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Electronic Theses, Treatises and Dissertations The Graduate School
2012 The Fabrication of Self-Healing Cellulose Triacetate Polymer Composites and Dicyclopentadine Polymeric Foam Artrease Spann
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COLLEGE OF ARTS AND SCIENCES
THE FABRICATION OF SELF-HEALING CELLULOSE TRIACETATE POLYMER
COMPOSITES AND DICYCLOPENTADINE POLYMERIC FOAM
By
ARTREASE SPANN
A dissertation submitted to the Department of Chemistry and Biochemistry in partial fulfillment of the requirements for the degree of Doctor of Philosophy
Degree Awarded: Fall Semester, 2012
Artrease Spann defended this dissertation on September 24, 2012.
The members of the supervisory committee were:
Harold Kroto Professor Directing Dissertation
Emmanuel Collins University Representative
Ken Knappenberger Committee Member
Lei Zhu Committee Member
The Graduate School has verified and approved the above-named committee members, and certifies that the dissertation has been approved in accordance with university requirements.
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To my mom, my darker side of the moon – always in the shadows leaving me to shine
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ACKNOWLEDGEMENTS
My doctoral research would not have been possible without God and all the wonderful people He put in my life. I owe my family a big thank you for always encouraging me. Mom, thank you for showing me college was possible and always encouraging me in my pursuit of more and more education. Thank you to the women in my family. My mom, grandmother, and aunts, have all been an inspiration throughout this process. I have realized how strong and wonderfully made I am. So much of who I am comes from the amazing women in my life. If I become only half as amazing as you all I know I will have a wonderful life ahead of me. Thank you for your strength, wisdom, and prayer. Not to be outdone the men of my family deserve an equally big thank you. My brother, uncles, and grandfather are always so proud of me. Your support is appreciated. It keeps me encouraged. A big THANK YOU goes to my big little brother, Arthur, who is and always has been my greatest cheerleader. If it was not for all of your constant pep talks I might not have tried and tried again. Thank you for always being there for me. You have grown to become a great man. I feel truly blessed that you are my brother.
I would like to thank my committee, Sir Harlod Kroto, Dr. Emmanuel Collins, Dr. Ken
Knappenberger, and Dr. Lei Zhu. Thank you for your knowledge and advice; I will carry it on to my next stage in life. I would like to extend an extra thank to my advisor, Harry, not only for his scientific insight and research expertise, but for his encouragement to pursue my interest outside of the lab. I am truly grateful for the opportunity to pursue the Florida Gubernatorial Fellowship and work on science education. Special thank you to our research scientist and resident genius,
Steve, I definitely could not have done this without you. I hope this gets me in the ark. Thank you to Darryl being an awesome lab mate and preventing lab craziness.
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Graduate school would not have been enjoyable without my wonderful all friends. I owe
Casey an extra special thank you for all the great moments over the past four years. Thank you for all the dinners, wine tastings, and dancing. Thank you to Robin, who has been encouraging me since my freshman year. Thank you for waking me up to study, when I didn’t want to, and thank you for keeping me focused. Thank to Crystal, my pif, for the constant foolishness and being the best PIF ever. Lastly, thank you to Rey for being an amazing friend, helping me find solutions to all my problems, and making sure I didn’t stress out too much, especially during this last year. Srn.
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TABLE OF CONTENTS
List of Tables ...... x
List of Figures ...... xi
List of Abbreviations ...... xvii
Abstract ...... xviii
1. CHAPTER ONE: INTRODUCTION AND LITERATURE REVIEW ...... 1
1.1 Introduction ...... 1
1.2 Background: Self-Healing Materials ...... 2
1.2.1 Concept and Design Strategies ...... 3
1.2.2 Microcapsule-Based Self-Healing System ...... 9
1.2.3 Self-Healing System Applications ...... 17
1.3 Background: Carbon Nanotubes ...... 18
1.3.1 Carbon Nanotube Structure...... 18
1.3.2 Properties ...... 19
1.3.3 Polymer/CNT Composites ...... 21
1.3.4 Polymer/CNT Composite Applications ...... 22
1.4 Background: Foams ...... 23
1.4.1 Microcellular Foams ...... 23
1.4.2 Microcellular Foams Applications ...... 27
1.5 Dissertation Overview ...... 27
2. CHAPTER TWO: MATERIALS AND FABRICATION METHODS ...... 29
2.1 Microencapsulation ...... 29
2.1.1 Urea-Formaldehyde Microencapsulation with Polymeric Core Materials ...... 29
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2.1.2 Urea-Formaldehyde Microcapsule with Polymer Core Fabrication Process ...30
2.1.3 Urea-Formaldehyde Microcapsules with Carbon Nanofiber Incorporated Shells and Polymer Core Fabrication Process ...... 31
2.2 Self-Healing Cellulose Triacetate Composite Films...... 32
2.2.1 Self-Healing Coatings and Film Materials ...... 32
2.2.2 Self-Healing Film and Coating Fabrication ...... 33
2.3 Dicyclopentadiene Microcelluar Foam ...... 34
2.3.1 Dicyclopentadiene Composite Foam Materials ...... 34
2.3.2 Dicyclopentadiene Microcelluar Foam Fabrication Process ...... 34
2.3.3 Dicyclopentadiene Microcelluar Foam with Carbon Nanofibers Fabrication Process ...... 36
2.4 Instrumentation ...... 36
2.4.1 Spincoater ...... 36
2.4.2 Scanning Electron Microscopy ...... 38
2.4.3 Everhart-Thornley Dectector ...... 42
2.4.4 FEI Nova 400 Nano Scanning Electron Microscope ...... 43
3. CHAPTER THREE: THE FOMATION OF CARBON NANOTUBE COATED MICROCAPSULES VIA IN SITU POLYMERIZATION ...... 45
3.1 Introduction ...... 45
3.2 Experimental Section ...... 46
3.2.1 Materials ...... 46
3.2.2 Microcapsule Fabrication Process ...... 46
3.3 Results ...... 47
3.3.1 Optimizing the Microcapsules Fabrication Process ...... 47
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3.3.2 Carbon Nanofibers Incorporation into the Microencapsulation Fabrication Process ...... 53
3.3.3 Carbon Nanotubes Incorporation into the Microencapsulation Fabrication Process ...... 69
3.4 Discussion ...... 73
4. CHAPTER FOUR: THE FORMATION OF SELF-HEALING FILMS AND COATINGS UTILIZING MICROCAPSULE SELF-HEALING SYSTEMS ...... 74
4.1 Introduction ...... 74
4.2 Experimental Section ...... 76
4.2.1 Materials ...... 76
4.2.2 Film and Coating Fabrication ...... 77
4.3 Results ...... 77
4.3.1 Self-Healing System Reactions: Ring Opening Metathesis vs Epoxy/Amine Reactions ...... 77
4.3.2 Self-Healing System Composite Films ...... 81
4.3.3 Self-Healing System Composite Coatings Self-Healing Evaluations ...... 100
4.4 Discussion ...... 107
5. CHAPTER FIVE: POROUS CARBON NANOFIBER DICYCLOPENTADIENE FOAM SYNTHESIS VIA HIGH INTERNAL PHASE EMULSION ...... 110
5.1 Introduction ...... 110
5.2 Experimental Section ...... 119
5.2.1 Dicyclopentadiene Foam Materials ...... 119
5.2.2 Sidewall Modification of MWCNTs ...... 119
5.2.3 Microcellular DCPD Foam Process ...... 119
5.2.4 DCPD Foam with CNF Incorporation Process ...... 121
5.2.5 Microscopy Analysis Instrumentation ...... 121 viii
5.3 Results ...... 122
5.3.1 Fabrication of DCPD Foam with Open-Celled Morphology ...... 122
5.3.2 DCPD Foam Formed Through Increased Emulsion Stability ...... 127
5.3.3 Incorporating CNFs into DCPD Foam ...... 132
5.3.4 Sidewall Modification of MWCNTs for Polymer Integration ...... 134
5.3.5 Thermal Removal of DCPD from CNF Network ...... 137
5.4 Discussion ...... 141
6. CHAPTER SIX: CONCLUSIONS AND FUTURE WORKS ...... 143
6.1 Conclusions ...... 143
6.1.1 Carbon Nanotube Coated Microcapsules Fabrication ...... 143
6.1.2 The Fabrication of Self-Healing Cellulose Triacetate Films and Coating Using Microcapsule Self-Healing Systems ...... 144
6.1.3 Porous Carbon Nanofiber Dicyclopentatdiene Foam Fabrication via High Internal Phase Emulsions ...... 146
6.2 Future Works ...... 147
7. REFRENCES ...... 149
8. BIOGRAPHICAL SKETCH ...... 155
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LIST OF TABLES
1.1 Tensile strength of common materials and carbon nanotubes ...... 20
4.1 Self-healing system comparison chart ...... 81
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LIST OF FIGURES
1.1 Schematic of extrinsic pipeline self-healing systems ...... 7
1.2 Steps in microcapsule self-healing: ...... 10 a) crack damage b) healing initiation c) healing agent polymerization
1.3 Molecular structure of dicyclopentadiene: ...... 12 a) endo- dicyclopentadiene b) exo-dicyclopentadiene
1.4 Molecular structure of Grubbs catalyst ...... 13
1.5 Ring opening metathesis polymerization of dicyclopentadiene ...... 14
1.6 Reaction of urea and formaldehyde forming urea-formaldehyde...... 16
1.7 Schematic of carbon nanotubes: ...... 18 a) single-walled carbon nanotube b) graphite sheet c) hemispherical end cap d) multi-walled carbon nanotube
1.8 Examples of microcellular foams: ...... 25 a) PVC b) PC c) ABS d) PET
1.9 Steps involved in chemically induced phase separation ...... 26
2.1 Chemical structure of encapsulated bisphenol A diglycidyl ether healing agent, EPON 828®...... 29
2.2 Steps involved in the microcapsule fabrication process ...... 30
2.3 Microencapsulation method process set up ...... 31
2.4 Chemical structure of cellulose triacetate ...... 32
2.5 Chemical structure of dichloromethane ...... 32
2.6 Chemical structure of dicyclopentadiene ...... 34
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2.7 Steps involved in the fabrication of DCPD foam ...... 35
2.8 Schematic of spincoating stage ...... 37
2.9 Spincoat G3-8 from Specialty Coating Systems ...... 37
2.10 Schematic of scanning electron microscope ...... 40
2.11 FEI Nova 400 Nano Scanning Electron Microscope ...... 44
2.12 SEM stub used mount samples ...... 44
3.1 Structure of endo-dicyclopentadiene (DCPD) ...... 46
3.2 Structure of bisphenol A diglycidyl ether (BADGE)- EPON 828 ...... 46
3.3 Steps for fabricating microcapsules with encapsulated monomer ...... 47
3.4 DCPD microcapsules with urea-formaldehyde shells: ...... 49 a) microcapsules prior to drying b) microcapsules after 24hr drying in room temperature
3.5 SEM images of DCPD self-healing microcapsules: ...... 50 a) SEM image of DCPD encapsulated microcapsules b) higher magnification of microcapsules
3.6 SEM image of microcapsules formed after 120 min ...... 51
3.7 Microcapsules generated using modified method of decreased reagents ...... 52
3.8 CNFs aggregate without integration into microcapsule shell ...... 58
3.9 SEM image of microcapsules with 50% increase DCPD concentration for CNF coating ....55
3.10 Higher magnification SEM image of DCPD coated CNF microcapsule ...... 55
3.11 50% Excess polymer coating on CNFs in DCPD encapsulated microcapsule ...... 57
3.12 SEM image of polymer coated microcapsules and a layer of cured polymer from the addition of 50% excess polymer ...... 58
3.13 Excess polymer creates DCPD coated CNF, but prevents the formation of microcapsules ..58
3.14 SEM image of polymer curing atop fully formed microcapsules ...... 59
3.15 SEM image of microcapsules formed with excess polymer: ...... 60
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a) excess polymer covers fully formed self-healing capsules b) crack in polymer covering shows underlying microcapsules
3.16 50% increase in CNFs concentration prevents the formation of microcapsules ...... 61
3.17 Urea formaldehyde reaction to form microcapsule shell ...... 62
3.18 Magnified SEM images of 20% increase in CNFs preventing the formation of microcapsules ...... 63
3.19 SEM images of product resulting from 20% increase in DCPD and CNF concentration with sonication: ...... 64 a) shows excess DCPD hardened in a solid mass, b) shows CNFs agglomeration in addition to polymerized DCPD
3.20 SEM image of fabricated product after 20% increase in CNFs and DCPD concentrations showing pointed edges of CNFs and polymer coated CNFs ...... 66
3.21 SEM of microcapsules with a 5% increase in CNFs and DCPD concentrations ...... 67 a) shows large aggregates of CNFs and large tube of polymer coated CNFs b) shows DCPD polymerized into a polymer ribbon, microcapsules smaller than 0.5 μm, and CNF aggregates
3.22 SEM image of microcapsules with a 5% increase in CNFs and DCPD concentrations indicating the fabrication of microcapsules ranging from 1 μm to less than 0.5 μm ...... 68
3.23 SEM image of CNTs vs CNFs ...... 69 a) image of CNTs b) image of CNFs51
3.24 SEM images of microcapsules with integrated CNTs shells ...... 71
4.1 Illustration of ring opening metathesis polymerization54 ...... 78
4.2 Illustration of ring opening metathesis polymerization of dicyclopentadiene54 ...... 78
4.3 ROMP reaction of DCPD ...... 79
4.4 Epoxy amine mechanism55 ...... 80
4.5 Schematic of fabricated self-healing systems ...... 82
4.6 Cellulose triacetate (CTA) dissolved in dichloromethane (DCM) ...... 84
4.7 CTA with p-Phenylenediamine (PPD) ...... 84
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4.8 Structure of BADGE- EPON 828 ...... 85
4.9 EPON 828®/EPIKURE 3140® reaction ...... 86
4.10 Unmodified CTA polymer on a steel substrate ...... 87
4.11 Images of a CTA spun thin film ...... 88 a) film after curing b) side image of film removed from substrate c) SEM image of film surface d) SEM image thin film edge
4.12 CNTs sonicated in CTA ...... 90
4.13 CNTs stirred into CTA ...... 91
4.14 CNT incorporated into CTA polymer ...... 92 a) CTA/CNT film b) SEM image of the surface of the CTA/CNT film.
4.15 SEM image CTA/CNT film with the CTA structure intact ...... 93
4.16 Image of CTA film ...... 94 a) CTA film with incorporated microcapsules b) SEM image of microcapsule aggregation within CTA film
4.17 SEM image of holes created after the addition of microcapsules with CNT incorporated shells ...... 95
4.18 SEM image CTA film with alternating layers of CNT and microcapsules ...... 97
4.19 SEM images CTA film with alternating layers after decreasing microcapsules concentrations ...... 98
4.20 Image of film after corrosion testing ...... 100 a) Film without self-healing capsules b) Film with self-healing capsules healed at 35oC
4.21 Image of self-healed film after 72 hours of corrosion testing ...... 101
4.22 SEM image of microcapsules self-healed film ...... 102 a) Film after crack was induced b) Film post healing
4.23 SEM images of a film healed at 35oC ...... 104
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a) After damage b) 12 hours c) 24 hours
4.24 SEM image of a CNT coated microcapsules self-healed film ...... 106
4.25 Diagram showing the testing of a cellulose film containing self-healing capsules and palladium nanoparticles...... 108
4.26 Picture of a cellulose film with embedded capsules containing the congo-red dye ...... 109
5.1 ROMP reaction of DCPD utilizing a Grubbs catalyst ...... 111
5.2 SEM image of nonporous DCPD42 ...... 111
5.3 SEM image of porous DCPD62 ...... 112
5.4 Traditional process plan used to create DCPD foam 42,62 ...... 113
5.5 Process plan of newly developed HIPE process for generating DCPD foam ...... 116
5.6 Urea formaldehyde reaction used to assist in stabilizing cell walls during polymerization ...... 119
5.7 SEM image of DCPD foam generated via HIPE indicates three distinct sections of morphology ...... 120
5.8 SEM image under higher magnification of closed-cell section of DCPD foam ...... 121
5.9 SEM image of poorly formed cell walls due to cell wall coalescence ...... 121
5.10 SEM image of open-celled DCPD foam via HIPE shows polyhedron pores with pore sizes ranging from 0.5 μm to 5 μm...... 122
5.11 SEM image of DCPD foam created with increased surfactant shows a closed-cell porous structure ...... 124
5.12 SEM images of DCPD foam with temperature adjustment stabilization ...... 125 a) layer of closed-celled pore structure covering open-celled pores b) complete opened-celled pore structure.
5.13 SEM image of DCPD foam with temperature stability ...... 127 a) DCPD with complete open-cell structure b) magnified SEM image
5.14 SEM image of closed-cell morphology seen in DCPD foam. The incorporated CNF are
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disbursed throughout the foam ...... 129
5.15 SEM image of DCPD foam with CNF incorporated into the cell walls ...... 130
5.16 Schematic of the sidewall modification of MWCNT-COOH with 5-norbornene-2- yl(ethyl)chlorodimethylsilane for the covalent attachment to DCPD based foam ...... 131
5.17 SEM image of DCPD foam with norbornene functionalized MWCNTs ...... 132
5.18 SEM Image of DCPD foam with 10 mg of norbornene functionalized MWCNTs ...... 133
5.19 SEM images of DCPD foam with after a) 1hr, b) 2 hr, c) 3 hr, and d) 4hr at 400oC ...... 135
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LIST OF ABBREVIATIONS
SHC ...... Self-healing capsules
DCM ...... dichloromethane
DCPD ...... dicyclopentadiene
PPD ...... 1, 4 phenylenediamine
CNT ...... carbon nanotube
CNF ...... carbon nanofiber
SEM ...... scanning electron microscopy
ROMP ...... ring opening metathesis polymerization
HIPE ...... high internal phase emulsion
BADGE ...... bisphenol-A diglycidyl ether
EMA ...... poly(ethylene) maleic anhydride
CTA ...... cellulose triacetate
SWCNT...... single-walled carbon nanotube
MWCNT ...... multi-walled carbon nanotube
PVC ...... polyvinyl chloride
PC ...... polycarbonate
ABS ...... acrylonitrile-butadiene-styrene
PET ...... polyethylene terephthalate
CIPS ...... chemically induced phase separation
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ABSTRACT
Damage continues to plague the structural integrity of polymer composites. Continued stress leads to cracking and weakening of a composite system. Engineering polymer composites with carbon nanotube reinforcements and self-healing capabilities can potentially enhance the composites properties and extend the service lifetime. Carbon nanotubes (CNTs) promise to improve the electrical and mechanical properties of composite materials into which they are incorporated. These hollow tubes comprised completely of carbon, which have nanometer scale diameters, as well as low density, possess unique paradigm-shifting mechanical and electrical properties. The major problem prohibiting the widespread use of enhanced polymer composites has been the difficulty fabricating these composites. Research in developing methods capable of fully integrating the components is essential. This study sought to develop fabrication strategies to overcome the problem of integration and create polymer composites with advanced behavior useful in multiple applications.
Engineering polymer composites with CNT and self-healing capabilities could enhance the composites’ properties and extend the lifetime. Polymer composites are prone to degradation and damage. The damage leads to major system failure. The strategy developed here was inspired by living systems that evolved to mend damage and regain function. Engineering
“smart” synthetic composites with infrastructures which can respond to damage is necessary for improving polymer usefulness. The molecular processes that alleviates damage aids in avoiding total mechanical failure. Such self-healing materials repair damage and continue function post repair.
There several methods for creating composites with self-healing capability. The one studied here involves embedded microcapsules whose envelopes burst open releasing their
xviii contents in response to the material cracking. Microcapsules containing a reactive liquid
“healing” core are embedded in the polymer during fabrication. A mechanical defect in the polymer breaks the shells of the microcapsules releasing the “healing” core. The core material polymerizes, thus healing the crack. As defects are repaired, functionality should be restored and the serviceable lifetime extended. The results obtained in this study indicate microcapsules can be modified to incorporate CNT. Also self-healing capabilities can be extended to new polymer systems in a cost-effective manner.
In addition to enhancing polymers for sustainability, CNT reinforced structural foams could also prove to be beneficial in numerous industrial applications, where mechanically strong foams are needed. CNTs are already being investigated as structural enhancements for other foams, such as polyurethane - the most common and widely used polymer. High internal phase emulsion foams, highly porous emulsion template foams, are emerging as new and important forms of microcellular foams with numerous advantages, such as high impact strength and high stiffness-to-weight ratio. This research developed a novel method for creating carbon nanotube reinforced microcellular foam.
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CHAPTER ONE
INTRODUCTION AND LITERATURE REVIEW
1.1 Introduction
Since World War II polymers have been one of the fastest growing industries in the
United States1. Polymers, large molecules created via covalent bonds between smaller structural units, can be natural or synthetic and have a variety of properties. A polymer is a macromolecule composed of repeating units. Polymers can be natural, such as cellulose and proteins, or synthetic, such as polyethylene and polypropylene. Polymer applications range from fields of automobiles to toys to health care, because they can have tunable mechanical and physical properties. Polymers are also used to increase strength of material by cross- linking them. Cross-linking polymers around a fiber, usually glass or carbon fibers, can create a stronger polymer matrix3.
The disadvantage of polymers or using polymers is its ability to degrade upon exposure to light or as a process of tear and wear. Polymer degradation, noted by changes in the physical and/or chemical properties, can cause scission in the polymer chain causing cracking and disintegration of the polymer. While some instances of polymer degradation are wanted, i.e. biodegradation and recycling, most polymer degradation is undesired because it leads to product failure2. Long-term durability and reliability remains problematic for applications that depend heavily on the use of polymers and polymer composites.
The first sign of damage many structural polymers suffer is microcracking. These microcracks often lead to larger cracks, material weakening, and system failure. Microcracks also allow other degradative processes to occur, i.e. introducing moisture and outside debris
1
into the system, further damaging the structural integrity of the system. Since cracks within a
system are usually mended by hand, microcracks deep within a polymer can be difficult to
detect and almost impossible to repair. If these microcracks go unrepaired they often lead
macrocracking, which ultimately leads to catastrophic failure of the polymer and
application.4
Engineers and materials scientist are constantly looking for improvements to current
materials. With many of today’s materials falling short of their optimal utility due to
degradation, scientists are researching methods and materials that stop or retard such
damage. One school of thought involves research into polymers capable of repair with little
to no human intervention, thus eliminating the need for repair and extending the lifetime for
manufactured items. The ability of a material to adapt to environmental stress is crucial for
extended lifetimes. Self-healing mechanical damage could lead to longer lifetimes yielding
lower production cost. It would also prevent cost due to material failure and reduce
inefficiency5. The goal is to have materials heal any damage before catastrophic failure, and
recover its original function.
1.2 Background: Self-Healing Materials
Self-healing materials are a class of smart materials designed to repair mechanical damage.
Naturally occurring segments of plants and animals already have self-healing adaptability. In contrast to synthetic materials, naturally existing materials are able to repair themselves and reestablish the activity in the previously damaged area. For instance, a broken bone induces a fibrin clot at the break site via internal bleeding. The fibrin clot forms a fiber mesh and calcifies into fibrous cartilage. The fibrous cartilage converts to fibrous bone and lamellar bone upon
2 further calcification, healing the break5. After the broken bone is healed, the bone regains much, if not all, of its original strength. Over the past few decades scientist have been trying to mimic these features in man-made materials. Inspired by biological systems these “smart” synthetic systems would have the capability to heal damage and reestablish activity in the damaged area6.
1.2.1 Concepts and Design Strategies
Engineering synthetic materials with “smart” characteristics creates materials with stimuli- responsive functions. The mechanical function of polymers is dependent on the chemical and physical structure of the polymer chains. Based on these properties, polymers fall into one of three broadly defined categories, thermoplastics, themroset, or elastomers. Thermoplastics can have a linear or branched chain and can be amorphous or semicrystalline. Thermoplastics have a mechanical response highly dependent on their molecular mass, chain entanglements, chain alignment, and degree of crystallinity7. Thermosetting polymers are highly cross-linked three- dimensional networks, with mechanical properties dependent on their molecular mass and cross- link density. The final category of polymers, elastomers, is highly elastic with light cross-linking.
Early research in crack healing discovered healing to be a generic property of synthetic polymers8. Wool, et. al. found that certain semicrystalline polymers, block copolymers, and filled elastomers all had the ability to heal, noting four important aspects and consequences of polymer healing:
1. Crack healing occurred in numerous polymer systems and must be considered when
evaluating the polymers mechanical properties.
2. The rate of healing is affected by temperature, strain history, environmental factors, and
the type of structural damage. Healing can occur instantaneously or be time-dependent.
3
3. Crack healing offered another theory to polymers ability to behave as viscoelastic
materials.
4. A rate of healing curve could be created based on the rate of the crack disappearance8.
While it was known to be a general phenomenon of polymers early healing research looked into the healing of thermoplastics. The healing of these polymers could be observed optically when film that was previously white because of polymer material bending, returned to their original clarity post-healing. On a molecular level thermoplastic polymers were shown to heal after fractured surfaces rejoined and the polymer chains re-entangled8. The re-entanglement of the polymer chains usually occurred between secondary chains and not the primary chains broken in the crack8.
Most polymers exhibit a viscoelastic response, showing both elastic solid and viscous liquid characteristics. Indicating the polymer displays both a reversible and irreversible mechanical response. This viscoelastic property causes the polymers mechanical response to stress to be not only depend on its structure, but also the time and temperature7. These early polymer healing process required an external stimulus, often times heat. The crucial temperature for polymers is the glass transition temperature, Tg, the temperature at which the polymer had a reversible transition from hard to molten. Above the polymer’s Tg the bulk materials is able to flow due to the polymer chain’s increased mobility. Below the Tg, polymer chain’s mobility is slowed and the polymer behaves as an elastic solid. Tg is dependent on the polymer’s chemical structure, molecular weight, and degree of chemical/physical cross-linking. A low Tg indicates a more elastic response caused by insufficient mobility for viscous mechanisms to take place.
Correspondingly, higher Tg leads to a more viscous response. Heat above the Tg enhanced healing; while no healing occurred at be temperatures below the Tg. Healing took place at all
4 points along the crack; however, the rate of healing varied8. While the crack tip healed instantaneously due to thermodynamic surface tension, the remainder of the crack saw slower
8 healing, which depended on the polymer’s Tg .
The healing also depended on factors other than heat, such as type/extent of structural damage, molecular rearrangement, ect. If sufficient driving forces existed the polymer could heal via molecular rearrangement. Post-healing the crack would disappear and partial strength would return to the polymer. Early healing was considered successful if any of the material’s original mechanical properties were recovered after cracking. Wool et. al. also noted crack healing and its effects on the polymer mechanical properties were largely underestimated.
As healing research advanced two distinct classes of healing methods were developed, intrinsic and extrinsic5. Intrinsic healing systems involve the cracked polymer itself being healed, whereas extrinsic systems contain a secondary system embedded in the polymer. Intrinsic healing systems are polymer specific. The polymer enables crack healing under certain conditions, mainly increased heat. Due to the large increases of heat needed autonomous automatic healing is not yet available in intrinsic systems. Based on the predominant molecular mechanisms there are two modes for intrinsic systems: 1) physical interactions and 2) chemical interaction5.
Thermoplastics, the first to be examined as potential self-healing materials, showed five phases for the physical interaction mode intrinsic healing process. Wool et al. defined the phases as : 1) surface rearrangement, 2) surface approach/ healing patterns, 3) wetting, 4) diffusion, and
5) randomization9. The last two phases controlled the recovery of properties after healing and ensured crack healing9, 10. Jud et al. found poly(methyl methacrylate) and poly(methyl methacrylate-comethyl ethylacrylate) samples could regain full short term resistance after
5 healing; however, to induce healing at the crack site heat above the glass transition temperature of the polymer had to be applied11. The chemical mode of intrinsic self-healing focuses on nanoscopic deteriorations, instead of microcracking. Bonds are broken and “healed” via recombination of broken molecular bonds. Real-world applications are improbable for the intrinsic chemical mode healing due to the high temperature and other rigorous conditions needed for healing. Despite the lack of healing autonomy intrinsic systems have the possibility of multiple healing. Since the polymer itself heals future cracking can be healed via the same process.
Unlike intrinsic systems, in extrinsic systems the polymer itself does not heal. Healing agents embedded throughout the polymer matrix react to fill in the crack. Extrinsic systems have garnered the most success in the development of healing systems, becoming the main focus of most the healing research and will be the focus of this work from this point forward. In extrinsic systems the polymer itself is not healing, it is merely the host material. Healing agents are encapsulated and embedded in the polymer host. The fragile encapsulating agent is destroyed by the same mechanical damage destroying the polymer host. The encapsulated healing agent is released into the crack and begins the healing process. By utilizing the crack as a trigger for healing, extrinsic systems can be autonomous and automatic.
Extrinsic systems open a new area for research in self-healing. In autonomous automatic self-healing, the materials begin the healing process at the start of damage and can progress through the entire healing process without human intervention. These self-healing systems do not require heat, beyond environmental conditions, or other rigorous conditions. Automatic self- healing materials should recover a portion of their strength and functionality after a crack has occurred. Each material will have its own methodology to accomplish the self-healing.
6
The two categories for extrinsic systems are based on the types of containers: 1) pipelines and 2) microcapsules5. Pipelines and microcapsules work in much of the same way. Both techniques contain brittle-walled vessels filled with a healing agent that can be cured. The two methods vary on vessel structure. Pipelines are hollow tubes filled with a healing agent via vacuum assisted capillary action5. There are three types of healing systems using pipelines5 shown in Figure 1.1: 1) single part adhesive, 2) two-part adhesive both in pipelines, 3) two-part adhesives, one in a pipeline and the other in a microcapsule.
polymer
pipeline
polymer pipeline
polymer
pipeline
microcapsule
Figure 1.1: Schematic of extrinsic pipeline self-healing systems
7
Single-part adhesive systems contain pipelines filled with the same healing agent. The healing agent flows upon pipeline cracking and is polymerized with a hardener, usually a catalyst covering the outside of the pipeline. In a pipeline only two-part system the healing agent and its polymerizing agent are stored in parallel pipelines. Both pipelines must be broken in order for this system to work. In the two-part adhesive system containing pipelines and microcapsules, one component, usually the pipeline, is filled the healing agent and the other component, usually the microcapsule, encapsulates the polymerizing agent5.
Dry et al. proposed the use of pipelines made from hollow glass capillary tubes with millimeter-scale diameter. The tubes, filled with various chemical healing agents (polyesters, two-part epoxy, vinyl monomers), were embedded in a polymer matrix13. This early work with pipelines showed limited success in healing. Eventually micron-sized fibers were used to delivery epoxy to the damaged area for healing. Adding a fluorescent dye in the system gave a visual of the crack and subsequent healing. Pipeline healing has also been achieved using optical fiber infused with a healing agent. When the material ruptures, so does the fiber yielding a significant reduction in light intensity output. The healing agent is released and heals the crack.
The change in light intensity helps locate the crack creating a material that can simultaneously monitor, diagnose, and heal itself. Yang et al. used ethyl cyanoacrylate as a healing agent in this system achieving recovery of 1/3 the initial tensile strength5.
An important property of extrinsic systems utilizing pipelines is choosing the appropriate pipeline material. The pipeline must break when the external polymer matrix breaks. Zhao et al. observed no healing in an epoxy/polyamide matrix when plastic hollow fibers failed to break at the first sign of polymer matrix cracking. Continued cracking of the polymer matrix did not
8 crack the pipeline material. In addition to utilizing a pipe material that will break upon cracking of the polymer composite, the size and wall thickness of the embedded pipeline may be tuned to control fracture mode and rate of healing agent release. The release rate depends on the healing agent viscosity and internal pressure within the pipeline. Zhao found 95% crack healing could occur at ~2 atm internal pressure5.
While pipelines can be tuned to control the rate of healing multiple healing are not attainable. A system with multiple healings is highly desirable, since multiple cracks can occur overtime. However, ruptured pipelines are depleted of their healing agent after initial damage. To create an extrinsic healing system with multiple healings Toohey et al. proposed a 3-D microvascular system. This system is analogous to human skin, where a cut triggers blood flow in a capillary network leading to rapid clot formation and healing. Inspired by this biological system, the 3-D microvascular network is capable of healing minor damage multiple times by delivering the healing agent to the crack site via channels. The healing agent polymerizes in the crack with the aid of embedded curing agent. Under repeated damage, the same crack healed as more healing agent arrived at the crack site. This system provides multiple healings a long as more healing agent was pumped into the system and curing agent was available at the damage site. While multiple healings are desired the system did not provide a practical solution for repeated cracking. The Toohey et al. system relied on a constant supply of healing agent being pumped into the system, yielding a system constantly tethered to a healing agent supply system.
1.2.2 Microcapsule-Based Self-Healing System
Extrinsic systems based on microcapsules, shown in Figure 1.2, are the most promising at the moment since the idea is relatively simple and easy to implement. Pipeline systems require
9 careful placement into the polymer host; however, microcapsule placement is less cumbersome.
Microcapsule systems contain three major components, the microcapsule, the curing agent, and host polymer matrix. Microcapsules are the end product of microencapsulation.
Microencapsulation involves enclosing a micron sized particle of solid, liquid, or gas in an inert shell14. The encased material is the healing agent, which will react with a curing agent to heal matrix cracking. The microcapsules are embedded within the polymer matrix. Since the release of the inner healing agent is triggered by matrix cracking automatic autonomous self-healing is achieved.
a) polymer matrix curing agent microcrack microcapsule
b)
c)
Figure 1.2: Steps in microcapsule self-healing: a) crack damage b) healing initiation c) healing agent polymerization.
10
There are three main steps for self-healing function. In the first step the polymer matrix sustains mechanical damage and begins to crack, figure 1.2a. Self-healing is initiated when the crack ruptures the microcapsule shell, initiating the self-healing process. The healing polymer fills the crack via capillary action, figure 1.2b. When the healing polymer interacts with the curing agent polymerization begins. This polymerization binds the crack and prevents further crack propagation. In the final step, the chemical reaction between the healing agent and the curing agent causes the healing polymer to harden, completely healing the crack. The polymer matrix’s properties are recovered. Ideally full functionality is resorted, extending the service lifetime extended.
For microcapsule systems with liquid healing agents to work effectively six critical parameters, 1) the encapsulated healing agent, 2) the curing agent, 3) the polymerization/healing process, 4) the composite material, 5) the cost and 6) the microcapsule,14 must be examined and fully developed. The encapsulated healing polymer is a part of one of the three major components of a microcapsule based self-healing system. The first successful autonomous self- healing material was reported by Prof. Scot White’s group at the University of Illinois in 20014.
White et al. created a man-made epoxy with autonomous self-healing capabilities using microencapsulation of a liquid healing agent. This liquid healing polymer was analogous to a bleeding agent in biological system. The healing polymer would “bleed” into a crack caused by damage. The crack would “heal” after the healing agent underwent polymerization with the aid of an embedded curing agent. Similar to the chemical reactions occurring in biological systems during reconstructing of damaged area. The liquid healing agent, which is encapsulated in a microcapsule, must be a low viscosity, low volatility monomer. The liquid monomer should flow
11 into a crack via capillary action, providing complete coverage15. Analogous to biological systems, where the bleeding healing agents need to fill in the damaged area rapidly, the released healing agent must be efficient and quick to prevent further damage. Low volatility is essential to allow sufficient time for the monomer to fill the crack before there is complete polymerization.
After polymerization the monomer should form a tough highly cross-linked polymer. In the system developed by White, the liquid healing agent used was dicyclopentadiene (DCPD), shown in Figure 1.3. a) b)
Figure 1.3: Molecular structure of dicyclopentadiene: a) endo- dicyclopentadiene and b) exo- dicyclopentadiene.
DCPD-based polymers are inexpensive and readily available making it an ideal healing polymer. DCPD is also liquid at room temperature with low viscosity. The low viscosity of
DCPD allows it to flow into microcracks. DCPD is also environmentally stable leading to a long- shelf life. DCPD also has a longer shelf life due to its relative insusceptibility for radical polymerization. White et al. used the endo-isomer of DCPD, as oppose to the exo-isomer, due to its greater reactivity.
Another critical parameter and second major component of the microcapsule system is the curing agent. The curing agent must react quickly with the healing agent. It must also withstand integration into the polymer host, dispersing throughout the polymer host with little to no aggregation. While the curing agent needs high reactivity with the healing agent it must not react with the polymer matrix in which it is embedded. In the 2001 White experiment a Grubbs
12 curing agent was used. The Grubbs catalyst, figure 1.4, is a ruthenium based carbine complex. It is readily stable in air and soluble in a wide range of solvents making the compound ease to handle.
Figure 1.4: Structure of Grubbs catalyst
The polymerization process between the encapsulated healing monomer and the curing agent should be relatively fast, occur at room temperature ideally, prevent further crack propagation, and not shrink the material at the crack site. If shrinkage occurs after polymerization the healed area will be more vulnerable to subsequent cracking. The total healing process, from initial crack to final polymerizations, must happen quickly and efficiently. The autonomous self-healing system developed by White utilized ring opening metathesis polymerization (ROMP), where
DCPD is catalyzed by the Grubbs catalyst to fully polymerization.
The monomer DCPD readily undergoes ROMP, as seen in figure 1.5, to yield a tough, highly cross-linked polymer. There are several advantages to ROMP. The reaction polymerizes at room temperature. During ROMP of DCPD the Grubbs catalyst relieves ring strain causing the ring to open. The ROMP of DCPD is a highly exothermic process yielding a highly cross-linked final polymer with minimal shrinkage.
13
Figure 1.5: Ring opening metathesis polymerization of dicyclopentadiene
The polymer matrix, the third critical component and major component of microcapsule systems, should readily incorporate the microcapsules, with sufficient bonding interaction between the outside of the microcapsule and the polymer matrix. In addition to having a strong bonding interaction with microcapsule and curing agent, the polymer host must maintain its structural integrity throughout the manufacturing process. Incorporation of the microcapsules and curing agent should not negatively influence the properties of the polymer matrix. Ideally, the composite polymer matrix will show property enhancement post microcapsules and curing agent incorporation. The method for creating self-healing microcapsule systems is specific to the host material; and the proper fabrication technique must be utilized to create a usable composite structure. The polymer matrix must also be compatible with the healing agent after healing. In addition to system compatibility, the polymer matrix should have the ability to regain function post healing.
Another huge factor in considering a self-healing system is cost. The system should not cost more to produce than the manual repair/replacement cost. All system components should be cheap and widely available. The White, et. al system used a 1st generation Grubbs catalyst in a
ROMP reaction, instead of the more reactive 2nd generation Grubbs. The 1st generation Grubbs catalyst is the precursor to other ruthenium-based catalyst, therefore less expensive16. While
14 autonomous systems are preferred, assisted self-healing systems are still desired due to cost concerns. Repairing a system via human intervention is cheaper and more environmentally- friendly than replacing an entire failed system. Polyurethane networks developed for self-repair utilize exposure to ultraviolent light as the curing agent to repair17. These systems were able to repair themselves in less than an hour, when exposed to bright sunlight17.
The final critical parameter, the microcapsule, is the most influential component in microcapsule systems. The capsule, made of an active core reagent and inert outer shell, should remain intact during integration with the polymer matrix. High bond strength between the capsule shell and polymer matrix is necessary for proper integration. This bond strength will ensure the microcapsule does not move around throughout the polymer matrix. Capsules should also distribute evenly throughout the polymer, avoiding large aggregates. Large aggregates would not only decrease healing in areas lacking microcapsules, but could also cause weak points within the polymer matrix. In addition to compatibility between the microcapsule shell and the polymer matrix, the polymer matrix must be compatible with the encapsulated polymer.
When the shell ruptures the encapsulated polymer will need to flow into the crack via capillary action; therefore, there must be sufficient intermolecular attraction forces for this to occur.
In order to create a complete self-healing system both the polymer matrix and the capsules must withstand processing and maintain function. The capsule must withstand integration in the polymer matrix, but rupture when a cracking occurs. While rupturing is necessary during polymer host cracking, the capsule shell must remain intact with a long shelf life to avoid leakage or diffusion of the encapsulated polymer. The polymer encapsulation is a crucial element in maintaining the shelf life of the reactive core. Without the protection of the microcapsule shell interaction of the core healing polymer with the embedded curing agent and
15 other environmental components could cause the healing polymer to become nonreactive when the healing process is needed.
One method for creating self-healing microcapsules with liquid healing agents involves dissolving shell precursors in water. The polymer to be encapsulated is then added to this precursor solution creating an oil-in-water emulsified mixture. The emulsification influenced by a number of factors, such as stirring rate and volume ratio of the two phases, will determine the size and size distribution of the capsules created. The shell precursors will self-condense around the core monomer. This encapsulation is influenced by surface activity and interaction of the shell precursors at the interface. In the White system, the microcapsules where made of a poly
(urea-formaldehyde) inert shell and a DCPD core. The urea-formaldehyde shell withstands processing, with good compatibility between poly(urea-formaldehyde) and the epoxy matrix used in the self-healing material.
Figure 1.6: Reaction of urea and formaldehyde forming urea-formaldehyde
The microcapsules encapsulated DCPD as the healing polymer inside a urea-formaldehyde shell. These microcapsules were embedded throughout the epoxy matrix along with the Grubbs catalyst. Cracking ruptured the urea-formaldehyde shell and released of the DCPD. During mechanical damage to the host polymer material the DCPD filled the crack via capillary action.
Upon contact with the Grubbs catalyst a ROMP reaction begins, creating a highly cross-linked tough polymer, polycylcopentadiene. Significant self-healing occurred within 1 hour at room
16 temperature, with complete polymerization occurring after 12 hours. In the early stages of research this system recovered 75% of its original fracture toughness4. This system has since been optimized and shown one of the highest strength recovery percentages, at 90% toughness recovery18. This initial report of autonomous self-healing opened new avenues for applications.
Self-healing was no longer tied the use of large amounts of heat, requiring human intervention.
The widespread use of polymer materials means the impact of self-healing could reach to all field. Any material prone to damage could potentially become self-healing. While each material will have to have a tailored self-healing mechanism, creating a self-healing material using any polymer material could be possible.
1.2.3 Self-Healing System Applications
The area of self-healing material systems opens new avenues in material applications. With the wide-spread use of polymers the need for materials with abilities to monitor and repair damage is growing evermore necessary. Recently the New York Times reported cracking in the wings of the double-decker Airbus airplane. These planes, which seat 555 passengers, showed hairline fractures and had to be taken out of service while the planes were inspected and repaired.
Where commercial airplanes have a service lifetime exceeding 25 years, with tens of thousands of flight hours and thousands of takeoff-landing cycles, the stress on structural materials is enormous. Damage may lead to product failure and replacing parts can be a long expensive process. In some instances, such as space shuttles, structural damage can fatal. Current solutions for mechanical damage include “over-engineering” a material. This over-engineering means more material is used than required. The added material increases weight and other cost associated with increased materials. Engineering materials capable of repair extends the service
17 of the material, decreases the need for “over-engineering”, decreases manufacturing cost, and decrease operating cost. The self-healing concept is already employed in certain car paints, decreasing the need to have starches repaired professionally.
1.3 Background: Carbon Nanotubes
Self-healing restores considerable damage; additional nanoparticles can enhance the existing properties. The physical properties of carbon nanotubes allow them to be used as reinforcing fibers in polymer composites,19 yielding ultra-light composites with enhanced mechanical, electrical, thermal, and optical properties.20, 21 Carbon nanotubes were discovered in 1991 by
22 Sumio Iijima, following the 1985 discovery of C60 by Sir Harold Kroto, Richard Smalley, and
Robert Curl.23
1.3.1 Carbon Nanotube Structure
Figure 1.7: Schematic of carbon nanotubes: a) single-walled carbon nanotube, b) graphite sheet, c) hemispherical end cap and d) multi-walled carbon nanotube19.
18
Carbon nanotubes (CNTs), hollow tubes comprised completely of carbon, have unique electrical, mechanical, and thermal properties. Carbon nanotubes can be described as sheets of graphite rolled up into a seamless cylindrical tube, sometimes end capped with a half fullerene.
24, 19 Carbon nanotubes are composed of hexagonal rings of carbon ending with 12 pentagonal rings. Carbon nanotubes with similar chemical composition to graphite are highly isotropic. This uniformity in properties gives CNTs their unique properties, setting them apart from other carbon structures.19 There are two types of CNTs: 1) single-walled nanotubes (SWNTs), individual cylinders, and 2) multi-walled nanotubes, several concentric cylinders bound together by Van der
Waals forces.25 A single-walled CNT (SWNT) consists of one layer of carbon atoms and can be either metallic or semiconducting, depending on the tubes final confirmation. Multi-walled
CNTs (MWNTs) consist of concentric sets SWNTs and are generally metallic. CNTs range in their length (20-200μm) and diameter (1.38 – 50nm) with an interlayer spacing in multi-walled tubes (~ 3.4Å). The chemical bonding of CNTs involves covalent bonds with sp2 hybridization, as in graphite, which are among the strongest bonds in nature. Each carbon atom bonds strongly with its neighbors via sp2-hybridization sigma-bonds. The fourth electron is in the pi-orbital with has lobes perpendicular to the plane. This arrangement yields a material with a high strength-to- weight ratio19.
1.3.2 Properties
The properties of carbon nanotubes have been heavily researched experimentally and theoretically. CNTs’ tensile strength, the force required to pull a material to its breaking point, is two orders of magnitude higher than other high-strength carbon fibers (100-600GPa). 19, 26, 27 If produced as bundles CNTs would be 100 times stronger than steel.28
19
Table 1.1: Tensile Strength of Common Materials and Carbon Nanotubes29.
Material Tensile Strength (GPa)
Kevlar™ 3.5
Tungsten Carbide 0.3448
Titanium 0.24 – 0.37
Iron 0.350
Copper 0.22
Aluminum 0.04 – 0.05
Lead 0.012
Carbon Nanotubes 300
CNTs’ hollow lightweight design yields a density of 1.3 g/cm3, giving CNTs strength unmatched by steel with only 1/6 the weight. CNTs have shown a compressive strength greater than two orders of magnitude higher than any know fiber.30 In addition to this remarkable stiffness, CNTs are extremely flexible. They deform reversibly with fracture strains between 10-
30%, compared to 0.1-2% for most carbon fibers.3, 19, 31 MWCNTs will behave as elastic rods and bend; however, under high bending caused by compression, MWCNTs will collapse forming kinks on the compressed side of the bend.30
The production of CNT leads to a few disadvantages. The main problems plaguing CNTs wide-spread use in polymer composites are relatively high cost, difficulty in mass production and purification, and the need for more toxicity testing. Additionally, during production many other carbon forms are also produced. The other carbon forms can be hard to separate. When
20 incorporating CNTs into a polymer oftentimes additional modification is needed in order to increase the interaction between the nanotubes and polymer.32
Carbon nanofibers are larger than CNTs, typically 50-200nm in diameter and 50-100μm in length. These fibers have a lower production cost than CNTs, making them more commercially attractive; however, the increased production comes at a cost. Carbon nanofibers have a larger number of microstructural defects compared to CNTs. They have a tensile strength of 240 GPa19 slightly lower than that of CNTs. Carbon nanofibers’ most remarkable feature is the presence of numerous edges, making these fibers ready for chemical or physical interactions.
When compared to CNTs carbon nanofibers (CNFs) have an increased chemically active surface area (300-700 m2/g)19.
1.3.3 Polymer/CNT Composites
To enhance the properties of polymer systems CNTs and CNFs are used as reinforcing agents. Several processing methods exist for producing polymer/CNT composites. Post processing properties of the polymer/CNT composite depends on two variables: 1) load transfer and 2) nanotube dispersion. These composite materials need bonding between the carbon species and the polymer to achieve load transfer from polymer to CNT.25, 33 High interfacial adhesion is necessary for a high degree of load transfer between the polymer and nanotubes. Poor interfacial adhesion leads to CNTs created holes, inducing localized stress and losing fiber reinforced enhancements CNTs provide. CNFs hold an advantage over CNT by improving interfacial bonding due to the increased surface area. This increased bonding leads to improved mechanical interlocking, chemical bonding, and van der Waals interactions. CNFs obtain optimal values of surface area and surface energy to provide significant reinforcement19.
21
The effective use of CNTs in composites also depends on dispersing the nanotubes throughout the composite, without destroying the nanotubes’ integrity34. To achieve this dispersion CNTs often undergo a preprocessing procedure, such as sonication. Ultrasonication, the most common preprocessing method, aids in the control of aggregation27, 35; however, sonication could potentially produce CNT defects.25, 32 Despite the potential risk of sonication the use of poorly dispersed CNTs is often the greater problems since it leads to failure of CNT properties being utilized.19, 36
The processing of polymer/CNT composites varies widely. In-situ polymerization improves dispersion and integration between the polymer and CNT.19, 21,37. In in-situ polymerization CNTs are dispersed in a low viscosity matrix. Following the disbursement the composite undergoes curing. This method could also be used to align CNT arrays,19 providing additional properties, such as increase strength in the direction of the CNT alignment. Crack propagation also decreases in polymer materials where nanotubes align perpendicular to a crack38. In in-situ polymerization the wide disbursement leads to increased impact strength with small additions of CNTs. Previous experiments show tensile strength increased by 25% with 1%
MWNT added to polysterene25.
1.3.4 Polymer/CNT Composite Applications
In effort to better tune polymers to provide properties specific to an applications the integration of CNTs and/or CNFs into the polymer matrix open addition avenues of possible applications. Currently CNTs are most often used as reinforcement in polyepoxides, also known as epoxies. Epoxy, a thermosetting polymer, already possesses mechanical properties equipping
22 this polymer to be useful as a structural material. Further integration of CNF with will yield a fiber reinforced plastic, which has been used in aerospace applications.
While the use of CNT as reinforcers in polymer composites can be more expensive in comparison to other reinforcement options such as glass or aluminum. High quality pure CNTs can cost up to $100/gram, but as research continues in the production of CNTs the price will continue to drop and quality increase. These polymer/CNT composite structure properties suggest CNTs capability to reinforce structural composites. In applications where strength and low weight are important considerations, CNTs provide huge advantages. The possibilities for carbon-nanotube composites are numerous, from space structural panels to ultra-light-weight thin- walled aquatic vehicles. CNT reinforcements enable thinner flatter structures to be produced19. A new Boeing 787 Dreamliner will use carbon fiber reinforced polymer in over half of its structural materials, making the plane lighter and 20% more fuel-efficient. The reinforced composites will be lighter with a higher tensile strength than the steel currently used.
1.4 Background: Foams
Polymeric foams are widely used as structural materials. First used in the 1940s polymeric foams are broadly defined as structures with material domain separated by voids39.
Polymeric foams can be classified based on pore size yielding three different classes: macrocellular, microcellular, ultracellular. Macrocellular foam, commonly referred to as conventional foam, has a cell size of 50 - 100 μm. Microcellular foams have an average cell size of 0.1-10 μm. Ultracellular foams contain voids < 0.1 μm.
1.4.1 Microcellular Foams
23
In the early eighties the field of microcellular foam exploded after a challenge to reduce the amount of polymer needed in food and film packaging.40 Previous packaging plastics could not produce solid, thin-walled, inexpensive plastics, thus the need for microcellular foams. Since the beginning researchers explored new methods to produce and use these foams. While conventional foam falls into either a low density region (less than 0.1 g/mL) or structural foam category (density greater than 0.5 g/mL) microcellular foams range in relative density between
0.1 and 0.5 g/mL. The density range presents new properties not available with conventional foams,40 such as high impact strength, high stiffness-to-weight ratio, high fatigue life, and high thermal stability. The fatigue life of foam increases with the lower density microcellular foams.
In polycarbonate foam, decreasing the density by less than 1%, increases the fatigue life thirty- fold when compared to solid polycarbonate 40.
Microcellular foams are created via different processing techniques. The solid-state batch process occurs in two stages. First, the polymer is placed in a pressure high-pressure vessel with a non-reacting gas at room temperature. As the gas diffuses into the polymer it becomes uniform throughout the materials. The second stage takes the “supersaturated” polymer from the high- pressure vessel and heats it above Tg. The glass transition temperature of the polymer-gas mixture is typically lower than the glass transition temperature of the polymer. The reduction in gas due to the heat increase causes bubble nucleation. The polymer remains in the solid state, giving this process its name, solid-state batch processing. The solid-state batch processing is used to create microcellular foams from many amorphous and semi crystalline polymers, such as polyvinyl chloride (PVC), polycarbonate (PC), acrylonitrile-butadiene-styrene (ABS), polyethylene terephthalate (PET)40.
24
Figure 1.8: Examples of microcellular foams: a) PVC, b) PC, c) ABS, and d) PET40.
The most common process for creating microcellular foams is chemically induced phase separation (CIPS). This process involves polymerizing a mixture of precursors, nonreactive solvent, and a low molecular weight liquid polymer with a low boiling point. The mixture, which is solvent for the precursors, is not a solvent for the final polymer foam. As the solution heats the initial homogenous solution separates into a two-phase system, solvent-rich phase and polymer- rich phase. Heating the system continues to cure the polymer. Once polymerization and phase separation are complete, the foam can be dried removing all solvent41.
25
Figure 1.9: Steps involved in chemically induced phase separation41
Despite the wide-spread use of the chemically-induced phase separation technique for creating microcellular foams, a new class of emulsion-derived foam is emerging. High internal phase emulsion (HIPE) yields foams that have a:
-internal void volume greater than 75%,
-controllable cell diameter,
- and, fully interconnected structure42.
The internal void volume can be as great as 90%, 90% pore volume to 10% polymer. Using a method with a controllable cell diameter enables specific pore sizes to be created, ranging from 2
μm to 100 μm. All pores are connected creating a fully connected structure, often not seen in
CIPS. Most emulsion-derived foams studied use styrene-divinylbenzene as the polymer; however, other commercially available polymers are being examined to determine if they can be used to create microcellular foams via HIPE. Previous studies have paired HIPE with ROMP to create microcellular DCPD foams; however, the desired open-cell structures were not obtained42.
One of the critical issues in HIPE foams is finding emulsions stable enough to withstand processing until the polymer has cured43. Additionally, HIPE is generally easier to perform than
CIPS. Dispersing CNTs within polymer foams can enhance the foam’s mechanical properties if
26 wall pores are reinforced44. The embedded fibers in polymer matrices allows for load transfer giving the material enhanced strength properties45.
1.4.2 Microcellular Foam Applications
The use of microcellular foams has expanded over the years. While conventional foam is widely used, the use of microcellular foam has not expanded in the same way. The major challenge is to create a processing method that is both reliable and cost effective. Currently the microcellular foam technology cannot create low cost foam that can compete with cost of conventional foam. Despite the cost disadvantage there are niche markets were microcellular foam provides superior advantages to conventional foam that the increase in price is warranted.
Organic microcellular foams with incorporated carbon fibers have proven useful as toxic gas absorbers. These foams have also been developed to act as bacterial filters. As research is continued and the technologies advance a greater number of applications will be able to take advantage of properties provided by microcellular foam.
1.5 Dissertation Overview
This research focused on the fabrication of self-healing carbon nanotube reinforced composite materials. Polymer self-healing system development is in its infancy. The method for developing a viable self-healing system varies depending on the materials utilized in self- healing; therefore, new methods need investigation when adjusting any component. Also the integration of carbon nanotubes, to enhance the properties of the final material, must be taken into consideration when developing reinforced materials. Previous research has not developed a process for integrating CNTs into the microcapsule process. Self-healing composites could
27 potentially to solve the problem of microcracking. This healing would mitigate the effects of harsh environments and increase the service lifetime of materials. While the use of carbon nanotubes as polymer reinforcements has been investigated, the incorporation into self-healing polymer composites has not.
Chapter 2 will cover the materials and processes used to create both the carbon nanotube integrated microcapsules, self-healing cellulose triacetate coatings, and carbon nanotube reinforced dicyclopentadiene foam. This chapter covers the process for fabrication of self- healing materials utilizing a natural based polymer matrix, instead of a completely synthetic polymer seen in previous research4, 7, 15, 18, 46, 47. The process for incorporating microcapsules in a new material is not trivial. Chapter 3 discusses fabrication for CNT incorporated microcapsules, while chapter 4 discusses the fabrication process for the creation of self-healing coatings.
Chapter 5 discusses the development of carbon nanotube reinforced foam. The ability to create new carbon nanotube reinforced composites continues to expand the uses of carbon nanotube materials and microcellular foam. This work combines previously investigated concepts of self- healing and microcellular foam with the carbon nanotubes to create carbon nanotube reinforced polymer composites.
28
CHAPTER TWO: MATERIALS AND FABRICATION METHODS
2.1 Microencapsulation
2.1.1. Urea-Formaldehyde Microencapsulation with Polymeric Core Materials
The microcapsules created consist of an outer inert shell and a reactive inner core. The reactive core resin used for self-healing was a bisphenol-A diglycidyl ether of (BADGE), EPON
828. The resin was obtained from Sigma Aldrich. The structure of the core material is shown in
Figure 2.1.
Figure 2.1: Chemical structure of encapsulated bisphenol A diglycidyl ether healing agent, EPON 828®.
The microcapsule walls were formed using resorcinol (C6H4 – 1, 3-(OH) 2), ammonium chloride
(NH4Cl), poly (ethylene-alt-maleic anhydride) (EMA), and formalin (37% formaldehyde in water) solution purchased from Sigma-Aldrich; and urea (NH2CONH2) was purchased from
Fisher Chemicals. In studied incorporating carbon nanofibers (CNFs) and carbon nanotubes
(CNTs) CNTs which were purchased from Nanosutructured and Amorphours Materials Inc.
29
2.1.2 Urea-Formaldehyde Microcapsule with Polymer Core Fabrication Process
Figure 2.2: Steps involved in the microcapsule fabrication process.
The process used to create the self -healing microcapsules was based on the Blaiszik, et al. method46 for urea-formaldehyde in situ polymerization. Figure 2.2 shows the steps for the final optimized process. A poly (ethylene) maleic anhydride (EMA) surfactant (0.125 g) was combined with 5 g urea, 0.5 g NH4Cl, and 0.5 g resorcinol in 200 mL deionized water to create the microcapsules precursor solution. The precursor solution, pH ~2.6, was made more basic by adding NaOH to attain pH 3.5. The precursor solution was heated in a hot oil bath to 35oC at
1oC/min then 60 mL of the core monomer EPON 828 was added to the precursor solution. The two solutions were agitated for 15 minutes at 35 oC, creating an oil-in-water emulsification. The solution was then heated to 55 oC at 1 oC/min and 13 mL formaldehyde was added. The solution 30
was sonicated with a sonication probe and stirred (320 rpm) at 55 oC for 4 hours. The system set
up is shown in figure 2.3. The solution was allowed to cool in the hot oil bath overnight.
Sonication probe
Figure 2.3: Microencapsulation system process set up.
2.1.3 Urea-Formaldehyde Microcapsules with Carbon Nanofiber Incorporated Shells and Polymer Core Fabrication Process
In studies creating microcapsules with CNTs incorporated into the shell The precursor
solution was made by dissolving 0.125 g (EMA), 5 g urea, 0.5 g NH4Cl, and 0.5 g resorcinol in
200 mL deionized water. This precursor solution pH was adjusted to pH 3.5; then heated in an
oil bath to 35oC at 1oC/min. After reaching 35 oC, 6 mL of the core polymer BADGE was added
to the precursor solution. The two solutions mixed at 35oC for 15 min, creating an oil-in-water
31 emulsification. Post emulsification the solution was heated to 55oC at 1oC/min. A solution of
0.05 g CNTs sonicated with 3 mL formaldehyde was then added to the solution. After combination of the two solutions the mixture was then sonicated with a sonication probe and stirred (320 rpm) at 55oC for 4 hours. The solution was allowed to cool in the hot oil bath overnight.
2.2 Self-Healing Cellulose Triacetate Composite Films
2.2.1. Self-Healing Coatings and Film Materials
Self-healing composite materials were created using cellulose triacetate (CTA), figure
2.4, purchased from Sigma-Aldrich.
Figure 2.4: Chemical structure of cellulose triacetate.
Dichloromethane (DCM), figure 2.5, was also purchased from Sigma-Aldrich.
Figure 2.5: Chemical structure of dichloromethane.
32
In order to induce curing of the healing agent a catalyst was added to the films. EPIKURE
3140®, purchased from Miller Stephenson, was the amine-catalyst used for the self-healing process.
2.2.2 Self-Healing Film and Coating Fabrication
CTA used the base polymer matrix was prepared using 16 g CTA and 250 mL DCM. The
CTA was dissolved into a heated (40oC) stirred DCM in ~1 g aliquots until all CTA was added.
This solution was the CTA base for all the film and coating fabrication. For film and coating fabrication 20 mL of the CTA polymer base was combined 0.5 mL of the catalyst, EPIKURE
3140®. The mixture was stirred at 500 rpm for 48 hours. Following the combination of CTA dn
EPIKURE 3140® varying concentrations of microcapsules were added. Prior to combining the microcapsules into the CTA base small aliquots of microcapsules, both CNT coated and uncoated depending on the study, were rinsed with DI water, and allowed to dry for 24 hours.
The varying concentrations of microcapsules, discussed in more depth in chapter 4, were stirred
(500 rpm) into the CTA solution for 48 hours. In studies using additional CNTs, the CNTs were mixed into the CTA solution after the microcapsules. The CNTs were stirred at 500rpm for 48 hours.
After stirring, thin films were created on steel substrates on a spincoater. All films contained an initial layer of virgin CTA. Once one layer was spun another layer was applied on top of the previous layer and spun under the same conditions. Two minutes of drying time was allowed after each layer was spun. Four layers of film containing the composite matrix were spun on top on the initial CTA layer to create a film with a total of five layers. If the film needed to be removed from the substrate the final layer was allowed to dry completely before the film
33 was removed. After spincoating the films were placed in a 40oC oven for 24 hours to ensure complete polymer base curing.
2.3 Dicyclopentadiene Microcellular Foam
2.3.1. Dicyclopentadiene Composite Foam Materials
The base foam used to create the composite foam was dicyclopentadiene (DCPD), shown in figure 2.6. The DCPD was purchased from Sigma-Aldrich.
Figure 2.6: Chemical structure of dicyclopentadiene.
The composite foam created utilized resorcinol, ammonium chloride, EMA, formalin, urea, and in certain studies CNFs. All materials were supplied by the same vendor used for creating the microcapsules.
2.3.2 Dicyclopentadiene Microcellular Foam Fabrication Process
The DCPD microcellular foam fabrication process followed the steps illustrated in Figure
2.7.
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Figure 2.7: Steps involved in the fabrication of DCPD foam.
The composite foam was created by dissolving 0.125g of EMA, 0.5g of urea, 0.5g of
NH4Cl, and 0.05g of resorcinol in 25mL of ultra pure H2O. The solution was covered with parafilm and stirred at 320 rpms overnight in order to ensure all components were completely dissolved. Once the solution was clear the pH was adjusted to 3.5 using 1N NaOH. The solution was then placed in a hot oil bath and allowed to heat to 55oC, while stirring at 320rpm. While the solution heated to 55oC, 6g of DCPD was added. After adding DCPD the solution stirred (320 rpm) for 15 minutes at 55oC. The mixture was sonicated and stirred at 320rpm at a constant heat of 55oC for 1.5 hours. Afterwards the solution cooled overnight in the hot oil bath. Subsequent drying was done in air.
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2.3.3 Dicyclopentadiene Microcellular Foam with Carbon Nanofibers Fabrication Process
The DCPD foam with incorporated CNFs was created by dissolving 0.125g of EMA,
0.5g of urea, 0.5g of NH4Cl, and 0.05g of resorcinol in 25mL of ultra pure H2O. The solution was covered with parafilm and stirred at 320 rpms overnight in order to ensure all components were completely dissolved. Once the solution was clear the pH was adjusted to 3.5 using 1N
NaOH. The solution was then placed in a hot oil bath and allowed to heat to 55oC, while stirring at 320rpm. While the solution heated to 55oC, 6g of DCPD was added. After adding DCPD the solution stirred (320 rpm) for 15 minutes at 55oC. Prior to sonication a mixture of 0.05 g of CNF and 3 mL of formaldehyde were added. The mixture was sonicated and stirred at 320rpm at a constant heat of 55oC for 1.5 hours. Afterwards the solution cooled uncovered overnight in the hot oil bath.
2.4 Instrumentation
2.4.1 Spincoater
Spincoating created a uniform thin film and coating of the self-healing polymer system after all components were integrated. The spincoating process occurs in four stages. The first stage consists of depositing the composite solution onto the spincoater substrate, figure 2.8. The viscous polymer composite solution is placed to the middle of a substrate and applied until the substrate has nearly covered in solution. As the substrate spins at a high rate of speed, the solution fills the substrate, while excess solution spins off the sides of the substrate. During spinning the volatile solvent, DCM, evaporates out of solution. The final thin film remains after spinning.
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Figure 2.8: Schematic of spincoating stage.
The spincoating system, figure 2.9, was a Spincoat G3-8 from Specialty Coating
Systems. The system was set to run for 2 minutes with acceleration and deceleration time of 20 seconds and a maximum spin time of 700 rpm. This creates a thin film about 20 μm. Each thin film layer rested for two minute before spinning another layer.
Figure 2.9: Spincoat G3-8 from Specialty Coating Systems.
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2.4.2. Scanning Electron Microscopy
Scanning electron microscopy (SEM) shows sample’s surface features on the micron scale. This interaction produces a signal containing information on the samples properties, such as surface topography, composition, and electrical conductivity. When creating a SEM image an electron beam scanning a sample’s surface, slightly penetrates the sample. The electrons interact with atoms at or near the sample’s surface producing secondary and back-scattered electrons, from which an image is produced. Specialty microscopes having detectors for the other signals, such as the x-rays, current electrons, and transmitted electrons, also produced. The signal resulting from the SEM’s electron beam interaction with sample’s surface can create very-high resolution images of the samples surface. The standard detector, scanning electron image detector, can reveal details down to the nanometer scale. SEM imaging is useful to understand the surface topography and characteristics of the samples surface.
Figure 2.10 shows a schematic of a SEM. The SEM’s electron beam is emitted from an electron gun with a tungsten filament cathode. The beam is achieved through thermionic emissions. Thermionic emission occurs when the thermal energy is larger than the binding potential allowing the heat-induced flow of electrons. Tungsten is most widely used due to cost and tungsten’s high melting point paired with low vapor pressure. The electron beam produced by an electron gun ranges in energy from 0.2 keV to 40 keV. The beam is focused to a 0.4 nm to
5 nm area on a condenser lens. As the beam travels under vaccum in the electron column it passes through scanning coils and an objective lens. The final lens deflects the beam in the x and y directions. The deflections in x and y allows for raster scanning over the samples surface.
Raster scanning is a scanning method for generating images via a line-by-line sweep.
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The SEM produces back-scattered electrons. These back-scattered electrons are the electrons reflected by the same through elastic scattering. The back-scattered electrons signal intensity relates to the atomic number of the samples. This signal intensity provides information on the distribution of elements within samples, providing details that would otherwise be undetectable light microscopy. Back-scattered electron beam is used in conjunction with characteristic X-rays. The characteristic X-rays are emitted when the incoming electron beam removes an inner shell electron from the sample. A high-energy electron replaces the electron that was removed and there is a release in energy. The characteristic X-rays identify the composition and abundance of element within samples.
When the electron beam comes in contact with the samples, the electrons lose energy.
The energy lose is caused by repeated random scattering and absorption in a volume of the samples known as the interaction volume. The interaction volume extends about 100 nm to 5 μm into the samples surface, depending on the electron’s landing energy, the sample’s atomic number, and the sample’s density. The interaction between the sample and the electron beam results in an energy exchange, reflection of high-energy electrons via elastic scattering, emission of secondary electrons through inelastic scattering, and electromagnetic radiation. All of these emissions can be detected by specialized detectors. The part of the electron beam absorbed by the sample can also be used to generate an image. Electron amplifiers enhance the signal. The signal is then displayed as a variation of brightness through a computer with each pixel synchronized with the electron beam’s position. The resulting image is an intensity distribution map of the signal emitted by the electrons in the scanned area.
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Figure 2.10: Schematic of scanning electron microscope.
The high-energy electrons that are reflected out of the sample, after interacting with the incoming electron beam, are referred to as back-scattered electrons (BSE). The electrons arise from the elastic scattering interaction between the original electron beam and the samples surface atoms. The BSE are used to detect contrasting areas of different chemical compositions48. BSE detectors are positioned in a “doughnut” arrangement above the sample to maximized solid angle collections. When the detector collects the electron symmetrically about the beam an atomic
40 number contrast is produced. Elements with high atomic numbers will backscatter electrons more strongly than low atomic number elements; therefore, the high atomic number elements will appear brighter in the SEM image. Strong topographical contrast is also produced using asymmetrical BSE collection. The contrast is an illumination of the topography from one side.
The SEM image magnification is controllable 10 to 500,000 times magnification. In
SEM, magnification is not a function of objective lens power, as in optical microscope or transmission electron microscopes. SEMs with objective lenses have these lenses to focus the beam to a spot. The lens is not used to create images of the samples. In theory, if the SEM’s electron gun could produce a beam of sufficiently small diameter, the SEM image could be produced without an objective lens. The objective lens does, however, provided versatility and very high resolution through the focusing of the electron beam. The SEM magnification is derived from the ratio of raster scanning of the sample and the raster scanning of the display screen. With a fixed display screen the higher magnification is the result of raster scanning the samples. The sample raster scanning is controlled by the current or voltage supplied to the x and y scanning.
The spatial resolution of the image is dependent on the electron beam spot. The beam spot depends on the electron-optical system and beam wavelength. The resolution also depends on the beam-sample interaction, both the volume size of the interaction and the extent of interaction. Since the interaction volume and spot size are much larger than the distances between atoms the resolution achieved via SEM is not high enough to see individual atoms, as seen with the shorter wavelength, higher energy transmission electron microscope. However, the advantage of SEM is its ability to image a larger sample area and examine bulk materials.
41
The samples used in SEM must be tailored to fit the sample chamber. Chambers can be up to about 6 inches and some are set up to tilt a sample up to 45o. Samples must have an electronically conductive surface. The sample must also be electronically grounded to prevent accumulation of electrostatic charge on the sample’s surface. Metal objects can be mounted onto the sample stub and imaged without special preparation; however, nonconductive samples are coated with an ultra thin coating of electronically conducting material, such as gold, palladium, platinum, or iridium. Nonconductive samples will charge when scanned by the electron beam causing scanning faults and image artifacts to appear. The electronically conductive material is applied by low-vacuum sputter coating or high-vacuum evaporation. The coating may also enhance the signal to noise ratio in samples with low atomic numbers.
2.4.3 Everhart-Thornley Detector
The Everhart-Thornely (E-T) detector is a scintillator-photomultiplier system used to detect secondary electrons. E-T detectors increase the efficiency of secondary electron detection collecting the signal in the sample chamber and converting it to light outside the sample chamber. The light guide and high efficiency photomultiplier enhances the weak signal from secondary electrons. E-T detectors are insufficient at collecting BSE since few of those electrons are emitted in the solid subtended angle suitable for detection. Additionally, the positive detection grid does not have the ability to attract higher energy BSE electrons.
The detector is the most common detector for low energy (<50 eV) secondary electrons.
These low energy secondary electrons are ejected from the samples atoms via inelastic scattering interaction between the electron beam and sample’s atoms. The low energy electrons originate only a few nanometers from the sample’s surface48. The detector consists of a scintillator inside
42 of a Faraday cage within the sample chamber. In order to collect the secondary electrons, a low positive voltage, about +400 V, attracts the low energy secondary electrons toward the electrically biased grid. Higher energy electrons are not attracted to the detector and only come in contact with the detector if their travel path takes them there. The scintillator, with a high positive voltage of about +2000 V, then accelerates the low energy electrons towards it. The increased acceleration produces sufficient energy to cause the scintillator to emit cathodoluminescense, flashes of light, which are collected for the photomultiplier via a light pipe. The photons then travel to a photomultiplier tube outside the microscope’s vacuum, where the signal is amplified. The amplified signal emitted by the photomultiplier is shown as an intensity distribution via raster scanning. The signal contrast is determined by the number of secondary electrons reaching the E-T detector. If the beam interacts with a sample perpendicular to the surface a number of secondary electrons are emitted. Various angles of incidence will yield various numbers of secondary electrons. The higher the angle on incidence the more secondary electrons emitted; therefore, the steep surfaces and edges yield brighter signals than flat surfaces.
2.4.4. FEI Nova 400 Nano Scanning Electron Microscope
The samples examined in these studies used a FEI Nova 400 Nano Scanning Electron
Microscope in the Florida State University Biology Imagining Resource Laboratory, seen in figure 2.11.
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Figure 2.11: FEI Nova 400 Nano Scanning Electron Microscope.
Images were recorded using an Everhart Thornley Secondary Electron Detector. Samples were mounted on a SEM specimen stub, figure 2.12, using electrically conductive double-sided adhesive tape. The tape was attached to the top of the stub and the sample was placed on the tape. The samples are nonconductive and were all sputter-coated with Iridium (Ir), which is electronically conductive, to prevent charging by the electron beam. Left uncoated charging causes complications, such as scanning faults and image artifacts, during scanning.
Figure 2.12: SEM stub used mount samples 49.
The procedures described in this chapter were the final optimized processes. A more detailed approach to discovering the optimal process and the results are discussed in future chapters.
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CHAPTER THREE: THE FOMATION OF CARBON NANOTUBE COATED MICROCAPSULES VIA IN SITU POLYMERIZATION
3.1 Introduction
Self-healing polymers are able to heal without human intervention, reducing maintenance cost and increasing application lifetime. During research to produce self-healing polymer composites, numerous reactions and models are being researched14. Microencapsulation systems have shown the most success and have become the major focus of research in this area. In microencapsulation systems, microcapsules are engineered to contain a reactive polymer and/or catalyst. The microcapsules are embedded in a polymer composite. The microcapsules’ shell will break if the polymer composite is strained and/or fractured. Further crack propagation is prevented when the encapsulated polymer fills the crack and a reaction with the catalyst hardens the polymer; thus, healing the crack.
A major component of these self-healing systems is the microcapsule. Microcapsule fabrication must be tailored to the polymer in which they are embedded and the self-healing reaction that will be used. This chapter examines the process for the formation of microcapsules that incorporates CNFs in to the microcapsule shell. Carbon nanofibers have unique properties they would greatly enhance self-healing polymer composites. Previous research has shown CNT can greatly enhance physical properties of a polymer compound; however, their use in composites has been little research. Research is ongoing in the fabrication process incorporating
CNFs and self-healing microcapsules. The studies in this chapter discuss the newly developed system for incorporating CNTs into the shell of the microcapsule while continuing to encapsulate a reactive polymer that can be used for self-healing.
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3.2 Experimental Section
3.2.1 Materials
The CNFs and multi-walled CNTs were supplied by Nanostructured and Amorphours
Materials Inc. The shell precursors used were resorcinol, ammonium chloride, poly (ethylene-alt- maleic anhydride) (EMA), formalin, and urea. Two distinct reactive monomer cores were used as the inner core polymer. Endo-dicyclopentadiene (DCPD), seen in figure 3.1, was first encapsulated. A bisphenol A diglycidyl ether (BADGE) was also encapsulated. The BADGE used was EPON 828, seen in figure 3.2. EPON 828 was purchased from Miller-Stephenson.
Figure 3.1: Structure of endo-dicyclopentadiene (DCPD)
Figure 3.2: Structure of bisphenol A diglycidyl ether (BADGE)- EPON 828®.
3.2.2 Microcapsule Fabrication Process
46
Figure 3.3: Steps for fabricating microcapsules with encapsulated monomer.
Microcapsules were prepared via in situ polymerization utilizing an oil-in-water emulsion in the manner described in chapter 2 and seen in figure 3.3. The surface morphology of samples was analyzed using the FEI Nova 400 Nano Scanning Electron Microscope (SEM). Samples were mounted on a conductive stage and sputter coated with a thin layer of Iridium (Ir) to avoid charging prior to being examined.
3.3 Results
3.3.1 Optimizing the Microcapsules Fabrication Process
47
The microcapsule encapsulation system developed by Blasik, et. al. created microcapsules with poly(urea-formaldehyde) shells and DCPD cores. This process generates large batches of microcapsules that must be stored for later use. Storing the microcapsules cuts the effective lifetime of the capsules. Over time the reactive core can diffuse out of the microcapsules shell. In large batches the microcapsules can aggregates. In addition to core leakage the aggregation prevents serration of the microcapsules inhibiting their incorporation into a polymer composite. These studies examined ways to minimized aggregation and reduce material consumption and processing steps in the fabrication process.
In the Blasik, et. al. procedure EMA is used by first dissolving it in water and adding the
EMA solution to the precursor solution. The EMA solution is used as a surfactant to aid in the emulsification process. Elimination of the surfactant solution by directly adding EMA to the precursor solution reduces the process time and additional steps. The 0.125 g of EMA was added directly into the precursor solution and allowed to dissolve alongside the other precursor reactants. Then the 200 mL precursor solution was combined with 60 mL of DCPD at 35oC. The solution emulsified for 15 minutes prior to 13mL of formaldehyde being added. The solution was stirred, sonicated and heated for 4 hours at 55oC.
After the process was completed the microcapsule solution, show in figure 3.4 was created. The solution produced prior to drying contained a mix of additional solvent and microcapsules. The microcapsules had an urea-formaldehyde shell and DCPD core. The fabrication process generated ~ 150mL of microcapsules.
48
a)
b)
Figure 3.4: DCPD microcapsules with urea-formaldehyde shells: a) microcapsules prior to drying, b) microcapsules after 24hr drying in room temperature
Figure 3.5a shows a SEM image of the microcapsules; and figure 3.5b show a higher magnification of the microcapsules. The microcapsules ranged in size from 4 μm to 0.2 μm, with a spherical in shape. The microcapsules are largely aggregated together. The fabrication process typically yielded product greater than 60%, based on the ratio of mass of the recovered microcapsules mass to the total mass of monomer and shell components. Eliminating the step in which surfactant solution was first created did not prevent the production of microcapsules nor did it alter the morphology of the microcapsules.
49
a)
b)
Figure 3.5: SEM images of DCPD self-healing microcapsules: a) SEM image of DCPD encapsulated microcapsules b) higher magnification of microcapsules
50
The direct addition of surfactant to the precursor eliminated surface bubbles in the precursor solution, seen in the Blasik, et. al. system. Without the presence of surface bubbles, octanol - commonly used to eliminate surface bubbles in microcapsules formation, was not needed.
Surface bubbles can interfere with the interaction between the aqueous precursors and the polymer to be encapsulated, disrupting microcapsule formation. With the lack of surface bubbles during production additional modifications the use of octanol could be eliminated.
No surface bubbles and direct addition of surfactant would allow for quicker formation of microcapsules; thus a reduction in production time. When adjusting the production time from four hours to two hours the resulting solution did not form complete capsules. These poorly formed capsules, shown in figure 3.6, were produced after 2 hours of sonication. Inhibiting full production of the microcapsules produces a gel-like structure likely to be caused by polymer diffusing out of the microcapsule shell in addition to incomplete shell formation. The release of polymer and poorly formed shells makes the capsules unable to be separated. The production time of 4 hours was found to be the most optimal for production of fully formed microcapsules.
Figure 3.6: SEM image of microcapsules formed after 120 min.
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In order to reduce material consumption and produce smaller microcapsule yields to prevent polymer diffusion, along with preventing the aggregation of microcapsules, the concentration of reactants was reduced. Unused microcapsules begin to show signs of degradation if not incorporated into a polymer composite within two weeks of production. Decreasing the amount of microcapsules produced would not only decrease materials used, but also cut waste from microcapsules degradation. The modification included a decrease in reagents up to a 75%. The
SEM images, shown in figure 3.7, indicate microcapsules did not fully form. Without full encapsulation of the DCPD, the polymer will polymerize outside the microcapsule shells forming a solid mass. Attempts to scale up the fabrication process, generating more than about
200 mL of microcapsules, were made. Fully formed microcapsules could be developed for reactant increases generating about 1 L of microcapsules. Based on these concentration modification studies, the threshold for reactant concentrations was determined to be 200 mL of precursors, 6 g DCPD, and 13 mL formaldehyde generating about 150 mL of microcapsules.
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Figure 3.7: Microcapsules generated using modified method of decreased reagents
3.3.2 Carbon Nanofibers Incorporation into the Microencapsulation Fabrication Process
Strong interest has been in incorporating CNFs into polymer composites to tailor the composites properties. CNFs were chosen over other carbon nanofibers, such as multi-walled carbon nanotubes (MWCNTs) because of their increased surface area in comparison to other carbon nanofibers. The goal was to integrate CNFs microcapsules’ shells. The increased surface area of the CNFS provided increased interaction sites. CNFs were added to the formaldehyde prior to its addition to the emulsification solution. The intended result CNFs integrated into the microcapsule shell, providing increased additional protection to polymer degradation. Figure 3.8 shows the SEM image of CNFs integration. While the process generated microcapsules the
CNFs aggregated away from the microcapsules and did not integrated into the capsule shell. The aggregation could be caused by a number of factors, such as the forces between shell
53 components and CNTs were weaker than the interactions forces between the CNFs. The stronger forces caused the aggregation.
Figure 3.8: CNFs aggregate without integration into microcapsule shell.
Since the integration of CNTs into polymer matrices can be enhanced by coating the
CNTs in polymer, additional DCPD was added into the emulsified solution. Also sonication was eliminated to ensure the CNFs would be coated. The encapsulated polymer, DCPD, was also used as the coating polymer. The DCPD concentration was increased 50% to ensure enough polymer was available for coating the CNFs and be encapsulation into the microcapsules. The additional polymer would coat the CNFs, reducing the likelihood of CNF aggregation caused by van de Waals forces. Figures 3.9 and 3.10 show the SEM images of this modification.
54
Figure 3.9: SEM image of microcapsules with 50% increase DCPD concentration for CNF coating.
55
Figure 3.10: Higher magnification SEM image of DCPD coated CNF microcapsule.
The increased DCPD concentrations produced microcapsules with coated CNF on the outside shell. Without sonication the microcapsules produced were five times larger than those produced with sonication. The coated microcapsules ranged in size from 15 to 25µm. While coating provided a means for integrating CNFs, the excess polymer is not easily removed causing the microcapsules to aggregate, which inhibits capsule integration into a polymer composite. Having capsules aggregate, as shown in Figure 3.9, with no means of separation hinders future use and can cause decomposition of unused microcapsules. Ideally, within the polymer matrix, the microcapsules should distribute evenly to ensure self-healing ability throughout the polymer; therefore, any microcapsules aggregation could be detrimental to the self-healing ability of the polymer composite.
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Figure 3.11: 50% Excess polymer coating on CNFs in DCPD encapsulated microcapsule.
In some instances the polymer coating masked the microcapsules produced, seen in figure 3.12.
Microcapsules were able to be produced; however, they were below a CNF coated DCPD sheet.
The layer of polymer cured on top of fully formed microcapsules. In order to use the microcapsules the layer would need to be removed. In other instances the ability to form microcapsules was blocked by the polymer. Figure 3.13 shows the SEM image of such a scenario. The increase in DCPD coated the CNFs while also creating microcapsules; however the microcapsules were over five times larger than those created without the excess polymer.
This increase in size was mostly due not only to the increase in encapsulating polymer concentration, but also the removal of sonication during the final stage of fabrication.
Microcapsules ranging from 15 to 20µm are larger than the ideal microcapsules to be used for integration into thin film.
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Figure 3.12: SEM image of polymer coated microcapsules and a layer of cured polymer from the addition of 50% excess polymer.
Figure 3.13: Excess polymer creates DCPD coated CNF, but prevents the formation of microcapsules.
58
In order to generate microcapsule on the size scale needed for thin films, the excess polymer was used along with sonication in the final fabrication step. The resulting microcapsule formation, shown in figure 3.14, shows only harden polymer and no microcapsule formation. The excess polymer has hardened into a solid structure with no visible microcapsule formation.
Figure 3.14: SEM image of polymer curing atop fully formed microcapsules.
SEM images, seen in figure 3.15, shows the polymer covering fully formed microcapsules. The
CNFs aggregated away from the microcapsules formed, with excess polymer. The excess polymer polymerized outside the microcapsule shell forming a cover over formed microcapsules.
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Figure 3.15: SEM image of microcapsules formed with excess polymer: a) excess polymer covers fully formed self-healing capsules b) crack in polymer covering shows underlying microcapsules.
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Adding the sonication back to the fabrication process caused the CNFs to migrate away from the polymer prior to coating. In order to achieve polymer coating and microcapsule formation with sonication, the concentration of CNFs was increased. With the increase in CNF concentration it was thought that not all CNFs would be able to aggregate prior to polymer coating, and with the polymer coating, the CNFs would integrate in the microcapsules shell. The concentration of
CNFs was increased 50%. Figure 3.16 is a SEM image of the fabricated material.
Figure 3.16: 50% increase in CNFs concentration prevents the formation of microcapsules.
Increasing the CNFs’ concentration caused a disruption microcapsule formation. This disruption in formation is thought to be caused by the breaking of the CNF during microcapsule formation. Sonicating the CNFs can cause the fibers to break, resulting in sharp pointed end.
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These ends then penetrate the microcapsule shell during formation. In addition to the sharp ends, increasing the CNFs may cause aggregation of the CNFs during the sonication stage. Disruption to the emulsion during this stage would cause a disruption in the reaction of the poly (urea- formaldehyde) reaction preventing the microcapsule shell from forming around the DCPD. The poly (urea-formaldehyde) shell if formed follows the reaction seen in figure 3.17. The reaction links the urea and formaldehyde to first forms a urea derivative. As the reaction continues the urea and formaldehyde in solution continue to react, condensing round DCPD. This reaction forms the microcapsule shell. If excess CNFs were aggregating it would block urea- formaldehyde reactions from forming poly(urea-formaldehyde).
Figure 3.17: Urea formaldehyde reaction to form microcapsule shell.
Figure 3.18 shows a higher magnification of the fabricated material. It can be see that only half the microcapsule shell is formed. The urea-formaldehyde reacts until the shell is formed; however, with the additional CNF the urea-formaldehyde was unable to coalesce around the
DCPD. CNFs are found throughout the structure. CNFs are both integrated in the partially formed shell walls and coated with polymer. However, without fully formed shell walls polymer
62 could not be encapsulated. The polymer cured to the shell walls of the partially formed microcapsules.
Figure 3.18: Magnified SEM images of 20% increase in CNFs preventing the formation of microcapsules.
63
In effort to prevent the sonicated CNFs from disrupting the microcapsule shells formation and promote successful integration into microcapsules, the fabrication process was modified with a 20% increase in both polymer and CNF concentration. The resulting solution, figure 3.19, was unable to form microcapsules. In certain areas the excess polymer harden outside of a microcapsules shell, failing the form microcapsules, illustrated in figure 3.19a. Polymer curing created at hard solid with CNF integrated throughout. Figure 3.19b shows the excess CNFs aggregating within the polymerized polymer.
a)
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b)
Figure 3.19: SEM images of product resulting from 20% increase in DCPD and CNF concentration with sonication: a) shows excess DCPD hardened in a solid mass, b) shows CNFs agglomeration in addition to polymerized DCPD.
While some CNFs were coated with polymer, seen in figure 3.20, the excess CNFs prevented the formation of microcapsules. When exposed to sonication CNFs can break forming sharp edges19. These pointed edges could then pierce the shell of the newly formed microcapsules causing the encapsulated polymer to spill out, thus disrupting the fabrication of microcapsules, as seen in figure 3.20. The CNFs not coated in polymer will also aggregate together. Large aggregates of CNFs would also disrupt the emulsion, further disrupting the urea- formaldehyde coalescence around the DCPD.
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CNF with pointed edge
Figure 3.20: SEM image of fabricated product after 20% increase in CNFs and DCPD concentrations showing pointed edges of CNFs and polymer coated CNFs.
A reduction was made to the increased CNF and DCPD concentration to a 5% increase in
concentration instead of the 20% increase. Figures 3.21 and 3.22 show the SEM images of the
resulting material. The process did not yield microcapsules with integrated CNFs. Figure 3.19
shows while DCPD coated CNFs existed all CNFs it did receive polymer coating. The uncoated
CNFs continued to form large aggregates. The polymer also created large tubes around the
coating of CNFs, seen in figure 3.21a. This may be caused by polymer coating of a CNFs
aggregate. Figure 3.21b shows smaller aggregates of CNFs with completely formed
microcapsules and ribbons of polymer. The ribbons of polymer are similar to the sheet of
polymer that occurred in studies were excess polymer covered fully formed microcapsules,
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previously shown in figure 3.15. The process also formed small microcapsules. The small
microcapsules formed are about 0.5 µm or smaller in size. The increase in carbon nanofibers
continue to cause disruption in the fabrication of the microcapsules. The microcapsule shell
cannot fully form around DCPD. The increase in CNFs created smaller emulsion droplets
resulting in smaller microcapsules being formed. Figure 3.22 shows these smaller capsules
dispersed throughout the solution. Previous studies show microcapsules smaller than ~ 0.5μm do
not contain enough polymer to initiate full self-healing50. The smaller microcapsules formed
were below the threshold for microcapsules capable of self-healing. All of the components
created in this system, tubes, ribbons, small microcapsules, aggregate together. Removing the
unwanted components i.e. ribbons of polymer, microcapsules too small for use, and aggregated
CNF could not be achieved due to the cured polymer binding them together.
a)
Large tube of DCPD coated CNFs
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b)
Ribbon
Figure 3.21: SEM of microcapsules with a 5% increase in CNFs and DCPD concentrations: a) shows large aggregates of CNFs and large tube of polymer coated CNFs, b) shows DCPD polymerized into a polymer ribbon, microcapsules smaller than 0.5 μm, and CNF aggregates.
Figure 3.22: SEM image of microcapsules with a 5% increase in CNFs and DCPD concentrations indicating the fabrication of microcapsules ranging from 1 μm to less than 0.5 μm
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3.3.3 Carbon Nanotubes Incorporation into the Microencapsulation Fabrication Process
The use excess CNFs and excess polymer did not produce a fully integrated system of fibers and microcapsules. Instead of incorporating CNFs, multi-walled carbon nanotubes
(MWCNTs) were used. MWCNTs, shown in Figure 3.23a, used had an outside diameter of 60-
100 nm and length of 6-15 µm. The CNFs, shown in figure 3.23b, which had been previously used, had an outside diameter of 200-500 nm and length 10-40 µm.
a)
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b)
Figure 3.23: SEM image of CNTs vs CNFs a) image of CNTs b) image of CNFs51.
MWCNTs were used in order to prevent disruption of microcapsule formation. With the smaller size in tub diameter and length the disruption of microcapsule shell wall formation would be reduced, thus creating fully formed microcapsules. The 0.05 g MWCNTs were combined with the formaldehyde and added to the fabrication process prior to sonication, in the same manner as the addition of CNFs. Excess DCPD polymer was not added to the solution and only the original
6 g were utilized. The resulting microcapsules, shown in figure 3.24, yields microcapsules with
CNTs integrated into the capsule shell. The capsules formed were 4 μm to 0.2 μm, similar to the microcapsules generated without CNTs, indicating sufficient DCPD was encapsulated into the microcapsules. Excess polymer was not needed to first coat the CNTs to promote integration.
The tube size allowed for the urea-formaldehyde reaction to fully condense around the DCPD and the emulsion was not disrupted. The dispersal of the MWCNT throughout the solution added in the microcapsule shell formation; and adding the MWCNT into the solution along with the 70 formaldehyde, one of the shell components, aided in shell integrations. The microcapsules could also be separated from aggregation, since there was no excess polymer outside the shell.
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Figure 3.24: SEM images of microcapsules with integrated CNTs shells.
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3.4 Discussion
This chapter discussed the formation of microcapsules with the elimination of a surfactant solution, but adding the surfactant directly to the precursor solutions. The direct addition of the surfactant eliminated surface bubbles that can arise when a surfactant solution is added. This direct addition also eliminated time and materials from the fabrication process. The chapter also discussed the fabrication of microcapsules with CNTs integrated in the shell. Microcapsules were created via an oil-in-water in situ polymerization, where the aqueous phase components, urea and formaldehyde, reacted forming a polymer shell around an emulsified DCPD. The urea- formaldehyde polymer reaction occurred around drops of emulsified DCPD, creating a highly cross-linked shell52. The development of a procedure combining the production of microcapsules with CNTs sought to enhance the effectiveness of the microcapsules in preventing microcrack propagation. The use of CNTs, instead of CNFs, was necessary to prevent puncture of the shell wall during formation. The urea-formaldehyde was unable to fully condense into a cross-linked shell around the DCPD due to the aggregation CNFs. The incorporation of the CNTs into the microcapsules shell has given the shell a rough exterior. Previous studies have shown microcapsules with a rough exterior have improved shelf-life and improved integration ability18.
With a method capable of producing microcapsule shell with CNTs these microcapsules can be embedded into the polymer matrix to create a self-healing material.
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CHAPTER FOUR: THE FORMATION OF SELF-HEALING FILMS AND COATING UTILIZING MICROCAPSULE SELF-HEALING SYSTEMS
4.1 Introduction
It is envisaged that the next generation of smart materials will have self-healing capabilities. Such a move will clearly have an impact in engineering and construction. Repairing damage, increasing efficiency, reducing cost, increasing safety, and adding protection against environmental stresses are just some of the advantages of these “smart” materials. Self-healing materials would repair damage with no human intervention saving on labor and material cost material and ensure longer operational lifetimes for the materials. Corrosion repair alone cost nearly $300 billion per year53. It is estimated the US navy spend nearly 60% of its time fighting environmental corrosion of its fleets. There are many way in which this problem is being addressed, but one of the more recent methods of investigation involves the addition of a thin protective coating that can also self-heal, reducing the extent of repairs during scheduled maintenance.
Various approaches have been employed to construct self-healing materials. Exploiting the intrinsic self-healing capabilities found in many polymers has not proven successful for many applications subjected to environmental stresses. For intrinsic polymer systems to self-mend damage an outside stimulus, commonly high temperatures, is required. Current research has shifted the focuses to the development of extrinsic systems. In these materials, secondary systems are embedded in the polymer. These secondary systems utilize various chemistries to achieve repair in damaged areas.
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Microcapsules systems, the most widely explored and most successful extrinsic system, have had nearly 100% strength recovery. While the components of a microcapsule system, the microcapsule and curing agent, may both change depending on the material intended for self healing, these systems have limitations hindering their use in self-healing coatings.
Coatings are the first line of defense against damage. These typically thin layers coatings are thin coverings over a material, referred to as a substrate, which protects the substrate from environmental exposure. Coating failure can lead to rapid substrate degradation. Typically coatings are in direct contact with the substrate and the environment. Unlike self-healing systems in bulk materials, self-healing polymer coatings are in direct contact with environmental contaminates, i.e. O2 and H2O. All components must be stable and functional in environmental conditions. Utilizing a microcapsule self-healing system, the microcapsule parameters become more important in a coating than in bulk material. Coatings are typically think layers; therefore, any embedded microcapsules must be fit within the layers and not destroy the structural integrity of the film. In addition to microcapsule size, the even distribution of microcapsules is of great importance with coatings. While the use of smaller capsules results in less encapsulated “healing agent”, more microcapsules can be used as long as the spacing between the capsules is sufficient.
It is essential that the capsules do not aggregate within the coating.
This chapter examines two approaches to self-healing, one using a dicyclopentadiene and
Grubbs catalyst and another using a bisphenol A polymer with an primary amine to cure the polymer. The self-healing systems offered different benefits. The dicyclopentadiene structure has been the most successful self-healing system used in bulk materials. The process has been optimized to achieve near 100% fracture recovery. While the use of bisphenol A provides a financial benefit making the use of this system more attractive in future larger scale productions.
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This chapter also will discuss a generalized approach to the development of self-healing thin films containing a cellulose based polymer and carbon nanotube coated microcapsules. Due to the scale of these self-healing systems and aspect ratio, fabrication and investigation of these composite systems present considerable challenges. Three fabrication structures were proposed, shown in figure 4.5: 1) four layers of microcapsules with carbon nanotubes embedded in the shell, 2) microcapsules without carbon nanotubes embedded shells, and 3) alternating layers of microcapsules and carbon nanotubes. The proposed structures offer unique advantages and disadvantages to fabricating coatings. CNT embedded shell microcapsules have a rough exterior.
Previous studies have shown increased interaction between rough exterior microcapsules and polymer verses microcapsules with smooth shells. The increased interaction minimized aggregation and promotes even dispersal throughout the polymer, which is a crucial component in creating self-healing coatings. Using smooth microcapsules with addition carbon nanotubes embedded in to the coating not only provided self-healing capabilities, but the carbon nanotubes would function as stops for preventing continued crack propagation. The alternating layers structure would provide specific layers for handling specific problems. The microcapsule layer would work to heal the cracking, where as the carbon nanotube layer would work to prevent/retard crack propagation.
4.2 Experimental Section
4.2.1 Materials
Cellulose films were created using cellulose triacetate (CTA) and dichloromethane
(DCM) purchased from Sigma-Aldrich. Derived from cellulose and acetate esters, CTA is commonly used in coating electronics and packaging. The use of a cellulose derivative creates a film able to withstand environmental conditions, such as H2O and O2. The microcapsules used
76 were made according to fabrication process outlined in chapter three. The curing agent used varied. No curing agent was modified from the condition in which it was received. Carbon nanotubes, incorporated in certain films, were left unmodified and used as received.
4.2.2 Film and Coating Fabrication
A stock solution of CTA dissolved in DCM was made using 16 g of CTA dissolved in
200 mL of DCM. The CTA was added in ~1g aliquots in stirring DCM set at 35oC until all CTA dissolved. The resulting solution was used to create the polymer matrix for self-healing coatings.
The coatings created used 20 mL of the CTA/DCM polymer matrix. The microcapsules and curing agents were stirred at 500 rpm into the polymer matrix for 48 hours. After stirring, the coatings were fabricated using a spincoater. The spincoater was set to accelerate for 20 sec in order to reach a top speed of 700 rpm. The spincoater would then spin at 700 rpm for 80 sec, finally decelerating to 0 rpm within 20 sec. One layer was spun and allowed to set for 2 minutes prior to another layer being applied. All subsequent layers were applied under the same spinning conditions, until a five layer coating was created. The coatings were spun onto a 4cm diameter stainless steel substrate. If the film needed to be removed from the substrate, the final fifth layer was allowed to dry completely before the film was removed. The coatings were then placed in a
40oC oven for 24 hours.
4.3 Results
4.3.1 Self-Healing System Reactions: Ring Opening Metathesis vs Epoxy/Amine Reactions
The initial self-healing chemistry designed for microcapsule systems was the ring opening metathesis polymerization (ROMP) of dicyclopentadiene (DCPD). Previous research has optimized this system to achieve nearly 100% fracture recovery and is the best system available for creating self-healing systems4. This DCPD/ROMP system is now being integrated
77 into various polymer matrices because of its wide range of compatibilities. In a ROMP reaction, illustrated in figure 4.1, the curing agent aids in a ring strain relief. ROMP reactions use strained cyclic olefins and a metathesis curing agent to generate polymers. The ring strain is a major factor in the reaction and polymerization of the polymer. The type of curing agent chosen affects the rate of reaction, as well as the polymer size ranges.
Figure 4.1: Illustration of ring opening metathesis polymerization54
DCPD is well suited for ROMP reactions, due to the its two double bonds. The two double bonds have unequal reactivity. The strained narbornene bond will undergo rapid metathesis leading to ring opened polymerization, seen in figure 4.2.
Figure 4.2: Illustration of ring opening metathesis polymerization of dicyclopentadiene54.
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The ROMP of DCPD creates a highly cross-linked polymer. The polymerized DCPD is a strong chemically resistant environmentally stable polymer. DCPD/ROMP reactions for self-healing utilize a Grubbs compound as the curing agent, because the Grubbs compound provides greater environmental stability. In self-healing system using this mechanism, the encapsulated DCPD is embedded into a polymer along with a Grubbs compound. The DCPD flows into a crack upon polymer damage after the microcapsules shell walls are broken. DCPD reacts with the Grubbs compound, as seen in figure 4.3, under environmental conditions.
Figure 4.3: ROMP reaction of DCPD.
Despite the low cost of the DCPD, the Grubbs compound was expensive for use in self- healing material. Since cost is one of the major factors in the consideration self-healing materials, the use of a Grubbs compound commercialization less feasible. Ideally, self-healing systems should not cost more to produce than replacing the damaged material. The expense of the Grubbs compound along with the amount of curing agent needed for the reaction to occur in a 1” square sample would be ~$500. While the process of creating DCPD encapsulated microcapsules is inexpensive and the incorporation of the capsules is feasible, without the
Grubbs curing agent the system would not function. When evaluating the entire system cost, utilizing DCPD/Grubbs system would not be feasible in future large scale productions. The high cost of the DCPD/Grubbs system required another system to be examined.
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An alternate system employs the use of epoxy based polymers. Epoxy based polymers are created in two steps. First a diepoxy is created. Then the diepoxy is cured through cross-linking with an amine. Generating the diepoxy is a step-growth polymerization using a bisphenol A base. These diepoxy prepolymers can be a variety of molecular weights, effecting characteristics such as viscosity. The diepoxy is then cured using an amine, usually a diamine. The epoxy amine reaction, seen in figure 4.4, results in a cross linked network. The reaction only requires the epoxy and amine be mixed in order to react. The reaction can occur at room temperature and can be accelerated a higher temperatures.
Figure 4.4: Epoxy amine mechanism55.
Epoxy polymers are thermosetting polymers with a wide range of applications in variety of fields. The coatings industry the number one consumer of epoxy resins, mainly focused in special purpose coatings. Since the epoxy/amine polymerization reactions can occur in room temperature, the reaction is ideal for self-healing applications. The resistance of epoxy resins to chemicals and corrosions makes the durable thin-layer ideal in industries from appliances to ships to chemical plants. The low volatility and superior adhesion of epoxy resin coatings have made them popular coatings for steel and aluminum applications.
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A coatings main function is protection against damage and coatings are modified to
perform depending on the environmental conditions. In addition to the performance, the system
needs to be cost efficient. Table 4.1 compares the two systems noting the difference in cost of the
two curing agents. Creating a cost efficient system on a small scale will promote large scale
production.
Table 4.1: Self-healing system comparison chart
Self-healing System Ring Opening Metathesis Epoxy/Amine Polymerization
Shell poly(urea-formaldehyde) poly(urea-formaldehyde)
Encapsulated polymer Dicyclopentadiene (DCPD) Bisphenol A diglycidyl ether (BADGE)
Fiber Reinforcement Carbon nanotubes Carbon nanotubes
Curing agent Grubbs amine
Fracture recovery 90%4 70%
Cost of Curing agent $108.00/g $1.10/g
4.3.2 Self-Healing System Composite Films
The base polymer, CTA, incorporated the BADGE/amine system. CTA is has been used
as a coating in other application and the cellulose base makes it compatible with various
substrates and the environment; however, no research has been done to determine the viability of
CTA as a polymer able to withstand self-healing system fabrication and the self-healing process.
The use of a new polymer matrix and new microcapsule system involving carbon nanotubes has
81 not been previously investigated. Figure 4.5 shows the various schemes self-healing coatings fabricated.
a)
b)
82
c)
Figure 4.5: Schematic of fabricated self-healing systems
In figure 4.5amicrocapsules with CNTs infused into the microcapsule shell are embedded in the polymer matrix along with the amine curing agent. Figure 4.5b contains self-healing capsules that are not coated in CNTs. The CNTs are added as a separate component. The final schematic, figure 4.5c, involves a layered structure. This system embeds only one component in a layer and stacks the layers. One layer contained CNTs. The following layer contained the epoxy/amine, microcapsules and curing agent.
The components of the epoxy/amine system were added one at a time to 20 mL of CTA.
The microcapsule system being utilized only involved the encapsulation of the epoxy. The amine curing agent was not encapsulated. Integration of the self-healing system curing agent is also crucial to creating a viable self-healing material. Self-healing takes place as long as the curing agent is available for the polymerization reaction. Double encapsulation systems encapsulate both the polymer and curing agent. Encapsulating the curing agent preserves its reactivity; however, these two capsules systems require both capsules be broken and components released for polymerization. A double encapsulation system is more feasible in bulk systems where
83 microcracking leads to larger cracks, breaking more capsules. In coatings, allowing a microcrack to expand poses an even greater risk than in the bulk polymer. With limited material for protection, the underlying substrate has increased exposure; therefore, the curing agent was directly integrated into the CTA polymer.
The initial amine chosen for the epoxy/amine reaction chosen was p-Phenylenediamine
(PPD). This amine is a common curing agent for epoxy. The CTA was dissolved in DCM, shown in figure 4.6. Then PPD was added in small aliquots to 20 mL of CTA, seen in figure 4.7. PPD was added to create a 1:1 ratio of PPD to epoxy.
Figure 4.6: Cellulose triacetate (CTA) dissolved in dichloromethane (DCM)
Figure 4.7: CTA with p-Phenylenediamine (PPD)
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The resulting solution of the PPD addition to CTA was not homogenous. The PPD formed crystal-like structures with the CTA. The PPD did not completely dissolve in solution after continued attempts to decrease concentration, from a 1:1 epoxy:amine ratio to a 2:1 ratio and 4:1 ratio. Attempts to promote PPD mixing stirring time was increased from 48 hours to up to 96 hours. Despite these modifications the PPD did not completely mix and stay in solution.
Due to the lack of mixing another amine has to be examined for the epoxy/amine self healing system. Bisphenol A diglycidyl ether (BADGE), seen in figure 4.8, was used as the encapsulated epoxy. The BADGE used, EPON 828®, is commonly used as in fiber reinforced composites. Like DCPD, BADGE is also an inexpensive monomer.
Figure 4.8: Structure of BADGE- EPON 828.
Previous studies using BADGE with an amine curing agent shows 70% strength recovery in the polymer matrix46. The new curing agent, EPIKURE 3140®, is an polyamine with a low viscosity. This curing agent has numerous benefits necessary for thin film coatings. It is chemical and corrosion resistance. It is often used with EPON 828® in maintenance coatings. The polymerization curing reaction between the EPON 828® and EPIKURE 3140®, seen in figure
4.9, occurs at room temperature, a characteristic needed for self-healing. For 20 mL CTA polymer aliquot 0.5 mL of curing agent was incorporated via stirring. The low viscosity liquid
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EPIKURE 3140 did not increase the viscosity of the CTA; therefore, additional DCM was not needed.
Figure 4.9: EPON 828®/EPIKURE 3140® reaction
Since the epoxy/amine reaction is an irreversible polyaddition the resulting crosslinked polymer is durable enough to withstand environmental conditions. The resulting polymerization is chemically resistant and the polarity of the –OH bonds provides hydrogen bonding aiding in adhesion to the CTA polymer matrix. While the strength recovery of the EPON 828®/EPIKURE
3140® system may not be as high as the recovery seen in the DCPD/Grubbs system, utilizing a cheaper system with an environmentally stable species would prove more practical for commercial use. The epoxy/amine system provides large mechanical strength, which can be upwards of 80MPa56, mainly due to the minimal shrinkage during the polymerization process.
Additionally, the resulting epoxy polymer is chemically resistant and considered watertight, allowing them to be used as a protection against water. Its resistance to heat degradation after polymerization also makes epoxy polymers ideal for the self-healing processes. Room cured epoxy can remain intact in temperatures up to 70oC, while heat cured, curing at temperatures above 20oC, will maintain heat resistance up to 250oC56.
The ability to combine CTA and EPIKURE 3140® provided a base for creating the coatings. A steel substrate was coated with the self-healing material. The first layer of all
86 coatings was unmodified CTA was first applied to the substrate. Figure 4.10 shows the initial
CTA layer.
Figure 4.10: Unmodified CTA polymer on a steel substrate.
CTA thin films, shown in figure 4.11a, are 4 cm wide. Each layer of the film was ~20
μm. Figure 4.11b shows the film can be removed from the substrate after drying and cured as a standalone films. The film is flexible enough to bend without cracking. There are five layers for each film, the first of which is an unmodified CTA layer followed by four layers of CTA combined with system components, yielding a film thickness of ~100 μm. Figure 4.11c illustrates the scanning electron microscope (SEM) image of the native CTA polymer structure.
The structure is similar to that of wood, which is fibrous cellulose. The Figure 4.11d is the SEM side view image of the native CTA structure.
87 a) b)
c)
88
d)
Figure 4.11: Images of a CTA spun thin film: a) film after curing b) side image of film removed from substrate c) SEM image of film surface d) SEM image thin film edge.
With the base structure of CTA established incorporating the self-healing system components into the polymer involved mixing the components one at a time until even distribution was achieve. The incorporation of the components should not destroy the native
CTA structure and should distribute evenly throughout. When incorporating the CNTs in a polymer system sonication is generally used. CNTs were incorporated into the CNTs and sonication via a sonication probe was applied for 15min. Figure 4.12 shows the result of CNTs sonicated into the CTA.
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Figure 4.12: CNTs sonicated in CTA
The use of the sonication probe resulted in a phase separation of the CNT and CTA, with the
CNTs aggregating at the bottom. The incorporation of CNTs using the sonication probe did not create evenly distributed CNTs. Sonication using a sonication bath was also attempted on the
CNT/CTA mixture. The sonication bath yielded the same results as the sonication probe, where the CNTs aggregated to the bottom. When combing the CNTs into the CTA using sonication, the energy, in the form of heat began to polymerize the CTA. The DCM, in which the CTA was dissolved, began to evaporate out of solution, due to its volatility. The decrease in solvent caused an increase polymer viscosity. The increases in viscosity lead to problems in dispersing the
CNTs throughout the CTA matrix. In order to control the amount of DCM evaporating sonication was discontinued. The solution was stirred and only exposed to air when the components were added. Figure 4.13 shows the result of stirring the CNTs in CTA at 500 rpm for 48 hours.
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Figure 4.13: CNTs stirred into CTA
After 48 hours of stirring, the CNTs are able to distribute throughout the matrix. The CNT were able to stay suspended in the CTA. Previous studies have shown an increase in viscosity of polymer matrices with even small amounts, around 1%, of CNT are added to a polymer system25.
The CTA system showed a similar increase in viscosity after the addition of CNTs. The increase in viscosity adversely affects the potential to form a thin homogeneous film when spin-coating.
In order to decrease the viscosity and enable uniformed coating be generated the solvent concentration was increased from 150mL to 200mL of DCM.
After creating a CTA/CNT solution where the CNTs were disbursed throughout, the solution was then spun onto a substrate. Figure 4.14a shows a CTA/CNT film. At a CNT concentration of 0.006 g/mL, there was a destruction of the CTA structure, as seen in figure
4.14b. This type of polymer destruction often presents itself when the concentration of a component is too high.
91
a)
b)
Figure 4.14: CNT incorporated into CTA polymer a) CTA/CNT film b) SEM image of the surface of the CTA/CNT film.
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The native structure, seen in figure 4.11c above, was disrupted from its natural pattern by the CNT. Due to the method of incorporating the CNTs into the CTA, the CNTs are orientated.
The high concentration of CNTs interfered with the interfacial interaction between the CTA causing damage to the structure. The concentration of CNTs was adjusted to 0.003 g/mL. The
SEM image of the decreased CNT concentration in the CTA film, shown in figure 4.15, showed the native CTA structure intact
Figure 4.15: SEM image CTA/CNT film with the CTA structure intact
Upon achieving a concentration and method for incorporating CNTs, 0.2 g microcapsules were then stirred into the 20 mL of CTA. The microcapsules were allowed to stir for 48 hours.
The mixture was then spun into a thin film. The same process was completed for both with and
93 without a CNT infused shell. Both films exhibited similar behavior of microcapsule aggregation.
Figure 4.16a shows the created film of microcapsules not coated with a CNT infused shell.
While some microcapsules were dispersed throughout the films, many can be seen in large aggregates. These aggregates distort the polymer structure, seen in figure 4.16b.
a)
b)
Figure 4.16: Image of CTA film: a) CTA film with incorporated microcapsules, b) SEM image of microcapsule aggregation within CTA film
94
The microcapsules coated with CNTs exhibited larger quantities of aggregation. The large number of aggregates caused major distortions in the CTA polymer film. These distortions can be seen in figure 4.17.
a)
b)
Figure 4.17: SEM image of holes created after the addition of microcapsules with CNT incorporated shells.
95
The holes and weak points within the polymer matrix decrease the film’s mechanical properties, as well as decrease its usefulness as a protective coating. The use of modified CNTs was employed to increase the interaction between the microcapsule shell and the CTA. These carboxy-functionalized and thiol-functionalized CNTs from Nanostructured and Amorphours
Materials Inc were used in the same manner as the non-functionalized CNTs. The functionalized
CNT were infused into the shell of the microcapsules using the same method; however, despite the fuctionalization the microcapsules with CNTs infused in the shell showed increase aggregation as compared to those without CNTs in the shell. The functionalize-CNT shell increased the aggregation of the microcapsules leading to aggregates large enough to prevent the mixture from generating a coating onto the steel substrate. The large aggregates created large voids within the coating, leaving parts of the steel exposed. The large aggregates also prevented the film from being removed from the substrate to be cured as a film. With the large voids, removing the film caused the film to rip, destroying the coating.
In order to decrease the number of voids in the film caused by the CNT infused microcapsules the concentration only non-functionalize CNTs were used and the concentration on CNT infused shell microcapsules was decreased from 20 mg to 10 mg. Since the microcapsules without CNT infused shells showed less aggregation the concentration was held at
20 mg. The stirring rate was increased from 500 rpm to 600 rpm. Additionally, the time allowed for stirring was increased from 48 to 96 hours. While all aggregates could not be eliminated, the concentrations and stirring conditions provided a mixture that could be used to create a film and coating. Decreases in the microcapsules concentration improved the quality of the CTA films.
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The fabrications scheme shown in figure 4.5c, created by alternating a CTA/CNT layer with a CTA/self-healing system layer, was also produced. The film, shown in figure 4.18, resulted in a combination of complications.
Figure 4.18: SEM image CTA film with alternating layers of CNT and microcapsules.
The distortions seen in the other films, both those with only CNTs and those with only microcapsules, were present in the alternating layers film. Holes and weak areas resulted from the aggregation of the microcapsules. There was also an increase in aggregation of the CNTs resulting in destruction of the CTA structure in certain areas. With the abundance of defects in the film, coating protection would be decreased. The issues seen in an individual layer was
97 compounded as each additional layer was added. The holes and voids caused by the microcapsules became areas where the CNTs would aggregate when that layer was applied. The concentrations of both the CNTs and the microcapsules were decreased further until a film could be fabricated with no destruction of the CTA polymer. Figure 4.19 shows this adjustment in concentration with the film of alternating layers. Areas of aggregation remain; however, the main structure of CTA can be seen to remain intact.
98
Figure 4.19: SEM images CTA film with alternating layers after decreasing microcapsule concentrations
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4.3.3 Self-Healing System Composite Coatings Self-Healing Evaluations
After the fabrication of each scheme, the self-healing was evaluated through corrosion testing of a damaged steel sample. For each coating an unmodified CTA layer was applied before the self-healing material was coated on the substrate. In order to ensure CTA cured fully the film was place in an oven at 35 oC for 24 hours. After curing, matrix damage was induced by hand scribing with a razor. The damage, made to the top layers of the coating, did not penetrate the steel substrate. The samples healed for 24 hour either at 35oC or at room temperature. Post healing the steel was submerged in 5% LiCl salt water for 24 hrs at 35oC. Figure 4.20 show the results of this corrosion testing.
a) b)
Figure 4.20: Image of film after corrosion testing a) film without self-healing capsules b) film with self-healing capsules healed at 35oC
Figure 4.20a shows films without out microcapsules. All substrates coated with films lacking microcapsules rapidly corrode within 24 hours. These films exhibit rust formation across the substrate surface. Coatings where the microcapsules concentration was decreased in order to create undamaged CTA also showed rapid uniform corrosion; however, films with elevated concentrations of microcapsules showed sign of healing and substrate protection. Coatings with
100 elevated microcapsule levels which were allowed to heal at 35 oC showed the evidence of healing and protection. Figure 4.20b is the resulting film corrosion test using an elevated concentration of uncoated microcapsules, which underwent healing at 35 oC. The elevated concentration of microcapsules was increased from 20 mg of microcapsules, used to create undamaged CTA polymer film, to 200 mg of microcapsules. For self-healing the concentration of microcapsules needed to be increased to levels that would yield damage to the CTA polymer structure. Defects caused by the aggregated microcapsules inhibit accurate strength recovery testing. The increase in concentration released the epoxy, which spread through the crack in the polymer via capillary action. Both the microcapsules and curing agent are necessary for healing.
The removal of either of the components resulted in substrate corrosion. The self-healed polymer systems also increased the adhesion to the steel substrate. Self-healed thin films remained attached to the substrate, which could be seen in the films after the corrosion testing. Post healing coatings continued to adhere to the substrate, whereas coatings without microcapsules did not remain attached to the substrate. The self-healing films continued to show protection for up 72 hours after exposure to salt water. Figure 14.21 shows a self-healed coating after 72 hours.
Limited corrosion is visible and the coating remains attached to the steel substrate.
Figure 4.21: Image of self-healed film after 72 hours of corrosion testing
101
Figure 4.22 shows SEM images of self-healed films with 100 mg microcapsule concentrations healed at 35 oC. The coatings prepared for SEM were fabricated similar to the films used corrosion testing; however, these films were not submerged in saltwater. SEM images were taken after scoring the film and again after 24 hours. Figure 4.22 shows the film after the crack was induced, seen in figure 4.22a, and the film 24 hours after healing, seen in figure 4.22b.
a)
b)
Figure 4.22: SEM image of microcapsules self-healed film: a) film after crack was induced b) film post healing
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The self-healing reaction between the epoxy and amine is a factor of both the reactant’s concentrations and substrate temperature. With epoxy and amine available, the reaction progresses over the 24 hour period. The film allowed to heal at 35oC showed healing after a 24 hour period, while the healing of film left to heal at room temperature did not show evidence of healing. The exothermic heat caused by the epoxy/amine reaction will not increase significantly in coatings and the epoxy will assume the temperature of the substrate. The hardening time, the time it takes for the epoxy to form a harden plastic and achieve its strength and resistance properties, is about 7 days for epoxy cured at room temperature. The epoxy will have 70-80% of its properties after 24 hours. Increasing the temperature, decreases the time needed for curing.
Epoxy/amine reactions follow Arrhenius’s law, where an increase of 10 oC in reaction temperature will decrease the reaction time by half. Increasing the temperature over room temperature allowed the epoxy to flow into the crack and increased the reaction rate of the epoxy/amine reaction leading to healing within a 24 hour period.
The films were also examined for signs healing prior to the 24 healing process. Figure
4.23 shows a sample immediately after scoring, after 12 hours of healing at 35oC, and after 24 hours of healing at 35oC. Figure 4.23a is a SEM image after razor blade scoring. Figure 4.23b shows the coatings after 12 hours at 35oC. The coatings do not show complete healing at this stage. Figure 4.23c is the film healed after 24 hours at 35oC. The film shows complete healing after 24 hours.
103 a)
b)
104
c)
Figure 4.23: SEM images of a film healed at 35oC a) after damage b) 12 hours and c) 24 hours
after damage
CNT infused shell microcapsules did not show the same level of healing as the samples without CNT infused microcapsule shells. The corrosion testing indicated uniformed corrosion of the steel substrate for both room temperature healed samples and samples healed at 35oC.
Despite the corrosion healing on a microscopic level was observed. Figure 4.24 shows the CNT infused shell microcapsule film, which was allowed to self-heal at 35oC.
105
Figure 4.24: SEM image of a CNT coated microcapsules self-healed film
These samples were scored in the same manner as the corrosion testing and allowed to heal in room temperature, as well as at 35 oC. Again the concentration of microcapsules had to be at elevated concentrations to observe healing. Despite increased microcapsule concentration the healing is not uniform across the crack. The increased aggregation left some areas of within the film unprotected. The increased concentration leads to increased aggregation of microcapsules.
An evenly distributed microcapsule suspension is crucial to the success of the self-healing system. With the increase concentration of microcapsules, the microcapsules show increased levels of aggregation. The breaks in healing would decrease protection and allow corrosion to take place. In addition to the uneven healing, the increase in microcapsule concentration
106 increases other defects, such as holes and weak points, in the CTA film, which lead to decreased protection. Films healed at room temperature did not show observable healing.
4.4 Discussion
A method for fabricating self-healing CTA thin film was examined. In order to achieve observable self-healing the concentration of microcapsules had to be elevated to concentrations which would damage the CTA structure by creating voids and holes. While elevated microcapsule concentrations damage the CTA, the system showed signs of healing. The self- healing also increased adhesion to the substrate. Without self-healing an adhesion promoter would be needed to keep the coating attached to the substrate and increased coating thickness would be needed to offer additional protection. The increased coating thickness would increase the cost and not provide healing protection. True self-healing requires no external intervention including increased temperature. While the films did not show healing at room temperature, an elevated temperature of 35 oC makes this system available for warmer environments. The system’s ability protect while being submerged in saltwater opens a new area of protection.
Future studies for examining alternative methods of tracking the progress of self healing over time involved measuring the resistance of a film after a crack has been established. To increase the sensitivity of the results palladium nanoparticles were added to a film. The film was attached to a glass slide and 4 probes attached. A stream of hydrogen was passed over the film essentially creating a hybrid hydrogen sensor. Changes in the resistance of the film would be an indication of self-healing.
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Figure 4.25. Diagram showing the testing of a cellulose film containing self-healing capsules and palladium nanoparticles.
In a second approach, seen in figure 4.26, to study self-healing in these films would examine the migration of polymer into a crack. Congo red dye added to the polymer solution prior to capsule formation would track the polymer filling the crack induced defect. As the microcapsules broke the red polymer could be tracked under a light microscope.
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Figure 4.26: Picture of a cellulose film with embedded capsules containing the congo-red dye.
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CHAPTER 5: POROUS CARBON NANOFIBER DICYCLOPENTADIENE FOAM SYNTHESIS VIA HIGH INTERNAL PHASE EMULSION 5.1 Introduction
First used in the 1940s, polymeric foams play important roles in numerous applications.
Polymeric foam applications include construction materials, gas storage devices, materials for separation, and numerous other applications. Polymeric foams, classified based on pore sizes, can be macrocellular, microcellular, or ultracellular. Microcellular foams range in average cell size of 0.1-10μm57 and have numerous advantages over other classes of foam, such as high impact strength and high stiffness-to-weight ratio. Microcellular foams are typically rigid, closed-cell structures58; however, there is growing research focusing on the fabrication of open- cell porous structures. From this research a new class of emulsion-derived foam is emerging as a way of creating open-cell microcellular foam59. The high internal phase emulsion (HIPE) produces foams from oil-in-water or water-in-oil emulsions60. In oil-in-water emulsions, the aqueous phase of HIPE foam is greater than 74%, usually as high as 90%61. The emulsion creates interconnected holes in the polymer during polymerization yielding highly porous foams. This method also has the potential to generate foams with a controllable cell diameter 42.
Creating emulsion-template microcellular foams using commercially available polymers is of interest. HIPE foams are generally easy and cheap to produce. Dicyclopentadiene (DCPD) is cheap readily available structural foam used in commercial applications. After polymerization,
DCPD becomes a ridged thermoset polymer with high chemical corrosion resistance. Previous studies have paired HIPE with ring opening metathesis polymerization (ROMP) to create microcellular dicylcopentadine foams. The traditional processing for creating DCPD foam requires the use of a Grubbs catalyst to facilitate ring opening metathesis with the DCPD, seen in
110 figure 5.1. The reaction utilizes a Grubbs catalyst and occurs at 80 oC over two hours under nitrogen.
Figure 5.1: ROMP reaction of DCPD utilizing a Grubbs catalyst.
Utilizing this process the desired open-cell structures were not obtained42. Figure 5.2 shows a scanning electron microscope (SEM) image of a typical close-celled structure obtained when creating DCPD foam.
Figure 5.2: SEM image of nonporous DCPD42.
One critical issue for open-celled HIPE foams is achieving stable emulsions capable of withstanding the curing process 43. It was long thought DCPD could not create an open-cell
111 foam structure due to weak wall formations that would collapse during the curing process. In
2012, Kovacic et al., created an open-cell foam with DCPD, figure 5.3, utilizing the ROMP method and a Grubbs catalyst62 through the incorporation of a stabilizer, synperonic PEL121.
The process, figure 5.4, for this foam using ROMP requires the use of a Grubbs catalyst and for the process to occur under nitrogen.
Figure 5.3: SEM image of porous DCPD62.
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Figure 5.4: Traditional process plan used to create DCPD foam 42,62.
In Kovacic et al. a DCPD foam of 80% porosity was generated. The cell morphology resembled that of a sponge, with void sizes ranging from 2-4 μm. Despite the success of achieving an open- celled structure the use of the Grubbs catalyst increased the cost and difficulty to manufacture this foam. The price of the Grubbs catalyst creates a process where producing a HIPE is no longer cheap. Also the reaction must be carried out under nitrogen to keep the catalyst stable and prevent decompositions. Research continues to be ongoing for the creation of open-celled DCPD foam that is cheap and easily fabricated.
As researchers continue to alter the structure of polymeric foams in order to tailor the properties of certain applications a focus has be given to the incorporation of additional materials, such as fibers, into foams. The properties of HIPE foams can be altered by modifying the structure and/or materials. Fibers have been incorporated into foam materials to modify the foams mechanical and thermal properties. Embedding fibers within polymer composite matrices allows for load transfer altering the material’s strength and stiffness45. Research is ongoing to create carbon nanotube (CNT) and carbon nanofiber (CNF) reinforced polymer foam
113 composites, further expanding the applications of carbon fiber polymer composites. CNTs/CNFs have unique electrical, thermal, and mechanical properties. In applications where increased strength and low weight are important considerations, these carbon fillers provide huge advantages. CNT/CNF reinforcements enable thinner flatter structures to be produced19. Previous studies have shown significant polymer strength increases after the incorporation of CNTs, in concentrations as small as 1%63.
Pairing the properties of microcellular foam with the properties of carbon nanofibers could potentially create polymeric foams with enhanced properties. The integration of CNFs into porous microcellular foam is not a straightforward process and has had limited success. The effective use of CNFs in polymer composites requires dispersing the nanofibers throughout the composite, without destroying the fibers’ or polymers’ integrity34. Poor dispersion can lead to failure of the CNFs properties, as well as decreases in the polymer’s properties 19, 36. A method for creating porous microcellular DCPD foam with CNF reinforced walls could further expand the uses of these foams. The addition on carbon fibers modifies the properties, such as potential improvement of the thermal and mechanical properties64. Theoretical evidence shows the adsorption of particles onto oil-water interfaces depends on the aspect ratios65 , indicating the critical load for CNFs is required for emulsion stabilization in HIPE. This chapter discusses a novel process for creating porous microcellular dicyclopentadiene foam with CNF integrated reinforcement. The foam was also examined with the incorporation of CNFs into the cell walls.
The process discussed in this chapter did not require the use of a catalyst, greatly reducing cost to manufacture. The porous structure with embedded carbon nanofibers posed a new possibility creating an interconnected CNF network, by removing the DCPD.
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5.2 Experimental Section
5.2.1 Dicyclopentadiene Foam Materials
The graphitized CNFs and carboxy functionalized multi-walled carbon nanotubes
(MWNT-COOH), supplied by Nanostructured and Amorphours Materials Inc, were used without any additional purification or modification. The resorcinol, ammonium chloride, poly(ethylene- alt-maleic anhydride) (EMA), and formalin used in the precursor solution, were all purchased from Sigma-Aldrich. Urea was purchased from Fisher Chemicals and was used without additional modifications. The DCPD chosen for this experiment was the endo-DCPD, due to its greater reactivity when compared to the exo-DCPD isomer. Endo-DCPD purchased from Sigma
Aldrich was not modified.
5.2.2 Sidewall Modification of MWCNTs
The modification of MWCNTs was achieved by first subjecting MWCNT-COOH to thionyl chloride under reflux at 60 °C for 3 days. After the acylation reaction was complete the tubes were washed with excess DMF and dried for 24 hours. 200 mg of the MWCNT-COCl were placed in a mixture of 100 ml ethylene glycol and 2.5 ml of pyridine. The mixture was refluxed at 70 °C for 48 h, washed with excess THF and dried. The resulting nanotubes were placed in 60 ml of 5-norbornene-2-yl(ethyl)chlorodimethylsilane (Sigma-Aldrich) and 2.5 ml of pyridine and refluxed at 70 °C for an additional 48 h before being washed with THF and dried.
5.2.3 Microcellular DCPD Foam Process
Figure 5.5 shows a process plan of the optimized process for creating DCPD foam via
HIPE. The approach to the process development will be explained further in this chapter.
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15 min @ 55oC
1.5 hours @ 55oC
Figure 5.5: Process plan of newly developed HIPE process for generating DCPD foam.
The DCPD foam was created via oil-in-water HIPE. The aqueous phase consisted of dissolving 0.125 g of poly(ethylene-alt-maleic anhydride), 0.5 g of urea, 0.5 g of NH4Cl, and
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0.05 g of resorcinol in 25 mL of ultra pure H2O. Once dissolved the pH was adjusted to 3.5 using 1 N sodium hydroxide. The solution was then heated to 55o C. Six grams of DCPD was then added. The DCPD was allowed to stir creating an oil-in-water emulsion. The mixture was sonicated and stirred at 55 oC for 1.5 hours. Afterwards the solution was then allowed to cool and dry overnight.
5.2.4 DCPD Foam with CNF Incorporation Process
The process for the incorporation of CNTs into the DCPD foam utilized similar steps as the DCPD. The foam components were prepared using 0.125 g of poly(ethylene-alt-maleic anhydride), 0.5 g of urea, 0.5 g of ammonium chloride, 0.05 g of resorcinol and 25 ml of ultra pure water. The mixture was adjusted to pH 3.5 with sodium hydroxide before the addition of 6 g of DCPD. This mixture was stirred at 35 °C to prevent the DCPD from solidifying. In studies involving CNF interaction with the DCPD foam, a CNF/formalin mixture (0.05 g of CNF with 3 mL of formalin) was then added to the solution. In studies incorporating multi-walled carbon nanotube via covalent bonding, 70 mg of norbornene functionalized MWCNTs were added to 3 ml of formaldehyde and added to the DCPD solution. The solution was then stirred with a stir bar while being dispersed with an ultrasonic probe at 55 °C for 1.5 hours.
5.2.5 Microscopy Analysis Instrumentation
The samples were analyzed using Scanning Electron Microscopy (SEM). Samples of the dried foam were taken and sputter coated with Iridium to avoid charging. The foam was examined using a FEI Nova 400 Nano Scanning Electron Microscope. Low magnification images were recorded using an Everhart Thornley Secondary Electron Detector (ETD).
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The cell density, the number of cells per unit volume, was determined from SEM images of the sample. Micrographs of the pores were analyzed and the number of pores, , along with the area of the micrograph, cm2, and the magnification factor, , was used to estimate the cell density66 utilizing equation 1:
(1)
The cell size is estimated utilizing equation 2, where , the average cell diameter is determined by , the cell diameter of pore, .
(2)
5.3 Results
5.3.1 Fabrication of DCPD Foam with Open-Celled Morphology
The objective, to create open-celled structure DCPD foam without the use of an expensive catalyst or difficult manufacturing, process is based on the continued research to tailor polymer composites for application. The foam created utilized DCPD as a base polymer. HIPE was an oil-in-water emulsion. The foam’s precursors, dissolved in water, composed the aqueous phase, which 83% of the mixture. The aqueous phase threshold for oil-in-water HIPE foams is
74%42. The process utilizes an aqueous phase above the threshold.
In order prevent collapse of the cell-walls and aid in cell wall stabilization during the polymerization process, urea, added as a precursor, and formaldehyde, added in the final step, were used. The urea-formaldehyde reaction, seen in figure 5.6, aided in the formation of stable cell-walls, during the final polymerization of DCPD.
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Figure 5.6: Urea formaldehyde reaction used to assist in stabilizing cell walls during polymerization.
The pH of the aqueous phase was raised 3.5 in order to prevent large aggregates of urea- formaldehyde from forming. A pH between 3.5 and 4.0 prevents large agglomerations from forming50 and maintain adequate urea-formaldehyde dispersion throughout the mixture. A higher pH causes degradation of the urea-formaldehyde preventing its use as cell-wall stabilizer during the polymerization.
The polymerization of DCPD occurred after the addition of formaldehyde. The solution was heated sonicated and stirred for 1.5 hours. After 1.5 hours the foam had little to no solvent in solution and the foam formed was ridged and solid. The sonication step enhances the emulsion beyond stirring alone. Figure 5.7 shows a SEM image of the dicyclopentadiene foam. The morphology of dicyclopentadiene foam shows distinct sections. While the side sections show the open-cell DCPD foam desired. The middle section shows closed-celled structure, indicating the middle section had a pore collapse in cell during drying, which resulted in a closed-cell morphology.
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Figure 5.7: SEM image of DCPD foam generated via HIPE indicates three distinct sections of morphology.
Figure 5.8 shows a SEM image under higher magnification of a middle closed-cell structure. The closed-cell foam suggested that the emulsion phase did not stabilize enough to withstand the polymerization process. The middle section indicates a stark contrast from the foam ends. The left end of the foam, figure 5.9, shows open-celled foam; however, the cell walls are poorly developed. This is observed in other open-cell polymer foams when the cell walls coalesce.
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Figure 5.8: SEM image under higher magnification of closed-cell section of DCPD foam.
Figure 5.9: SEM image of poorly formed cell walls due to cell wall coalescence.
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While the left side showed poorly formed cell walls and the middle section contains a layer of closed-cell foam, the right side displayed open-cell foam with well defined pores. The
SEM image, Figure 5.10, shows the HIPE did not destabilize and collapse into closed-cell foam, as seen previously and with other DCPD foam process. The pores are polyhedron in shape. The
DCPD’s porous sections exhibit voids similar to pores seen in soap foam. The diameter of the voids ranges from 0.5 μm to 5 μm, typical of HIPE structures. The foam’s average cell size is 2.5
μm, with a cell density on the order of 1012 cell/cm3. The production of well structured voids indicates an enhanced stability during the emulsions phase.
Figure 5.10: SEM image of open-celled DCPD foam via HIPE shows polyhedron pores with pore sizes ranging from 0.5 μm to 5 μm.
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5.3.2 DCPD Foam Formed Through Increased Emulsion Stability
In effort to produce a system comprised solely of well defined open-cell foam the foam process was adjusted and optimized, by adjusting reactant concentrations and enhancing emulsion temperature stabilization. In polymeric foam, pore definition is achieved through stable emulsions. For increased emulsion stability, first the level of surfactant was increased in the precursor solution. Surfactants are commonly used in creating foam via emulsification.
Surfactants aid in lowering the interfacial tension between the oil and water phases. The surfactant help generate increased stability in the emulsion by dissociating oil aggregates allowing for improved emulsification. The surfactant, EMA, was increased up to 25%; however, with this increase in surfactant the foam did not yield a well defined open-cell system. The foam created with this increase surfactant, shown in figure 5.11, did not contain open-cell pores. The closed-celled morphology is most likely attributed to the dissociation of oil aggregates being too great. Increased in surfactant caused increased coalescence of the emulsion droplet before polymerization. Further images of foam below the surface also did not indicate an open cell system existed. The surfactant increase caused a closed-cell structure throughout the foam. Since the increase in EMA did not yield open-cell foam the surfactant concentration originally used,
0.125 g, is believed to be optimal.
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Figure 5.11: SEM image of DCPD foam created with increased surfactant shows a closed-cell porous structure.
Another factor contributing to emulsification stability besides reactant concentrations is temperature. Temperature is a factor in the rate of polymerization. The polymerization rate determines helps determines the foams structure. Poorly formed cell wall that polymerize too quickly will fail to form open-celled structures. Also a polymerization that does not happen while the cell walls are stable will not be able to form open-cell structures. The rate of polymerization for the family of cyclopentadiene polymer, which DCPD is included, increases rapidly at higher temperature. The rate of polymerization for cyclopentadiene, the precursor to
DCPD, contributes to the polymerization and fabrication of DCPD foam. Based on the dimerization rate of cyclopentadiene, a rage of temperatures, 35-60oC, could be utilized; however, 55oC was found to be the optimal temperature for generating open-celled DCPD foam utilizing this process.
Controlling the temperature during emulsion and post polymerization is crucial in the production of an open-cell foam structure. While the concentration of reagents remained 124 unchanged, a focus was placed on achieving and maintaining stable temperature for emulsion stabilization. Temperature stability throughout the process yields a completely open-celled structure. In the previous foam, the temperature was raised from room temperature to 35 oC prior to the addition of DCPD. For a more stable emulsion process, the DCPD was added to the aqueous phase solution then the mixture was heated to 35 oC at 1 oC/min. After 35 oC was reached, the mixture was then allowed to emulsify, without sonication, for 15 minutes.
Following the emulsification the original procedure was followed.
Figure 5.12 shows the SEM images of DCPD foam formed with this temperature modification. The foam has well defined open-cell foam. The open-cell foam was found throughout the entire material; however, a top layer of foam continues to show a closed-cell system, seen in Figure 5.12a. Below the surface of the closed-cell foam, the foam has a completely open-celled morphology. Figure 5.12b shows the foam below the surface of the closed-cell polymer.
a)
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b)
Figure 5.12: SEM images of DCPD foam with temperature adjustment stabilization: a) layer of closed-celled pore structure covering open-celled pores, b) complete opened-celled pore structure.
Further studies were done to eliminate the closed-cell foam atop the open-celled structure. With the temperature adjustments showing increased emulsion stabilization and generating increased open-celled foam. DCPD was added then the temperature was increased to 55 oC, eliminating the
35oC emulsion. The oil-in-water emulsion was allowed to mix for ~15 minutes before continuing the procedure. Figure 5.13 shows the SEM images of the resulting foam.
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a)
b)
Figure 5.13: SEM image of DCPD foam with temperature stability: a) DCPD with complete open-cell structure b) magnified SEM image.
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By slowly increasing the temperature of the both the oil and aqueous phases simultaneously and having the emulsification at 55 oC for the entire process the foam shows a completely open-cell system. The surfaces, as well as the inner layers, contain a stable emulsification.
5.3.3 Incorporating CNFs into DCPD Foam
Further research incorporating CNFs was examined. Carbon nanofibers properties have made it the focus of intensive studies. Previous research on creating high-performance polymer composites by incorporating single-walled carbon nanotubes (SWCNTs) has shown significant improvement to the composites properties. A 10 wt% addition of SWCNTs to poly(pphenylene benzobisoxazole) has shown 50% increase in tensile strength, reduced shrinkage, and increased thermal stability67. The incorporation of fibers into foam has been the focus of research efforts in order to tailor materials to better fit applications. Research in this filed continues, in order to find processes for incorporating carbon nanofibers into other polymer composites.
For these studies the CNFs were incorporated into the emulsion during the formaldehyde addition. The CNFs were incorporated in with an emulsion stabilizer to move the fibers into the cell wall of the foam pores. The CNFs’ addition occurred prior to the final polymerization. This allows the fibers to best opportunity to be incorporated in the cell-walls. Initial foam created prior to emulsion stabilization via temperature research shows CNFs are embedded with the foam, figure 5.14.
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Figure 5.14: SEM image of closed-cell morphology seen in DCPD foam. The incorporated CNF are disbursed throughout the foam.
A crack in the DCPD foam with closed-cell morphology shows CNFs protruding out into the crack, indicating CNF dispersion. This process utilizes an in situ polymerization method for the
CNFs’ integration. in situ polymerization is an effective process for dispersing nanofibers into a material. The nanofibers are dispersed and suspended in solution. The solution is then polymerized. After polymerization the final product, nanofiber incoproated materials, is produced. Previous research as used in situ polymerization to improve dispersion and integration between polymers and CNFs 19, 21,37. With in situ polymerization the CNFs disperse in a low viscosity polymer matrix and the polymer is subsequently cured. Polymer curing post CNF dispersion traps the CNFs in place. As the polymer hardens, the CNFs remain suspended in the pore’s cell walls. This cracked section reveals fibers integrated throughout the foam. With additional emulsion stabilization the DCPD foam was created with CNFs. A SEM image, figure
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5.15, of the DCPD foam shows the integrated CNF into the cell walls. The CNF, circled in black, are extending out the cell walls. The foam maintains its open-cell morphology and continues to have the same polyhedron shaped pores as the foam without the CNF.
Figure 5.15: SEM image of DCPD foam with CNF incorporated into the cell walls.
5.3.4 Sidewall Modification of MWCNTs for Polymer Integration
The integration of CNTs with polymers is important to confer the intrinsic properties of the carbon nanotubes to polymer. While physical integration can be achieved the best way to achieve successful transference of properties is through chemical attachment. Chemically attaching a molecule to the sidewalls of the CNTs aids in the attachment of the CNTs to the polymer composite. This study examined forming an open cell DCPD foam with covalently attached norbornene MWCNTs. MWCNTs were modified to create norbornene functionalized
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MWCNTs. The norbornene functionalized MWCNT were incorporated into the foam process during the final step, similar to the incorporation of the CNFs. The incorporation was done prior to the polymerization, but after the emulsion began. After the addition of the norbornene functionalized MWCNTs the solution was stirred with a stir bar while being dispersed with an ultrasonic probe at 55 °C for 1.5 hours. Figure 5.16 shows a schematic of the reaction.
Figure 5.16: Schematic of the sidewall modification of MWCNT-COOH with 5-norbornene-2- yl(ethyl)chlorodimethylsilane for the covalent attachment to DCPD based foam.
The results showed indications of cell structures, but the carbon nanotube concentration may have been too high and caused more cross-linking with the polymer than foam synthesis, figure
5.17.
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Figure 5.17. SEM image of DCPD foam with norbornene functionalized MWCNTs.
Failure to form a complete foam structure could have also been due to alteration in the pH of the solution. Small amounts of unreacted 5-norbornene-2-yl(ethyl)chlorodimethylsilane immobilized on the surface of the nanotubes can cause changes in the pH effecting the urea-formaldyehyde reaction used to stabilize the pore walls. The reaction steps were modified to use 10 mg of norbornene functionalized MWCNTs and the addition of the tubes to the aqueous solution of using 0.125 g of poly(ethylene-alt-maleic anhydride), 0.5 g of urea, 0.5 g of ammonium chloride,
0.05 g of resorcinol and 25 ml of ultra pure water before sonication to separate the tubes and adjustment to pH 3.5. Figure 5.18 shows the SEM image of the foam after this procedure. There is a slight improvement here with a coated CNT and the inference of a cell structure.
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Stabilization of the cells may be required with the addition of a stabilizer, such as synperonic
PEL121.
Figure 5.18: SEM Image of DCPD foam with 10 mg of norbornene functionalized MWCNTs.
5.3.5 Thermal Removal of DCPD from CNF network
The development of a porous carbon nanomaterial has recently been the focus of numerous research efforts68. These porous carbon nanomaterials are being examined for applications as adbsorbents, gas storage, sensors, and other applications where high surface area and unique physical and chemical properties of carbon nanotubes would be an advantage. Porous carbon nanomaterial is usually formed using a polymer template. Templating is an ideal process for controlling the morphology of the final material. The inner/outer dimensions and shape of the pores can be tailored by selecting various molds. The structure’s shape is dependent on the template. Latex templates are often used to obtain spherical pores, whereas amphiphilic block
133 copolymers are used often used for cylindrical pores. Final porosity is also determined by the template. The template acts as a cast for the final structural material. Controlling the dimensions and shape of the final structure enables the final properties of the nanomaterial. For example control over the pore size and shape controls the materials surface area69.
The template process involves first incorporating the carbon precursor into the pores of polymer composite foam, which is the template. The template is then removed thermally or chemically. More success has been seen in fabricating porous carbon nanomaterials using physical verses chemical methods for removing the polymer composite. The removal of the template leaves a carbon network, like that of the polymer template. The removal of the template can be a difficult process oftentimes failing to result in a carbon network70.
The foam formed with CNFs, via the fabrication process described above, was placed in a furnace at 400 oC for 1, 2, 3, and 4 hours. The thermal decomposition was done under argon to aid in removal of the DCPD and enhances formation of the CNF network. The aim was thermal removal DCPD leaving the CNFs behind the polyhedron porous structure. The following studies examined the feasibility of obtaining a porous CNF network from the thermal decomposition of
DCPD foam to fabricate porous CNF materials in one-step. The fabricated materials were characterized via SEM.
Figure 5.19 a-d shows the SEM images resulting after the foams were placed in the oven for 1, 2, 3, and 4 hours, respectively.
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b)
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c)
d)
Figure 5.19: SEM images of DCPD foam with after a) 1hr, b) 2 hr, c) 3 hr, and d) 4hr at 400oC.
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As the foam began to burn off the CNFs did not stay in the shape of the foam and began to interact before the network was thermal secure. The DCPD had been the factor overcoming the
CNFs’ van der Waals forces, preventing the CNF aggregation. With the removal of the DCPD the CNFs were no longer suspended throughout the network. The carbon network began to breakdown as a result. Utilizing one-step to both remove the DCPD and thermally fuse the CNFs into a pours network was not feasible using this DCPD template. Attempts made to create a carbon network at higher temperatures did not yield a nanofiber networks and resulted in large aggregations of CNF.
5.4 Discussion
Dicyclopentadiene’s exothermic behavior allows for polymerization without the use of a catalyst, when combined with heat and low solvent concentrations71. The heat in the process and that generated by the polymer cannot dissipate causing curing of the polymer. While the mechanism has not been fully elucidated this process has been observed in other procedures involving DCPD71. However, this thermal curing allowed for the creation of highly porous
DCPD foam utilizing high internal phase emulsions without the use of a catalyst. The synthesis also successfully integrated carbon nanofibers into the cell walls. With increased temperature control throughout the emulsion and polymerization process all closed-cell sections could be eliminated and defined cell walls could be created. Under these conditions, dicyclopentadience organized in a manner resembling open-celled foam. The resulting solid is microscopically highly porous foam.
Unlike other methods of creating microcellular dicyclopentadiane using HIPE this method was performed in air and did not need to be carried out under nitrogen. The process
137 described in this chapter did not use a catalyst. The Grubbs catalyst used in previous research needed to be kept under nitrogen42. Similarly when DCPD foam is created using an alternative catalyst, such as a tungsten based catalyst, the reaction much occur under argon72. Utilizing this method instead of ring opening metathesis to polymerize DCPD removes the need for a Grubbs catalyst and thus creates a cheaper method with a stable emulsions phase at a lower temperature.
The pores were generated via an oil-in-water emulsion of the polymer in aqueous precursors, with sufficient stabilization of the emulsion throughout polymerization to yield a cell wall able to withstand polymerization. The open-celled structure was also achieved with the addition of
CNFs, indicating the CNF loads did not disrupt the emulsion. The production of porous microcellular foam with highly interconnected pores is intrinsic to this method using DCPD, as this structure could be achieved with and without the introduction of CNFs.
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CHAPTER SIX: CONCLUSIONS AND FUTURE WORKS
6.1 Conclusions
6.1.1 Carbon Nanotube Coated Microcapsules Fabrication
This research presented a new self-healing polymeric composite incorporating the use of carbon nanotubes. The field of self-healing polymers, which is relatively new, has garnered much interest in the wake of catastrophes due to material failure and the ever increasing pile of material waste. The development of materials capable of monitoring its structure for damage and healing the damage has become imperative. These materials would reduce the amount of waste and extend the lifetime of materials. One approach to solving both the problems of material failure and material waste is to create self-healing materials.
Chapter 3 examined the formation of microcapsules with the elimination of a surfactant solution and the creation of microcapsules with carbon nanotubes (CNTs) integrated into the microcapsules shells. The elimination of surfactant, through direct addition into the precursor solutions, not only eliminated process time, but also eliminated material cost associated with removing surface bubbles. The process alteration did not change the morphology of the capsules.
The process was able to create about 150 mL of microcapsules from 200 mL of precursor solution. The microcapsules were 0.2 – 4 μm in size and contained a urea-formaldehyde shell with a dicyclopentadiene core.
The chapter also discussed the creating of microcapsules with CNTs integrated shells via an oil-in-water in situ polymerization. The use of CNTs sought to enhance the effectiveness of the microcapsules in preventing microcrack propagation in polymer composites. Carbon nanofibers (CNFs) were originally used because of the larger fibers contained a larger surface
139 area thought to enhance interaction with the microcapsules and aid integration. However, CNFs caused disruption in the formation of the shell wall for the microcapsules, thus preventing microcapsule formation. The CNFs longer and larger fibers not only aggregated together, but produced pointed edges, which can break the microcapsule shell. Various studies examining the effect on excess dicyclopentadiene (DCPD) and excess to coat the CNFs resulted in a multitude of problems, including shells that were not fully formed, microcapsules smaller than the 0.5 μm threshold for use in self-healing systems, sheets of polymer cured atop fully formed microcapsules, and tubes of CNFs coated with polymer. The excess concentrations caused disruptions in the urea-formaldehyde cross-linking and microcapsules could not be formed.
Utilizing multi-walled carbon nanotubes (MWCNT), which have shorter lengths and smaller diameters than CNFs, allowed for microcapsules fabrication, because the shell components were able to polymerize around DCPD. The MWCNTs were also able to be incorporated into the shell walls. The incorporation of the CNTs gave the microcapsules a rough exterior, which has been seen to improve shelf-life and integration ability18. With a method capable of producing microcapsule with CNT embedded shells these microcapsules can be embedded into the polymer composite to create a self-healing polymer.
6.1.2 The Fabrication of Self-Healing Cellulose Triacetate Films and Coating Using
Microcapsule Self-Healing Systems
There are various systems used to create self-healing systems. While many polymers have an intrinsic self-healing capability extreme conditions are necessary for these intrinsic systems to react; therefore, extrinsic systems, in which a secondary “healing” system is implanted into the primary material, have become the main method for self-healing. The most
140 researched extrinsic self-healing systems involve the use of microcapsules. In self-healing systems utilizing microcapsules the microcapsules encapsulates a healing agent, usually a liquid polymer. The outside microcapsule’s shell will break in response to material cracking. The previously encapsulated polymer fills the crack via capillary action. The polymer will react with an embedded catalyst for a polymerization reaction. The polymerized polymer “heals” the materials crack. This type of healing system was first successful created in 2001. The system utilized encapsulated dicyclopendatiene reacting with a Grubbs catalyst to heal an epoxy material without human intervention.
The use of the microcapsules system is relatively simple and easy to implement.
Problems arise in the need to tailor the microcapsule system to the polymeric system in which it will be embedded. Chapter 4 discussed the pairing of a commercially viable self-healing system with a natural-based polymer to create a self-healing system. The research examined the feasibility of using a ring-opening metathesis polymerization (ROMP) reaction and using an amine-epoxy polymerization. While the amine-epoxy reaction has a fracture recovery of only
70%, compared to the dicyclopentadiene ROMP reaction with a fracture recovery of 90%, the use of the amine-epoxy reaction yields a cost-effective material and has a greater commercial viability.
Producing self-healing coatings adds to the complexity of the creating a self-healing system. The majority of research in the self-healing field has been in the self-healing of bulk polymers, where self-healing systems do not have to operate exposed to environmental conditions, nor do those systems have to be into thin layers, where aggregation can cause major defects in the polymer composite. Cellulose-triacetate, a natural based polymer, has numerous
141 uses including electronics’ coatings. The microcapsules encapsulated bisphenol A polymer,
EPON 828®, and an amine catalyst, EPIKURE 3140®, was embedded directly into the polymer.
Observable self-healing was achieved through the increased concentration of microcapsules and increased temperature above room temperature. Without self-healing an adhesion promoter would be needed to keep the coating attached to the substrate and increased coating thickness would be needed to offer additional protection. The healing also showed increased adhesion to the steel substrate by protecting the substrate from corrosion damage longer than unhealed coatings. Despite being able to observe self-healing the increased concentration caused damage to the CTA structure by creating voids and holes. These voids created would results in weak points in the film. The holes decrease the strength recovery of the coating; however, a coating’s main objective is to protection from environmental damage. While the films did not show healing at room temperature, an elevated temperature of 35 oC makes this system available for warmer environments. Carbon nanotube coated microcapsules showed more signs of damage to the cellulose triacetate structure than microcapsules without; and CNT coated microcapsules did not show signs of healing throughout the crack. The additional voids and minimal healing is thought to be due to the increase aggregation seen in the CNT coated microcapsules.
6.1.3 Porous Carbon Nanofiber Dicyclopentatdiene Foam Fabrication via High Internal
Phase Emulsions
In chapter 5 the process for creating DCPD foam with and without CNFs via high internal phase emulsion (HIPE) was examined. Dicyclopentadiene, as structural polymer, was previously thought to not be able to create porous foam; however, utilizing high internal
142 emulsion polymerization with decreased solvent concentrations, scanning electron microscopy shows porous dicyclopentadiene. The use of this technique is also more cost-effective than previous techniques used to make dicyclopentadiene foam. Dicyclopentadiene’s exothermic behavior allows for polymerization without the use of a catalyst, when combined with heat and low solvent concentrations71. While the mechanism has not been fully elucidated this process has been observed in other procedures involving DCPD71. The process discussed in chapter 5 used thermal curing to create highly porous DCPD foam without the use of a catalyst. The fabrication also integrated carbon nanofibers into the cell walls. Temperature controlled emulsion and polymerization eliminated all closed-cells structures and produced defined cell walls in the open- celled DCPD.
Unlike other methods of creating microcellular dicyclopentadiane using HIPE this method was performed in air and did not use a catalyst; thus providing a cheaper method with a stable emulsions phase at a lower temperature. The pores generated via an oil-in-water emulsion maintained sufficient stabilization throughout polymerization, even with the addition of CNF, indicating the CNF loads did not disrupt the emulsion.
6.2 Future Works
The future research in fabricating carbon nanotube integrated microcapsules includes possible functionalizing carbon nanotubes. Carbon nanotubes functionalized with a carboxyl group would add additional polarity creating stronger bonds with polymer in which the capsules will be embedded. The stronger bond between the capsules and polymer could potentially decrease the aggregation of microcapsules. In a thin film decreased aggregation is crucial for decreasing film defects. The functionalize carbon nanotube microcapsules could also extend the
143 application of the microcapsules beyond use of the microcapsules for self-healing. Enhanced interaction could allow properties of the carbon nanotubes to enhance the properties of the polymer thin film.
Future works in the development of self-healing coatings using cellulose triacetate includes different methods observing self-healing over time. One method would include measuring the changes in film resistance after healing. Using palladium nanoparticles to increase the sensitivity the film would be attached to four probes attached and hydrogen passed over the film. Changes in the resistance of the film would be an indication of self-healing. Another approach would be to examine the migration of polymer into a crack. Adding congo red dye to the encapsulated polymer prior to capsule formation. As the polymer fills a crack the red polymer would be tracked under a light microscope.
With the process for creating open-cell DCPD foam, future works include adjusting the pore size of the foam. The one advantage of using HIPE for creating polymeric foams it the ability to alter the pore size. Pore size of foam will affect the foam properties, such as surface area. Being able to adjust the pore size, usually done by varying the sonication or stirring rates, will provide a wider range of application available for this foam.
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BIOGRAPHICAL SKETCH
Artrease Spann was born on June 16, 1983 in Albany, GA, where she spent most of her childhood. After moving to Fayetteville, GA she graduated from Fayette County High School in
2001 with honors. In 2001, she attended Spelman College in Atlanta, GA and engaged in undergraduate research in the laboratory of Dr. Lisa Hibbard conducting cataract research investigating the affect of various stresses, such as UV light, on the structure of α-crystallin, a major eye protein. Upon graduating from Spelman College in 2005 with a B.S. in biochemistry, she began her graduate career at the Georgia Institute of Technology in the research group of Dr.
Joseph Perry, where she created a viable polymer host-system for optical limiting materials. She obtained a M.S. in chemistry from Georgia Institute of Technology in 2007 and began her doctoral research at Florida State under the advisement of Nobel Laureate Sir. Harold Kroto. Her doctoral research involved creating novel self-healing carbon nanotube reinforced polymers composites. During her tenure, she held a Florida Gubernatorial Fellowship, where she worked for the Executive Office of the Governor assigned to the Florida Department of Education making recommendations and writing policy on improving Florida’s K-12 science education.
She is currently working as a packaging engineer with Heinz in Pittsburgh, PA; and she will continue in that position upon completion of her PhD.
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