Design and Properties of SWCNT-Polyetherimide
Nanocomposites
Proefschrift
ter verkrijging van de graad van doctor
aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus prof. ir. K.C.A.M. Luyben,
voorzitter van het College voor Promoties,
in het openbaar te verdedigen op dinsdag 28 januari 2013 om 10:00 uur
door
Maruti HEGDE
Master of Science in Chemistry with Nanotechnology,
University of Hull, Hull, United Kingdom.
geboren te Honaver, India.
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Dit proefschrift is goedgekeurd door de promotor: Prof. dr. T. J. Dingemans
Samenstelling promotiecommissie:
Rector Magnificus voorzitter
Prof. dr. T. J. Dingemans Technische Universiteit Delft, promotor
Prof. dr. E. T. Samulski University of North Carolina at Chapel Hill, Verenigde Staten Prof. dr. C. E. Koning Technische Universiteit Eindhoven
Prof. dr. ir. R. Benedictus Technische Universiteit Delft Prof. dr. ir. S. van der Zwaag Technische Universiteit Delft Prof. dr. S. J. Picken Technische Universiteit Delft
Dr. H. van der Werff DSM Dyneema B. V.
Prof dr. F. M. Mulder Technische Universiteit Delft, reservelid
The research carried out in this thesis is funded by NWO Vidi grant, project no. 07560
ISBN: 978-94-6259-042-7 Copyright@2014 by Maruti Hegde
All rights reserved. No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without written permission from the author.
Published by Ipskamp Drukkers
Acknowledgements
i After& having& spent& the& past& five& years& at& Novel& Aerospace& Materials,& it& is& fitting&that&I&start&by&acknowledging&the&people&who&have&made&this&thesis&possible.& Without&their&help,&I&probably&would&still&be&trying&to&finish&the&research&written&in& this&book.&&
I& want& to& thank& my& promoter,& Prof.& dr.& Theo& Dingemans& for& his& wisdom,& support,& excellent& research& guidance& and& commitment& to& the& highest& standards.& I& owe&him&gratitude&for&taking&the&time&to&discuss&ideas&and&results&no&matter&the&time& of&day&and&for&forcing&me&to&learn&to&write&succintly.&He&has&been&a&father&figure&and& someone& I& can& look& up& to.& I& thank& Prof.& dr.& ir.& Sybrand& van& der& Zwaag& for& all& his& support.& His& help& in& understanding& materials& from& the& viewpoint& of& a& material& scientist&has&been&invaluable.&His&doors&have&always&been&open&for&questions&depite& an& endless& stream& of& visitors& and& students.& I& owe& gratitude& to& Prof.& dr.& Stephen& J.& Picken&and&Prof.&dr.&Ed&Samulski&for&their&assistance&in&understanding&complex&ideas& related& to& polymer& physics& and& nanocomposites.& Reading& sections& of& our& papers& after&edits&by&Ed&has&certainly&been&one&of&the&highlights.&I&appreciate&the&assistance& from&dr.&Ugo&Lafont&for&the&excellent&TEM&work&that&was&critically&important&for&this& thesis& and& for& sharing& his& extensive& knowledge& on& XRD& and& spectroscopic& techniques.& I& thank& Prof.& dr.& Michael& Rubinstein& for& his& critical& comments& on& our& work& that& helped& shape& our& papers& and& the& thesis.& I& greatly& appreciate& the& advice& from& Prof.& dr.& Cor.& E.& Koning& and& dr.& Daniel& Vlasveld& on& dispersing& nanotubes& in& solution.&
The&SEM&images&and&XRD&figures&that&appear&in&this&thesis&were&possible&due& to& the& technical& expertise& of& Franz& Oostrum& and& Ben& Norder.& Without& their& help,& measuring& and& analysing& data& would& have& been& that& much& harder.& I& also& thank& Lixing& Xue,& Berthil& Grashof,& Bob& Vogel& and& Johan& Boender& for& helping& with& the& machines& in& the& laboratory& and& Aerospace& hangar.& Thanks& to& Marcel& Bus& and& Piet& Droppert& for& their& help& while& working& at& NanoStructured& Materials.& I& warmly& appreciate& the& help& from& Shanta& Visser& and& Laura& Chant& in& taking& care& of& the& administrative&work&and&for&generally&being&warm&and&friendly&even&if&admonishing& me& for& something& was& Laura’s& daily& morning& ritual.& I& owe& a& lot& to& Arek& Stephen,&
& ! ! ! Acknowledgements! ii Chris&and&Alwin&for&teaching&me&about&organic&synthesis,&polymers,&and&training&me& to& use& the& instruments& in& the& laboratory.& Thanks& to& Michiel& and& Ricardo& for& their& help&and&the&discussions&over&coffee&or&drinks.&Despite&their&persistent&‘stealing’&of& my& glassware,& their& knowledge& and& sense& of& humor& made& fixing& instruments& and& sonication&experiments&seem&easier&and&fun.&Thanks&to&Krishna&for&the&discussions& on& polymer& nanoparticles& and& nanocomposites.& I& thank& Mladen,& Ranjita,& Nora,& Jie,& Zeljka,&Hong&Li,&Mina,&Nijesh,&Jimmy,&Srikanth,&Martino,&Santiago,&Pedro,&Hao&Chen,& Xiaojun,& Wouter,& Jesus,& Wei& Xu,& Mazhar,& Renee,& Marcus,& Christian,& my& officemates& Qingbao& and& Jian& Wei& and& the& rest& of& NovAM& colleagues& for& their& help& and& for& making&the&last&5&years&a&memorable&experience.&Thanks&to&my&housemates&Sarav,& Chai,& Vera,& Kanag& and& my& friends& for& making& life& interesting& outside& of& the& lab.& I& especially& thank& Maria& for& her& unfailing& support& during& the& tough& times& and& for& being&patient&during&the&latter&stages&of&the&thesis.&& I&am&grateful&to&my&relatives&in&India,&their&support&made&it&possible&for&me&to& focus&on&my&research.&Lastly,&I&thank&my&parents&as&none&of&this&would&have&been& possible&if¬&for&the&sacrifices&made&by&them.&& & &
Contents
iii 1"""""""Introduction"""""" " " " " " " " " """"1" 1.1# Carbon#nanotubes#and#nanocomposites## # # # ####2# 1.2# Nanoparticles#and#Nanocomposites# # # # # ####3# 1.3# Route#towards#polymer:CNT#nanocomposites############################################6# # # 1.3.1########Nanotube#dispersions#and#nanocomposite#synthesis# ####6# " " 1.3.2########Ultrasonication#and#solvent:cast#nanocomposite#films### ####7# ## # 1.3.3""""""""Changes#in#polymer#morphology#due#to#CNT#inclusion"""""""""12" " " 1.3.4########Interfacial#polymer# # # # # # ##13# 1.4# High:performance#polyimide:CNT#nanocomposite#### # # ##14# 1.5# Scope#and#outline#of#the#thesis# # # # # # ##16# 1.6## References# # # # # # # # # ##17# # 2""""""In#situ"polymerization:"A"synthetic"route"towards"SWCNT>Polyetherimide" nanocomposite"" " " " " " " " " ""21" 2.1## Introduction## # # # # # # # ##22# 2.2# Experimental# # # # # # # # ##25# 2.3# Results#and#Discussion# # # # # # # ##25# # # 2.3.1# #####Exfoliation#of#SWCNTs#in#NMP#using#ultrasonication.## ##25# # # # 2.3.2########Determining#the#quality#of#exfoliation## # ######### ##30# # # # 2.3.3# !!!!!In$situ#polymerization# # # # # # ##38# # 2.4.# Conclusions# # # # # # # ############################41# # 2.5# References# # # # # # # ############################41#3." SWCNT" Induced" Crystallization" in" an" Amorphous" All>Aromatic" Polyetherimide" " " " " " " " " ""45" 3.1# Introduction## # # # # # # ###############46# 3.2# Experimental# # # # # # # ###############48# 3.3# Results#and#Discussion# # # # # # # ##50# 3.3.1########Molecular#weight#of#the#polyamic#acids# # ############ ##50## 3.3.2# #####Nanocomposite#morphology;#the#melting#transition# ##51# 3.3.3# #####Nanocomposite#morphology;#the#glass#transition# # ##61 # 3.3.4# #####Dynamic#Mechanical#Thermal#Analysis# # ###############63#
# # ! Contents" iv ######## 3.3.5# #####SWCNT#dispersion#quality#in#the#matrix# # # ##66# # # 3.3.6# #####Stress:strain#behavior#of#the#nanocomposites# ###############68# 3.4# Conclusions# # # # # # # # ###############72# 3.5# References# # # # # # # # ###############73# #
4" SWCNT" Induced" Crystallization" in" Amorphous" and" Semi>Crystalline" All> Aromatic"Polyetherimides" " " " " " " ""75" 4.1## Introduction## # # # # # # # ##76# 4.2# Experimental# # # # # # # # ##79# 4.3# Results#and#Discussion# # # # # # # ##80# # # 4.3.1# #####Preparation#of#nanocomposite#films## ### # # ##80# # # # 4.3.2########Thermal#imidization;#Optical#Microscopy# # # ##81## # # # 4.3.3# !!!!!Nanocomposite#morphology;#X:ray#diffraction# # ##83# # # # 4.3.4# #####Transmission#electron#microscopy# # # # ##88# # # # 4.3.5# #####Nanocomposite#morphology;#the#glass#transition# ## ##90# 4.3.6# #####Dynamic#Mechanical#Thermal#Analysis# # ###############92# 4.3.7# #####Stress:strain#behavior#of#the#nanocomposites# ###############94# 4.4# Conclusions# # # # # # # ## # ##99# # 4.5# References# # # # # # # ### # #100#
5"" SWCNT" reinforcement" in" Amorphous" All>aromatic" Polyetherimide" nanocomposites." " " " " " " " " 102" 5.1## Introduction## # # # # # # # #103# 5.2# Experimental# # # # # # # # #104# 5.3# Results#and#Discussion# # # # # # # #104# # # 5.3.1# #####Nanocomposite#synthesis## ### # # # #104# # # # 5.3.2#########Nanocomposite#morphology;#the#glass#transition# # #105# # # # 5.3.3# !!!!!Dynamic#Mechanical#Thermal#Analysis# # # #109# # # # 5.3.4# #####Stress:strain#behavior#of#nanocomposites# # # #112# # 5.4# Conclusions# # # # # # # # # #116# 5.5# References# # # # # # # # # #116#
# #
"" """"""""""""""Contents"
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v
6" " " C60" and" SWCNT" Induced" Crystallization" in" an" Amorphous" All>aromatic"
Polyetherimide" " " " " " " " """""""""""""118" 6.1## Introduction## # # # # # # # 119# 6.2# Experimental# # # # # # # # 120# 6.3# Results#and#Discussion# # # # # # # 120# # # 6.3.1# #####Nanocomposite#synthesis# # # # ## 120# # # # 6.3.2########Thermal#imidization;#Optical#microscopy# # # 122# # # # 6.3.3# #####Nanocomposite#morphology;#the#melt#transition# # 124# # # # 6.3.4# #####Nanocomposite#morphology;#the#glass#transition# # 128# # # # 6.3.5# #####Dynamic#Mechanical#Thermal#Analysis# # # 130######## # # 6.3.6# #####Stress:strain#behavior#of#nanocomposites# # # 132# 6.4# Conclusions# # # # # # # # # 137# 6.5# References# # # # # # # # # 138# " Summary" " " " " " " " " " 139" " Samenvatting"" " " " " " " " " 142" " Appendix-A" " " " " " " """""""""""""""" " 146" """"""" Appendix-B" " " " " " " """""""""""""""" " 149" """"""" Appendix-C" " " " " " " """""""""""""""" " 157" """"""" Appendix-D" " " " " " " """""""""""""""" " 160" " Curriculum"vitae""" " " " " " " " " 163" " List"of"publications"and"presentations" " " " " " 164#
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Introduction
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1.1 Carbon nanotubes and nanocomposites
The$ need$ for$ strong$ and$ lightweight$ materials$ for$ structural$ automotive$ and$ aerospace$ applications$ has$ put$ polymer$ nanocomposites$ in$ the$ spotlight.$ The$ key$ drivers$ for$ the$ use$ of$ polymer$ nanocomposites$ in$ said$ industries$ are$ reduction$ in$ vehicle$weight,$improved$engine$efficiency$(fuel$saving),$reduction$in$CO2$emission$and$
superior$performance$(safety,$increased$comfort$and$improved$vehicle$control).$ The$discovery$of$carbon$nanotubes$(CNT)$of$the$multiAwalled$type$(MWCNT)$by$ Iijima$in$19911$and$subsequent$isolation$of$singleAwalled$carbon$nanotubes$(SWCNT)$
by$two$groups,$Iijima$et!al2!and$Bethune$et!al3$in$1993$has$spurred$the$investigation$of$ these$ carbon$ allotropes$ as$ fillers$ in$ a$ wide$ variety$ of$ polymeric$ matrices.$ The$ properties$alluded$to$nanotubes$that$render$them$with$tremendous$potential$as$filler$ materials$include$their$extremely$small$dimensions$(average$diameter$of$a$SWCNT$~$ 1.2$ nm),$ large$ aspect$ ratios$ (can$ be$ around$ 1000$ depending$ on$ the$ production$ method)4,$ superlative$ strength/toughness$ (Young’s$ modulus$ of$ approx.$ 1$ TPa)5$and$
electronic$ ballistic$ transport$ capabilities$ due$ to$ their$ oneAdimension$ like$ electronic$ structures.6$Over$ the$ years,$ the$ field$ of$ nanotubeApolymer$ nanocomposites$ has$
predominantly$focused$on$achieving$electrical$and$mechanical$improvements$over$the$ neat$ polymer$ matrix.$ $ Because$ the$ theme$ of$ this$ thesis$ is$ SWCNT$ reinforcement$ of$ polymers,$and$polyetherimides$in$particular,$the$rest$of$the$chapter$is$written$in$this$ context.$ One$ of$ the$ first$ articles$ to$ propose$ the$ use$ of$ carbon$ nanotubes$ as$ reinforcement$ fillers,$ due$ to$ their$ large$ aspect$ ratio$ and$ small$ diameters,$ was$ published$in$Nature$in$1992$by$Paul$Calvert.7$Since$the$first$reports$on$CNTApolymer$
nanocomposites$ by$ Ajayan$ et! al8$more$ than$ thirty$ different$ types$ of$ polymeric$ structures$ have$ been$ explored$ as$ matrices$ for$ polymerAbased$ CNT$ nanocomposites.9$
However,$ success$ in$ achieving$ ‘significant’$ improvements$ in$ thermoAmechanical$ performance$over$the$neat$polymer$matrix$has$only$been$minimal.$The$properties$that$ mark$ them$ as$ promising$ reinforcement$ fillers$ also$ make$ nanocomposite$ inherently$ difficult$to$prepare.$$
What$are$the$main$challenges$in$this$field$and$how$has$this$been$addressed$so$ far?$ Relevant$ literature$ will$ be$ reviewed$ in$ the$ following$ paragraphs.$ The$ scope$ and$ outline$of$the$thesis$will$be$discussed$at$the$end$of$this$chapter.$$
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1.2 Nanoparticles and nanocomposites
It$was$the$pioneering$work$by$the$research$group$at$the$Toyota$Central$Research$ and$ Development$ Center$ (Japan)$ on$ NylonA6$ and$ clay$ nanoparticles$ that$ led$ to$ the$ advent$ of$ polymeric$ nanocomposites.10$Using$ very$ small$ amounts$ of$ montmorillonite$
clay$ particles$ (2–5$ wt.%)$ significant$ improvements$ in$ thermal$ and$ mechanical$ properties$ over$ the$ neat$ polymer$ were$ obtained.$ The$ reason$ that$ nanoparticles$ reinforce$ a$ polymer$ more$ efficiently$ as$ compared$ to$ microAparticles$ is$ mostly$ due$ to$ their$ high$ surface$ area$ and$ high$ stiffness.$ $ The$ large$ surface$ area$ to$ volume$ ratio$ of$ these$ nanoparticles$ implies$ the$ presence$ of$ a$ larger$ amount$ of$ interfacial$ polymer$ adhered$to$the$nanoparticles.$This$is$represented$schematically$in$Figure$1.$$
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$ Figure!1.$$The$distribution$of$microA$and$nanoAparticles$at$the$same$vol.%$loading$and$ unit$volume$(1$mm3).$A$and$B$represent$micronAsize$particles$of$Al2O3$and$carbon$fiber.$ C$ and$ D$ represent$ nanoparticles$ of$ graphitic$ nanoplatelets$ and$ carbon$ nanotubes,$ respectively.$ $ The$ presence$ of$ a$ large$ number$ of$ nanoparticles,$ as$ shown$ in$ the$ schematic,$ also$ makes$ obtaining$ good$ dispersions$ challenging.$ (Reproduced$ with$ permission$from$reference$11)$
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The$ advantage$ of$ using$ nanoparticles,$ such$ as$ well$ dispersed$ montmorillonite$ clay,$ over$ traditional$ microAfibers$ such$ as$ glass$ fiber$ for$ obtaining$ composites$ with$ improved$moduli$is$shown$in$Figure$2$as$published$by$Fornes$et!al.12$$
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Figure! 2.$ Reinforcement$ gain$ (Y/Ym)$ plotted$ as$ a$ function$ of$ wt.%$ organically$ modified$montmorillonite$nanoparticles$vs.$short$glass$fiber$(aspect$ratio$=$20)$filler$in$ a$Nylon$6$matrix.$Y$and$Ym$denote$Young’s$modulus$values$of$the$nanocomposite$and$ the$neat$matrix.$(Reproduced$with$permission$from$reference$12)$$$
$
Over$ the$ past$ two$ decades,$ a$ host$ of$ nanoparticles$ based$ on$ carbonaceous$ and$ nonAcarbonaceous$ particles$ has$ been$ explored$ as$ fillers$ in$ polymeric$ matrices$ for$ a$ variety$of$field$applications.13$Carbon$nanotubes$and$carbon$nanofibers$represent$two$
of$ the$ strongest$ nanofillers$ used$ in$ polymeric$ composite.$ Carbon$ nanofibers$ are$ typically$ grown$ using$ chemical$ vapor$ deposition$ technique$ (CVD),$ with$ lengths$ generally$ lower$ than$ 100$ microns$ and$ diameters$ of$ ~100$ nm.14$Their$ structure$ is$
composed$ of$ multiple$ nested$ tubes$ with$ walls$ inclined$ at$ 20o$ with$ respect$ to$ the$
longitudinal$ axis.$ They$ are$ different$ from$ nanotubes$ in$ the$ sense$ that$ CNFs$ are$ not$ composed$of$continuous$tubes.15$$Nanofibers$can$have$strengths$in$the$range$of$2.5$to$
3.5$GPa$and$Young’s$moduli$in$the$range$of$100–1000$GPa.16$
In$ an$ attempt$ to$ compare$ the$ improvements$ in$ mechanical$ properties$ due$ to$ different$ particle$ types,$ we$ have$ compared$ the$ results$ obtained$ using$ a$ common$ thermoplastic$polymer,$polypropylene$(PP).$As$a$guide$for$comparing$the$reinforcement$ efficiencies,$we$have$used$the$yardstick$devised$by$J.$N.$Coleman$et!al.,$namely$dY/dVf$
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5 nanoparticle$ in$ the$ matrix.17$It$ must$ be$ noted$ that$ nanocomposite$ processing$
techniques$ and$ surface$ treatment$ of$ nanoparticles$ can$ strongly$ influence$ the$ nanocomposite$ mechanical$ properties.$ Therefore,$ only$ the$ nanocomposites$ produced$ using$a$similar$approach,$primarily$based$on$an$injection$molding$technique,$have$been$ compared.$The$addition$of$vapor$grown$carbon$nano$fiber$(VGCNF)$having$an$average$ diameter$ of$ 200$ nm$ to$ PP,$ prepared$ using$ ballAmilling$ and$ injection$ molding$ led$ to$ a$ doubling$of$the$tensile$strength$(from$25$to$50$MPa)$at$a$loading$of$8$wt.%.$The$Young’s$ modulus$ (Y)$ quadrupled$ from$ 1.2$ to$ 4.8$ GPa$ at$ 30$ wt.%$ loading,$ which$ constitutes$ a$ dY/dVf$of$~23$GPa.18$$The$ballAmilling$step$was$found$to$be$very$important$in$separating$
the$nanofibers$and$to$allow$polymer$infiltration.$If$this$procedure$wasn’t$followed,$only$ low$ loadings$ and$ low$ improvements$ in$ properties$ could$ be$ obtained.$ Surface$ modification$to$a$‘small$extent’$was$deemed$advantageous$for$obtaining$reinforcement.$ The$ addition$ of$ montmorillonite$ clay$ to$ PP$ using$ melt$ compounding$ and$ injection$ molding$ increased$ the$ modulus$ from$ 1.5$ GPa$ to$ 2.4$ GPa$ at$ 7$ wt.%$ loading.$ But$ no$ improvement$in$yield$strength$could$be$obtained.19$In$terms$of$enhancement$of$stiffness,$
both$ nanofiber$ and$ clay$ display$ similar$ reinforcement$ efficiencies$ of$ ~28$ GPa.$ In$ comparison,$ oxidized$ MWCNT$ nanocomposites$ prepared$ using$ injection$ molding$ resulted$in$a$dY/dVf$of$~150$GPa$$(estimated$since$modulus$values$were$not$published)$
at$0.3$wt.%$loading.20$This$represents$a$significant$increase$in$reinforcement$efficiency$
compared$ to$ clay$ or$ nanofibers.$ A$ comparison$ of$ the$ results$ with$ those$ obtained$ for$ short$ carbon$ fiber$ (CF)$ and$ glass$ fiber$ (GF)$ displaying$ dY/dVf$~65$ GPa$ and~37$ GPa$
respectively$ alludes$ to$ the$ superior$ reinforcement$ efficiencies$ that$ can$ be$ obtained$ using$ nanotubes.21$The$ results$ for$ both$ nanocomposite$ types$ were$ identical$ with$ the$
Young’s$modulus$values$increasing$by$30%$over$the$neat$polymer$at$14$wt.%$loading.$$ While$ comparing$ traditional$ polymeric$ composite$ materials$ to$ polymeric$ nanocomposites$ (PNC)$ one$ has$ to$ realize$ that$ nanocomposites$ not$ just$ offer$ a$ route$ towards$achieving$improvements$in$mechanical$and$electrical$properties.$The$value$of$ adding$ these$ nanoparticles$ lies$ in$ the$ ability$ to$ modify$ or$ add$ to$ the$ resin$ properties$ and$ to$ fineAtune$ the$ final$ application$ without$ sacrificing$ the$ processability$ or$ mechanical$properties$of$the$resin$material.22$$
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1.3 Routes towards polymer-CNT nanocomposites
In$this$section,$the$topics$most$relevant$to$preparing$CNTAbased$nanocomposites$ with$ enhanced$ mechanical$ properties$ will$ be$ discussed.$ In$ their$ review$ article,$ Coleman$et!al.$pointed$out$four$main$system$requirements$that$have$to$be$met$in$order$ to$obtain$effective$reinforcements$in$polymer–nanotube$composites,$namely$nanotube$ aspect$ ratio,$ good$ dispersion$ quality,$ alignment$ and$ a$ strong$ polymer$ nanotube$ interface.17$ For$ sake$ of$ brevity,$ we$ will$ skip$ the$ discussion$ on$ the$ production$ and$synthesis$ of$ nanotubes.$ There$ are$ excellent$ review$ articles$and$ books$ available$ that$
cover$this$topic$in$great$detail.9,$23$The$issue$of$nanotube$alignment$is$also$skipped,$as$
the$ focus$ in$ this$ thesis$ is$ on$ nonAaligned$ nanocomposites.$ Instead,$ we$ will$ focus$ our$ attention$ on$ topics$ relating$ to$ dispersing$ the$ nanotubes,$ the$ synthesis$ of$ nanocomposites$and$CNT–polymer$interactions.$
1.3.1!Nanotube!dispersion!and!nanocomposite!preparation.!
Debundling$and$dispersing$SWCNTs$into$polymer$matrices$is$extremely$difficult$ due$ to$ the$ high$ Van$ der$ Waals$ interaction$ between$ nanotubes$ (500$ eV/µm$ of$ tube$ length24).$The$effective$shear$modulus$of$SWCNT$bundles$was$found$to$be$much$lower$
than$that$of$a$single$SWCNT$(~1$TPa)$due$to$shearAinduced$slippage$of$individual$tubes$ within$the$bundles.25$Therefore$improvements$in$mechanical$properties$are$always$far$
below$theoretically$predicted$values$due$to$inefficient$load$transfer$from$the$matrix$to$ the$SWCNTs.$This$is$a$major$hurdle$that$experimentalists$must$first$overcome$in$order$ to$ obtain$ improvements$ in$ mechanical$ properties$ in$ nanocomposites.$ Table$ 1$ summarizes$the$different$methods$available$to$disperse$nanotubes$in$polymer$matrices.$ $ $ $ $ $$
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! 7 Table! 1.! Various$processing$techniques$for$mechanically$dispersing$CNTs$in$polymer$ matrices$(Table$reproduced$with$permission$from$reference$[11])$$ Technique! Damage!to!CNTs! Suitable!polymer!matrix! Governing!factors! Availability!
Ultrasonication Yes
Soluble polymer, low viscous polymer or oligomer
Power and mode of sonicator, sonication time
Commonly used in lab. Easy operation and cleaning after use
Calendaring No Liquid polymer or
oligomer
Rotation time, distance between adjacent screws
Operation training is required, hard to clean after use
Ball milling Yes Powder (polymer) Milling time, rotation speed, size of balls, ball/CNT ratio
Easy operation, cleaning required after use
Shear Mixing No
Soluble polymer, low viscous polymer or oligomer
Size and shape of the propeller, mixing speed and size
Commonly used in lab, easy operation and cleaning after use
Extrusion No Thermoplastics
Temperature,
configuration and rotation speed of the screw
Large-scale
production, operation training is required, hard to clean after use
As$ presented$ in$ the$ table,$ the$ choice$ of$ the$ method$ often$ depends$ upon$ the$ processing$requirements$of$the$polymer$matrix$and$the$scale$of$the$operation.$The$next$ section$presents$an$overview$of$the$results$obtained$for$solutionAbased$techniques.$
1.3.2!Ultrasonication!and!solventGcast!nanocomposite!films!$$
The$agitation$induced$by$ultrasonic$sound$waves$results$in$the$debundling$of$the$ nanotube$aggregates$in$solution.$Using$the$appropriate$solvent,$CNTs$can$be$debundled$ with$ ultrasonication$ and$ mixed$ with$ a$ soluble$ polymer$ either$ during$ sonication$ or$ after$ sonication$ to$ obtain$ nanocomposites$ after$ evaporation$ of$ the$ solvent.$ A$ host$ of$ polymerACNT$ nanocomposites$ based$ on$ matrices$ such$ as$ polyvinyl$ alcohol$ (PVA),$ polystyrene$(PS),$and$highAdensity$polyethylene$(HDPE)$have$been$produced$over$the$ years.9,17$ Using$ solutionAbased$ fabrication$ techniques,$ MWCNTs$ in$ PVA$ improved$ the$
Introduction
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dY/dVf$ of$ 754$ GPa.17, 26$In$ comparison,$ the$ Young’s$ modulus$ for$ MWCNTAPS$
nanocomposites$increased$from$1.2$GPa$to$1.7GPa$at$1$wt.%$(dY/dVf$=$74$GPa).17,27$$
Over$ the$ past$ few$ years,$ researchers$ have$ identified$ that$ amongst$ organic$ solvents,$the$amideAbased$(aprotic)$solvents$are$perhaps$the$best$for$dispersing$carbon$ nanotubes28$with$NAmethylA2Apyrrolidone$(NMP)$yielding$the$largest$concentration$of$
individual$SWCNTs$at$0.02$mg/ml.29,$30,$31$Also,$solutions$of$SWCNTs$in$NMP$have$been$
shown$to$be$stable$for$at$least$a$period$of$1$week$at$25$oC,$showing$no$signs$of$CNT$
sedimentation.$ By$ employing$ techniques$ such$ as$ UVAVis$ in$ combination$ with$ ultraA centrifugation,$ the$ concentrations$ of$ SWCNTs$ debundled$ in$ solution$ can$ be$ easily$ measured$ and$ quantified.$ For$ analyzing$ debundled$ solutions,$ i.e.$ to$ check$ for$ the$ presence$of$large$populations$of$isolated$SWCNTs,$a$combination$of$techniques$has$to$ be$ employed$ and$ includes$ atomic$ force$ microscopy$ (AFM),$ transmission$ electron$ microscopy$ (TEM)$ and$ spectroscopic$ techniques$ such$ as$ UV/NIR/Vis$ and$ Raman$ spectroscopy. 32 $Studies$ based$ on$ fluorescence$ of$ isolated$ nanotubes$ –the$
photoemission$being$absent$in$bundled$nanotubes$is$also$often$used$as$a$technique$to$ study$ nanotube$ dispersions.33$There$ are$ alternate$ strategies$ towards$ obtaining$ good$
dispersion$ qualities$ in$ instances$ where$ amideAbased$ solvents$ cannot$ be$ readily$ employed.$ Generally,$ surfactants$ such$ as$ sodium$ dodecyl$ sulfate$ (SDS),$ lithium,$ dodecyl$ sulfate$ (LDS),$ sodium$ cholate$ (SC),$ dodecyltrimethylammonium$ bromide$
(DTAB),$ tetradecyl$ trimethyl$ ammonium$ bromide$ (TTAP),$
hexadecyltrimethylammonium$ bromide$ (HTAB)$ and$ Triton™ XA100$ are$ used$ as$ dispersion$ aids$ to$ obtain$ isolated$ tubes$ in$ aqueous$ solutions.34$The$ mechanism$ of$
debundling$in$the$presence$of$these$surfactants$was$proposed$by$Strano$et!al.35$They$
demonstrated$ that$ by$ applying$ ultrasonication$ the$ surfactants$ adsorb$ onto$ the$ dangling$ends$of$the$CNT$bundles$and$prevents$reAaggregation.$Further$application$of$ ultrasonic$ energy$ causes$ the$ adsorbed$ surfactant$ to$ fully$ coat$ the$ sidewalls$ of$ the$ partially$ individualized$ nanotubes$ and$ separate$ the$ nanotube$ from$ the$ bundle.$ In$ a$ comparative$study$on$surfactants$by$Islam$et!al.,$it$was$shown$that$Triton™ XA100$was$ more$effective$than$surfactants$such$as$SDS.$AFM$studies$confirmed$that$nearly$65%$of$ the$nanotubes$at$concentrations$of$20$mg/ml$existed$as$single$tubes.36$
Chapter(1
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!
9 Small$ molecules$ based$ on$ conjugated$ rigid$ aromatic$ cores$ such$ as$ perylene$ functionalized$with$aliphatic$chains$are$capable$of$dispersing$CNTs$by$forming$strong$ πAπ$stacking$interactions.37$Using$fluorescence$spectroscopy,$Debnath$et!al.$studied$the$
interactions$ between$ SWCNTs$ and$ aromatic$ compounds.38$ A$ comparison$ between$
acene$ compounds$ (e.g.$ napthalene)$ and$ linear$ phenyl$ compounds$ (e.g.$ biphenyl)$ having$various$molecular$weights$was$made$with$respect$to$their$interaction$capability$ with$SWCNTs.38$From$the$results$obtained,$it$ was$found$that$lower$molecular$weight$
compounds$ such$ as$ napthalene$ disperse$ SWCNTs$ more$ efficiently$ than$ higher$ molecular$ weight$ aromatic$ compounds$ such$ as$ tetracene.$ The$ acene$ compounds$ performed$better$then$their$phenyl$counterparts$(Figure$3A).$However,$binding$energy$ studies$ indicated$ stronger$ interactions$ with$ SWCNTs$ for$ larger$ molecules$ such$ as$ pA quinquephenyl$compared$to$smaller$molecules$such$as$naphthalene$or$biphenyl.$In$a$ separate$ paper,$ Yoo$ et! al.$ measured$ the$ affinity$ of$ fused$ aromatic$ compounds$ to$ SWCNTs$ using$ a$ HPLC$ column$ packed$ with$ silicaAgel$ coated$ with$ a$ SWCNT$ monolayer.39$The$ results$ matched$ with$ what$ is$ shown$ in$ Figure$ 3B.$ Interestingly,$
linear$ acene$ molecules$ have$ a$ stronger$ affinity$ to$ SWCNTs$ than$ branched$ acene$ molecules$of$comparable$molecular$weight,$for$example:$Triphenylene$<$Tetraphene$<$ Tetracene.$$ In$the$past$decade,$groups$led$by$Nakashima$from$Kyushu$University,$Japan$and$ Hirsch$from$the$University$of$Erlangen,$Germany$have$focused$on$the$design$of$organic$ small$molecules$tailored$for$nonAcovalent$functionalization$of$CNTs$for$a$wide$variety$ of$applications.40,$34$ $ $
Introduction
!
10
$
$
Figure! 3.! $ AA" Dispersion$ limit$ of$ HiPCO$ SWCNT$ as$ a$ function$ of$ molecular$ weight$ of$ aromatic$ hydrocarbon$ compounds" BA" Binding$ energy$ between$ HiPCO$ SWCNT$ and$ aromatic$compounds$as$a$function$of$molecular$weight.$Reproduced$with$permission$ from$reference$[38].$
$ $
Chapter(1
!
! 11 The$stabilization$and$debundling$of$CNTs$can$also$be$achieved$using$polymers$in$ solution.34$Natural$polymers$such$as$DNA$can$help$in$dispersing$and$stabilizing$CNTs$ (the$maximum$reported$SWCNT$concentration$is$0.04$mg/ml)$in$aqueous$solution$by$ mechanisms$ such$ as$ polymer$ wrapping. 41 $In$ synthetic$ polymers$ such$ as$polyvinylpyrrolidinone$(PVP)$and$polystyrene$sulfonate$(PSS)$carrying$pendant$polar$ groups,$CNTs$could$be$dissolved$in$aqueous$solutions$by$polymer$wrapping$and$CNT$ concentrations$ as$ high$ as$ 4$ mg/ml$ could$ be$ obtained.42$The$ authors$ proposed$ a$
thermodynamic$ drive$ to$ eliminate$ the$ hydrophobic$ interface$ between$ the$ tubes$ and$ their$aqueous$surrounding$as$the$reason$for$polymer$wrapping.$The$choice$of$solvent$ is$crucial;$in$alternative$solvents,$such$as$tetrahydrofuran$(THF),$the$phenomenon$of$ polymer$ wrapping$ is$ not$ observed.$ $ The$ favorable$ interaction$ between$ PVP$ and$ MWCNTs$ was$ utilized$ by$ Zhang$ et! al.! in$ preparing$ a$ nanocomposite$ with$ improved$ mechanical$ properties.43$Using$ PVA$ along$ with$ PVP,$ SDS$ and$ 5$ wt.%$ MWCNTs,$ the$
nanocomposite$ film$ prepared$ by$ solution$ casting$ exhibited$ an$ increase$ in$ Young’s$ modulus$from$1.9$GPa$to$4.0$GPa$(dY/dVf$=$~75$GPa)$in$relation$to$the$neat$polymer.$
The$tensile$strength$also$increased$from$83$MPa$to$~150$MPa.$$
In$ rigid$ conjugated$ aromatic$ polymers$ such$ poly(arylene$ ethylene)s$ (PPEs),$ where$ polymer$ wrapping$ is$ not$ possible,$ solubilization$ of$ nanotubes$ occurs$ through$ interactions$ between$ the$ aromatic$ moiety$ and$ tube$ surface$ through$πAπ$ stacking$ interactions.44$Nanocomposite$ fibers$ prepared$ using$ 10$ wt.%$ SWCNTs$ in$ rigidArod$
polymeric$systems$such$as$poly(pAphenyleneA2,6Abenzobisoxazole)$(PBO)$have$shown$ remarkable$ improvements$ in$ properties.$ The$ Young’s$ modulus$ improved$ from$ 138$ GPa$ to$ 167$ GPa$ (dY/dVf$ =$ 550$ GPa)$ and$ the$ strength$ improved$ from$ 2.6$ GPa$ to$ 4.2$
GPa.45$$
The$use$of$surfactants$in$aqueous$solutions$has$obvious$advantages$in$obtaining$ good$dispersions$of$CNTs,$however$in$the$context$of$CNTApolymer$nanocomposites,$the$ presence$ of$ a$ surfactant$ can$ complicate$ polymer$ processing$ and$ minimize$ the$ reinforcement$ efficiency.$ This$ makes$ the$ field$ of$ nonAcovalent$ functionalization$ of$ CNTs$ using$ small$ molecules$ or$ polymers$ interesting$ from$ a$ perspective$ of$ nanocomposite$ preparation.$ An$ exhaustive$ compilation$ of$ the$ work$ done$ on$ nonA covalent$functionalization$is$listed$here$as$reference$[34].$$
Introduction
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12
An$extension$of$the$nanotube$dispersion$in$solution$using$ultrasonication$is$the$ method$ of$ in0situ$ polymerization.$ This$ method$ has$ been$ utilized$ by$ Park$ et! al.$ to$ prepare$polyimideAbased$CNT$nanocomposites,$which$exhibit$a$60%$improvement$in$ storage$modulus$over$the$neat$unAfilled$polymer.46$This$method$involves$sonicating$a$
mixture$of$the$nanotube$in$an$amideAbased$solvent$(such$as$Dimethylacetamide)$in$the$ presence$ of$ the$ diamine$ monomer$ and$ subsequently$ polymerizing$ the$ mixture$ by$ addition$ of$ the$ dianhydride$ monomer.$ Such$ methods$ have$ also$ been$ utilized$ in$ the$ synthesis$ of$ poly(methyl$ methacrylate)$ (PMMA)$ based$ nanocomposites$ using$ MWCNTs.$This$resulted$in$an$improvement$of$the$Young’s$modulus$by$1$GPa$–$from$1.5$ GPa$to$2.5$GPa$at$1$wt.%$MWCNT$(dY/dVf$=$175$GPa).47$
An$ alternative$ route$ to$ debundling$ the$ nanotube$ aggregates$ that$ has$ attracted$ attention$ is$ chemical$ functionalization$ of$ the$ nanotube$ surface.$ The$ SP2$ surface$ of$ nanotubes$is$modified$by$covalently$linking$reactive$functionalities,$which$in$turn$can$ react$with$polymer$endAgroups$and$so$form$a$covalent$link$between$the$polymer$and$ nanotube.11$The$mechanical$properties$of$PP$nanocomposites$prepared$using$MWCNTs$ grafted$with$PP$were$significantly$better$with$respect$to$strength,$storage$modulus$and$ toughness$as$compared$to$nanocomposites$prepared$with$pristine$MWCNTs.48$In$this$ direct$comparison,$functionalized$MWCNTs$had$reinforcement$efficiencies$(dY/dVf)$of$
65$ GPa$ compared$ to$ 29$ GPa$ for$ pristine$ MWCNTs.$ Interestingly,$ the$ tensile$ strength$ using$ functionalized$ MWCNTs$ doubled$ over$ the$ neat$ polymer$ while$ nanocomposites$ with$ pristine$ MWCNTs$ were$ comparable$ to$ the$ neat$ polymer$ matrix.$ $ In$ a$ separate$ report$by$Blake$et!al.,$similar$results$were$obtained$with$MWCNTs$grafted$with$ClAPP$ in$a$PP$matrix.$The$reinforcement$efficiency$obtained$was$72$GPa,$with$an$increase$in$ tensile$strength$from$12$MPa$to$49$MPa.49$$$$$
1.3.3!Changes!in!polymer!morphology!due!to!CNT!inclusion!!!
The$ incorporation$ of$ nanotubes$ in$ thermoplastics$ with$ good$ CNT$ wetting$ capabilities$ can$ have$ consequences$ with$ respect$ to$ the$ polymer$ morphology.$ The$ incorporation$ of$ nanotubes$ into$ conjugated$ polymers$ such$ as$ poly(mA phenylenevinyleneAcoA2,5AdioctyloxyApAphenylenevinylene)$ (PmPV)$ nucleated$ crystalline$ growth$ in$ the$ polymer,$ with$ crystalline$ polymer$ growing$ coaxial$ to$ the$ nanotube$axis$and$dendritic$crystal$growth$from$defect$sites$on$the$nanotube.50$Such$
Chapter(1
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!
13 nanotubeAnucleated$crystallinity$has$been$observed$in$semiAcrystalline$polymers$such$ polyethylene$(PE)51$and$polycarbonate$(PC)52.$In$polymers$such$as$PE,$the$nanotubes$
were$found$to$form$shishAkebabAlike$structures$around$the$nanotubes$with$the$crystal$ growth$ transverse$ to$ the$ nanotube$ long$ axis.51$ For$ polypropylene$ (PP),$ the$ onset$ of$
crystallization$was$found$to$decrease$by$~5$oC$and$crystallinity$was$dominated$by$the$
β −form$at$the$expense$of$the$α−form$iPP.53$$In$polyvinyl$alcohol$(PVA),$MWCNTs$were$
found$to$increase$crystallinity$linearly$from$5%$in$the$neat$polymer$to$~17%$at$0.6$vol.%$ nanofiller$ loading.$54$From$ analysis$ of$ the$ fracture$ edges$ using$ scanning$ electron$
microscopy$ (SEM),$ the$ thickness$ of$ crystalline$ coating$ around$ a$ nanotube$ having$ radius$of$~7.5$nm$was$measured$to$be$~21$nm.$$
Incorporation$ of$ nanotubes$ in$ amorphous$ polymers$ can$ have$ an$ effect$ on$ the$ polymer$ glass$ transition$ temperature$ (Tg).55$In$ polymers$ such$ as$ poly(9Avinyl$
carbazole)$ (PVK),$ the$ incorporation$ of$ CNTs$ results$ in$ the$ appearance$ of$ two$ glass$ transition$temperatures.56$In$addition$to$the$‘neat’$polymer$Tg,$a$certain$fraction$of$the$
amorphous$chains$will$be$in$contact$with$the$nanotube$surface,$which$results$in$slower$ relaxation$ dynamics$ and$ hence$ a$ higher$ Tg$ is$ observed.55.$ In$ general$ it$ can$be$ stated$
that$ in$ case$ of$ good$ wetting$ of$ the$ nanotubes$ by$ the$ polymer$ matrix,$ the$ Tg$ can$ be$ expected$to$increase$and$broaden.57$How$much$the$Tg$will$increase$will$depend$upon$ the$strength$of$the$interaction$and$the$number$of$polymer$chains$in$direct$contact$with$ the$CNTs.$$ 1.3.4!Interfacial!polymer! A$crucial$requirement$for$any$fiberAreinforced$composite$material$is$the$transfer$ of$external$stresses$from$the$matrix$to$the$much$stronger$fiber.$The$fracture$surface$of$ nanocomposite$ sample$ should$ not$ display$ large$ numbers$ of$ clean$ nanotubes,$ as$ this$ would$be$indicative$of$a$weak$nanotubeApolymer$interface.4$As$Coleman$explains$in$his$
review$ article$ (reference$ [17]),$ upon$ the$ application$ of$ an$ external$ stress$ on$ a$ nanocomposite$ film,$ the$ matrix$ undergoes$ a$ larger$ strain$ compared$ to$ the$ nanotube.$ This$ creates$ a$ shear$ stress$ field$ radiating$ outward$ from$ the$ nanotube$ to$ the$ matrix.$ This$interfacial$shear$stress$(IFSS)$controls$the$efficiency$of$the$stress$transfer$from$the$ matrix$to$the$nanotube$and$is$ultimately$responsible$for$the$mechanical$properties$of$ the$ final$ composite.58$The$ inclusion$ of$ nanotubes$ into$ matrices$ with$ good$ wetting$
Introduction
!
14
characteristics$can$be$expected$to$display$strong$binding$energies.$However,$a$study$by$ Yao$ et! al.$ using$ force$ field$ molecular$ mechanics$ suggests$ that$ binding$ energies$ play$ only$a$minor$role$in$determining$the$strength$of$the$interface.$59$It$is$the$strength$of$the$
polymer$ at$ the$ interphase$ that$ ultimately$ governs$ the$ mechanical$ properties$ of$ the$ final$nanocomposite.$It$appears$that$the$presence$of$a$highly$ordered$polymer$around$ the$CNTs$results$in$large$reinforcement$efficiencies$due$to$the$intrinsic$stiffness$of$the$ ordered$ polymer.$ Cadek$ et! al.$ were$ able$ to$ prove$ this! by$ comparing$ the$ mechanical$ results$ obtained$ for$ nanocomposites$ based$ on$ semiAcrystalline$ PVA,$ where$ CNT$ nucleates$ crystal$ growth$ in$ the$ vicinity$ of$ the$ nanotube,$ to$ amorphous$ PVK$ nanocomposites.56$ The$ rate$ of$ modulus$ (from$ nanoAhardness$ tests)$ increase$ in$ PVA$
composites$ was$ found$ to$ be$ more$ than$ twice$ that$ of$ PVK$ composites.$If$ the$ polymer$ matrix$is$unable$to$crystallize,$covalent$functionalization$of$the$nanotubes$may$help$in$ strengthening$the$interphase$leading$to$better$mechanical$properties.60$!
1.4 High-performance polyimide-CNT
nanocomposites
HighAperformance$ polymers$ (HPPs)$ are$ typically$ composed$ of$ all$ SP2Abased$ rigid$ aromatic$ and$ heterocyclic$ monomers.$ For$ this$ reason,$ HPPs$ exhibit$ high$ decomposition$ temperatures$ in$ excess$ of$ 450$oC,$ high$ glassAtransition$ temperatures$
(Tg$$>200$oC)$and$outstanding$mechanical$properties$(E’$~3–4$GPa$and$tensile$strength$
of$70–90$MPa).61$WellAknown$subsets$of$this$class$of$polymers$are$polyimides$(PIs)$and$
polyetherimides$ (PEIs).$ Amongst$ highAperformance$ polymers$ as$ matrices$ for$ CNTA based$ nanocomposites,$ polyimides$ have$ probably$ received$ the$ most$ attention$ due$ to$ their$ease$of$processing$via$solutionAbased$processing$techniques.$Most$papers$report$ the$ synthesis$ of$ PEIAbased$ nanocomposites$ by$ mixing$ a$ solution$ of$ exfoliated$ nanotubes$in$an$aprotic$solvent,$such$as$N,!NAdimethylacetamide$(DMAc)$or$NAmethylA 2Apyrrolidone$(NMP),$with$the$intermediate$polyamic$acid$solution.62$By$simply$casting$
a$ film$ on$ a$ glass$ substrate,$ evaporation$ of$ the$ solvent$ and$ subsequent$ thermal$ imidization,$a$polyimideACNT$nanocomposite$film$can$be$obtained.$Using$this$method,$ the$Young’s$modulus$increased$from$6.4$GPa$to$9.3$GPa$at$3.3$vol.%$MWCNT$(dY/dVf=$
93$GPa)$and$the$tensile$strength$increased$by$$~60$MPa$$(strength$for$the$neat$polymer$ was$ 150$ MPa)$ in$ a$ polyimide$ synthesized$ from$ 3,3’,4,4’Abiphenyltetracarboxylic$ acid$
Chapter(1
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!
15 (BPDA)$ and$ 3,3’AdihydroxyA4,4’Adiaminobiphenyl$ (HAB)$ (scheme$ 1)63$.$ Park$ et! al.$
reported$ a$ 60%$ increase$ in$ the$ storage$ modulus$ (from$ 1.0$ GPa$ to$ 1.6$ GPa)$ of$ the$ polyimide$ nanocomposite$ using$ APBA6FDA$ matrix$ and$ 1$ vol.%$ SWCNTs$ prepared$ using$an$in0situ$polymerization$technique.46$However,$most$reports$on$polyimideACNT$
nanocomposites$ focus$ on$ using$ a$ commercially$ important$ polyimide$ based$ on$ pyromellitic$dianhydride$(PMDA)$and$4,4Aoxydianiline$(ODA)$–commercially$known$as$ KaptonTM.$Neat$PMDAAODA$exhibits$a$Tg$of$420$oC$and$an$elastic$modulus$in$the$range$
of$0.9A1.0$GPa.64$The$elastic$moduli$for$the$corresponding$nanocomposite$films$range$
from$ ~1.0$ GPa$ at$ 0.3$ wt.%$ MWCNTs$65$and$ 1.0–1.2$ GPa$ at$ 5$ wt.%$ MWCNTs66,$ which$
represent$extremely$small$improvements$in$elastic$modulus$at$best.$ Scheme!1.$Structures$of$polyimides$used$as$matrices$for$CNT$nanocomposites$ $ O O N O O O N N O O O O n Semi-crystalline, PMDA-ODA, Tg ~ 420 oC, Tm > Td N N O O O O OH HO HAB-BPDA Amorphous, APB-6FDA, Tg ~ 199 oC N O O F3C F3C
Introduction
!
16
1.5 Scope and outline of the thesis
The$focus$of$this$thesis$is$on$the$design$of$polyetherimide$(PEI)$nanocomposites$ using$oneAdimensional$(1AD)$singleAwalled$carbon$nanotubes$(SWCNTs)$and$whether$ it$is$possible$to$significantly$improve$the$thermoAmechanical$properties$of$this$class$of$ highAperformance$polymers.$A$suitable$method$towards$dispersing$nonAfunctionalized$ SWCNTs$ in$ a$ polyetherimide$ matrix$ has$ been$ developed$ and$ we$ investigated$ the$ effects$on$how$the$SWCNTs$affect$the$final$polymer$morphology$and$the$thermalA$and$ mechanical$properties.$The$results$are$contrasted$with$a$series$based$on$PEI–C60$(0AD)$ nanocomposites.$All$SWCNT–$and$C60–PEI$nanocomposites$are$based$on$nonAcovalent$ interactions.$$$
In$ Chapter! 2! we$ will$ introduce$ the$ materials$ and$ the$ method$ used$ for$ dispersing$nanotubes$in$NAMethylA2Apyrrolidone$(NMP)$solvent$using$sonication$in$the$ presence$of$a$monomer.$The$methods$used$to$disperse$and$characterize$exfoliation$of$ SWCNT$ in$ solution$ are$ described.$ The$ process$ of$ in0situ$ polymerization$ to$ produce$ polyetherimideASWCNT$films$is$presented.$
The$ results$ obtained$ for$ nanocomposites$ using$ two$ allAaromatic$ amorphous$ ODPAAP3$and$aBPDAAP3$polyetherimides$are$presented$in$Chapter! 3.$In$this$chapter$ we$will$discuss$the$effect$of$molecular$geometry$on$the$final$nanocomposite$properties.$ The$thermal$and$thermoAmechanical$properties$obtained$for$the$nanocomposite$films$ are$ compared.$ The$ results$ obtained$ from$ optical$ microscopy,$ XRD$ analysis,$ TEM$ imaging$ and$ mechanical$ properties$ from$ stressAstrain$ behavior$ are$ analyzed$ and$ compared$for$the$two$polymeric$matrices.$$
In! Chapter! 4,! the$ results$ obtained$ for$ nanocomposites$ based$ on$ amorphous$ ODPAAP3$ matrix$ are$ compared$ to$ an$ allAaromatic$ semiAcrystalline$ BPDAAP3$ polyetherimide$ matrix.$ The$ effect$ of$ matrix$ morphology$ on$ the$ final$ nanocomposite$ properties$will$be$discussed.!The$thermal$and$thermoAmechanical$properties$obtained$ for$ the$ nanocomposite$ films$ are$ compared.$ The$ results$ obtained$ from$ optical$ microscopy,$XRD$analysis,$TEM$imaging$and$mechanical$properties$from$stressAstrain$ behavior$are$analyzed$and$compared$for$the$two$polymeric$matrices.$$
Chapter(1
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!
17 The$ results$ obtained$ for$ semiAcrystalline$ ODPAAP3$ SWCNT$ and$ BPDAAP3$ SWCNT$nanocomposites$are$compared$to$amorphous$ODPAAP3$SWCNT$and$BPDAAP3$ SWCNT$in$Chapter!5.$The$procedure$used$to$obtain$amorphous$nanocomposite$films$is$ presented.$ The$ resulting$ changes$ in$ thermal,$ thermoAmechanical$ and$ mechanical$ properties$from$stressAstrain$tests$are$discussed.$The$reinforcement$efficiencies$of$the$ amorphous$ nanocomposites$ are$ compared$ to$ the$ corresponding$ semiAcrystalline$ nanocomposite$films.$$
$In$Chapter! 6,$we$will$compare$results$obtained$for$ODPAAP3$nanocomposites$ containing$ 1AD$ SWCNTs$ and$ 0AD$ fullerene$ C60.$ The$ morphological$ changes$ in$ amorphous$ ODPAAP3$ matrix$ due$ to$ inclusion$ of$ C60$ is$ presented.$ The$ thermal$ and$ thermoAmechanical$properties$of$the$two$series$of$nanocomposite$films$are$compared.$ A$ comparison$ of$ the$ mechanical$ properties$ for$ the$ two$ nanocomposites$ series$ as$ a$ function$of$crystalline$content$is$presented$in$the$chapter.$
1.6 References
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13 Ryan, K. P. Phd Thesis, Trinity College, Dublin, 2005, p3.
Introduction
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15 Kang, I.; Heung, Y. Y.; Kim, J. H.; Lee, W. H.; Gollapudi, R.; Subramaniam, S.; Narasimhadevara, S.; Hurd, D.; Kirikera, G. R.; Shanov, V.; Schulz, M. J.; Shi, D.; Boerio, J.; Mall, S.; Ruggles-Wren, M.
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16 Tibbetts G. C. in Carbon Fibers, Filaments and Composites, Figueiredo, J. L.; Bernardo, C. A.; Baker, R. T. K.; Huttinger, K. J., Kluwer Academic Publishers, Dordrecht, 1990. p73.
17 Coleman, J. N.; Khan, U.; Blau, W. J.; Gun’ko, Y. K. Carbon , 2006, 44, 1624. 18 Tibbetts, G. C.; McHugh, J. J. J. Mater. Res. 1999, 14, 2871.
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20 Xiao, Y.; Zhang, X.; Cao, W.; Wang, K.; Tan, H.; Zhang, Q.; Du, R.; Fu, Q. J Appl. Poly. Sci., 2007, 104, 1880.
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23 Harris, P. J. F. Carbon nanotube and related structures: New materials for the Twenty-first century, Cambridge University Press, 2001.
24 Gojny, F.H.; Wichmann, M.H.G.; Köpke, U.; Fiedler B.; Schulte, K. Compos. Sci. Technol., 2004, 64, 2363.
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26 Coleman, J. N.; Cadek, M.; Blake, R.; Nicolasi, V.; Ryan, K. P.; Belton, C.; Fonseca, A.; Janos, B.; Gun’ko, Y. K.; Blau, W. J. Adv Func. Mater., 2004, 14, 791.
27 Qian, D.; Dickey, E. C.; Andrews, R.; Rantell, T. Appl. Phys. Lett., 2000, 76, 2868.
28 K. D. Ausman, R. Piner, O.Lourie, R.S. Ruoff and M. Korobov, J. Phys. Chem. B, 2000, 104, 8911. 29 Furtado, C. A.; Kim, U. J.; Gutierrez, H. R.; Pan, L.; Dickey, E. C.; Eklund, P. C. J. Am. Chem. Soc. 2004,
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30 Giordani, S.; Bergin, S. D.; Nicolosi, V.; Lebedkin, S.; Kappes, M. M.; Blau, W. J.; Coleman, J. N. J.
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31 Bahr, J. L..;Mickelson, E. T.;Bronikowski, M. J.;Smalley, R. E.; Tour, J. M. Chem. Commun., 2001, 2,193. 32 Wang, H. Curr. Op. Coll. & Inter. Sci. 2009, 14, 364.
33 O’Connell, M. J.; Bachilo, S. M.;Huffman, C. B.; Moore, V. C.; Strano, M. S.; Haroz, E. H.; Science, 2002, 297, !593.
Chapter(1
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Introduction
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54 Coleman, J. N.; Cadek, M.; Blake, R.; Nicolasi, R.; Ryan, K. P.; Belton, C.; Fonseca, A.; Nagy, J. B.; Gun’ko, Y. K.; Blau, W. J. Adv. Funct. Mater., 2004, 14, 791.
55 Oh, H.; Green, P. F. Nat. Mater. 2009, 8, 139.
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Chapter(2
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CHAPTER
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In-situ polymerization: A synthetic route
towards SWCNT–Polyetherimide nanocomposites
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In# this# chapter,# we# will# describe# a# method# towards# preparing# well5dispersed# SWCNT# solutions# in# N5methyl525pyrollidinone# (NMP)# using# high# intensity# probe# sonication.# The# process# of# exfoliation# was# followed# by# UV5Vis# spectroscopy# and# the# concentration# of# SWCNTs# in# solution# could# be# quantified# by# measuring# the# absorbance# of# a# known# concentration# of# nanotubes# and# subsequently# calculating# the# extinction# coefficient# by# Beer5Lambert’s# law.# # Raman# spectroscopy# was# used# to# check# the# structural# integrity# of# nanotubes#after#exfoliation#using#a#probe#ultrasonicator.#By#probe#sonicating#SWCNTs#in# solution#and#in#the#presence#of#an#all5aromatic#arylether5based#diamine#monomer#it#was# possible#to#debundle#the#nanotubes#effectively#and#prepare#well#dispersed#polyamic#acid# solutions,# which# were# subsequently# cast# into# films# and# thermally# imidized# to# form# free# standing#SWCNT5PEI#nanocomposite#films.##
!
In$situ!polymerization:!A!synthetic!route!towards!SWCNT$!PEI!nanocomposites!!
(
2.1 Introduction
A# critical# step# towards# the# production# of# carbon# nanotube# (CNT)–polymer# nanocomposites# is# the# dispersion# of# CNT# bundles# into# individual# nanotubes.1#The#
most# common# production# methods# of# single# wall# carbon# nanotubes# (SWCNTs)# include# chemical# vapor# deposition# (CVD)2,# carbon# arc# discharge3#and# the# high#
pressure# carbon# monoxide# process4#(HiPCO).# All# methods# produce# nanotube#
bundles#that#exist#in#order#to#minimize#the#surface#energy#of#the#individual#tubes.5#
But# the# high# Van# der# Waals# interaction,# 500# eV/µm# of# tube# length,# between# nanotubes#primarily#arises#from#the#high#non5reactive#surface#area#and#makes#the# inclusion# of# these# nano5fillers# in# polymer# matrices# difficult.6#Aggregation# of#
nanotubes#is#perhaps#the#biggest#problem#when#it#comes#to#preparing#polymer5CNT# nanocomposites# with# enhanced# properties.7#Nanotube# aggregation# prevents# the#
formation# of# uniform# products# and# prevents# efficient# transfer# of# mechanical# loads# from#the#polymer#matrix#to#the#CNTs.8#
In# the# past# decade,# a# large# number# of# methods# for# dispersing# nanotubes# in# fluids#have#been#investigated#and#have#met#with#varying#degrees#of#success.1,#5,#9,#10#
Solution5based# processing# of# nanocomposites# has# emerged# as# the# most# practical# method#due#to#the#inherent#simplicity#of#this#technique.#Dispersion#aids#such#as#the# use#of#surfactants11#or#functionalization#of#the#tube#surface9#with#the#aim#to#modify#
the#surface#energy#and#effectively#reduce#or#eliminate#agglomeration#are#routinely# employed# in# nanocomposite# synthesis.12#Although# ultrasonication# is# commonly#
employed#to#assist#dispersion,#there#is#always#the#danger#of#destroying#nanotubes# by#applying#too#much#energy.13#While#it#is#beyond#the#scope#of#this#thesis#to#dwell#
on#all#the#mechanical#methods#available#to#researchers,#it#is#important#to#note#that# the# polymeric# matrix# dictates# the# method# of# exfoliation.# Examples# include# processing# of# nanotube# composites# using# polymeric# resins# such# as# PP14,# PE15#or#
epoxy16.#By#exploring#combinations#of#polymer#processing#and#nanotube#dispersion#
techniques,# researchers# in# academia# and# industry# attempted# to# accommodate# nanotube# exfoliation# into# the# flowchart# used# for# polymer# processing.# But# researchers# soon# found# out,# exfoliation# of# carbon# nanotubes# in# a# fluid# is# anything# but#trivial.#
Chapter(2
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23 In#this#thesis#several#all5aromatic#polyetherimides#(PEIs)#were#selected#as#the# matrix# for# preparing# CNT5based# nanocomposites# because# PEIs# are# among# a# high5 performance#class#of#polymers#with#reported#affinity#towards#CNTs.17#Moreover,#the#
solution5based#synthetic#route#used#for#PEIs#can#be#easily#modified#to#prepared#PEI5 CNT# nanocomposites.# Improving# their# thermo5mechanical# performance# is# of# interest# as# PEIs# are# routinely# used# in# aerospace# applications# where# high# strength# over#a#wide#temperature#range#can#be#of#critical#importance.#In#this#thesis#we#will# describe# a# modified# in%situ# polymerization# method# for# the# preparation# of# SWCNT# nanocomposites# using# polyetherimides# as# the# polymeric# matrix.# The# method# was# inspired#by#a#similar#method#previously#reported#by#Park#et!al.18#In#this#chapter#we#
will#discuss#all#aspects#of#the#synthetic#process#to#produce#PEI–CNT#nanocomposite# films.# An# all5aromatic# polyetherimide# based# on# 3,3’,4,4’5biphenyl# dianhydride# (sBPDA)# and# 1,45bis[45(45aminophenoxy)phenoxy]# benzene# (P3)19## is# used# as# a#
representative#example#for#all#SWCNT5PEI#nanocomposites#presented#in#this#thesis.# A# traditional# two5step# process# is# employed# for# the# synthesis# of# most# polyetherimides# including# the# neat# BPDA5P3# polymer.20#The# first# step# involves#
preparing#the#intermediate#polyamic#acid#by#adding#a#dianhydride#to#a#solution#of# the# diamine# monomer# in# an# aprotic# solvent# such# as# N5methyl525pyrrolidinone# (NMP).#The#second#step#involves#doctor#blading#the#intermediate#polyamic#acid#on#a# glass# plate# followed# by# evaporation# of# the# solvent# and# subsequent# thermal# cyclo5 dehydration#of#the#intermediate#polyamic#acid#to#form#the#final#polyetherimide.#The#
in%situ# polymerization# of# a# BPDA5P3# SWCNT# nanocomposite# is# summarized# in#
Scheme# 1.# # The# methods# and# parameters# used# for# exfoliating# nanotubes,# characterizing#nanotubes#in#solution#and#the#in%situ#polymerization#process#will#be# discussed#in#this#chapter.# # # # !
!
In$situ!polymerization:!A!synthetic!route!towards!SWCNT$!PEI!nanocomposites!!
(
Scheme! 1.# Preparation# of# BPDA5P3# SWCNT# nanocomposites# using# an# in%situ# polymerization#method# #
O O O O H2N NH2 O O O O N H H N
1. Disperse carbon nanotubes by probe sonication in NMP
Bath sonication for 3 h.
Polymerization under Ar / 24 h. at r.t. (BPDA) bundled SWCNTs debundled SWCNTs (P3) (BPDA-P3)
2. polyamic acid intermediate with dispersed SWCNTs
3. BPDA-P3 SWCNT nanocomposite film
Film casting (target thickness: ~25 µm) Vacuum drying at 60 oC / 1.5 h. Thermal imidization (1 h./100 oC, 1 h./ 200 oC and 1 h./ 300 oC) SWCNTs O O O O N O O N O O n O O O O O O O HOOC O n COOH
Chapter(2
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25
2.2 Experimental
Materials:! SWCNTs# of# the# HiPCO# type# with# catalytic# content# <15%# were# purchased#from#Unidym#Inc.,#USA.#According#to#the#manufacturer,#the#diameter#was# between#0.851.2#nm#and#lengths#ranged#between#10051000#nm.#99%#pure#3,3’,4,4’5 biphenyl# dianhydride# (BPDA)# was# purchased# from# Marshallton# Laboratories# Inc.# while# 1,45Bis[45(45aminophenoxy)phenoxy]# benzene# (P3)# was# synthesized# according# to# a# previously# reported# method.18# Extra# dry# N5methyl525pyrrolidone#
(NMP)#(H2O#content#<#0.01%)#was#purchased#from#Acros#Organics.!
Equipment:!The#probe#sonicator#used#was#a#Cole5Parmer#Homogenizer#with# amplitude#control,#pulse#modes,#frequency#of#20#kHz#and#maximum#power#output#of# 750# Watt.# A# high# gain# extender# probe# with# face# diameter# of# 19# mm# was# used# for# sonicating# samples.! The# bath# ultrasonicator# used# was# a# Cole5Parmer# ultrasonic# cleaner# with# an# operating# frequency# of# 40# kHz# and# bath# volume# of# 1.5# l.# Ultracentrifugation#was#performed#with#an#Eppendorf#microcentrifuge#M5430#using# vials# of# 1# ml# volume.# UV5Vis# was# performed# using# a# Perkin# Elmer# Lambda# 35# spectrophotometer# with# quartz# cuvettes# having# path# length# (l)# of# 1# mm.# Raman# measurements# were# performed# using# a# spectrometer# equipped# with# a# laser# excitation#at#660#nm.#GPC#measurements#on#the#polyamic#acid#intermediates#were# performed# using# a# Shimadzu# GPU# DGU520A3,# equipped# with# a# Shodex# LF5801# column#and#refractive#index#detector.#NMP#solvent#containing#5#mM#LiBr#was#used# as#the#eluent#at#a#flow#rate#of#0.5#ml/min#and#at#a#temperature#of#60#oC.#The#machine#
was# calibrated# using# polystyrene# standards.# All# polyamic# acid# intermediates# were# filtered#through#a#0.45#µm#filter#prior#to#a#GPC#run.####!
2.3 Results and discussion
!2.3.1 Exfoliation of SWCNTs in NMP using ultrasonication
Dispersion! by! ultrasonication:! One# of# the# most# commonly# used# methods# for# dispersing# SWCNTs# in# a# solvent# is# ultrasonication.10# Ultrasound# induces#cavitation# or# voids# in# a# fluid# by# separating# the# fluid# molecules# more# than# the# distance# required# to# keep# the# liquid# intact.21#This# occurs# during# the# negative#
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In$situ!polymerization:!A!synthetic!route!towards!SWCNT$!PEI!nanocomposites!!
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pressure#cycle#of#the#travelling#sound#(ultrasound)#wave.#These#cavitation#induced# bubbles# coalesce# and# collapse# violently# releasing# a# large# amount# of# mechanical# energy,# which# is# in# turn# absorbed# by# neighboring# carbon# nanotube# aggregates# to# overcome#attractive#forces#and#form#smaller#bundles.22#Pressures#from#the#collapse#
can# rise# up# to# a# 1000# atmospheres,# while# temperatures# can# rise# up# to# several# thousand#degrees#centigrade.23!!
Factors# that# affect# ultrasonication# are# amplitude# or# intensity# of# sonication,# frequency,#solvent,#pressure#and#temperature.24#!
• The#amplitude#of#vibration#of#an#ultrasonic#device#is#directly#related#to# the#intensity#of#the#ultrasonic#energy.#While#increasing#the#amplitude#increases#the# amount# of# mechanical# energy# released# during# the# sonication# process,# cavitation# occurs#after#a#minimum#intensity#threshold.#It#is#important#to#note#that#the#power# output# of# the# sonicator# is# directly# proportional# to# the# resistance# of# the# probe’s# movement.# For# a# given# amplitude# setting,# the# power# output# will# change# with# a# change# in# viscosity# of# the# solution,# depth# of# probe# immersion,# probe# diameter# or# pressure#settings.25##
• As# sound# waves# travel# through# a# medium# causing# rarefaction# and# compression# cycles,# the# shorter# the# difference# in# time# i.e.# larger# the# frequency,# between#the#two#parts#of#the#cycle,#the#harder#it#becomes#to#induce#cavitation#in#the# liquid.#Generally,#equipment#for#ultrasonication#operates#at#frequencies#in#the#order# of#kHz.21### • Solvent#also#plays#a#large#role#in#enabling#efficient#ultrasonication#effects.# As#a#rule#of#thumb,#high#solvent#viscosity#and#high#surface#tension#inhibits#cavitation# due#to#presence#of#stronger#cohesive#forces#in#the#liquid.21,#24##
• The# collapse# of# cavitation# bubbles# during# ultrasonication# releases# energy# in# the# form# of# heat# and# thereby# increasing# the# temperature# of# the# liquid.# Depending#on#the#vapor#pressure#of#the#liquid#medium,#vapors#can#fill#these#voids# and# reduce# the# impact# of# their# collapse.# Conversely,# higher# temperatures# help# weaken# cohesive# forces# such# as# Van# der# Waals# forces.# Generally,# the# operating# temperature#is#dictated#by#the#solvent#system#being#used.24#
Chapter(2
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27 • Application# of# external# pressure# increases# the# amount# of# energy# required#to#break#intermolecular#forces#in#the#solvent,#but#also#leads#to#an#increase# in# intensity# during# cavitational# collapse.# By# bubbling# gas# through# the# liquid# the# effectiveness# of# ultrasonication# can# be# increased# as# monoatomic# gases# act# as# nucleating#sites#for#cavitation.21#
The#most#common#sources#of#ultrasound#in#a#laboratory#environment#are#the# bath# ultrasonicator# and# probe# ultrasonicator.# For# the# purpose# of# exfoliating# SWCNTs# in# an# organic# solvent,# a# probe# ultrasonicator# was# used# by# immersing# the# probe#into#the#solvent#containing#SWCNTs#to#be#exfoliated.#The#probe#ultrasonicator# was# preferred# over# a# bath# sonicator# because# of# faster# rates# of# exfoliation# due# to# higher# ultrasonic# intensities23,# easier# measurement# and# control# of# the#
energy/power# output# into# the# solution,# better# temperature# control# and# better# reproducibility#of#the#experiments.21#
Probe! Ultrasonication.! Probe:#The#shape#and#diameter#of#the#probe#along# with# the# amplitude# of# the# probe# vibration# dictates# the# intensity# of# the# ultrasonication#at#the#probe#tip.#For#example,#using#a#very#small#tip#at#relatively#high# amplitudes# can# lead# to# very# intense# cavitation# in# the# liquid.# However,# higher# ultrasonic#intensity#near#the#tip#alone#does#not#necessarily#imply#better#exfoliation# quality.21#Exfoliation#quality#is#also#dependent#on#the#shape#of#the#flask#being#used#
and#the#volume#being#sonicated.26#Also,#a#power#output#greater#than#20#W#is#known#
to# damage# SWCNTs.27#Therefore# by# choosing# the# correct# probe# and# the# right#
amplitude,# damage# to# SWCNTs# can# be# minimized# or# eliminated# altogether.# An# extender#with#a#probe#diameter#of#19#mm#has#been#used#for#the#work#presented#in# this#thesis#as#it#gave#the#best#results#for#a#volume#of#~20#ml.#The#probe#was#used#by# immersing#it#at#a#depth#of#approximately#1.5#cm#into#the#liquid#being#sonicated.#
Flask! vessel:! Assisting# the# process# of# dispersion# are# factors# such# as# the# shape#of#the#flask,#the#material#of#the#flask#and#the#volume#of#the#flask.#A#flask#with#a# flat#bottom#or#with#a#concave#bottom#helps#in#reflecting#the#ultrasonic#waves#into# the#liquid.#In#a#round#bottom#flask,#the#waves#impinge#the#base#at#an#angle#and#are# therefore#scattered#into#the#surrounding#thereby#reducing#efficiency.21#Elimination#
