synthesis of carbon nanotubes and polymers to increase the performance of polymer blends

Carbon nanotubes introduced into a suitable polymer matrix can be engineered to produce the desired characteristics.. The approach to produce the polymer matrices reported herein employ

By

Shavesha Lavette Anderson Rutledge

Submitted to the Faculty of the College of Arts and Sciences

of American University

in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy

In Chemistry

AMERICAN UNIVERSITY LIBRARY &

%¢-UMI Number: 3211689

Copyright 2006 by Rutledge, Shavesha Lavette Anderson

All rights reserved

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Shavesha Lavette Anderson Rutledge

2006 ALL RIGHTS RESERVED

DEDICATION

To my sister Debra Jean Anderson who instilled in me the importance of education

To my parents who always believed in me

To my husband who continues to love and support me

By 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

ii

studied 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

11

without 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]

iv

TABLE OF CONTENTS ABSTRACCT án HT TH TH TH TH TH HH Họ 0016 ii ACKNOWLEDGEMENTTS HH HH HH HH TH Tà Hi TH nà iv LIST OF TABLES HH HH HH HT TH TH TH HH nkp Vii LIST OF ILLUSTRATIONS 155 vii Chapter

1 INTRODUC TIƠONN . nà HH HH HT TH TH HH vn 1

Carbon Nanotues - - 4 HH ng TT Họ gi gi 1 Polymer Background - - ‹s- «s9 TT Ti 3e 8

2 STATEMENT OF PURPOSE HH TH HH HH nà 13

3 POLYMERIZATION HH HH HT HH TH HH tt hệt 17

POlySỈYT€T€ - G TH HH Họ HH HH 00 090 001 17 PolyethyÏene - ng ng nọ HH 21 PolyCaprỌaCtOn€ -. sọ TY HT nọ Hi HT ke 23 PolymethylmethacryÌat€ SH 99g ng gu ng g0 0k6 24 v99) 6 27 EXẨTUSIOHA HH HH HH Họ To HT TT 0903p 27 ElecCffOSDInDINE cu nọ nọ 04 04 28

5 MATERIALS AND METHODS - Án HH HH ng giờ 30

Carbon Nanotube PreparatiOn - «s9 HH ng ng ng iên 30

Miniextruder .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 GÓT 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

vi

Table

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

Vil

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 MoleCuÌes§ . - 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

Viii

18 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 có 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

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

1x

First Electrospinning Apparatus Sefup co HH HH HH ng Hư 57 Schematic Setup of the 2™ Electrospinning Apparatus «se seo 38 PCL Film on an Aluminum Backing and Carbon Double Sticky Tape 59 PCL Film on an Aluminum Backing and Carbon Double Sticky Tape, Magnified VICW HH HH HH HT HT TH TH HH To TH T000 110 59 SEM Image of the PCL/CNT Film on an Aluminum Backing - 60 SEM Image of CNTs Dispersed Before Adding Them to the PCL Solution .61 SEM Image of the PCL/CNT FIbers - - HH HH 61 SEM Image of the PCL/CNT Fibers Ác HH HH ng ngưp 62 SEM of Polycaprolactone Carbon Nanofibers Spun at Bore Radii 64 Atomic Force Microscopy and Force Integration to Equal Limits Elasticity Maps

of Relative Elasticity for PCL Nanofibers 20.0 essesssesecsncecsreeesnesseeseeeeseeeaee 68

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

Xi

xu

CHAPTER 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

1

Carbon 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 ©?

Carbon nanotubes can be classified into two group’s: single-walled carbon

nanotubes (SWNT) and multi-walled carbon nanotubes (MWNT) When Iijima first reported the observation of carbon nanotubes it was of the MWNT type The discovery

of SWNTs followed less than two years later The diameter of a carbon nanotube is of nanometer size and length of micrometer size “ Carbon nanotubes have captured the

interest 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)

Carbon nanotubes can be either metallic or semi-conducting, depending on the configuration, provided that there is no bond alternation of carbon-carbon bond distances

band 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 @

Direct electrical conductivity and ESR measurements were used to confirm the metallic properties of SWNTs Because the C=C bond in graphite is one of the strongest chemical bonds in nature, carbon nanotubes are commonly viewed as the ultimate fiber when it comes to their strength along the nanotube axis Also, SWNTs are relatively flexible in the direction normal to the nanotubes curved surface The following three kinds of forces between carbon atoms in carbon nanotubes produce their characteristic elastic properties: strong sigma bonding forces, strong pi bonding forces and weak interlayer interactions The total energy of the nanotube is increased by the strain energy related to the way the nanotube is curved As the diameter of the nanotube decreases the strain energy increases This indicates that a nanotube with a small diameter may be less stable than a nanotube with a larger diameter SWNTs are thought to possess many of the desirable mechanical properties of carbon fibers as well as other properties such as ability to endure cross-sectional and twisting distortions, their flexibility, extensibility and their ability to endure compression without fracture © Scientists have estimated the Young’s modulus for carbon nanotubes and have found them to be equal to or greater

than 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 °!®

Carbon nanotubes can be grown by catalytic as well as non-catalytic methods The following methods will be discussed: Arc-discharge, Laser Ablation, Chemical Vapor Deposition (CVD), Gas Phase Catalytic Growth, and the Non-Catalytic Arc Welding Method The Arc-Discharge Method (See Figure 3) evaporates carbon atoms by

a plasma of helium gas, which is ignited by high currents that pass through opposing carbon anodes and cathodes

The arc discharge is dependent on keeping a 1mm gap between the carbon

electrodes Keeping the arc discharge stable and the electrode cool will increase the product quantity of MWNTs Even under the most suitable conditions a considerable amount of graphite is produced in the cathode deposit © This method does have the ability to produce both multi-walled and single-walled carbon nanotubes MWNTs can

crystallinity 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

produced 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

He aes helium arc welder §

nozzle

Figure 5: NASA Goddard Space Flight Center (GSFC) Non-Catalytic Method for

Producing Carbon Nanotubes (CNTs) (12)

a 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

strength polyamide fibers are used for lightweight bullet proof vests, polyethylene plastic

is used for milk bottles, polyurethane plastic is used for an artificial heart and rubber is used for automobile tires Polymers can be defined as large molecules that are made up

of repeating units They are synthesized from monomers which are simple molecules For example, ethylene is a monomer that can be polymerized to form polyethylene (See Figure 6) Polymers can be linear or branched (See Figure 7) Linear polymers consist of

a 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

by their difference in viscosity, light scattering behavior or by their lower tendency to crystallize Polymers can be thermosetting, thermoplastic, or elastomers The

thermosetting plastics decompose when they are heated and therefore these plastics are

not 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

termination In the initiation process, initiators such as free radicals, cations, or anions open the double bond of the monomer, which then becomes active and bonds with other monomers This propagates a rapid chain reaction and the reaction is terminated by another free radical or another polymer In addition reactions the molecular weight may

be difficult to control and undesirable branching products can form The characterization

of polymers can be achieved using a number of analytical instruments To observe the structure and dynamics of polymer chains both in solution and in the solid state, Nuclear Magnetic Resonance (NMR) spectroscopy is most effective Another powerful technique used to characterize polymers is Infrared Spectroscopy (IR) This method is used to investigate the polymer structure and the analysis of functional groups Samples in the

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)

Gel Permeation Chromatography (GPC) or Size Exclusion Chromatography (SEC) is used to determine the molecular weight and molecular weight distribution of polymer

samples 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

polymers such as X-Ray Diffraction, Optical Microscopy, tensile strength measurements and more This research used a number of techniques to properly identify the properties

of the polymers that are produced

CHAPTER 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

13

Figure 9: Image of a NASA Balloon Used to Collect

flight missions consist of a balloon, a parachute, and a payload that is carrying

instruments used to conduct scientific experiments The payload can weigh up to 8,000 pounds The balloons can reach altitudes of 115,000 ft to 130,000 ft and each mission can last anywhere from 8 to 26 days The missions are fairly simple; the balloon is partially filled with helium and launched with the payload suspended below

it Once the balloon begins to rise, the helium expands and fills the balloon until it is fully inflated The balloon will reach a peak altitude and as it drifts across the sky the scientific instruments within the payload begin to collect data When the experiment

is complete a tear is created in the balloon material from a radio command that is sent

to separate the payload from the balloon The balloon is destroyed by the rip created

by the radio command A parachute opens and floats the payload back to the ground The balloons provide a quick and inexpensive method of doing scientific

15

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

nanopolymer blends are synthesized by benchtop level chemistry, polymer extrusion,

and electrospinning techniques to produced homogenous nanocomposites and aligned nanocomposite fibers A comparison of the properties of the polymer matrix to the nanopolymer matrix can be analyzed using various analytical techniques such as Scanning Electron Microscopy (SEM), X-ray Diffraction, Raman Spectroscopy, Thermal Gravimetrical Analysis (TGA), and mechanical testing instrumentation.

CHAPTER 3 POLYMERIZATION Polystyrene Polystyrene is synthesized from the monomer styrene (Figure 10) Styrene can be polymerized by free-radical, cationic, anionic and coordination mechanisms

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

17

applications of polystyrene can be found in Table 1

Polystyrene was chosen for this research to help understand polymer chemistry and because a large body of data about this polymer exists which serves as a ready source

of comparison Synthesized polystyrene was compared with commercially produced polystyrene Synthesized polystyrene was combined with carbon nanotubes and analyzed

by TGA/DTA and SEM The polymerization of polystyrene can be achieved by a number

of polymerization processes such as the free radical polymerization process and

19

Advantages of Disadvantages of Some Applications of

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

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,

sodium dodecyl benzenesulfonate, or dodecylamine hydrochloride), also called detergent, and a water soluble free radical generator (hydroperoxides) The surfactant has two ends

of different solubility: A long hydrocarbon end (the tail) that is soluble in nonpolar

organic compounds and a water soluble end (the head) The surfactant forms micelles which provide the site needed for polymerization

Polystyrene 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

bicarbonate, and distilled water were mixed together for 15 minutes After 15 minutes the inhibitor-free styrene was added to the mixture and placed in a water bath at 80°C and potassium persulfate (K2S2Ox) was then added The solution was allowed to heat for 1 hour and 30 minutes The test tube was taken out of the hot water bath and the product

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 190°C) This polymer can also be prepared by the Ziegler-Natta polymerization of ethylene shown in Figure 12." It has a specific gravity of 0.92 to 0.93 g/cm’ LDPE is branched Polyethylene/carbon nanotube rods were produced in this research by the extrusion process Low density polyethylene beads were donated by Wallops Flight Facility, Code 541 The LDPE 2056G is manufactured by Dow Chemical and has the following properties: melt index — 1 g/10 min; density — 0.917 g/cc; melting point — 250°F (121°C); softening point - 223°C (106°C) The LDPE beads were heated above the

softening point to obtain a melt and the carbon nanotubes were added to the melt The polymer mixture was extruded by pressure into rod shaped specimens

23

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

parameters such as the distance between the capillary and collection screen, the capillary

tilt, 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

properties PMMA polymer beads were used as received and were placed in a 50/50 % mixture of tetrahydrofuran (THF) and DMF The solution was heated until the PMMA appeared to be dissolved The solution was then electrospun into fibers PMMA fibers were produced by electrospinning PMMA with an average molecular weight of 120,000 g/mol was obtained from Sigma-Aldrich Tetrahydrofuran (THF) is used to dissolve the PMMA Dimethylformamide is used to assist in the electrospinning

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