%¢-UMI Number: 3211689
Copyright 2006 by
Rutledge, Shavesha Lavette Anderson All rights reserved
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Trang 3Shavesha Lavette Anderson Rutledge 2006
Trang 4DEDICATION
To my sister Debra Jean Anderson who instilled in me the importance of education To my parents who always believed in me
Trang 5By
Shavesha Lavette Anderson Rutledge ABSTRACT
Nanotechnology has the potential to meet the need for stronger, lighter polymeric
materials Carbon nanotubes introduced into a suitable polymer matrix can be engineered to produce the desired characteristics Two polymers were focused on in this research, polycaprolactone (PCL) and polyethylene (PE) Polycaprolactones have an extraordinary blend of properties that generate good physical characteristics and low temperature flexibility Due to its rubbery properties it has been widely used for improving elasticity PCL has the tendency to form compatible blends with multiple polymers PCL was chosen primarily because of its ability to be electrospun into fibers Polyethylene was chosen because of its use in spaceflight applications such as Mars balloons used to conduct scientific research NASA is in search of new composite materials that will enhance these spaceflight missions The approach to produce the polymer matrices reported herein employs a range of chemical methods using bench-top level polymer chemistry and carbon nanotubes Two techniques were studied to produce the desired polymer blends, i.e., electrospinning and extrusion These techniques were used to obtain homogenous nanopolymer blends and aligned nanopolymer fibers The samples were
Trang 6studied by scanning electron microscopy and their tensile properties were measured The properties of the electrospun fibers were studied and an increase in fiber elasticity was observed as the fiber diameter decreased The extruded polymers displayed over 60% increase in tensile strength with small amounts of carbon nanotubes incorporated within the polymer matrix
Trang 7without the help of others I would like to acknowledge Dr Nina Rosher for believing in me and for giving me the opportunity to study at American University I would like to acknowledge Dr Paul Waters for providing me with exceptional advice to complete my research Thank you to Harry Shaw for being an excellent mentor, for including me in your multiple projects, and for introducing me to many forms of science and engineering I would like to acknowledge Dr Monika Konekleiva for serving on my committee and for being an excellent professor To the Harriett G Jenkins Pre-doctoral Fellowship program, thank you for supporting my education financially I must acknowledge NASA Goddard Space Flight Center Code 562 for supporting me during these years A special thank you goes to Darryl Lakins for providing me the opportunity to Co-Op in your organization I would like to thank Dr James Girard and the AU chemistry department for helping me during my graduate school years A special thanks goes to my husband for his love and support Jason Rutledge you are the greatest husband and friend I could ever have Thanks to my family and friends for always being there for me And finally, to my Lord and Savior Jesus Christ whom I have learned to totally trust in throughout this entire process Trust in the Lord with all your heart and lean not towards your own
understanding, in all your ways acknowledge Him and He shall direct your path [Proverbs 3:5, 6]
Trang 8TABLE OF CONTENTS
Trang 9Miniextruder .cecsscssscssesseecssesscecsesssceseeseesesoaseaeseesessessesssensenses 34 6 RESULTS AND DISCUSSION - LH HH HH HH HH 36 Carbon Nanotube Charact€r1Zaf1OT s- si TH HH nan 36 Polystyrene CharaCf€TIZA{1OTI - <G- ng ni ng ng 44 Polycaprolactone Film Characfer1ZatiOI - Ác 111.1951515 x2 50 Electrospinning of Polycaprolactone Fibers .ccssssessssssssescesnessceseeseeees 56 Electrospinning of Aligned PCL/CNT FiberS cccescssssessesssesteeseneensees 77 Electrospinning of PMMA Fibers eessssssceeesesscesessscteesasescesetsasenseneeees 79 Extrusion of LDPE and Carbon Nanotubes - 5S 80 COnCÏUSIOII GĨT TH TH TH TH TH nọ TH 000 98 Future R€s€arCH: HH TH nh Chu 99 REFERENCES - HH HH HH HT no TT TH TH 00 K00 101
Trang 10Table
LIST OF TABLES
Page Advantages, Disadvantages, and Applications o£ Polystyrene - 19 Molecular Values of Polystyrene SamtpÏÌe - -‹- 2G SH nn 49 Results of Tensile Strength Test of PCL and PCL/CNT Composites 55 Optimal Voltage for Electrospinning - -«- sưng nu 62 Results of Tensile Testing for Each Group of Polymer Samples 89
Trang 1110 11 12 13 14 15 16 17,
Types of Carbon Materials 2 Computer Generated Image of Multiwalled Carbon Nanotubes (MWNT) 3 Schematic Diagram of the Arc-Discharge Method - se 5 Schematic Diagram of the Laser Ablation Method ca 6 NASA Goddard Space Flight Center (GSFC) Non-Catalytic Method for
Producing Carbon Nanotubes (CNÏTS) - (ch HH ng ngư 7 Reaction of Ethylene to form Polyethylene (PE) LH nen 9 Structure Of PỌYITI€TS - cọ TH th 9 Addition ReaC(IOH - Gà TH HH HT TH CC TH HC 0g 11 Image of a NASA Balloon Ủsed to Collect Scientific Data - 14 Free Radical Polymerization of StyT€n . - ch ng Hư, 17 Types of Polystyrene MoleCes§ . - c1 9H HH như 18 Ziegler-Natta Polymerization of Ethylene - cong hưu 22 Ring Opening Polymerization of e-CaproÌaCtOn€ s5 sen, 25 Polymerization of Methyl Methacrylate «HH Hàn 26 Single Screw EXITU€T - Ác HH TH TH Hà K00 0080150 28 GSFC Samples of Carbon Nanotubes Immersed in Toluene - ‹- 31 Carboxylic Acid Group Functionalization of CN T§ ccccnieeee 33
Trang 1218 Simulated Image of the Electrospinning Apparatus .cccssssscesscsseeesesseesenes 33 19 Actual Image of the Electrospinning ApparafUS «coi, 34 20 The Miniextruder and its CompOn€rIS - on ng ng gen 35 21 Images of Scanning Electron Microscopes (SEM) He 37 22 SEM Sample PreparatiOn cĩ 1 T110 11 0T vàn ng 094 38 23 SEM Image of GSFC Produced CNTs After Grinding .ccccecsssseetscseeteees 38 24 SEM Image of GSFC Produced CNTs After Further Grinding with a Dremel 39 25 SEM Image of GSFC Produced CNTs After Refluxing in Nitric Acid 39 26 SEM Image of GSFC Produced CNTs After Refluxing in Nitric Acid with
Increased Magnification .escsscsscssessscssecssessssssessessscssecsnseessesessseessnseneesensens 40 27 SEM Image of the Carbon Nanotubes Produced by the Johnson Space Center
High Pressure Carbon Monoxide (JSC/HiPCO) Process 7c 41 28 SEM Image of the Carbon Nanotubes Produced by the JSC/HiPCO Process 41 29 Scanning Transmission Electron Microscope (STEM) Bright Field Image of the
JSC/HiPCO Produced SWNTS Hình ng 42 30 STEM Dark Field Image of the JSC/HIPCO Produced SWNTs§ 42 31 Transmission Electron Microscope (TEM) Images of GSFC Produced CNTs 43 32 Gel Permeation Chromatography (GPC) Chromatogram of Synthesized
IlJà/)À/(: 0n" 5 45
33 GPC Overlay Plot of Cumulative Weight Fraction vs Log Molecular Weight 46 34 GPC Overlay Plot WF/dLog MWvs Log Molecular Weight 47 35 GPC Mark-Houwink PPÏOI «sọ HT ng tà 48 36 Fourier Transform Infrared (FTIR) of Synthesized Polystyrene 50 37 Image of Polycaprolactone (PCL) Thin FiÌm: G5 HH cớ 31
Trang 1340 41 42 43 44 45 46 41 48 49 50 51 52 53 54 55 56 51 Image of PCL/0.20% CNT (disperS€d) ĩc HH ng ng 10 11g16 51 SEM Image of PCL Matrix without CNTS 0 cscsscscsessssssceseseereeceereseeseees 52 SEM image of 0.20% CNÏT and PC L, 5 <6 c1 ng , 53 SEM image of 0.07% CNT and PCL scsscssesesessesscsseeessceesssssssessessessessssesees 53 SEM Image of PCL Film at 500X 0 ccccessssssescssessseesseeeesssseesseseeeres "— 54 SEM Image of PCLUCNT 0.20% Film at 5O0X QQ SH, 54 SEM Image of PCL/CNT 0.07% Film at 5ƯŨX Án HH, 55 Chart of the Results of Tensile Strength Measurements of PCL and PCL/CNT
®0i)2 5) TƯ ẲẢỘỪ£Ồ 56
Trang 1458 59 60 61 62 63 64 65 66 67 68 69 70 71 ?2 73 74 75 76 Mean Value Force Distance Curves for Data Analyzed Along Several PCL/CNT NanOfTDTS - HH HH TH TT TH Ti Hi 000001090 69 Schematic Setup of the 3™ Electrospinning Apparafus -. -scccsc<ccsee 70 SEM Image of Aligned PCL Fibers Produced by Electrospinning with an Electric FFl€ÏC HH TH HH HT TT Họ TH 0i 0080 71 SEM Image of Aligned PCL Fibers Produced by Electrospinning with and
3 cvsv 0110 71
Image of the 4h Electrospinning ÀApDaräfUS dc ng n4 kớ, 72 Schematic Setup of the 5" Electrospinning Apparatus seo 73 SEM Image of Aligned PC FÏD€FS G1 ng V0 ng 74 SEM Image of Aligned PC, FIDe€TS 76 HH ng kg 74 SEM Image of Aligned PC, FI€TS - s5 s1 ng nu mg 75 SEM Image of Aligned PCL FIbeTs G5 HH ng ng nếp 75 SEM Image of Aligned PCL FIbers 0 G HHgnH ngnt 76 SEM Image of the Edges of Aligned Fibers Torn off the Semicircle Aluminum
COll€CfOTE PPÏ4É€ 7G GV 76
Polycaprolactone and Carbon Nanotube Electrospun Rope Fiber 78 Polycaprolactone and Carbon Nanotube Electrospun Rope Fibers with Increased
MagnIfiCAfÍOTA HT TH 0 78
SEM Image of Randomly Ortented PMMA FElibers - - sec ằseeee 79 SEM Image of Randomly Oriented PMMA Fibers se 79 Image of Extruded PE ROd SH" TH HH TY TH ng HH HH HH in 82 Image of Jet Press Used to Press Polymer Samples .- sec 83 Jet Press with PTFE Sprayed Metal Bottom Plate cesssescssscteeesesseeeees 84
Trang 1579 80 81 82 83 84 85 86 87 88 89 90
Image of PE/CNT Pressed SampÌe .- ship 85 Image of Instron 4442 Testing Machine HH kg mm 86 Image of Dog-bone CẮT . ch HH TH HH TH HH th ng 87 Dog-bone Shape with Sample DimenSiO'S s5 ke 88 Graph of Force vs Time for GTOUD Í 5 4 ng ke 90 Graph of Force vs Time for GTOUD 2 . -s- <5 Án ng ng 91 Graph of Force vs Time for Group Ổ - 0 HH ng nh nHt 92 Graph of Force vs Time for GTOUD 4 HH HH ng rưệp 93 Graph of Force vs Time for GrOUD Õ, 5 <- càng ng 94 Graph of Force vs Time for GTOUD Ố án ng ng net 95 Graph of Force vs Time for Group 7 - ¬ 96 Graph of Force vs Time for Group Ư - TH ng ng 97
Trang 16CHAPTER 1 INTRODUCTION
The outstanding properties of carbon nanotubes incorporated into a polymer matrix have numerous applications Carbon nanotubes can be used for reinforcing
structures to create high strength composites because of their mechanical properties They can also be used for multifunctional purposes such as increasing electrical conductivity in polymers Scientists have studied poly(m-phenylenevinylene-co-2,5-dioctoxy-p-
phenylenevinylene) (PPV) composites which have shown a large increase in electrical conductivity by approximately 8 orders of magnitude compared to the pristine
polymer.“ Carbon nanotubes were also incorporated in Polyacrylonitrile (PAN), commonly used as carbon fiber precursors In this study, PAN/SWNT fibers that
contained a 10 wt% of nanotubes were used to increase tensile modulus, reduce thermal shrinkage, and increase the glass transition temperature when compared to PAN fibers @) The research presented herein utilizes small mole fractions to increase the tensile strength of low density polyethylene (LDPE) commonly used for space flight applications This research also introduces a technique to produce aligned nanofibers that can be used to make woven nanopolymer materials
Carbon Nanotubes
An electron microscopist, Sumio Iijima, discovered carbon nanotubes in 1991 (3) lijima accidentally discovered them while studying the carbon cathode used for the arc
Trang 17Carbon materials can be found in a number of forms such as graphite, diamond, carbon fibers, fullerenes and now carbon nanotubes (See Figure 1).© Many structural forms can be simulated by carbon because a carbon atom can form several distinct types of valence bonds It is the lightest atom in column IV of the periodic table and is an element with very unique properties © diamond Figure 1: Types of Carbon Materials ©?
Trang 18interest of many physicists, chemists, and material scientists They are attractive because of their astonishing electronic properties, amazing stiffness, strength and resilience MWNTs (See Figure 2) are made up of 2 to 30 graphitic layers whose diameters range from 10 to 50 nm and have a length of more than 10 microns It is assumed that the tubes are not scroll like but are concentric They are useful because they are stabilized by their large number of layers and are available in relatively large quantities ) Aithough they are structurally stable, they often contain regions of structural imperfection This occurrence of defects degrades the materials’ properties such as its strength ” Figure 2: Computer Generated Image of Multiwalled Carbon Nanotubes (MWNT) ® SWNTSs are usually more homogenous than MWNT and contain fewer obvious defects The tubes have a much smaller diameter of typically 1 nm and are frequently curled and looped rather than straight 6)
Trang 19band in an atom or molecule The bonding of atoms distinguishes metals from non- metals Metallic materials have electrons that move freely and therefore tend to be good electrical conductors The electron spin resonance (ESR) and the 3¢ nuclear magnetic resonance (NMR) measurements have proven that the MWNT can show metallic properties based on the spin susceptibility Semiconductive MWNTs have also been shown by the ESR measurement @
Trang 20than the accepted value for a graphene sheet of 1.06Tpa (Tera-Pascals — 10’) © At moderate temperatures, diamond-diamond like materials demonstrate the highest measured thermal conductivity of any known material °!®
Trang 21crystallinity In order to grow SWNTs by this process, a metal catalyst such as cobalt, nickel, or iron is needed Typical by-products of this method include fullerenes, graphitic particles, metal particles, and amorphous carbon in the form of particles or coating over on the sidewalls of the nanotubes This process uses high temperatures that range from
2500 - 3000°C
Figure 4 illustrates a schematic diagram of the Laser Ablation Method developed by Richard Smalley’s group at Rice University in 1995 A laser is used to vaporize a graphite target that is held in a controlled environment oven Helium or argon carrier gas is used and the temperature of the oven is approximately 1200°C Carbon nanotubes are collected on a cooled target and contain nanotubes and nanoparticles that are graphitized and have a perfect structure Went: cachet Cu Cetecion ¬ _ Figure 4: Schematic Diagram of the Laser Ablation Method f?
Trang 22produced and this process is known as Chemical Vapor Deposition (CVD) The main parameters that affect this process are the hydrocarbons, catalysts, and growth
temperatures ‘'” A process was developed by Smalley’s group at Rice University that had the ability to produce large quantities of SWNTs The process is known as the High Pressure Carbon Monoxide (HiPCO) process The nanotubes are produced from gas phase reactions of iron carbonyl] at high pressure (10 - 100atm) in carbon monoxide In the process, CO is added to a small amount of iron pentacarbonyl Fe(CO)s and is heated to produce SWNTs The products of thermal decomposition of Fe(CO)s react to produce iron clusters in the gas phase and these metal clusters act as nuclei where the nanotubes nucleate and grow
Dr Jeannette Benavides and group at NASA Goddard Space Flight Center (GSFC) developed a non-catalytic method for producing carbon nanotubes (See Figure 5) helium in
tube height control
current in, coolan He aes helium arc welder § nozzle distance “d” carbon rod d= imm
Figure 5: NASA Goddard Space Flight Center (GSFC) Non-Catalytic Method for Producing Carbon Nanotubes (CNTs) (12)
Trang 23a cathode that is place in a cooled water bath Once the current is applied and the arc is formed, carbon nanotubes are then deposited on the carbon cathode This process is advantageous because it does not use a metal catalyst, is inexpensive and much simpler than some of the previously mentioned processes This process was patented in 2004
(Patent No.: US 6,740,224)
The fascinating materials of single-wall and multi-wall carbon nanotubes are said to become “one of the most important materials in the 21* century” Carbon nanotubes may be the strongest, toughest, stiffest structures that have ever been produced and therefore the potential of the material is vast The possible applications for carbon nanotubes are numerous This research focuses on their ability to be used to increase the performance of polymers to create nanopolymer composites
Polymer Background
Polymers are used on a daily basis Plastics, fibers, elastomers, coatings, adhesives, rubber, protein and cellulose are all a part of polymer chemistry High
Trang 24a long chain of skeletal atoms that have the substituent groups attached to it The linear and branched polymers are usually soluble in some solvents and are in the solid state at normal temperatures The crosslinked polymers are not soluble Linear polymers exist as elastomers, flexible materials, or glasslike thermoplastics H, H, H Initiator _†C-~ _~C _.C + H,C=CH, —— * C Cc C©Cln He H_ 2 Ethylene - Polyethylene - Single unit Repeated units Figure 6: Reaction of Ethylene to form Polyethylene (PE) _—_=_
Linear: uninterrupted Branched: occasional Crosslinked or network: many
chain branches off the long interconnected linear chains
chain but one large molecule Figure 7: Structure of Polymers
Branched polymers can be thought of as linear polymers with branches of the same basic structure as the main chain They can be distinguished from linear polymers by their difference in viscosity, light scattering behavior or by their lower tendency to crystallize Polymers can be thermosetting, thermoplastic, or elastomers The
Trang 25not recyclable Thermoplastics flow when heated and can be easily reshaped and
recycled; this is usually because they have long chains with no crosslinking Crosslinked (See Figure 7) or network polymers have chemical linkages between the chains
Polymers can also be amorphous or semi-crystalline Tacticity, which is the arrangement of substituents around the backbone, determines the degree of crystallinity Isotactic (R groups on the same side of the backbone) and syndiotactic (alternating R groups on the backbone) may crystallize The degree of crystallinity is dependent upon the size of the side groups and the regularity of the chain Increased crystallinity will enhance the mechanical properties of the polymer
Polymers are typically synthesized by either condensation or addition methods Condensation is a stepwise addition where two monomers will react to form a covalent bond by eliminating a small molecule such as HCl, water, or CO2 Addition (See Figure 8) occurs when monomers react through stages of initiation, propagation, and
Trang 2611 RO-OR Leal Ro + ‹0R a peroxide alkoxy radicals HH Initiation steps Hy H | | fat, +!OR —> © C=C-OR H M 7 H H HH H HHH CHp=CH) H HH H ‘exc? +e bebop ae dodid kop oy se k_ Loa Propagation H H 11 Liu i 1 | steps HH HHHH mpszed HATA A many times đ polymer Figure 8: Addition Reaction
gaseous, liquid, and solid state can be studied by IR spectroscopy Thermogravimetric Analysis (TGA) is used to provide a quantitative measurement of any mass change in the polymer or material associated with a transition or thermal degradation TGA is defined as “a technique in which the mass of a substance is measured as a function of time or temperature while the substance is subjected to a controlled temperature program” 04) Additives, previous heat treatment, and the inclusion of other substances can affect the thermal stability of polymers TGA should be complimented with another analysis tool because it is very difficult if not impossible to identify the material of an unknown using this method alone The two primary applications of the technique are qualitative
identification and compositional analysis Since different polymers have different thermal stabilities a qualitative identification can be achieved (14)
Trang 27samples This method separates polymers and provides the relative molecular weight of the polymer There are so many other techniques that can be used to characterize
Trang 28CHAPTER 2
STATEMENT OF PURPOSE
The purpose of this research is to develop processes to produce polymer nanocomposites that are applicable for space flight applications NASA space flight applications are requiring larger, higher strength, lower mass gossamer structures such as solar sails, deployable antennas, Mars balloons and solar shields Many of these
structures can be made up of durable polymer composites that meet the necessary
requirements for the applications The requirements are that the materials be flexible and display a unique combination of physical and mechanical properties for each mission CNTs introduced into a suitable polymer matrix can be engineered to produce the desired characteristics Due to the outstanding properties of carbon nanotubes and the common use of polymers in space this research has the potential to substantially enhance
spacecraft missions A number of polymers were chosen for this research but only two are intensely focused on Polyethylene for example was chosen because of its current use in spaceflight applications For decades NASA has used balloons to conduct scientific studies These balloons (Figure 9) are made of a thin polyethylene material, 0.8 mil in thickness “>,
When the balloons are fully inflated they range up to 40 million cubic feet in volume and 600 feet in diameter and are taller than a 60 story building The balloon
Trang 3015
investigations.’ Data regarding the atmosphere, the universe, the sun and the near earth space environment can all be collected using the balloon missions NASA balloon missions are in search of new composite balloon materials that will enhance the balloon operations for use in planetary exploration The current goal for the ballooning technology development is a material with an areal density < 10 g/m? while meeting other properties and requirements These requirements may include but are not limited to properties such as yield strength, dimensional stability, tear resistance and environmental durability
Carbon nanotubes introduced into a polymer matrix such as polyethylene can produce the desired characteristics For example, the strength of the carbon nanotubes can be used to increase the yield strength and tear resistance of the polymer while other unique properties of carbon nanotubes such as thermal stability can be used to heighten the environmental durability Polyethylene , made up of carbon and
hydrogen, is also considered as an excellent material for radiation shielding When compared to aluminum, which is commonly used for radiation shielding, PE is better at shielding both solar flares and cosmic rays The heavier atoms tend to produce more secondary radiation than the lighter atoms such as carbon and hydrogen Since carbon is considered to be one of the lighter atoms the addition of carbon nanotubes in the polymer matrix should not affect the shielding properties of polyethylene
Polystyrene, polyethylene, polycaprolactone, and polymethylmethacrylate were among the polymers chosen for this research Polystyrene was chosen to become familiar with polymer chemistry and polymer characterization The
Trang 32CHAPTER 3 POLYMERIZATION
Polystyrene
Polystyrene is synthesized from the monomer styrene (Figure 10) Styrene can be polymerized by free-radical, cationic, anionic and coordination mechanisms free radical H ‘ } H vinyl polymerization i lrati H | | ‘c= —= “ar H H styrene polystyrene Figure 10: Free Radical Polymerization of Styrene
The polymerization occurs due to resonance stabilization of the reactive polystyryl species in the transition state that lowers the activation energy of the propagation reaction The styrene monomer has low polarity which aids in the polymerization
process The low polarity facilitates attack by free radicals, differently charged ions, and metal complexes From the free-radical polymerization of styrene the following
information was developed: styrene polymerizes thermally and oxygen hinders the polymerization of styrene (16) Polystyrene is a vinyl polymer Vinyl monomers, making up the largest family of polymers, are small molecules that contain carbon-carbon double bonds and are used to form vinyl polymers Polystyrene can be isotactic, syndiotactic or
Trang 33HỆ CC 4 ee Ồ HỆ H H = H a= TS St Figure 11: Types of Polystyrene Molecules atactic A polystyrene molecule with all of the phenyl groups on the same side of the chain is said to be isotactic If the phenyl groups are on alternating sides of the chain the molecule is syndiotactic and if the phenyl groups are on both sides, in a random fashion then the molecule is atactic (See Figure 11) The importance of the tacticity of polymers was previously discussed in the introduction The advantages, disadvantages and applications of polystyrene can be found in Table 1
Trang 3419
Advantages of Disadvantages of Some Applications of
polystyrene polystyrene polystyrene
Cheap, rigid, Brittle, poor chemical Light diffusers, beakers,
transparent, easy to resistance especially to cutlery, general household mold and has good organics, susceptible to appliances, video/audio dimensional stability, | ultraviolet (UV) cassette cases, electronic good electrical degradation, Flammable _| housings, refrigerator
properties, low liners, structural foam
dielectric loss, polystyrene moldings,
excellent resistance tools, cases and boxes,
to gamma radiation dishes, and egg boxes Table 1: Advantages, Disadvantages, and Applications of Polystyrene the emulsion polymerization process Free radical polymerization of polymers is one of the most commonly used reactions for making addition polymers The process begins with an initiator Acyl peroxides, hydroperoxides, or azo compounds are frequently used initiators for this method The initiator is added in small quantities and is decomposed by heat or light which produces a free radical (R’) Free radicals are easily formed from oxygen or peroxides because the O-O covalent bond is weak Emulsion polymerization is widely used in commercial processes and the method yields high molecular weight polymers." The process consists of water and a surfactant (sodium lauryl sulfate,
Trang 35Polystyrene was prepared by using the free radical polymerization process and the emulsion polymerization process The styrene monomer (55 ml) was washed twice with 25 ml portions of 25% aqueous sodium hydroxide to remove the inhibitor The monomer was then washed twice with 25 ml of distilled water to remove any residual reagents Then 50 grams of the inhibitor-free styrene was added to a test tube The test tube was flushed with nitrogen and 1.0 gram of benzoyl peroxide was added The solution was mixed by gently shaking the tube The test tube was placed in an oil bath at 80°C for 1 - 2 hours When the solution became viscous the contents were dissolved in 50 ml of toluene and then poured into 500 ml of methanol which precipitated the polystyrene that formed The polymer produced was isolated by filtration “*?,
The majority of emulsion polymerization recipes are developed by attempting multiple formulas “” The ingredients for an emulsion polymerization reaction include some or all of the following: a monomer, water, an initiator, a surfactant, a chaser, a chain transfer agent, and a buffer Various formulations were tried and a successful recipe was developed for this research The recipe consisted of approximately 32% of styrene monomer, 66% of water (H20), 1% of sodium dodecyl sulfate (SDS), 0.1% sodium bicarbonate (NaHCO3) buffer used to control the pH, 0.1% of potassium persulfate (K2S2Og) and 0.1% of toluene In a large test tube SDS, toluene, sodium
Trang 3621
was washed with distilled water and sodium chloride (NaCl) solution The sample had two phases, a clear portion on the top and a cloudy white portion on the bottom The white portion was separated and varying percentages of carbon nanotubes were added to of the solution Once the polymer composite was formed, it was reheated with pressure to achieve a uniformed thickness of the PS/CNT composite Polystyrene was initially considered for this research because the polymer has simple processing
procedures which served as a learning tool for understanding polymer reactions It was also used because there is a wealth of readily available information on the polymer which served as a source for comparison This polymer is quite brittle and would not be
acceptable for space flight gossamer structures; therefore the polymer research on polystyrene was not carried out throughout the research but served merely as a learning tool
Polyethylene
Low Density Polyethylene (Figures 6) is a thermoplastic that is prepared by polymerizing ethylene at high pressure (1,000 to 4,000 atm) and high temperature (180 to
Trang 3823
Polycaprolactone
Polycaprolactone, an aliphatic polyester, is made from the ring opening polymerization of e-caprolactone E-caprolactone is synthesized from the addition of acetic acid and hydrogen peroxide shown in Figure 13.” The ring opening
polymerization of PCL is also shown in Figure 13 The unique process of the synthesis of e-caprolactone monomer was developed by Solvay Polycaprolactones have an extraordinary blend of properties that generate good physical characteristics and low temperature flexibility Due to its rubbery properties it has been widely used for improving elasticity °” This polymer has been considered to be of great interest in biomedical applications, in the areas of tissue engineering and controlled drug delivery due to its biocompatible and biodegradable properties.?’” PCL has the tendency to form compatible blends with multiple polymers.” PCL was primarily chosen because of its ability to be electospun into fibers K.H Lee et al characterized nano-structured polycaprolactone nonwoven mats by electrospinning.”” Polycaprolactone beads were purchased from Sigma Aldrich The carboxylic-acid attached CNTs, produced by
oxidation with nitric acid, were immersed in dimethylformamide (DMF) to make varying solution concentrations, and were ultrasonicated for 3 - 6 hours each A 15% solution of PCL beads and methylene chloride (MC) was heated to create a viscous solution The PCL/CNT films were produced by taking the PCL/MC/DMF solution and adding JSC HiPCO produced nanotubes as received or immersed in DMF The sample was then placed in a thin sample well and allowed to dry
Nanopolymer fibers were produced by electrospinning Electrospinning
Trang 39tilt, the capillary bore size, and the contour of the collector plate were altered to obtain alignment Alignment of the polymer matrix is important to achieve crystallinity as opposed to producing the amorphous form of the polymer blend This process is inexpensive; the fibers are produced rapidly and efficiently
Polymethylmethacrylate
Polymethylmethacrylate (PMMA) or polymethyl-2-methyl propanoate is a thermoplastic synthesized from methyl methacrylate (Figure 14) It is a polymer with good hardness and stiffness; however it has poor solvent resistance PMMA is often used as a replacement for glass The difference between glass and PMMA is that PMMA is lighter, its density is about half that of glass and it does not shatter PMMA and carbon nanotubes thin films have been used for gas sensing applications “’ Their responses to different organic vapors were evaluated by monitoring the change in the resistance of the thin films when they were exposed to gases These fibers as well as the other polymer fibers discussed herein can offer an alternative to carbon fibers because of their small diameter, their large aspect ratio and their potential for outstanding mechanical
Trang 4025
H,C-COOH + H,O, ——= CH,COOOH + OH,
Acetic Hydrogen Peracetic Water