol soluble in polar organic solvents including DMSO and DMF, and the addition

of 15 or more hydroxyls results in a fullerenol with high water solubility.16, 17 This allows

for improved processability while preserving the unique properties of the nanoparticle.

[60]Fullerene derivatives bearing outstanding structural,18 magnetic,19 superconducting,20

electrochemical,21 and photophysical properties22 have been published. The addition of

polar functional groups to the non-polar fullerene core results in an amphiphillic

molecule that can be tuned to self-assemble into supramolecular structures, in which the

fullerene derivatives form larger structures via non-covalent interactions, including

spheres,23 nanorods,24 vesicles, and bilayers.25

Due to its character as a soft electrophile,10 [60]fullerenes participate in a variety

of reactions with nucleophiles.26-28 The [6,6] bonds behave as dienophiles, allowing

cycloadditions such as Diels-Alder and Bingel reactions.29-31 A variety of other reaction

types including hydrogenation,32, 33 hydroxylation,16, 34-36 chlorination,37 and carbene

addition38 have been reported to occur with the fullerene nucleus, where multiple

additions commonly occur. Friedel-Crafts fullerenation reactions of aromatic molecules

including solvents benzene and toluene can occur in the presence of a Lewis acid, such as

AlCl3, to yield fullerenes bearing aromatic adducts.39 Strategic reactions of fullerene

derivatives can aid in the preparation of novel molecules. Nucleophilic substitution of

chlorofullerenes with variety of thiols has been studied as an efficient and selective

method to prepare highly selective and biologically interesting water-soluble fullerene

3

derivatives.40 Retro-Bingel reactions have been performed for the selective removal of

addends offering a protecting strategy for fullerenes.41

The reactivity of [60]fullerenes with radicals of C, Si, O, P, N, H, halogens, and

several metals have been documented and recently summarized in a detailed review by

Tzirakis & Orfanopoulos.42 One of the earliest fullerene radical reactions reported was

the addition of alkyl carbon-centered radicals to a [60]fullerene nucleus, which was

studied by electron-spin resonance (ESR).43 Later studies revealed that multiple

photochemically-generated carbon-centered radicals readily add to C60, with up to 34

additions in the case of methyl radicals, as determined by mass spectrometry.11 Radical

trapping was used to determine the rate constants of the addition of some carbon-centered

radicals to C60 fullerene, where values obtained were two orders of magnitude greater

than the rate constants of compounds containing one unsaturation.44 This may be in part

to the increased stability of the delocalized fullerynl radical that is produced in this type

of reaction. The addition of thermal radical initiator 2,2’-azo-(bisisobutyronitrile) (AIBN)

to C60 has also been documented.45 The reactions of alkylthio radicals with C60 and C70

have also been studied by ESR.46, 47 Studies indicated that the addition of alkylthio

radicals to C60 were highly reversible, which was credited to a weak C-S bond.47

4

Figure 1. Empty cage fullerenes (A) C60 and (B) C70, (C) endohedral metallofullerene,

Sc2@C84, and (D) metallic nitride fullerene, Sc3N@C80.

Figure 2. Examples of metals and metal combinations that can be bound to nitrogen in

the trimetallic nitride cluster within C80 cage.

Figure 3. Illustration of the electrostatic relationship of the metallic nitride cluster with

the C80 cage in Sc3N@C80.

Shortly after the discovery of C60, it was hypothesized that fullerene cages could

contain atoms.1 This was confirmed with the discovery of lanthanum-containing C60

A B C D

A B C D

5

endohedral fullerene (denoted as La@C60) which was detected by time-of-flight mass

spectrometry and confirmed to possess metallic properties.48 Classical endohedral

fullerenes contain up to four atoms including lanthanides, Group I-III metals, and non-

metals.49-52 Reports of oxometallic and metal carbide fullerenes have also been

published.53-55 Figure 1C gives an illustration of Sc2@C84 endohedral fullerene.

Metallic nitride fullerenes (MNFs), first isolated by Dorn and Stevenson, consist

of a metal-nitride cluster (M3N) encapsulated within a carbon cage (Figure 1D).56-58 This

variety of endohedral fullerene is significantly more stable in air than free atom-

containing fullerenes. MNFs can contain a single type of metal or a mixture of metals

bonded to the nitrogen, as shown in Figure 2. In the case of Sc3N@C80, the most

abundant MNF, each scandium atom donates two electrons to the carbon cage and one to

the nitrogen atom, whereby the formula can be considered [Sc3N] 6+ @[C80] 6-, shown in

Figure 3.59

Functionalization of MNFs is more complicated than that of the empty cage C60

due to the stabilizing effect of the electrostatic relationship between the C80 cage and the

metal nitride cluster contained within. Derivatization of MNFs may not occur under the

same conditions as the empty cage fullerenes or endohedral metalofullerenes. Successful

functionalizations that have been reported include hydroxylation,60 Bingel,61 benzyne,62

and various other cycloadditions.63-65 Preparation of a water-soluble MNF derivative,

Sc3N@C80(OH)~10(O)~10 , was reported by Dorn and coworkers in 2002 through a radical

anion intermediate.66 Carbon-centered radical additions to MNFs have been documented

as well. The addition of CF3 to Sc3N@C80 was reported, where as many as 16 addends

were added to the MNF. 67-69 Other MNF derivatives have been synthesized by addition

6

of diethylmalonate catalyzed with manganese (III) acetate70 as well as a dibenzene

derivative formed from photochemically generated radicals.71

MNFs have been used in exciting applications. The use of water-soluble

gadolinium MNFs has been pursued as the basis for new contrast agents in magnetic

resonance imaging. Gd3N@C80 offers enhanced relaxivity and metal incarceration which

can prevent unwanted side effects that occur with Gd3+ ion exposure.72, 73 The use of

Sc3N@C80 has also been explored for use in photovoltaic applications74 and singlet-

oxygen generation.75

Fullerene-Containing Polymers

In order to harness the unique abilities of fullerenes, many efforts have been made

to incorporate them into a polymer matrix, giving the nanoparticles a three-dimensional

scaffold. There are numerous fullerene-polymer nanocomposites that have been explored.

The variety of possible polymer-fullerene combinations offers potential to tailor the

properties of each component to yield high-performance materials.

There are several different classifications of fullerene-containing polymers. The

two main categories are based on the incorporation method of the fullerene: blending and

covalent incorporation. Blending is commonly performed by dissolving fullerene or

fullerene derivative in a compatible solvent then combining the solution with the desired

polymer. Various applications of such films have been published recently, such as

fullerene-polymer blends combining C60 with polystyrene-block-polybutadiene-block-

polystyrene (SBS) and polystyrene-block-polyisoprene-block-polystyrene (SIS) that

were reported for their stimuli-responsive adhesive properties.76 The same adhesive films

were later investigated for their role in singlet oxygen production in the loss of

7

adhesion.77 SIS-C60 and SIS-Sc3N@C80 blends have also been explored for their anti-

microbial abilities based on singlet-oxygen generation.75 Self-cleaning photocatalytic

polyurethane (PU) coatings containing pristine fullerene and a fullerene derivative were

determined to be effective for chemical decontamination.78 Using blending, arylated

[60]fullerene derivatives were loaded into polystyrene (PS) and

poly(methyl)methacrylate (PMMA) in order to assess their thermal and thermo-oxidative

stability in air, where degradation of PS/C60 was shown by thermogravimetric analysis

(TGA) to increase in thermal stability by up to 45 ⁰C.79 One obvious limitation in the use

of blending fullerene-polymer nanocomposite films is the relatively low loading that can

be well dispersed. Previous examples for blended films contained a maximum of 5.0 wt

% fullerene. Due to the highly hydrophobic nature of pristine fullerene, aggregation can

quickly occur within a modestly polar polymer. Modification of the fullerene core can

improve its dispersion throughout a polymer solution by disrupting some of the

hydrophobic interactions and increasing the affinity of the nanoparticles for the solvent

and/or polymer.79

For the covalent incorporation of fullerene into a polymer matrix, many reports

have utilized the reactivity of the polymer to tether to C60 (Figure 4). For instance, side

chain fullerene-containing polymers can be prepared from linear polymers with side

chains containing groups known to react with fullerenes (Figure 4A). This has been

achieved through many routes including, but not limited to, the reaction of C60 with

azide-functionalized linear PS,80 fullerene cycloaddition with benzocyclobutenone-PS,81,

82 direct reaction of fullerene derivative with polycarbonate in the presence of AlCl3,83

and reaction of carbanion-containing side chain polymers such as PS with C60.84

8

Recently, Ladelta and coworkers reported polymethacrylate systems containing

[60]fullerene pendant groups with loadings of 41.4 wt % for potential organic

photovoltaic (OPV) applications.85

Figure 4. Examples of covalently incorporated fullerene-containing polymers: (A)

fullerene side-chain polymer, (B) fullerene main-chain polymer, (C) crosslinked

fullerene-containing network, (D) fullerene end-capped polymer, (E) fullerene star-

shaped polymer, and (F) all-C60 polymer.

Main-chain fullerene-containing polymers have also been reported, including the

C60 pearl-necklace polymer86 and a thermally recyclable C60-polymer prepared by

cycloaddition with bis-anthracene (Figure 4B).87 One difficulty in the preparation of

main-chain fullerene-containing polymers is the multifunctionality of the fullerene that

can result in multiple additions leading to crosslinking within the polymer. Several in-

depth reviews detail other types of fullerene-containing polymers, such as fullerene end-

9

capped polymers, star-shaped fullerene polymers, and all-C60 polymers (Figures 4D-

F).88-90

Crosslinked fullerene networks offer the ability to create a truly three-dimensional

network (Figure 4C). Several free-radical propagated fullerene-containing crosslinked

C60-containing polymers have been reported, with systems such as PS networks91 and

copolymers of 4-vinylbenzoic acid.92 C60 photo-crosslinked networks have been prepared

by means of a furan-polymer derivative, where polymerization occurred in the presence

of visible light.93 Another photo-crosslinked system is that of C60 with ethylene-

propylene diene monomer which yields a photoconductive film.94

A different strategy for the preparation of crosslinked fullerene-containing

networks utilizes the addition of reactive functionalities to the surface of the

nanoparticles. This allows for subsequent reactions with crosslinkers, resulting in

covalently bound fullerene-polymer composites. This was first reported by Chiang et. al

with the use of polyhydroxylated fullerenols and diisocyanate polyether prepolymer to

yield a C60-polyurethane (PU) nanocomposite.95 Many variations of C60-PU have been

synthesized and summarized in a review by Badamshina and Gafurova.96 More recent

works present composites with unique properties such as electromagnetic absorption for

C60-PU containing nano-barium particles97 and high dielectric permittivity and low loss

values in the case of Sc3N@C80-PU composites.98 Additional modification of fullerenols

has also been explored. Recently, the reaction of fullerenol with allyl isocyanate afforded

an ene-functionalized fullerene via urethane linkage which was subsequently crosslinked

into a thiol-ene thermoset using a tri-functional thiol.99 Films containing up to 27 wt %

10

C60 prepared by this method were also determined to possess high dielectric permittivity

(Scheme 1).

Scheme 1. Strategic functionalization of C60 fullerene to yield crosslinked fullerene-thiol-

ene network via fullerene-urethane linkage. 99

In many cases, the addition of fullerenes into crosslinked networks can enhance

mechanical properties. A fullerene-poly(urethane urea) network experienced a three-fold

increase in crosslink density with improved breaking strength, elastic modulus, and

elongation at break values compared to its control film.96 The addition of fullerenols to

natural rubber showed increased tensile strength, elongation at break, and modulus

11

values, with an observed anti-aging effect after heating that was credited to the radical-

scavenging ability of the fullerenol.100 The radical scavenging ability of the fullerenes is

also responsible for improved thermal stability of various nanocomposite films as well as

anti-oxidant behavior in biological applications.42

“Click” Chemistry and the Thiol-Ene Reaction

In 2001, Sharpless et al.101 coined the term “click chemistry” in reference to a

class of “quick and easy” reactions joined by a heteroatom link. “Click” chemistry is

comprised of efficient, stereospecific reactions prepared from readily available starting

materials. The reactions are insensitive to molecular oxygen as well as water, which is

commonly used as a solvent for “click” reactions, though solvent-free conditions are also

known. These “green” reactions produce little to no byproducts, and purification is

simple. “Click” reactions have revolutionized organic synthesis, as they have simplified

a variety of common classes of reactions, including cycloadditions of unsaturated

molecules, carbonyl chemistry, ring-opening nucleophilic substitution, and additions to

alkenes and alkynes. Scheme 2 demonstrates a fundamental “click” reaction, the Hüisgen

azide-alkyne 1,3-dipolar cycloaddition.102

Scheme 2. The Hüisgen azide-alkyne 1,3-dipolar cycloaddition.

Within the class of “click” reactions resides “thiol-ene” and “thiol-yne”

chemistry, the hydrothiolation of an olefinic bond (alkene and alkyne, respectively).

Although thiol-ene chemistry dates back to Charles Goodyear’s use of sulfur for the

vulcanization of natural rubber (poly(cis-isoprene)), modern resurgence of the field can

12

be attributed to the “click” reaction movement.103, 104 Thiol-ene chemistry offers mild

conditions and high yielding reactions with versatility in choice of monomers. Reactions

can occur under base catalyzed or radical mediated conditions.

Scheme 3. General radical mechanism for the photoinitiation and propagation of a thiol-

ene "click" reaction, where kp is the rate constant for propagation and kct is the rate

constant for chain transfer.

For the radical mediated mechanism, initiation can occur via thermal or

photochemical processes. Thiol-ene reactions can occur without the presence of initiator,

although the rate of the polymerization is significantly decreased. For example, using

tetrakis(3-mercapopropionate) and 1,3,5-triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione,

full conversion from monomer to polymer occurs in 100 s using photoinitiator 2,2-

dimethoxy-2-phenylacetophenone (DMPA), while the intiatorless reaction is complete in

800 s .105-107 The formation of a highly homogeneous network occurs exclusively through

a step-growth free-radical mechanism without the occurrence of homopolymerization or

chain growth.108 The general reaction mechanism for a photoinitiated reaction is show in

Scheme 3. The generation of radicals occurs by cleavage of the photoinitiator, whereby

the initiator radical abstracts a hydrogen atom from the thiol moiety. Propagation of the

reaction continues as the thiyl radical adds to a carbon-carbon multiple bond, resulting in

a carbon-centered radical. The propensity of the carbon-centered radical for hydrogen

13

atom abstraction drives the reaction to continue propagation until termination occurs via

coupling of a combination of thiyl- and carbon-radicals.109

A variety of thiol and ene monomers are commercially available. The most

commonly utilized thiols are the alkyl-3-mercaptopropionates, alkylthioglycolates, and

alkyl thiols, shown in Figure 5.110 The variety of ene structures capable of participating in

thiol-ene reactions is virtually unlimited as any unsaturated carbon-carbon bond will

react. The rate of the reaction depends heavily on the structure of the ene, where the

general trend is that electron-rich enes react more rapidly than electron-poor enes.

Exceptions are the rates of norbornene, which reacts very rapidly due to the relief of ring-

strain in the propagation step, with kp/kCT = 1, showing complete conversion in just 15 s

in the case of (1,6-hexanediol di(endo,exo-norborn-2-ene-5-carboxylate) with tetrakis(3-

mercapopropionate) and DMPA,111 and conjugated systems such as styrene, which result

in resonance-stabilized carbon-centered radicals (kp/kCT =5 x 108), retarding the rate of

chain-transfer.106

Figure 5. General structures of common commercially available thiol monomers: (A)

alkyl-3-mercaptopropionates, (B) alkylthioglycolates, and (C) alkyl thiols.

Perhaps the most unique benefit of thiol-ene chemistry is its insensitivity to

molecular oxygen. While most free radical polymerizations suffer from inhibition by

ambient oxygen, the oxygen scavenging nature of the thiol-ene reaction can transform

ambient oxygen into a reactant, as shown in Scheme 4.112 This gives thiol-ene chemistry

a distinct advantage over other oxygen sensitive polymer systems, such as acrylates.112,

113

14

Scheme 4. Utilization of ambient oxygen as a reactant in a free-radical thiol-ene reaction.

Thiol-ene networks offer excellent physical and mechanical properties.106

Coatings exhibit delayed gelation, which results in superb adhesion to a variety of

surfaces, including glass, wood, plastic, and metal.114, 115 High monomer conversion,

excellent network uniformity, and high crosslink density impart unique mechanical

properties to thiol-ene polymers, granting narrow glass transitions as observed by tan δ

and storage modulus plots determined by dynamic mechanical analysis (DMA).116 This

allows for a tailoring of mechanical properties that is unmatched by other polymer

systems.117The numerous benefits and tunability associated with thiol-ene polymers

renders them ideal candidates for the preparation of a fullerene nanocomposite network.

Polymer Microspheres

Polymer microspheres have a multitude of applications in a variety of fields,

including drug delivery,118-120 solid support catalysis,121 solid phase synthesis,122 solid

phase extraction,123 ion exchange,124, 125 and chromatography.126 Reports of

methodologies for the production of polymer microspheres include emulsion and

suspension,119, 127 precipitation polymerization,123-125 microfluidics,128-130 seed

polymerization,126 and jet break-up techniques.131 Several recent reviews have also

15

summarized these common microsphere production methods.131-134 Batch processes such

as emulsion and suspension techniques are commonly used in industrial settings, but

generally suffer from difficulty in producing monodisperse microspheres with diameters

<10 μm.

Microfluidic and jet break-up techniques have the benefit of producing

microspheres individually, allowing for a continuous production process.130 These

techniques are based on the breaking of the polymer liquid jet into a series of uniform

droplets. Interest in the breaking of a liquid jet into droplets dates back to the late 1800s

when Lord Rayleigh investigated the behavior of a liquid jet exiting a capillary.135 This

principle inspired research in the area of microfluidics, in which two flowing immiscible

phases are joined in a microchannel, resulting in small micron-scale droplets.130, 136-138

Later, techniques that combined a microfluidic flow with vibrational disturbance were

investigated by several groups.118, 128, 139, 140 These combination techniques offer greater

particle uniformity as microspheres are produced individually, with microsphere

diameters smaller than the device orifice credited to jet break-up, vibrational disturbance,

and controlled flow parameters. For instance, Berkland and coworkers applied acoustic

excitation ranging from 1-70 kHz to the nozzle delivering poly(D,L-lactide-co-glycolide)

to a flow-focusing carrier phase.128 This resulted in polymer microspheres with narrow

size distributions that were consistently smaller that the nozzle used for their production.

Choy et al. expanded on this technique through the use of a charged hydrogel polymer

solution, utilizing the electrostatic repulsion to prevent the coalescing of the produced

microspheres.140 Heinzen and company utilized nozzle vibration and polymer flow rate to

16

produce highly reproducible microspheres encapsulated with animal cells and organic

liquids.139

The use of “click” chemistry in the preparation of nano- and microspheres has

emerged as a popular interest due to high yields and versatility. Reports of thiol-ene,127,

141, 142 thiol-yne,129, 132, 143and azide-alkyne 1,3-dipolar cycloaddition144, 145 for the

preparation of functional particles have been published. Thiol “click” reactions have also

been utilized for post-modification of functional particles.146

To use a combination technique for the preparation of individual microspheres by

a continuous process, the construction of a device capable of combining the desired

vibrational disturbance with the choice of polymer and non-solvent carrier flow is

required. Construction of the device and optimization of available parameters must be

performed to prepare microspheres of the desired size dispersion.

Scope of Current Work

The scope of this work is multi-disciplinary, reaching into the areas of

nanoscience, polymer chemistry, physics, and engineering. The simplicity of “click”

chemistry, including straightforward syntheses with robust reaction conditions without

tedious purifications, served as the inspiration to incorporate C60 and Sc3N@C80

nanoparticles into thiol-ene architectures. While there is increasing interest in the area of

fullerene-containing polymers, there are few reports of fullerene-containing thiol-ene

polymer nanocomposites and their characteristics. Previous research within the Buchanan

research lab pursued the incorporation of C60 and Sc3N@C80 through hydroxylation of

the fullerene cage followed by crosslinking into a polyurethane network (Scheme 5).98

During my graduate studies, this work was expanded through the preparation and

17

mechanical and dielectric characterization of a thiol-ene-C60 crosslinked network

(Scheme 5).99 Hydroxylated C60 was reacted with allyl isocyanate to yield an alkene-

terminated fullerene derivative which was crosslinked via thiol-ene reaction using tris(3-

mercaptopropionate). Multiple synthetic steps and moisture-sensitive reaction conditions

were required in the preparation of the networks. During the preparation of these films, it

became evident that a more straightforward synthetic strategy of fullerene-containing

thiol-ene networks could be developed.

18

Scheme 5. Overview of crosslinked fullerene-containing polymer networks from previous

and current work.

In this study, a new series of crosslinked fullerene-thiol-ene networks were

prepared through a facile, one-pot photochemical pre-polymer method. The novel

strategy avoids the complicated purification and characterization steps often associated

with the synthesis of fullerene derivatives by linking the fullerene directly into the

19

polymer network. Preparation of nanocomposite thin films was pursued for the study of

physical and mechanical properties of the networks.

The development of a tunable acoustic-driven technique for the controlled

preparation of thiol-ene microspheres was another aspect of this study. To use a

combination technique for the preparation of individual microspheres by a continuous

process, the construction of a device capable of combining the desired vibrational

disturbance with the choice of polymer and non-solvent carrier flow is required.

Construction of the device and optimization of parameters was performed to ensure

preparation of microspheres with the desired size distribution. Upon thorough

characterization of the acoustic-driven technique for microsphere preparation, C60- and

Sc3N@C80-thiol-ene microspheres were synthesized and studied.

20

CHAPTER II

SYNTHESIS AND CHARACTERIZATION OF FULLERENE-CONTAINING

THIOL-ENE NANOCOMPOSITE THIN FILMS

Introduction

In this chapter, radical mediated "click" chemistry and fullerene’s well known

radical reactivity served as the basis for the one-pot pre-polymer synthesis of a novel line

of fullerene-containing, thiol-ene nanocomposites. A partially oxidized disulfide initiator

was prepared from a commercial tri-functional thiol monomer. This disulfide monomer

was photochemically cleaved to initiate the free-radical reaction of multifunctional thiol

with fullerene (C60 or Sc3N@C80) and multifunctional alkyl ene. This resulted in the

covalent linkages between fullerene and thiol as well as a “building-up” of the network

between the thiol and alkyl ene. Thermal cure of the concentrated pre-polymer resulted in

fullerene-containing thiol-ene films of a variety of compositions, with C60 loadings of 1,

5, 10, 20, and 30 wt % and 1 wt % Sc3N@C80 films. Films were characterized by a

variety of techniques including FT-IR spectroscopy, gel fraction, TGA, and DMA.

Transmission electron microscopy (TEM) was used to visually observe fullerene

dispersion through the thiol-ene matrix.

Materials, Method, and Characterization

Materials

Materials used in the preparation of the thiol-ene-fullerene nanocomposite films

were trimethylolpropane tris-3-mercaptopropionate (TMPMP, ≥95.0%) and

trimethylolpropane diallyl ether (TMPDE, 90%), pentaerythritol allyl ether (APE, 70%

triene; 30% monoene), 1,3,5-triallyl-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione (TTT, 98%)

21

[1,4]diazobicyclo(2.2.2)octane (DABCO, 98%), 2,2’-azobisisobutyronitrile (AIBN,

98%), anhydrous magnesium sulfate (≥97%), and benzene-d6 (99.6 atom % D) obtained

from Aldrich. Benzene (99.0%) and dichloromethane (DCM, ≥99.5%) were purchased

from EMD Chemicals. Irgacure 819 was purchased from Ciba Specialty Chemicals. C60

(>99%) was purchased from MER Corporation. Sc3N@C80 (>97%) fullerenes were

purchased from SES Research. All materials were obtained commercially and used

without further purification unless noted.

Method

Partial oxidation of trimethylolpropane tris-3-mercaptopropionate to disulfide (S-

S). Trimethylolpropane tris-3-mercaptopropionate was added to a dilute solution of

[1,4]diazobicyclo(2.2.2)octane in benzene in a 1:1 molar equivalence. The solution was

aerated at room temperature for 72 hours. Benzene was removed under reduced pressure

and the product was purified by liquid-liquid extraction with DCM and aqueous 0.1 M

HCl (2x) followed by deionized H2O (3x). The organic portion was dried over anhydrous

magnesium sulfate, filtered, and solvent was removed under reduced pressure.

Preparation of fullerene-containing thiol-ene films. Alkyl ene (TMPDE, APE, or

TTT) and S-S were added to a solution of fullerene (C60 or Sc3N@C80) in benzene. Some

films also contained Irgacure 819 commercial photoinitiator at 0.5 molar equivalence

relative to the moles of fullerene. The functional group equivalences were either 1:1

ene/thiol or 0.75:1.0 ene/thiol with respect to alkyl ene. Irradiation of the solution at 254

nm along with mechanical stirring occurred for 15 minutes up to 18 hours, depending on

composition, where compositions containing Irgacure 819 or low wt % C60 had shorter

reaction times. Upon the addition of 1 wt % AIBN, benzene was removed under reduced

22

pressure. Unless noted otherwise, the concentrated pre-polymer was transferred to a

PTFE dish and cured at 75 ⁰C for 24-72 hours. Fullerene mass was 1, 5, 10, 20, or 30 wt

% of the total mass of monomers for C60-containing films and 1 wt % for Sc3N@C80-

containing films.

Characterization

Nuclear magnetic resonance spectroscopy (NMR). 1H-NMR spectra were

obtained at 400 MHz in benzene-d6. Chemical shifts are reported in parts per million

(ppm) relative to benzene peak at 7.15 ppm. 1H-NMR TMPMP (ppm): 4.04 (s, 6H), 2.37

(q, 6H), 2.15 (t, 6H), 1.36 (t, 3H), 1.28 (q, 2H), 0.70 (t, 3H). 1H-NMR S-S (25% yield):

4.04 (m, 6H), 2.69 (t, 1H), 2.45 (t, 1H), 2.37 (m, 5H), 2.15 (m, 5H), 1.36 (t, 2H), 1.28 (q,

2H), 0.70 (m, 3H).

Fourier transform infrared spectroscopy (FT-IR). FT-IR spectra of films were

collected using a Nicolet Nexus 470 FT-IR spectrometer in the range of 500-4000 cm-1.

FT-IR TMPDE (cm-1): 3450 (v, O-H), 3080 (v, =C-H), 2967, 2853 (v, C-H), 1647 (v,

C=C), 1420, 1351 (δ, CH2 & CH3), 1100 (v, C-O), 923 (δ, =CH2). FT-IR 1 wt % C60 film

(cm-1): 3450 (v, O-H), 2967, 2853 (v, C-H), 1730 (v, C=O), 1463, 1351 (δ, CH2 & CH3),

1240, 1141 (v, C-O). FT-IR S-S (cm-1): 2967, 2853 (v, C-H), 2565 (v, S-H), 1730 (v,

C=O), 1463, 1351 (δ, C-H), 1240, 1150 (v, C-O), 673 (v, C-S).

Gel percent. The gel percent of each film was determined by dissolving a known

mass of each film in benzene, resting the sample at room temperature for 24 hours, and

then drying the insoluble portion under reduced pressure. The final mass of the dry,

insoluble portion divided by the initial sample mass is expressed as a percentage.

23

Thermogravimetric analaysis (TGA). The thermal stability of prepared networks

was evaluated using a TA instruments Q5000. Samples were analyzed in a nitrogen

environment using platinum pans over the range of 25-800 °C using heating rate of 10

°C/min. Onset degradation temperatures were calculated using Universal Analysis 2000.

Remaining mass was reported as the percentage of mass present at 600 ⁰C.

Transmission electron microscopy (TEM). Dispersion of fullerenes throughout the

thiol-ene matrix was determined using a Zeiss EM-900 transmission electron microscope

(TEM) operated at 50kV. Thin slices of films were prepared using glass knives in a Leica

EM FC6 ultramicrotome with cryochamber cooled to -60 ⁰C. Samples were applied to

Formvar carbon film on 300 mesh copper grids, purchased from Electron Microscopy

Sciences.

Results and Discussion

In order to avoid the use of commercially available photoinitiators that result in

carbon-centered radicals which are known to add to fullerenes, preparation of a disulfide

initiator, S-S, from the desired tri-functional thiol monomer, TMPMP, was pursued

(Scheme 6). The use of disulfide to produce alkylthio radicals for addition to C60 and C70

has previously been studied by ESR. 46, 47 Studies report the absorption of UV light at 254

nm results in the cleavage of disulfide bond.147 While irradiation at other wavelengths

may also be useful in cleaving S-S bonds, care must be taken to avoid side reactions with

fullerene, which has been reported to form fullerene oxides at 420 nm for C60 and 350 nm

for Sc3N@C80.148 Using thiyl radicals to initiate the thiol-ene reaction with fullerene and

alkyl ene monomers eliminates the need for additional photoinitiator, preventing the

formation of unwanted side products within the forming network.

24

Scheme 6. Oxidation of TMPMP to disulfide (S-S).

Figure 6. 1H-NMR spectrum of TMPMP and S-S, relative to benzene shift at 7.15 ppm.

Asterisk (*) indicates impurity.

4 3 2 1 0

a

* * **

ab c d

ef

ab'bc

c'

d

5H5H 3H2H

3H2H3H

2H1H 1H

6H6H

6H

6H

b'

b

d

c

c'

a

e

f

* **

d'

ef

d'

d

f

eac

b

ppm

25

Characterization of partially oxidized TMPMP/S-S product was performed by 1H-

NMR spectroscopy, shown in Figure 6. All shifts are reported relative to benzene shift at

7.15 ppm. Confirmation of S-S was determined by the appearance of two triplets at 2.69

(c’) and 2.45 (b’) in addition to signals at 2.37 (c) and 2.15 (b) upon the oxidation of the

S-H group to S-S. Integration of signals b & c were added to the remaining integration of

each respective downfield shift (b’ & c’) for a total of 6 hydrogens, the same number

observed with the tri-functional TMPMP monomer. In addition to the shifts in the protons

residing on the α and β carbons, a decrease in integration area occurs in the protons of the

thiol group (a). To estimate the % conversion to S-S, the relative integration of (a) in the

S-S product was divided by the relative integration of (a) in the stock TMPMP. This

value, indicative of the percentage of thiol protons remaining, was subtracted from 100 to

determine the percentage of disulfide bonds formed. This estimates that approximately

25% of the thiol groups were oxidized to disulfide bonds.

Figure 7. Multifunctional thiol and ene monomers and Irgacure 819 photoinitiator used in

the synthesis of C60-thiol-ene nanocomposites.

26

Scheme 7. Competition reaction for addition to C60 occurring between thiyl (RS•) and

photoinitiator radicals (PI•) upon cleavage by UV light, λmax = 254 nm.

The use of multifunctional monomer containing disulfide bonds to serve as

radical initiator was to avoid unwanted side products. C60 readily undergoes radical

reactions with many different radicals, including carbon-centered radicals, which are

formed upon cleavage of Irgacure 819 photoinitiator (Figure 7). This can results in an

unwanted competition reaction (Scheme 7). By using a monomer as initiator, this

competition reaction is eliminated, improving the likelihood of the addition of S-centered

radicals to the fullerene nucleus.

27

Scheme 8. General preparation of "click-inspired" C60-containing thiol-ene films with tri-

functional enes.

Investigation of the one-pot methodology for C60-thiol-ene films was pursued

with tri-functional alkyl enes APE and TTT. Scheme 8 details the synthesis of the APE-

TMPMP nanocomposites. A pre-polymer reaction was utilized for the synthesis of the

fullerene-thiol-ene films. The advantage for this step is two-fold. First, it allows for the

reaction of thiol groups with the fullerenes, improving the miscibility of the nanoparticles

in both the solvent and the monomers. Second, it forms a “building-up” of molecular

weight with thiol, ene, and fullerene monomers as the thiol reacts with both ene species.

28

After the pre-polymer reaction, addition of thermal initiator and removal of solvent yield

a viscous brown pre-polymer where a great enhancement of fullerene solubility and

dispersion were observed (Figure 8). Thermal cure times for fullerene-containing films

were significantly longer than for control thiol-ene samples, and cure time increased with

higher C60 loading. This is due to the radical-scavenging nature of the fullerene, where

the addition of a radical species to the fullerene cage results in a resonance-stabilized

radical, slowing further propagation.

Figure 8. Side-by-side comparison of 1 wt % C60-thiol-ene films prepared by (left)

blending in monomers in the absence of solvent, and (right) covalent incorporation by

pre-polymer photoreaction.

29

Figure 9. 20 wt % C60-thiol-ene films prepared with S-S and tri-functional alkyl enes

resulting in a buckled morphology, where ene monomer and cure conditions are as noted:

(A) APE, 40 ⁰C for 1 h, 60 ⁰C for 1 h, and 80 ⁰C for 12 h, (B) APE, 65 ⁰C 14 hr, (C)

APE, 100 ⁰C 14 h, (D) APE, cycled at RT 48 h, 70 ⁰C 1 h, for two cycles (E) APE,

cycled at 100 ⁰C 10 minutes, RT 14 h, for two cycles (F) TTT, 70 ⁰C 2 h then 85 ⁰C 14 h.

Films produced with the tri-functional enes consistently presented a wrinkled

topology, shown in Figure 9. This phenomenon is referred to as buckling and has been

the topic of much investigation.149-151 When the pre-polymer is heated for thermal curing,

a rigid film forms at the pre-polymer/air interface, which rests on top of the uncured pre-

polymer. This causes a compressive stress to build within the system until the point of

critical strain has been achieved. When critical strain has been reached, the stress is

relieved by buckling of the rigid surface film, which occurs in a sinusoidal pattern.150, 151

For this system, the cause of the compressive stress was investigated, where thermal or

mechanical expansion were investigated as possible causes. In the first attempt to relieve

stress and prevent buckling, a variety of thermal cure conditions were investigated. Films

cured at higher temperatures of 80-100 ⁰C resulted in greatest number of wrinkles, and

A B C

D E F

30

the identity of the tri-functional ene monomer (APE or TTT) did not decrease the degree

of buckling. UV cure conditions were not appropriate for the dark brown pre-polymer as

they resulted in cured skin on the top of the uncured pre-polymer. However, the

substitution of the tri-functional ene with di-functional ene TMPDE alleviated all

buckling (Figure 10). In the tri-ene system, the highly crosslinked network has a

significantly higher modulus value than the underlying pre-polymer.152 When difference

in the moduli values of the film and pre-polymer are great, the modulus mismatch creates

a compressive stress that builds within the system to the point of critical strain, at which

point buckling of the film occurs. With the substitution of the di-ene in place of a tri-ene,

less crosslinking occurs during polymerization lowering the modulus of the film formed

at the air interface. This decrease in the film/pre-polymer moduli difference prevents the

system from surpassing critical strain, thus eliminating mechanical buckling. Due to the

irregularities of the films containing tri-functional ene, only the fullerene-diene series was

selected for full characterization (Scheme 9).

Figure 10. 10 wt % C60-thiol-ene film prepared with diene, TMPDE, cured at 75 ⁰C for

72 hours.

31

Scheme 9. Preparation of "click-inspired" C60-containing thiol-ene films with di-

functional TMPDE.

32

Figure 11. Representative IR spectrum of a 1 wt % C60-thiol-ene fullerene film prepared

with TMPDE.

In Figure 11, IR spectrum of a 1 wt % C60 thiol-ene film prepared with TMPDE

and S-S monomers is shown. In the TMPDE spectrum, peaks at 3080 cm-1 and 924 cm-1

represent C=CH2 stretching and bending, respectively, while C=C stretching is observed

at 1647 cm-1. For the S-S spectrum, the peak present at 2565 cm-1 is due to S-H stretching

of the residual free thiol. The absence of peaks for C=CH2 and S-H in the 1 wt % C60

thiol-ene film demonstrates good monomer conversion. C60 is reported to exhibit weak

absorption at 1428, 1183, 576, and 527 cm-1, all of which are overshadowed by

absorptions present in the monomers.153 Additional strong peaks that are observed are

attributed to the C=O ester stretching observed at 1730 cm-1 and O-H stretching at 3438

cm-1 of which both functional groups are present in monomers and final films.

5001000150020002500300035004000

Wavenumbers (cm-1)

TMPDE

S-S

1 wt % C60 Film

33

Two series of fullerene-containing thiol-ene films were prepared to investigate the

role of C60 and Sc3N@C80 as ene monomers. Ene to thiol functional group equivalence

was maintained at 1:1 ratio for the “1.0” series, which represents an ideal thiol-ene

network. Several other film compositions were prepared with an “ene deficiency”, using

alkyl ene equivalences of 0.5, 0.6, 0.7, 0.8, and 0.9 relative to 1.0 equivalence of thiol in

order to determine the minimum amount of alkyl ene required for the formation of

networks with good mechanical integrity. Compositions prepared with less than 0.7:1

ene/thiol showed poor film quality and gel % values lower than 70%. For this reason, the

second series was prepared with a 0.75:1 ene/thiol functional group equivalence (“0.75”

series) to allow for the study of an ene deficient network with the maximum difference in

composition while still producing high quality films. The motivation for the preparation

of an “ene deficient” network containing fullerenes is to probe the ability of the fullerene

to act as a reactive ene component. A deficiency of alkyl ene is necessary when one

considers the kinetics of the reaction of a thiyl radical with enes of various structures and

electron densities (Scheme 10). As previously described, electron rich alkenes have

higher rates of propagation, where TMPDE has kP/kCT = 10.106, 111 Though rate studies of

the thiol-fullerene radical reactions have yet to be reported in literature, the addition of

thiyl radical to fullerene, which behaves as an electron deficient polyene, will result in a

resonance stabilized carbon-centered radical, where the kP/kCT is expected to be orders of

magnitude greater than that of TMPDE. Study of an ene deficient fullerene-containing

network was pursued to give initial insight into this large discrepancy of reaction rates.

34

Scheme 10. Competition reaction of thiyl radical between fullerene and alkyl ene, where

the rate of propagation with fullerene is expected to be significantly slower due to

resonance-stabilized radical.

Films containing a range of C60 were prepared to probe the maximum

nanoparticle loading that could yield high quality films, where C60 loadings ranging from

1 to 30 wt % were produced. Some films were also prepared using commercial

photoinitiator Irgacure 819 in addition to S-S to act as another radical source for the

initiation of the pre-polymer reaction decreasing required reaction times. Table 1

summarizes the compositions of the films that were prepared, indicating fullerene identity

and loading, photoinitiator source(s), and alkyl ene to thiol functional group

equivalences. Evaluation of the thermal and mechanical properties of the markedly

different film series was performed to determine if the fullerene component serves as the

necessary reactive ene monomer for the thiol groups.

The determination of gel percent of each film gave initial insight to the degree of

monomer incorporation into the network (Table 1). Overall, high gel percentages were

obtained for films of the 0.75 series containing up to 10 wt % C60, suggesting a high

molecular weight polymer network was formed with good incorporation of fullerene and

monomers into the network. This supports the theory that the fullerene can serve as the

reactive ene component for the alkyl ene deficient 0.75 film series. For the 1.0 series, the

35

control film and 1 wt % C60 film gave good gel fractions while the 5 wt % film

experiences a slight decrease in gel percent. The 10 wt % 1.0 film was determined to

have gel percent of only 64%. This is attributed to the excess of ene present due to high

loading of C60 in addition to a stoichiometric equivalence of alkyl ene to thiol monomer.

The soluble fractions obtained from these films were deep purple in color, characteristic

of dissolved C60 in benzene, suggesting that with the excess of alkyl ene and fullerene, a

large percentage of C60 fullerene was not covalently linked into the polymer. This can be

attributed to the faster reaction rate for the formation of a thiol-alkyl ene network over a

thiol-fullerene network, leaving large amounts of fullerene untethered to the polymer.

Analysis of films containing 20 and 30 wt % C60 possessed low gel fractions (< 60% gel),

so further characterization was not pursued. The gel fraction of a 1 wt % Sc3N@C80 1.0

film was collected, yielding a moderate 84 % gel suggesting good reactivity between S-S,

TMPDE, and Sc3N@C80.

Next, 1 wt % C60 and Sc3N@C80-containing thiol-ene films with commercial

photoinitiator, Irgacure 819, were also prepared to analyze the effect of an additional

radical source on the characteristics of the resulting networks. While both networks

experienced significantly faster reaction times of < 1 h, the C60-thiol-ene film containing

Irgacure 819 showed a relatively low gel of 71%, where as the analogous Sc3N@C80-

containing film demonstrated excellent gel of ≈100%. The differences in the electronic

structures of the two fullerenes play an important role in their chemical reactivity. In the

case of C60, up to 15 photochemically generated benzyl radicals add to the empty cage

fullerene.11 Sc3N@C80 behaves much differently in this photochemical reaction, with the

addition of only two benzyl groups.154 The reduction in reactivity observed in Sc3N@C80

36

is due to the stabilizing effect of the encapsulated scandium nitride cluster. The +6 formal

charge of the Sc3N cluster paired with the -6 formal charge of the C80 cage leads to a

larger HOMO-LUMO gap and lower reactivity than the empty C60 cage.59 However,

comparable reactivity has been observed for both C60 and Sc3N@C80 in thermally driven

reactions, as observed with the trifluoromethylation reactions, with the preparation of

C60(CF3)18 and Sc3N@C80(CF3)16.155,156 Using Irgacure 819, the benzoyl radicals from

the photoinitiator can rapidly add to the C60 cage, forming adducts at the molecule’s most

reactive sites. With these reactive sites occupied, C60 is less reactive with S-S monomers,

leaving a pre-polymer with fewer crosslinks. Upon thermal cure, the resulting network

suffers from poor monomer incorporation and ultimately low gel %. In the case of the

larger fullerene, Sc3N@C80, the addition of a small number of benzoyl adducts in the

photochemical reaction does not consume all reactive sites, but serves to improve

solubility and increase the reactivity of the Sc3N@C80 adduct with S-S monomers in the

thermal cure step. This yields a pre-polymer with greater crosslinking, which leads to a

thermally cured film with excellent monomer conversion.

37

Table 1

Sample Composition and Gel % of Thiol-Ene-Fullerene Films

Wt % Fullerene Pre-Polymer Initiator Ene/Thiol Ratio Gel %

0 wt % S-S 0.75 90

1 wt % C60 S-S 0.75 91

5 wt % C60 S-S 0.75 91

10 wt % C60 S-S 0.75 92

1 wt % C60 S-S & Irgacure 819 0.75 71

1 wt % Sc3N@C80 S-S & Irgacure 819 0.75 100

0 wt % S-S 1.0 99

1 wt % C60 S-S 1.0 93

5 wt % C60 S-S 1.0 88

10 wt % C60 S-S 1.0 64

1 wt % Sc3N@C80 S-S 1.0 84

Thermogravimetric analysis was performed to monitor weight loss as a function

of temperature with films with increasing C60 loading. TGA plots for 0.75 and 1.0 are

38

shown in Figures 12 & 13. Mass loss occurring before 200 ⁰C can be attributed to gradual

loss of residual solvent and alkyl ene monomers that were not tethered into the network.79

For both film series, decrease in thermal stability is observed with increased C60 loading.

This is due to a weak fullerene-thiol bond, where average C-S bond-dissociation energy

is 260 kJ/mol. Upon the breaking of the C-S bond, the fullerene in capable of re-

establishing aromaticity within the molecule, which is favorable considering bond-

dissociation of C=C is 728 kJ/mol. With the addition of C60, the calculated degradation

onset temperature decreases approximately 35-41 ⁰C for 1.0 series and 25-29 ⁰C for 0.75

series, though no considerable differences in onset temperature are observed amongst the

various loadings of C60 nanocomposite films (Table 2). A similar trend is observed for the

Sc3N@C80 sample, listed in Table 2. The degradation onset temperature is lower than the

control film while comparable to that of the 1 wt % C60 1.0 film. Reports of increased

thermal stability on polymer networks containing blended fullerene derivatives have

declared that the optimal stabilization occurs at low C60 loadings (0.4-0.8 wt %), where

higher loadings increase aggregation, thus deforming the morphological interactions

between polymer chains and negatively impacting the networks.79 Another study of the

thermal stability of C60-containing PS and PMMA concluded that increased thermal

stability of C60-containing networks occurred only with blended C60 due to its superior

ability of the π-system to “trap” radicals resulting from thermal degradation, while C60

networks containing multiple covalent attachments to the network lacked this

characteristic, thus demonstrating decreased thermal stability.157 The decreasing thermal

stability of the prepared fullerene-thiol-ene networks supports the presence of multiple

covalent linkages of fullerenes into the network.

39

Figure 12. Thermogravimetric analysis of 0.75 C60-thiol-ene film series in N2 atmosphere

where wt % C60 is noted.

Increasing C60 loadings resulted in higher residual mass for each loaded film, as

determined at 600 ⁰C. This remaining mass is attributed to the high thermal stability of

the C60 and Sc3N@C80 cage. 148, 158 It is interesting to note that the remaining mass of

each sample was greater than the combined mass remaining for the control sample plus

fullerene loading. This has been observed in other studies and is believed to be due to one

of the following possibilities: (1) strong interactions occurring between the fullerene cage

and the polymer or initiator in the system that require temperatures greater than 600 ⁰C to

effectively dissociate, or (2) aggregation of the nanoparticles which results in trapped

portions of the polymer.78 Because the thermal stability of the polymer nanocomposites

were negatively impacted by the addition of the fullerenes, it does not seem likely that the

0

10

20

30

40

50

60

70

80

90

100

50 150 250 350 450 550

Weig

ht

(%)

Temperature (⁰ C)

10%

0%

10%

0%

40

increase in remaining mass is due to strong interaction between the fullerene and

polymer. However, the thermal initiator AIBN is known to add to fullerenes, resulting in

a fullerene-carbon bond that is likely stable at high temperatures.45 This, paired with the

likelihood of polymer trapped by fullerene aggregates in samples with higher C60 loading,

could account for the increase in remaining mass beyond the added fullerene mass %.

Figure 13. Thermogravimetric analysis of 1.0 C60-thiol-ene film series in N2 atmosphere

where wt % C60 is noted.

0

10

20

30

40

50

60

70

80

90

100

50 150 250 350 450 550

Weig

ht (%

)

Temperature (⁰C)

10%

0%

10%

0%

41

Table 2

TGA Results of C60-Thiol-Ene Films Prepared With S-S Photoinitiator

Sample Ene/thiol ratio Degradation Onset

Temperature (⁰C)

% Residual Mass

at 600 ⁰C

0 wt% 0.75 318 1.6 a

1 wt% C60 0.75 289 5.5

5 wt% C60 0.75 294 11.1

10 wt% C60 0.75 291 13.2

0 wt% 1.0 329 1.3

1 wt% C60 1.0 294 4.7

5 wt% C60 1.0 291 9.0

10 wt% C60 1.0 288 17.2

1 wt% Sc3N@C80 1.0 295 4.7

a Remaining mass percentage for this sample was calculated by the remaining mass at 588 ⁰C.

Dynamic mechanical analysis of the fullerene-thiol-ene nanocomposites was

chosen to monitor the stress-strain relationship of the materials over a wide temperature

range at a given frequency. DMA is useful in the study of materials with viscoelastic

behavior as it monitors the change in modulus over temperature. By cyclically deforming

42

the material, information can be obtained about its relaxation behavior. The complex

dynamic modulus, E*, relates the properties of storage, E’, to loss, E”, in the following

Equation 1:

E* = E’ + iE” (1)

E’ is the measure of strain energy that is recoverable, whereas E” is the measure

of strain energy that is dissipated, and i is the square root of -1. Loss and storage are

related by the phase angle δ in the loss tangent (Equation 2), where:

tan δ = E”/E’ (2)

which describes the damping ability of the material at any given E’ and E” values as

related by the tan δ.

43

Table 3

Dynamic Mechanical Analysis of Fullerene-Thiol-Ene Films

Sample E’ @ 25 ⁰C

(MPa)

Tg, ⁰C

(DMA)

Tan δ Tan δ W @

½ h (⁰C)

Calculated XLD

@ 25 ⁰C (mol/cm3)

0 wt% 0.75 2.5 -14 1.7 11.3 338

1 wt% C60 0.75 4.6 -5 1.6 12.9 625

5 wt% C60 0.75 7.6 -5 1.3 15.8 1027

10 wt% C60 0.75 10.2 +5 1.0 18.0 1368

1 wt% C60 Irg 819 0.75 0.31 -4 2.1 17.2 41

1 wt% Sc3N@C80 Irg

819 0.75

2.1 +3 1.4 15.1 280

0 wt% 1.0 5.8 -6 1.6 11.7 782

1 wt% C60 1.0 0.9 -10 1.5 21.7 124

5 wt% C60 1.0 2.5 -9 1.3 18.2 331

10 wt% C60 1.0 4.5 -12 1.2 20.4 603

1 wt% Sc3N@C80 1.0 7.7 +10 1.5 13.6 1039

44

Figure 14. Dynamic storage moduli (E') of 0.75 series C60-thiol-ene films.

The storage modulus, E’, for 0.75 series is given in Figure 14. The materials

possess a high modulus at low temperatures, characteristic of glassy materials. As the

temperature is increased, a sharp decline in E’ occurs when the materials pass through

their glass transition temperature, Tg. During this transition, conformational changes

occur within the molecular chains. Beyond the glass transition, a lower modulus is

achieved in the rubbery plateau. For the ene-deficient 0.75 series, a sequential increase is

observed for E’ in the rubbery region, where the highest storage value of 10.2 MPa is

observed for the 10 wt % C60-thiol-ene film at 25 ⁰C (Table 3). This suggests an increase

in the rigidity of the 0.75 films with increasing amounts C60. The loss tangent plot for

0.75 film series is shown in Figure 15. The glass transition temperature can more clearly

0.001

0.01

0.1

1

10

-80 -40 0 40

log E

' (G

Pa)

Temperature (⁰C)

0%

10%

1%

5%

45

be identified by evaluating the temperature at which the maximum loss tangent value

occurs. Tg value increases substantially with the addition of C60, from -14 ⁰C for the

control sample to -5 ⁰C for films containing 1 and 5 wt %. The addition of 10 wt % C60

yields another large increase in Tg to +5 ⁰C. The increased Tg is attributed to the

formation of C60-thiol bonds, where the bound spherical nanoparticles impart stiffness to

the network, thus increasing the glass transition. Values for these transitions are also

noted in Table 3.

The maximum tan δ values are indicative of the material’s damping ability. The

greatest damping for the 0.75 series occurs within the control film, with a value of 1.7.

This film has a great excess of thiol groups relative to alkene functionalities, so

untethered ends are expected from this sample. The decrease in crosslinks gives this

sample high loss modulus during the transition from glassy to rubbery. As the percentage

of C60 in the samples is increased, a decrease in the tan δ value is observed. This can be

attributed to the ene functionalities contributed by C60, wherein the fullerene component

becomes incorporated as part of the network and behaves less like an additive, as

observed with the 10 wt % 0.75 film. This results in more complete crosslinking with the

tri-functional thiol groups but decreases the material’s damping ability. Additionally,

investigation of the width of the tan δ peak can give insight into the structure of the

crosslinked network. The control thiol-ene network has a narrow tan δ, as determined by

the peak width at ½ maximum height (values given in Table 3), indicating a uniform

network. The addition of C60 into the network causes a broadening of the tan δ peak,

indicative of increasing heterogeneity of the network, where the maximum network

heterogeneity is observed in the film with the highest C60 percentage.

46

Figure 15. Loss tangent (tan δ) plot of 0.75 series C60-thiol-ene films.

Dynamic storage modulus plot of the 1.0 film series is presented in Figure 16. All

materials show high modulus values consistent with glassy materials at low temperature

and lower storage values as the temperature increases through the glass transition into the

rubbery region. However, this film composition gives a significantly different trend than

that seen in the 0.75 series. It is important to recognize the 1.0 control sample as an ideal

thiol-ene network, unlike the 0.75 control film. Addition of only 1 wt % of C60 causes a

considerable decrease in the storage modulus in the rubbery plateau due to decrease in the

network linking density. Upon the increased addition of C60, E’ increases as a function of

increase in rigid component, although the 10 wt % addition of C60 does not return the

storage modulus value to that of the ideal network. E’ values are listed in Table 3.

0

0.5

1

1.5

2

2.5

-50 -30 -10 10 30 50

tan

δ

Temperature (⁰C)

0%

10%

1%

5%

47

Figure 16. Dynamic storage moduli (E’) of 1.0 series C60-thiol-ene films.

Figure 17 shows the tan δ plot for the 1.0 film series. Glass transition

temperatures in Table 3 are reported as the temperature corresponding to the maximum of

the tan δ peak. The 1.0 control film presents a Tg of -6 ⁰C, and a slight decrease in the Tg

to -10 ⁰C is observed for 1 wt % 1.0 film, which can be attributed to the lower amount of

crosslinking that occurs in the presence of the bulky C60 molecules. Increasing C60 content

to 5 and 10 wt % shows the influence of the competing factors of decreased crosslink

density with increase in rigid component where Tg values remain relatively close to the

value of the 1 wt % sample. This plasticizing effect may be caused by the formation of

fullerene-thiol bonds, where an insufficient amount of multifunctional thiol was available

to act as a crosslinker for both the C60 and alkyl ene, leaving “dangling” fullerene-capped

ends within the network. Similar findings were reported in fullerenol containing

0.0001

0.001

0.01

0.1

1

10

-80 -40 0 40

log E

' (G

Pa)

Temperature (⁰C)

1%

5%

10%

0%

48

polydimethylsiloxane networks where the polymer chains were functionalized with

fullerenols primarily on the side chains, leaving greater mobility in the network.159 The

more obvious effect of the increasing rigidity of the networks is observed with the

decrease in the tan δ values with the addition of C60, where 10 wt % 1.0 film has a tan δ

value of 1.2, indicating a network with lowered damping ability. The networks become

increasingly heterogeneous as C60 is added as determined by the broadening of the tan δ

peak. Increased heterogeneity is credited to the competition of the favored thiol-alkyl ene

reaction, which occurs predominately over the thiol-fullerene reaction, though this is also

present. Little covalent linking of the C60 molecules into the thiol-ene network is

expected since a stoichiometric equivalent of alkyl ene was available to react with the

thiol.

Figure 17. Loss tangent (tan δ) plot of 1.0 series C60-thiol-ene films.

0

0.5

1

1.5

2

-50 -30 -10 10 30 50

tan

δ

Temperature (⁰C)

0%

10%

1%

5%

49

Figure 18. Dynamic storage moduli of fullerene-thiol-ene films prepared with Irgacure

819 and/or Sc3N@C80.

E’ analysis of films containing Sc3N@C80 as well as those prepared with

commercial photoinitiator are displayed in Figure 18, with values listed in Table 3. All

films demonstrate glassy behavior at low temperatures and rubbery behavior at

temperatures beyond the glass transition. The 1 wt % Sc3N@C80 1.0 film shows the

highest E’ value in the rubbery plateau of all other films within the 1.0 series, indicating

the incorporation of Sc3N@C80 serves to increase the rigidity of the network to a greater

extent than C60, even at low loading. This can be attributed to an innate difference of the

Sc3N@C80 molecule compared to C60. A similar trend is seen with the 1 wt %

Sc3N@C80-Irgacure 819 film when compared to its C60 analogue, where the Sc3N@C80

sample gives higher E’ value than C60-containing sample. However, when the two

0.0001

0.001

0.01

0.1

1

10

-80 -40 0 40

log E

' (G

Pa)

Temperature (⁰C)

1% Sc3N@C80 + Irg 819

1% C60 + Irg 819

1% Sc3N@C80

1% C60

50

samples prepared with Irgacure 819 are compared to other films in the 0.75 series, it

becomes evident that the addition of commercial photoinitiator greatly depresses E’

values, indicating the networks are more elastic in nature. The lowered E’ values support

the addition of the Irgacure 819 radicals to the fullerenes which occupies some of the

reactive sites that would otherwise react with thiyl radicals. The decrease in available

reactive sites results in lower crosslinking throughout the network compared to the

networks initiated with S-S alone.

In Figure 19, tan δ plots of the films are displayed. Glass transition information

was obtained by the temperature corresponding to the maximum of the tan δ curve for

each sample. The sample containing 1 wt % C60 + Irgacure 819 was found to have a Tg of

-4 ⁰C, very similar to that of the film with 1 wt % C60 without Irgacure 819, -5 ⁰C,

suggesting comparable incorporation of C60. Both films containing Sc3N@C80

demonstrate higher glass transitions than their C60 counterparts as the Sc3N@C80

nanoparticles are larger in diameter and behave as a “harder” component. The sample

containing Sc3N@C80 + Irgacure 819 possessed a Tg of 4 ⁰C, while the 1 wt % Sc3N@C80

1.0 thiol-ene film that shows the highest Tg of all other prepared films with a value of 10

⁰C. Comparing the two Sc3N@C80-containing films, benzoyl radicals are expected to add

to the Sc3N@C80 cage during the pre-polymer step of the network containing Irgacure

819, decreasing the number of Sc3N@C80-S bonds when compared to the Sc3N@C80 film

initiated with S-S alone. The addition of the benzoyl addends increases the free volume

of the Sc3N@C80 within the network, causing lowered Tg value as well as depressed E’.

Evaluation of the height and width of the tan δ curves helps to elucidate the structure of

the networks. For the C60 + Irgacure sample, a high tan δ value of 2.1 is observed,

51

indicating a large dissipation of energy occurs upon stress due to more free volume

within the sample, i.e., more dangling polymer chains and/or C60 molecules. This is

consistent with information obtained from gel % information. The two samples

containing Sc3N@C80 have tan δ values comparable to other 1 wt % C60 films of their

respective compositions, at 1.4 for the composition initiated with Irgacure 819 and 1.5 for

the network with S-S alone. The width at ½ height for the tan δ curve of C60 + Irgacure

film shows a very broad base indicative of a non-uniform network, which is expected in a

network with sub-par monomer incorporation. Again, both Sc3N@C80-containing samples

show behavior more typical of a thiol-ene network with high conversion with narrow

glass transitions and symmetrical curves, implying greater homogeneity throughout the

network.

52

Figure 19. Tan δ plot for fullerene-thiol-ene films prepared with Irgacure 819 and/or

Sc3N@C80.

When small deformations are applied to a crosslinked network during modulus

measurements, conformational changes can occur without the cleavage of bonds. In this

case, the value of the E’ in the rubbery region is related to the crosslink density (XLD) of

the network:

ve = E’/3RT (3)

where ve is the crosslink density in mol/m3, T is the temperature in Kelvin, and R is the

gas constant, 8.314 J/K·mol.160 Using E’ values at 25 ⁰C, the crosslink density of each

film was calculated, with values provided in Table 3 and Figure 20. For the 1.0 series,

the control film demonstrates the highest crosslink density which is characteristic of

thiol-ene films prepared with monomers of low molecular weight. Addition of 1 wt % C60

0

0.5

1

1.5

2

2.5

-50 -30 -10 10 30 50

tan

δ

Temperature (⁰C)

1% Sc3N@C80

1% Sc3N@C80 +

Irgacure 819

1% C60 + Irgacure 819

1% C60

53

decreases the XLD significantly, though higher loadings of C60 serve to slightly increase

the value. Addition of C60 to the 1.0 ideal network serves to interrupt the rapid reaction of

the thiol and alkyl ene monomers by acting as a competing ene. This disruption decreases

the number of possible bonds formed between thiol and alkyl ene, as few C60-S bonds are

formed, effectively decreasing the network’s crosslink density. In contrast, the 0.75 films

show great increase in the XLD value upon the addition of 1 wt % C60. The trend

continues with each increase in C60 loading leading to XLD values much greater than that

of the ideal 1.0 thiol-ene control sample. This can be credited to the hypercrosslinking

ability of the C60 cage and the encouraged reaction of thiol with C60 in an ene-deficient

environment. The 1 wt % Sc3N@C80 1.0 film also surpassed the ideal control thiol-ene

film in crosslink density. The larger Sc3N@C80 cage contains a greater number of π-

bonds by which to react with thiol groups and link into the thiol-ene polymer. Stark

differences are observed for Fullerene + Irgacure films, where the addition of commercial

photoinitiator decreases the amount of crosslinking occurring in the samples. Addition of

the benzoyl radical initiator fragments react with the fullerene molecules, decreasing the

number of fullerene-thiol linkages they can form in the thiol-ene network.

54

Figure 20. Calculated crosslink density (XLD) for all films at 25 ⁰C.

Figure 21. TEM image of 1 wt % 0.75 C60-thiol-ene film which appears continuous

throughout and no aggregation of nanoparticles is observed.

0

200

400

600

800

1000

1200

1400

1600

XL

D (

mo

l/m

3)

0.75 C=C/SH

1.0 C=C/SH

55

Figure 22. TEM image of 1 wt % 1.0 C60-thiol-ene film which appears continuous

throughout and no aggregation of nanoparticles is observed.

Transmission electron microscopy was utilized for visual analysis of fullerene

dispersion throughout films containing 1 and 10 wt % fullerene.. Thin sections of each

film were imaged to obtain information about the nanoparticle dispersion. Figures 21 &

22 show 1 wt % C60 thiol-ene films of both 0.75 and 1.0 series, which appear uniform

throughout the sample. Figures 23-25 shows that no apparent aggregation is observable

within the C60 + Irgacure 819, Sc3N@C80 + Irgacure 819, or the Sc3N@C80 thiol-ene

films at 1 wt % nanoparticle loading. TEM microscopy indicates that the pre-polymer

synthesis step successfully improves the solubility of both C60 and Sc3N@C80 in the S-S

and TMPDE monomers resulting in thiol-ene networks with a good dispersion of the

fullerene component.

56

Figure 23. TEM images of 1 wt % 1.0 Sc3N@C80-thiol-ene film where no aggregation is

visible.

Figure 24. TEM image of 1 wt % 0.75 Sc3N@C80-Irgacure 819 thiol-ene film with no

visible aggregation.

57

Figure 25. TEM images of 1 wt % 0.75 C60-Irgacure 819 thiol-ene film where no

aggregation is observed.

Looking to compositions containing higher loadings of C60, the 10 wt % 0.75 C60-

thiol-ene film was visualized. Figure 26 shows evidence of C60 microstructure or ordered

aggregation of nanoparticles occurring continuously through the film, further supporting

the incorporation of C60 as a reactive network component in an ene-deficient

environment. This structure is consistent with the increased rigidity of the network with

the decreased height and broadening of the tan δ peak from the DMA analysis.

Comparing this to the 10 wt % 1.0 C60-thiol-ene film (Figure 27), significantly less order

of the nanoparticles is observed. Aggregation is evident, but appears more sporadic

throughout the network, with larger clusters of C60 in some regions and their absence in

others. This lends strong support to the behavior of C60 as an additive in an “ideal”

environment containing a stoichiometric equivalent of the more reactive alkyl ene.

58

Figure 26. TEM image of 10 wt % 0.75 C60-thiol-ene, where aggregation of C60 particles

appears consistently throughout the network.

Figure 27. TEM image of 10 wt % 1.0 C60-thiol-ene network, where aggregation of C60

particles is not continuous throughout the sample.

59

CHAPTER III

ACOUSTIC EXCITATION COAXIAL FLOW METHOD FOR CONTROLLED

PRODUCTION OF FULLERENE-CONTAINING THIOL-ENE MICROSPHERES

Introduction

As the list of applications for fullerene-polymer nanocomposites continues to

grow, it stands to reason that the architectures of such materials be expanded beyond

films. The development of C60 and Sc3N@C80-containing thiol-ene microspheres allows

for the development of a host of new functional materials.

The development of a microsphere preparation technique with multiple physical

elements to control resulting microsphere diameter was necessary (Figure 28).

Combination of several components was required to offer the desired characteristics to

the experimental system. Acoustic energy was utilized to disrupt the formation of

growing monomer droplets, which was made possible by implementing a waveform

generator to control acoustic perturbation. Addition of a continuous carrier solution was

also desired to flow past the point of monomer droplet formation, shearing away the

monomer droplets delivered by a syringe. This was made possible by means of a

peristaltic pump, allowing for control of a carrier flow rate. A simple device was

designed that would combine the delivery of acoustic energy to a needle containing the

flowing monomer solution. To prevent the coalescing of the monomer droplets, the

droplets were immediately exposed to UV irradiation upon exiting the device, initiating

photochemical polymerization. The microsphere dispersion was collected in a bath

contained within a photochemical reactor, where the completion of photopolymerization

can occur. The combined performance requirements of each component lead to the

60

development of the low-frequency, acoustic excitation coaxial flow (AECF) method

described herein. This method offers tuning of variables including acoustic wave

amplitude & frequency and non-solvent continuous carrier flow, along with continuous

microsphere production. Fast and convenient photopolymerization capability paves the

way for the production of microspheres for many different applications. Well-established

thiol-ene chemistry was utilized in the early stages of AECF method development to

optimize the device, eventually leading to the preparation of composite networks. Using

the “click inspired” preparation of fullerene-thiol-ene pre-polymer, C60 and Sc3N@C80-

containing microspheres were produced and studied.

Figure 28. Elements that were combined in the development of the AECF experimental

technique.

Materials, Method, and Characterization

Materials

Materials for the production of thiol-ene microspheres include: sodium dodecyl

sulfate (SDS, ≥90%), pentaerythritol allyl ether (APE, 70% triene; 30% monoene), 1,3,5-

triallyl-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione (TTT, 98%) trimethylolpropane tris-3-

61

mercaptopropionate (TMPMP, ≥ 95%) hexane (mixture of isomers, ≥ 98.5%), and

hydrogen peroxide (H2O2, 30 wt % in H2O) obtained from Sigma-Aldrich. Sulfuric acid

(ACS Certified Plus) was obtained from Fisher Scientific. Irgacure 819 was purchased

from Ciba Specialty Chemicals. C60 (>99%) was purchased from MER Corporation.

Sc3N@C80 (>97%) fullerenes were purchased from SES Research. Deionized water was

purified using Elix® Advantage water purification system. All reagents were used

without further purification.

Method

Acoustic Excitation Coaxial Flow Instrumentation and Methodology. Thiol-ene

microspheres were prepared using an acoustic excitation coaxial flow (AECF) method,

illustrated in Figure 29. The AECF body is composed of acrylonitrile butadiene styrene

shaped by means of a Makerbot® Replicator® 2X 3D printer. With an electric field

applied across the polarized material, induced dipoles align themselves with the electric

field and the transducer changes dimensions through a phenomenon known as reverse

piezoelectric effect, or electrostriction. The movement of the material driven by the

excitation energy produces molecular pressures at the boundary interface, thus creating

wavefronts of propagating pressures at the frequency of the driven force. The parameters

of the acoustic energy delivered to the AECF device are controlled with a Hewlett

Packard 33120A arbitrary waveform generator, with control of frequency and amplitude.

Delivery of the 0.5 wt % SDS non-solvent continuous flow solution to the AECF body is

controlled by the Rainin Dynamax peristaltic pump via 1/8 inch inner diameter Tygon

tubing at a rate of 50 mL/min unless noted otherwise. The experimental setup is given in

Figure 29.

62

Figure 29. Acoustic excitation coaxial flow (AECF) experimental setup.

Characterization of AECF method. A quantitative investigation of this novel

method was necessary to understand optimal settings for microsphere production,

specifically, applied frequency and amplitude. An investigation of the frequency response

of the AECF device was performed using a a piezoelectric PVDF polymer film with a

mass near the end of the cantilever. to measure the activity over a range of settings in

question. A Measurement Specialties LDT0-028K cantilevered piezoelectric transducer

was attached to the AECF housing, opposite the piezo excitation driver. The transducer

functioned as a cantilever-beam accelerometer and was oriented in the same vibrational

plane as the acoustic transducer. The output response given by the thin film provided a

direct correlation of electrical excitation energy with regard to resultant mechanical

oscillation. The measured variation in housing position thus implied a parallel variation

Signal Generator Photoreactor

Peristaltic

Pump

Tactile

Transducer

Carrier Solution

Monomers

+ Initiator

63

in the region where droplets form at the tip of the needle, due to the rigid nature of the

fixture. This disturbance was observed as a voltage output of the thin film, using the

Tektronix TDS 2012B oscilloscope. Experimental conditions were simulated during

testing to ensure that the collected measurements corresponded to those of an actual

experiment. The energy profile of the AECF device was derived by applying a sine wave

input with a frequency sweep from 50-170 Hz at amplitudes of 1, 2, 3, and 4 volts peak-

to-peak (VPP). Frequencies below 50 Hz and amplitudes above 4 VPP were omitted from

this study as they were determined to be damaging to the tactile transducer in this

application.

Preparation of Thiol-Ene Microspheres via AECF Method. Irgacure 819

photoinitiator (0.1 wt%) was dissolved into a tri-functional ene (APE or TTT) or ene

solution by sonication. Subsequently, addition of TMPMP to ene at 1:1 thiol-ene

functional group equivalence was mixed for 1 minute using a Fisher Scientific Vortex

Mixer. The monomer solution was immediately delivered to the AECF device through a

light-blocked BD Luer-Lok single-use 5 mL syringe equipped with a BD Luer-Lok

single-use 27 Gauge 1 ¼ inch needle. The monomer solution was gravity-fed through the

needle. A suspension of thiol-ene droplets was produced within the AECF device where

the monomer solution droplet is sheared away from the needle tip by the surfactant

continuous flow. The flowing suspension was collected in a vessel with mechanical

stirring inside a Rayonet Photochemical Reactor possessing UVA lamps centered at 350

nm for 20 minutes to ensure polymerization. The resulting thiol-ene microspheres were

collected via vacuum filtration with 0.45 μm polyamide filter membrane, rinsed

thoroughly with distilled water, and dried at 50 ⁰C for 1 hour.

64

Preparation of C60 and Sc3N@C80 thiol-ene microspheres. Preparation of a C60-

containing pre-polymer solution was completed by combining partially oxidized

TMPMP/S-S (synthesis described in Chapter II) and ene (TTT or TMPDE,1:1 functional

group equivalence) in a solution of C60 in benzene, where the fullerene component was 1

wt % of the total mass of monomers. The solution was mechanically stirred while

irradiated with lamps centered at λmax = 254 nm for 2 hours, in which time the solution

changed from purple to faint red. Upon the addition of 1 wt % Irgacure 819, the pre-

polymer solution was concentrated under reduced pressure and the desired dilution was

prepared by addition or removal of benzene. Pre-polymer solution was transferred to the

syringe of the AECF setup and microsphere production occurred as previously described,

with applied sine waveform frequency of 77 Hz at an amplitude of 4 VPP, where the SDS

continuous phase flowed at 50 mL/min. Prepared microspheres were irradiated at λmax

350 nm for 16 hours before collection by vacuum filtration.

Sc3N@C80 thiol-ene pre-polymer solution was prepared using 1:1 functional

group equivalence of TMPMP/S-S with TTT in a solution of Irgacure 819 and Sc3N@C80

(1:2 molar equivalence) in benzene, where Sc3N@C80 was 1 wt % of the total mass of

monomers. The solution was mechanically stirred while irradiated with lamps centered at

254 nm for 30 minutes at which point the pale brown solution became slightly cloudy. A

second molar equivalence of Irgacure 819 was added, the solution was concentrated by

rotary evaporation, and the % benzene dilution was determined to be 165% by mass.

Microsphere production occurring as previously described, with continuous flow rates of

25 and 50 mL/min at 4 VPP and 77 Hz, then irradiated at 350 nm for 16 hours before

65

collection by vacuum filtration. The rinsed collected microspheres were dried at 50 ⁰C for

1 hour.

Optical microscopy (OM). Optical microscopy images were collected using a

Keyence VHX-600 digital microscope. Dry microspheres samples were collected from

dry filter membrane and placed on a clean glass microscope slide or, in the case of the

dark colored fullerene-containing microspheres, observed directly on the filter

membrane.

Dynamic light scattering (DLS). Particle size analysis was obtained with a

Microtrac S3500 instrumentation suite. Thiol-ene microspheres were treated as

transparent spherical particles with a refractive index of 1.59. The mean number (MN)

distributions and standard deviations (SD) of the dried microspheres were reported.

Scanning electron microscopy (SEM). A Zeiss Sigma VP scanning electron

microscope was used to obtain high resolution images of thiol-ene microspheres. Images

were collected at 5-10 kV with 1.5-68k magnifications.

Differential scanning calorimetry (DSC). TA instruments modulated differential

scanning calorimetry (DSC) Q2000 instrument was used to determine the glass transition

temperature of collected MS over the temperature range of -50 to 50 ⁰C in a

heat/cool/heat cycle at 5 ⁰C/min. Tg information was obtained from the second heat cycle.

Thermogravimetric analysis (TGA). The thermal stability of prepared networks

was evaluated using a TA instruments Q5000. Samples were analyzed in a nitrogen

environment using platinum pans over the range of 25-800 °C using heating rate of 10

°C/min. Onset degradation temperatures were calculated using Universal Analysis 2000

program. Remaining mass was reported as the percentage of mass present at 600 ⁰C.

66

Results and Discussion

AECF experimental setup characterization. The results of the frequency response

for the AECF device are given in Figure 30 below. The study revealed a maximum output

voltage occurred at applied frequencies ranging from 77-80 Hz for each amplitude

investigated. The maximum output voltage increased linearly as amplitude is increased

from 1 to 4 VPP, as seen in the inset of Figure 30. Resonance of the device with the

applied frequency is observed in essentially the same frequency range for all values of

applied voltage. Aberrations in the system response curve were due, in part, to

constructive and destructive interference, which occurred when the body of the device

was mechanically overdriven at and around the resonant frequency.

Figure 30. Results of AECF characterization: Output voltage measured as a result of the

applied frequencies at different amplitudes. Inset: Maximum output voltage measured at

tested amplitudes.

67

Having determined settings for the AECF method that would subject the

formation of monomer droplets to a range of mechanical disturbances, several

experimental frequencies were chosen for the preparation of APE-TMPMP microspheres

at constant amplitude (4 VPP) to analyze the influence of applied frequency on the

resulting MS diameter. Excitation values were selected from the 77 Hz resonant

frequency to 130 Hz as the applied frequencies for the preparation of thiol-ene

microspheres. Driving frequencies less than 77 Hz were not applied, as they were

observed to negatively perturb the transducer housing and not be useful for efficient bead

production. Initial experiments were performed with APE and TMPMP as the chemistry

is well-understood and thoroughly characterized. The results for the preliminary

experiments are given in Table 4. Optimal experimental conditions would offer little

variation in microsphere diameter over the course of several experiments. This was

observed for experimental frequencies of 77 Hz and 90 Hz; however, experiments

performed at 90 Hz produced very low yields of the microspheres, complicating sample

characterization. For the other frequencies that were tested, large variation in the

microsphere diameter according to DLS mean number value and optical micrographs

eliminated these frequencies from further study.

Having determined an appropriate applied frequency, investigation of the effect of

amplitude on microsphere production began. Maintaining a constant input frequency of

77 Hz, the amplitude of the applied waveform was varied from 1 to 4 VPP, with results

listed in Table 4. While the amplitude of the acoustic waves did not greatly impact the

average diameter of the resulting microspheres, it did appear to affect the precision of the

size range according to SD values. In Figure 31, images of thiol-ene microspheres

68

collected by OM and SEM are shown to be in agreement with DLS measurements. It was

determined that the amplitude of 4 VPP combined with the frequency of 77 Hz offered

the most control of droplet formation and, ultimately, microsphere production.

69

Table 4

Study of Effect of Frequency and Amplitude on Microsphere Diameter Using APE-

TMPMP

Frequency (Hz) Amplitude (VPP) MN (μm) SD (μm)

77 4 3.4 – 4.1 a -

80 4 4.1 – 183 a -

90 4 2.1 – 3.8 a -

100 4 2.5 – 118 a -

130 4 3.7 – 13.5 a -

77 1 3.8 b ±1.0

77 2 4.7 b ±3.4

77 3 4.4 b ±1.4

77 4 3.7 b ±0.41

a Mean number range of three microsphere samples. b Average mean number based on three microsphere samples.

70

Figure 31. Effect of amplitude on microspheres prepared using APE-TMPMP at 77 Hz,

where OM images are displayed in series A, SEM images displayed in series B, and

number in upper left corner indicates amplitude in VPP.

71

Figure 32. DSC thermograms of APE-TMPMP and TTT-TMPMP network, where glass

transition temperatures are noted.

In order for the AECF method to be a useful technique, it must be able to produce

microspheres of many different compositions with various physical properties. The

investigation of a new monomer composition was pursued with triallyl-s-triazine-

2,4,6(1H, 3H, 5H)-trione (TTT) with TMPMP. TTT is a tri-functional ene monomer that

is similar in reactivity to APE, but due to its cyclic structure, yields a more rigid polymer

network with TMPMP, giving a Tg value of 19 ⁰C,107 compared to the -15 ⁰C for APE-

TMPMP (Figure 32).161 Initial experiments with TTT-TMPMP performed under

previously described conditions of 1:1 ene/thiol yielded 2-5 visibly large microspheres in

a 20 minute experiment, which are too few microspheres for characterization. Low

microsphere production was attributed to an increase in the dynamic viscosity value (μ)

of TTT, 14.2 mPa·s, when compared to APE, 4.61 mPa·s, as determined by TA

Instruments AR-G2 Rheometer. Though small, the increase in viscosity of the discrete

phase was enough to prevent the flow of the monomer mixture through the needle at a

rate necessary to produce a sufficient amount of microspheres. The solution to this

-30 -10 10 30 50

Hea

t F

low

(W

/g)

Temperature (⁰C)

TTT-TMPMP

19⁰C

APE-TMPMP

-15⁰C

72

problem came with an investigation of the viscosity of the discrete phase. Ene dilutions

were prepared by the addition of a specific mass of hexane- 1, 2.5, 5, 7.5, or 10 wt%-

holding all other experimental conditions constant. The miscibility of hexane with TTT

and TMPMP monomers along with its significantly lower viscosity, 0.326 mPa·s at 20

⁰C, make it a suitable diluent for this study.162 The various diluted samples were

subjected to 77 Hz at 4 VPP by AECF method and the results are presented in Table 5. A

decrease in microsphere diameter is seen with increased hexane content, with 10 wt%

hexane dilution yielding microspheres smaller than 1 μm. A narrowing of the standard

deviation is also observed as the solutions became more dilute. The decrease in

microsphere size and dispersity is consistent with reports from Berkland and coworkers

regarding preparation of poly(D,L-lactide-co-glycolide) microspheres for drug

delivery.128 Visual characterizations of microspheres performed by OM and SEM are

displayed in Figure 33.

73

Table 5

Dilution Study with Microspheres Composed of TTT-TMPMP

Sample ID Hexane w/w% in TTT MN (μm) SD (μm)

0% 0 N/A N/A

1%-HEX 1 4.9 1.7

2.5%-HEX 2.5 4.3 1.6

5%-HEX 5 3.5 0.66

7.5%-HEX 7.5 3.1 0.46

10%-HEX 10 0.66 0.087

74

Figure 33. Effect of viscosity on microsphere diameter, using HEX-TTT-TMPMP at 77

Hz and 4 VPP. A: SEM images with scale bars noted, B: Optical microscope images

where wt % hexane in TTT is (1) 1, (2) 2.5, (3) 5. (4) 7.5, and (5) 10 wt %.

The effect of the continuous flow rate on the microsphere diameter was also

explored. While prior studies were performed with a flow rate of 50 mL/min, additional

75

experiments were conducted at flow rates 25, 80, and 100 mL/min using 1%-HEX- TTT-

TMPMP system. Table 6 summarizes the findings. Increase of the carrier flow rate gives

smaller microspheres, as well as more uniform particle distributions.

Table 6

Flow Rate Study of Microsphere Prepared With 1%-HEX-TTT-TMPMP at 77 Hz 4 VPP

Flow rate MN (μm) SD (μm)

25 mL/min 5.2 ±2.2

50 mL/min 4.9 ±1.7

80 mL/min 3.0 ±0.75

100 mL/min 2.9 ±0.93

C60- and Sc3N@C80-thiol-ene microspheres

After thorough characterization of the AECF method and determination of the

effect of controllable variables, investigation of fullerene-containing microspheres was

pursued. Utilizing pre-polymer preparation methods detailed in the previous chapter,

microspheres composed of C60-thiol-ene were produced by standard AECF operating

procedure. Due to the building up of molecular weight that occurs during the pre-polymer

synthesis, higher dilution conditions were implemented to allow the flow of the solution

through the AECF device and decrease the diameter of the resultant microspheres.

Benzene, with a viscosity of benzene 0.652 mPa·s at 20 ⁰C,162 was utilized as the diluent

76

for the preparation of the fullerene-containing microspheres due to its use in pre-polymer

synthesis, as well as the good solubility of nanoparticles in this solvent.15

Initial investigation of C60-thiol-ene microspheres utilized TTT as a tri-functional

ene component, where wt % of benzene in the pre-polymer solution was < 10 wt % and

continuous flow rate was 50 mL/min. While high yields of nanocomposite particles were

collected and good dispersion of C60 throughout the matrix was achieved, the

microparticles that were collected and dried did not possess a smooth, spherical surface

as seen with control study microspheres (Figure 34). Rather, the prepared microparticles

appear to display the manifestation of buckling within a microsphere upon

photopolymerization.149 This can occur by the initial formation of a polymer skin

surrounding the pre-polymer droplet. Then, upon the polymerization of the remaining

internal pre-polymer, large difference in the internal modulus compared to that of the

external skin causes the microsphere to collapse upon itself to relieve the compressive

stress. Analysis of the microparticle diameters by OM revealed a large size distribution

ranging from approximately 100 μm to >400 μm. This may be attributed to the increased

viscosity of the pre-polymer solution due to a low percentage of diluent benzene, which

is higher in viscosity than the previously studied diluent hexane,162 though the irregular

shape of the microparticles complicates their measure.

77

Figure 34. OM of C60-containing microspheres prepared with TTT/S-S. Microspheres

exhibit a buckled architecture, as seen in the thin films with the same composition.

Efforts to correct the irregularly shaped microspheres were pursued by the

substitution of TTT for di-functional ene TMPDE. To correct large particle size

distribution and decrease microsphere diameter, the percent dilution of benzene for C60-

TMPDE/TMPMP microspheres was increased to 18 wt % of the total mass of monomers

and initiator. Though this solution was more dilute than the previously studied control

dilution series, the viscosity of the hexane-TTT-TMPMP solutions are not analogous to

the fullerene-containing pre-polymer samples; therefore, the size range of resultant

nanocomposite microspheres are not expected to be identical but, rather, are expected to

follow the same general trends in regards to dilution and surfactant continuous flow rate.

100 μm

78

Characterization of 1 wt % C60-TMPDE-TMPMP microspheres by OM revealed

smooth, spherical particles (Figure 35). This supports the theory that substituting a di-

functional TMPDE for tri-functional TTT can prevent buckling within a C60-thiol-ene

system by decreasing the modulus mismatch that occurs during polymerization. There is

a good dispersion of C60 throughout the thiol-ene matrix, as observed by the uniform

golden brown color. Improved uniformity of microsphere diameter was observed along

with a narrowing of size distribution with microspheres of approximately 70 μm in

diameter. The decrease in microsphere diameter of the TMPDE-TMPMP system can be

attributed to the higher benzene dilution of 18 wt % compared to the C60- TTT-TMPMP

microparticles prepared from pre-polymer dilution of < 10 wt % benzene.

Figure 35. OM of C60-containing microspheres prepared with TMPDE/S-S.

50 μm

79

Sc3N@C80-TTT-TMPMP microspheres were also prepared using pre-polymer

methodology with the addition of 0.5 molar equivalence of Irgacure 819 to increase pre-

polymer reaction rate and improve nanoparticle miscibility in the solution and pre-

polymer. The improved miscibility of the Sc3N@C80 in benzene and monomers is

credited to the capture of Irgacure 819 radical fragments and/or alkyl thiyl radicals from

TMPMP. The pre-polymer solution was diluted with benzene by a factor of 1.65x mass

of total reagents to aid in flow through the AECF device.

Sc3N@C80-TTT-TMPMP microspheres were characterized by OM (Figure 36).

Average microsphere diameter and size distribution is greater for microspheres prepared

with a slower continuous flow rate of 25 mL/min, as expected, with diameters

approximately 20-50 μm. Samples prepared with a continuous flow rate of 50 mL/min

demonstrate much greater uniformity along with decreased diameter, measuring 10 to 20

μm on average.

Interestingly, buckling is not observed for Sc3N@C80-containing TTT-TMPMP

microspheres. This can be attributed to the uptake of benzoyl addends from Irgacure 819

to the Sc3N@C80 cage. The addition of the initiator fragments results in fewer available

active sites, decreasing the number of Sc3N@C80-sulfur bonds. This would yield a

Sc3N@C80 derivative with improved solubility in solvent and monomers, though little

change would be observed in the crosslink density of the network, as the Sc3N@C80 is

bound to the network with few crosslinking components. As a lower elastic modulus

occurs in a network with decreased crosslinking, modulus mismatch with unpolymerized

pre-polymer does not occur, thus buckling is avoided.

80

Figure 36. OM images. (Left) 1 wt % Sc3N@C80 –TTT-TMPMP microspheres prepared

with 25 mL/min continuous flow rate. (Right) Sc3N@C80 –TTT-TMPMP microspheres

prepared with a greater continuous flow rate of 50 mL/min.

DSC of the Sc3N@C80 thiol-ene microspheres is shown in Figure 37 compared to

TTT-TMPMP control. A significant depression of the Tg to -10 ⁰C was observed relative

to published values.107 This supports the thought that a low number of Sc3N@C80-thiol

bonds are present, so there is in greater Sc3N@C80 mobility within the network, resulting

in a decreased Tg. A similar trend was observed by Ahmed et. al with the addition of C60

fullerenols to a thiol-ene network, where increased C60 contents resulted in a larger free

volume within the polymerized network due to untethered reactive groups.99 The

presence of a large amount of benzene in the pre-polymer solution may also influence the

Tg depression, acting as a plasticizer preventing the formation of a highly crosslinked

network.

50 μm 50 μm

81

Figure 37. DSC of Sc3N@C80-TTT-TMPMP microspheres compared to TTT-TMPMP

control, where glass transition is noted.

TGA analysis of the Sc3N@C80-TTT-TMPMP microspheres is given in Figure

38. Calculated degradation onset temperature of Sc3N@C80-TTT-TMPMP was

determined to be 362 ⁰C, which is a considerable increase compared to 349 ⁰C of the

control TTT-TMPMP sample. Kokubo and coworkers reported similar findings with PS

and PMMA nanocomposites containing arylated fullerene derivatives, where low

loadings of the fullerene component offered improvements of the thermal stability.79

Ginzburg et al. attributed improvements in thermal stability of fullerene-containing

nanocomposites to those possessing blended fullerenes, as the networks with multiple

covalent incorporations showed decreased thermal stability.157 The enhanced stability of

fullerene-containing polymer blends is attributed to the radical-scavenging ability of the

fullerene’s π bonds, and low loadings allow for a uniform dispersion of the nanoparticle

throughout the polymer network without disrupting the packing of the polymer. This also

supports the notion that there are few Sc3N@C80–S bonds linking the nanoparticles to the

-50 -30 -10 10 30 50

Hea

t F

low

(W

/g)

Temperature (⁰C)

Sc3N@C80

TTT/TMPMP

Microspheres

-10 ⁰C

TTT/TMPMP

Control

19 ⁰C

82

polymer network. Attributing this to the addition of the benzoyl radicals to Sc3N@C80 in

the pre-polymer reaction, which form stronger C-C bonds (BDE=350 kJ/mol), fewer

reactive sites were available to form weaker C-S bonds (BDE=260 kJ/mol) that would

break more readily upon exposure to thermal energy. An increase in the residual mass at

600 ⁰C is observed for microspheres containing 1 wt % Sc3N@C80 with 6.5% mass

remaining, up from the 4.8% mass remaining in the unloaded control sample. This is to

be expected based upon the high thermal stability of fullerenes, where Sc3N@C80 is stable

up to 850 ⁰C.148

Figure 38. TGA analysis of Sc3N@C80-containing TTT-TMPMP microspheres under N2

environment.

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800

Wt

%

Temperature (⁰C)

Control

TTT/TMPMP

1 wt % Sc3N@C80

TTT/TMPMP Microspheres

83

CHAPTER IV

CONCLUSION

A method for the covalent incorporation of C60 and Sc3N@C80 fullerenes into

thiol-ene polymer matrix was investigated in this work. The free-radical reaction of tri-

functional thiol monomer TMPMP with multifunctional alkyl ene and fullerene occurred

in a photochemical pre-polymer reaction, where the source of radicals was provided by

the cleavage of intramolecular disulfide bonds of the thiol monomer. C60-thiol-ene films

prepared with tri-functional ene monomers APE and TTT resulted in buckling of the film

surface as a mechanism to relieve compressive stress occurring from the differences in

pre-polymer and polymer elastic modulus values. Substitution of the tri-functional alkyl

ene monomers with di-functional TMPDE prevented the formation of buckled networks.

Using this combination of monomers, 1, 5, 10, 20, and 30 wt % C60 films were prepared,

as well as 1 wt % Sc3N@C80 films. The effect of additional commercial photoinitiator

Irgacure 819 on network structure was also investigated in 1 wt % C60 and Sc3N@C80

films.

Analysis of network behavior with 1:1 alkyl ene/thiol reactive groups (“1.0”

series) was compared to that of an alkyl ene deficient composition, with 0.75:1 alkyl

ene/thiol reactive groups (“0.75” series). Generally, films of the 1.0 series showed

decreasing gel % with increasing fullerene content, while the 0.75 series showed

consistently high gel % regardless of fullerene loading. Addition of fullerenes to the

thermally cured films showed a decrease in thermal stability as studied by TGA,

attributed to breaking of fullerene-thiol bonds to regenerate aromaticity of the

nanoparticles. Investigation of mechanical properties of the fullerene-containing

84

networks by DMA gave insight into the differences in the network structures. Films of

the 0.75 series displayed a positive correlation of storage modulus values with increased

C60 content, suggesting that the ene deficient films incorporated the nanoparticles into the

polymer network. Increasing glass transition temperatures occurred with successive C60

loading as well as a decrease in network damping, mobility, and homogeneity, as

observed by tan δ information. However, the 1.0 film series was impacted differently by

the presence of C60, showing a large increase in the viscoelastic behavior of the network

with the addition of only 1 wt % C60 and a general depression of the glass transition

temperature. This shows the ability of C60 fullerenes to act as an additive when sufficient

alkyl ene is available to react with thiol. Conversely, C60 fullerenes can be encouraged to

be linked into a thiol-ene matrix in alkyl ene-deficient conditions, resulting in a network

with markedly different characteristics. Interestingly, 1 wt % Sc3N@C80-containing films

showed enhanced mechanical properties when compared to analogous C60-containing

films. The use of commercial photoinitiator Irgacure 819 in production of 0.75 series

films lowered the rubbery storage modulus values for both C60 and Sc3N@C80 films.

Overall, this study showed the ability to tune the mechanical properties of a fullerene-

containing thiol-ene network for a desired application. Investigation of nanoparticle

dispersion throughout the thiol-ene matrix revealed no agglomeration of fullerenes in 1

wt % films. For the ene-deficient 10 wt % C60 -thiol-ene sample, the presence of

aggregated C60 appeared uniformly throughout the matrix. The 1.0 10 wt % C60-thiol-ene

network showed much different behavior, where larger C60 aggregates were present along

with areas of relatively low C60 content, supporting the behavior of C60 as an additive in

the presence of sufficient alkyl ene. Slow radical propagation of the aromatic cage in

85

comparison with the electron-rich TMPDE monomer is deemed responsible for this

behavior.

To apply the novel synthesis of fullerene-containing thiol-ene networks to

additional polymer architectures, production of nanocomposite microspheres was

pursued. Development of an acoustic driven microsphere preparation technique was

performed involving the fabrication of a novel device with tunable settings in a low

frequency range. A thorough characterization of factors impacting microsphere

production was investigated. The AECF method was determined to yield microspheres of

narrow size dispersion when maximum vibrational disturbance was applied to the novel

device, occurring at a frequency of 77 Hz and amplitude of 4 VPP. Monomer solution

viscosity and carrier solution flow rate were also determined to impact the size and size

dispersion of thiol-ene microspheres that were produced. For C60 and Sc3N@C80-

containing microspheres, preparation of the desired fullerene-containing pre-polymer was

delivered to the AECF device and fullerene-thiol-ene droplets were photochemically

polymerized. C60-thiol-ene microparticles prepared with TTT exhibited irregular shapes,

similar to that of films of the same composition. Substitution of TTT for di-functional

TMPDE produced spherical microparticles approximately 70 μm in diameter. Sc3N@C80

+ Irgacure 819 thiol-ene microspheres prepared with TTT did not exhibit the collapsed

shaped seen in the C60 microspheres. Investigation of the Sc3N@C80-microspheres by

DSC and TGA suggested a lower amount of fullerene-thiol bonds were formed,

significantly decreasing the Tg and overall improving the thermal stability of the

networks. The decrease in Tg may be attributed to a plasticizing effect of residual benzene

86

in the microspheres or perhaps the incomplete photopolymerization for the colored pre-

polymer droplets.

87

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