iMULTIFUNCTIONALNANCOMPOSITEFOAMSFORSPACEAPPLICATIONSByDiandraJRollinsADISSERTATIONSubmittedtoMichiganStateUniversityinpartialentoftherequirementsforthedegreeofMaterialsScienceandEngineering{DoctorofPhilosophy2016iiABSTRACTMULTIFUNCTIONALNANCOMPOSITEFOAMSFORSPACEAPPLICATIONSByDiandraJRollinsMaterialscombinedwithasmallamountofnanoparticlesnewpossibilitiesinthesynthesizingofmultifunctionalmaterials.Graphenenanoplatelets(GnP)aremultifunctionalnanoreinforcingagentsconsistingofstacksofgraphenesheetswithcomparablepropertiestoasinglegraphenelayeratanoveralllowercostinamorerobustform.Suchparticleshavebeenshowntohavegoodthermal,mechanicalandelectricalproperties.Inaddition,alowdensitymultifunctionalnanocompositefoamhasthepotentialformultipleapplicationsandpotentialusefortheaerospaceindustry.Thisdissertationinvestigatestwotmicroporous(foam)polymersthataremodbytheadditionofGnPtocombatthisdensitytoimprovethefoam'smacroscopicpropertiesThreesizesofGnPwithvaryingaspectratiowereusedtoimprovethepolymericfoams'dielectric,electricalandmechanicalproperties.GnPwasaddedtoawater-blownpolyurethane/polyisocyanurate(PUR/PIR)foamwithaneatdensityof0.16g=cm3.ThelargestaspectratioGnPpercolatedtoformaconnectednetworktoallowforthetransferofelectrons,whichresultedinadecreaseoftheelectricalresistivityby5ordersofmagnitude.Thissameelectricalcontactimprovedtheoveralldielectricproperties:increasedtherealpermittivitybyoverdoubletheamountoftheneatfoam,increasedtheelectromagneticinterference(EMI)shieldingeness(SE)byabouttentimesbyincreasethenanocomposite'sabsorbanceandcapabilities.However,theselargeparticlesfromagglomerationsreducingthemechanicalperformanceeventhoughthecellsizedecreasedanddemonstratedlittleinteractionoftheparticleswiththematrixbythesimilarglasstemperaturebetweenthesolidandthefoam.TheedgegroupsontheGnPweresuccessfullytreatedwitheitherpolymericormolecularisocyanatetoformurethanetypegroupsasbyx-rayphotoelectronspectroscopyiii(XPS).EdgetreatmentoftheGnPonlyshowedtadvantagesforthesmallestsizeofGnPwiththehighestedgedensity,andresultedinimprovedmechanicalstrength,andsomeimprovementsinthedielectricandshieldingproperties.Theseparticleswerealsofoundtohavetheleastonthemolecularstructure.TheoftreatingtheedgeswithpolymericormoleculargroupswasmostdependentonthesizeoftheGnP.Theshorterurethanemoleculesthatformedwhenreactedwithtoluenediisocyanate(TDI)ontheedgeshadlittletonoonthemechanicalstrength,butwasabletolowertheelectricalresistivitybyaboutanorderofmagnitudeoverthesameparticlesizetreatedusingthesamemethodbutwithapolymericisocyanate.Thelargerparticlestreatedwithisocyanatedemonstratednoimprovementinthecompressivestrengthovertheneatparticlesandingeneralincreasedtheelectricalresistivityaswellcausingthedielectricperformancetodecreaseinconjunction.GnPwasalsoaddedtoapolydimethylsiloxane(PDMS)matrix,whichiscommonlyusedinaerospaceapplicationsduetoitsxiblepropertiesatlowtemperaturesandenvironmentalresistance.ThenanocompositewiththelargestaspectratioandhighestloadingofparticlestlyimprovedtherealpermittivityandEMIshieldingpropertiesintheX-bandovertheneatPDMSincludingtheandabsorbancecapabilities,buttheGnPstillshowedtagglomerations.AsyntacticfoamwasusedtoimprovethepercolationoftheGnPbycoatingthehollowglassspheres(HGS)withsmallerGnPpriortoaddingtothematrix.AlthoughitisunclearfromtheSEMimagesiftheGnPwasabletostayadheredtotheparticlesduringprocessing,thedielectricandEMIshieldingpropertiesofthenanocompositesyntacticfoamwassimilartotheneatwhileproducingafoamthatwas20%lighter.ivCopyrightbyDIANDRAJROLLINS2016vThisdissertationisdedicatedtomyfamilywhohavenotonlysupportedandencouragedme,buthavebeenexcellentrolemodelsinshowingmehowtogetitdone.vviACKNOWLEDGEMENTSEternalgratitudetomySaviorJesuswhobyHisSpiritthatlivesinmehasgivenmejoyandpeacetohelpmetheracestrong.ThankyoutoDr.L.T.Drzalwhohasalwaysbeensopatientinhelpingmedevelopmyresearchskills.Mygratitudegoesouttomycommitteemembers:Dr.L.Matuana,Dr.R.OfoliDr.J.SakamotoandDr.Jayaraman,somehavebeenwithmealongtimeandothersmorerecent,butallhavebeensohelpfulingettingtotheendofthislongjourney.ToalltheCMSCPerAskeland,BrianRook,MikeRich,EdDrownandKarenLilliswhowerealwayssohelpfulindesigningmyexperimentsandansweringmymanyquestionsnomatterhowinanetheywere.SpecialthankstoDr.Rothwellandhisgraduatestudents,especiallyJon,forbeingsopatientwithmeandhelpingmelearnsomuchaboutsuchunfamiliarEEconceptsinashortamountoftime.IalsowanttoexpressmygratitudetoMattandhisteamatNASALaRC,theywereveryaccommodatingandgavegreatfeedbackinregardstomyproject.TothepastandcurrentgraduatestudentsoftheCMSCwhohavebeenmysupport,myfriendsandts,Ilookforwardtoseeingalltheamazingthingseachoneofyouwilldo.LastlyIwanttothankmyfamily.EachoneofyouisblessingwhotaughtmethejoyoflearningandIlookforwardtocontinuingtogrowwitheachofyou.viviiTABLEOFCONTENTSLISTOFTABLES..................................xLISTOFFIGURES.................................xiCHAPTER1INTRODUCTION.........................11.1Background....................................21.1.1PolymerNancomposites.........................21.1.2Graphene.................................31.2ResearchObjectivesandGoals..........................10CHAPTER2MATERIALSANDEXPERIMENTALTECHNIQUES...142.1Materials.....................................142.1.1PolymerMatrices.............................142.1.1.1RigidPolyurethane/PolyisocyanurateFoam.........142.1.1.2FlexiblePolydimethylsiloxane.................182.1.2HollowGlassSpheres...........................192.1.3.................................202.2ExperimentalProcedure.............................212.2.1PUR/PIRNancompositeFoamSynthesis................212.2.2Edge-FunctionalizationofGnP.....................222.2.3NanocompositePDMS..........................232.2.3.1CoatingGlassMicrosphereswithGnP............232.2.3.2PDMSSyntacticFoamProcedure...............242.3TestingProcedures................................252.3.1X-rayPhotoelectronSpectroscopy....................252.3.2ThermalAnalysis.............................262.3.3Microscopy................................262.3.4MechanicalandElectrical........................272.3.5ElectromagneticPerformance......................27CHAPTER3INTERACTIONSOFGNPINRIGIDPUR/PIRFOAM..303.1Introduction....................................303.2ExperimentalProcedure.............................323.2.1Materials.................................323.2.2ExperimentalProcedure.........................333.2.2.1RigidPUR/PIRSynthesis...................333.2.2.2Edge-functionalizationofGnP.................343.2.3TestingProcedures............................343.3Results.......................................353.4Discussion.....................................49viiviii3.4.1ComparisonofMonolithicRigidPUR/PIRandRigidPUR/PIRFoamwithnoGnP.............................493.4.2CharacterizationofMonolithicRigidPUR/PIRwithNeatGnP...503.4.3CharacterizationofFoamSampleswithNeatGnP...........513.4.4CharacterizationoftreatedGnP.....................553.5Conclusion.....................................61APPENDIX......................................64CHAPTER4MULTIFUNCTIONALPERFORMANCEOFRIGIDPUR/PIRFOAM................................754.1Introduction....................................754.2ExperimentalMethods..............................764.2.1Materials.................................764.2.2SynthesisofPUR/PIRNanocompositeRigidFoam..........784.2.3Edge-functionalizationofGnP......................784.2.4TestingProcedures............................794.2.4.1XPS...............................794.2.4.2MechanicalandElectrical...................804.2.4.3DielectricandEMISE.....................804.2.4.4Microscopy...........................814.3Results.......................................824.3.1MechanicalProperties..........................824.3.2ElectricalProperties...........................844.3.3ElectromagneticProperties........................844.4Discussion.....................................874.4.0.1MechanicalPropertiesofNancompositeRigidPUR/PIRfoam874.4.0.2ElectricalresistivityofNancompositePUR/PIRRigidFoam974.4.0.3DielectricPerformance.....................1004.5Conclusions....................................104APPENDIX......................................107CHAPTER5DIELECTRICANDEMISHIELDINGPROPERTIESOFPDMSANDPDMSSYNTACTICFOAMNANOCOM-POSITES...............................1105.1Introduction....................................1105.2Materials/Synthesis................................1125.2.1Materials.................................1125.2.1.1PDMS..............................1125.2.1.2GnP...............................1125.2.1.3HollowGlassSpheres......................1125.2.2ExperimentalProcedure.........................1135.2.2.1GnP/PDMS...........................1135.2.2.2CoatingGlassBubbles.....................114viiiix5.2.2.3PDMSsyntacticfoam.....................1155.2.3Testing...................................1165.2.3.1VectorNetworkAnalyzer...................1165.2.3.2Microscopy...........................1165.3Results.......................................1175.4Discussion.....................................1175.4.1ModesofDielectricResponse......................1215.4.2ProbablemodesofdielectricresponseinnanocompositePDMS...1225.4.3ofconductivityonpermittivity..................1245.4.4Modesoflossindielectrics........................1265.4.5EMIShielding...............................1275.4.6Improvements...............................1285.5Conclusion.....................................132CHAPTER6SUMMARYANDFUTUREWORK..............1356.1RigidPUR/PIRnanocompositefoam......................1366.2FlexibleNancompositePDMSSyntacticFoam.................1406.3FutureworkformakingnanocompositeswithGnP..............141REFERENCES....................................143ixxLISTOFTABLESTable2.1:RatioofconcentrationsofhydroxylliquidsusedinPUR/PIRrigidfoamformulation...................................18Table2.2:Concentrationoftcomponentsusedforfoamformulationnormal-izedtopartsperhundredofpolyol(pphp)..................18Table3.1:Atomic%ofchemicalgroupsonGnPafterreactionovernightwithpMDIasdeterminedbyXPS.............................36Table3.2:Atomicpercent(at%)ofchemicalgroupsonGnPafterreactionwithpMDIandTDIasdeterminedbyXPS.Mdesignatesthemonolayermethodofreaction,forallothersthereactantwasinexcess.........38Table3.3:GlasstransitiontemperatureofmonolithicPUR/PIRbefore("standard")andaftertheadditionofGnPfrom3specimens...............46Table3.4:Theaverageglasstemperature(Tg)andsampledeviationofrigidPUR/PIRfoamwithandwithout("standard")GnPandwithtreatedGnPasmea-suredbyDSCtakenfrom3specimens.....................47Table3.5:CellsizeofthemeanandmediandiameterofPUR/PIRrigidfoamwithandwithoutdittypesandloadingsofGnPversustheneatrigidPUR/PIRfoam("standard".)........................48Table4.1:Propertiesofttypesofas-receivedGnPusedinnanocompositefoams.77Table4.2:AspectratioforGnPaccordingtotheproductsheetsforlargestthickness.99xxiLISTOFFIGURESFigure1.1:SchematicforsynthesisofexfoliatedmultilayeredgrapheneesfromGICs8Figure1.2:SEMimageofXGSciences'graphenenanoplatelets(GnP)whichconsistsofmultiplelayersofgraphene.........................9Figure1.3:SEMimagesoftgradesofxGnP:(a)XGSciencesM-gradexGnPofaveragediameterof25mandthicknessof6-8nm(b)XGSciencesC-gradexGnPafterultrasonicationinacetone...............13Figure2.1:ChemicalreactionsforpolymerizationandgasevolutioninPUR/PIRfoam15Figure2.2:ChemicalstructuresofisocyanatesusedinthesynthesisofPURfoams:(a)Polymericdiphenylmethanediisocyanate(pMDI)(b)2,4'-Toluenediisocyante(TDI)...............................16Figure2.3:SEMimageof3MŠiM16Kglassbubblessprinkledwithamorphoussilicaonsurfaceandmeasureddiameters.....................19Figure2.4:Chemicalstructureoftrimethoxysilylpropylmopolylethenimine,thesilanecouplingagentusedtoadhereGnPtoHGS...........20Figure2.5:SEMimageofHGScoatedwithGnP-5andGnP(750)afterdrying.Thesamplewasnotcoatedpriortoimaging...................25Figure3.1:SEMimageofPUR/PIRrigidfoamhighlightingthetpartsinthecellularstructure..............................36Figure3.2:AnexampleofthenitrogenspectraofpMDItreatedGnP.AbroadnitrogenpeaksuggeststhatthebindingenergyisnotthesameforallthenitrogenatomsandusingFTTsoftwarethebroadspectracanbebrokendownintotheirindividualnitrogengroups.............37Figure3.3:ThermaldegradationprofPUR/PIRrigidfoamandthemonolithicpolymer.ThesolidlinesrepresentstherigidPUR/PIRfoamandthedashedlineistheforthemonolithicrigidPUR/PIR............39Figure3.4:GraphoverlayofthethermaldegradationofthemonolithicrigidPUR/PIRwithandwithoutGnP("standard")..............40Figure3.5:GraphoverlayofthederivativethermaldegradationofthemonolithicrigidPUR/PIRwithandwithoutGnP("standard")......40xixiiFigure3.6:ThermaldegradationofPUR/PIRrigidfoambefore("standard")andaftertheadditionofttypesandloadingsofGnP.......41Figure3.7:GraphoverlayofthederivativeweightofthethermaldegradationofPUR/PIRrigidfoambefore("standard")andaftertheadditionofttypesandloadingsofGnP......................42Figure3.8:ComparisonofthederivativeweightthermaldegradationofrigidPUR/PIRfoamoftheneat("standard")versusaftertheadditionof5wt%bakedGnP-25andedge-functionalizedpMDItreatedGnP-25....42Figure3.9:ComparisonofthederivativeweightthermaldegradationofrigidPUR/PIRfoamoftheneat("standard")versusaftertheadditionof8wt%bakedGnP-5andedge-functionalizedGnP-5withpMDIinexcessorTDIusingthemonolayermethod(M)...................43Figure3.10:ComparisonofthederivativeweightthermaldegradationofrigidPUR/PIRfoamoftheneat("standard")versusaftertheadditionof8wt%bakedGnP(750)andedge-functionalizedGnP-5withpMDIinexcess,pMDIusingthemonolayermethodorTDIusingthemonolayermethod(M)..................................44Figure3.11:SomechemicalreactionsthatoccurduringthefoamingofaPUR/PIRwater-blownfoam...............................45Figure3.12:TheamountofheatincreaseasthefoamcureswithoutGnP(solidline)andwith5wt%GnP-25(dashedline)....................47Figure3.13:DegradationofrigidPUR/PIRfoamwithnoGnPcomparedtofoamwithneatGnP..............................52Figure3.14:SEMimagesofGnPdispersioninstrutsinrigidPUR/PIRfoam:(a)FIBcutoffoamwith8wt%GnP(750);(b)FIBcutoffoamwith8wt%GnP-5.....................................54Figure3.15:SEMimageofGnPdispersionattheapexofstrutsinrigidPUR/PIRfoamwith5wt%GnP-25...........................55Figure3.16:SEMimagesofGnP-25reactedwithpMDIfor1hatelevatedtemperature57Figure3.17:FESEMimageofGnPdispersioninrigidPUR/PIRfoamwith8wt%pMDItreatedGnP(750).StrutwaspreparedwiththeFIBandaggre-gatesareoutlinedinblack...........................58Figure3.18:ThermaldegradationofedgetreatedGnP-5comparedtotheneatfoam.......................................60xiixiiiFigure3.19:Maxandmincelldiametersof60cellsinrigidPUR/PIRfoam:(a)Histogramofmaximumcellsize(m);(b)Histogramofminimumcellsize(m)...................................65Figure3.20:Maxandmincelldiametersof60cellsinrigidPUR/PIRfoamwith5wt%GnP-25:(a)Histogramofmaximumcellsize(m);(b)Histogramofminimumcellsize(m)..........................66Figure3.21:Maxandmincelldiametersof60cellsinrigidPUR/PIRfoamwith5wt%pMDIGnP-25:(a)Histogramofmaximumcellsize(m);(b)Histogramofminimumcellsize(m)....................67Figure3.22:Maxandmincelldiametersof60cellsinrigidPUR/PIRfoamwith8wt%GnP-5:(a)Histogramofmaximumcellsize(m);(b)Histogramofminimumcellsize(m)..........................68Figure3.23:Maxandmincelldiametersof60cellsinrigidPUR/PIRfoamwith8wt%pMDIGnP-5:(a)Histogramofmaximumcellsize(m);(b)Histogramofminimumcellsize(m)....................69Figure3.24:Maxandmincelldiametersof60cellsinrigidPUR/PIRfoamwith8wt%TDIMGnP-5:(a)Histogramofmaximumcellsize(m);(b)Histogramofminimumcellsize(m)....................70Figure3.25:Maxandmincelldiametersof60cellsinrigidPUR/PIRfoamwith8wt%GnP(750):(a)Histogramofmaximumcellsize(m);(b)Histogramofminimumcellsize(m)..........................71Figure3.26:Maxandmincelldiametersof60cellsinrigidPUR/PIRfoamwith8wt%pMDIGnP(750):(a)Histogramofmaximumcellsize(m);(b)Histogramofminimumcellsize(m)....................72Figure3.27:Maxandmincelldiametersof60cellsinrigidPUR/PIRfoamwith8wt%pMDIMGnP(750):(a)Histogramofmaximumcellsize(m);(b)Histogramofminimumcellsize(m)....................73Figure3.28:Maxandmincelldiametersof60cellsinrigidPUR/PIRfoamwith8wt%TDIMGnP(750):(a)Histogramofmaximumcellsize(m);(b)Histogramofminimumcellsize(m)....................74Figure4.1:CompressivestrengthoffoamwithnoGnP("standard")comparedtothenanocompositefoamwithsampledeviationsat10%ormaximumstrength...............................82Figure4.2:CompressionelasticmodulusoffoamwithnoGnP("standard")com-paredtothenanocompositefoamwithsampledeviation..........83xiiixivFigure4.3:TheresistivityofthefoamspecimensshownonalogarithmicscalecomparingthestandardwithnoGnPtothenanocompositefoamwithsampledeviation................................83Figure4.4:Theratiooftherealpermittivityofthefoamsamplesrelativetofreespacefrom8.2to12.0GHz..........................84Figure4.5:ThefractionofthetotalEMwavethatistransmittedthroughthetfoamsamplesfrom8.2to12.0GHz.................85Figure4.6:ThefractionofthetotalEMwavethatisbackfromthetfoamsamplesfrom8.2to12.0GHz.....................85Figure4.7:ThefractionofthetotalEMwavethatisabsorbedbythetfoamsamplesfrom8.2to12.0GHz.........................86Figure4.8:ThetotalEMISEofthestandardrigidPUR/PIRfoamascomparedtothenanocompositefoamfrom8.2to12.0GHz...............86Figure4.9:OpticalimagesofcellwallsinrigidPUR/PIRnancompositefoam:(a)Cellwallsaresothintheyarewrinkledfromgasexpansion;(b)Easytoidentifythincellwalls,butthesmallamountcouldbedotoahighernumberofthickercellwalls....................88Figure4.10:Standardstress-straincurvesofelasto-plastic(left)andelastomeric(right)foams..................................89Figure4.11:FESEMimageofFIBcutinrigidPUR/PIRfoamwith5wt%GnP-25:(a)FIBcutofstrut;(b)DispersionofGnP-25instrut..........90Figure4.12:FESEMimageofFIBcutofstrutinrigidPUR/PIRfoamwith8wt%GnP-5......................................92Figure4.13:FESEMimageofFIBcutstrutinPUR/PIRrigidfoamwith8wt%GnP(750)....................................92Figure4.14:FESEMimageofstrutcutwithFIBinPUR/PIRrigidfoamwith8wt%pMDItreatedGnP(750).ThepMDItreatedGnP(750)agglomeratesarehighlighted...................................93Figure4.15:FESEMimageofGnPdispersioninrigidPUR/PIRfoamwith8wt%pMDItreatedGnP(750).StrutwaspreparedwiththeFIBandaggre-gatesareoutlinedinblack...........................94Figure4.16:SEMimageofGnP(750)reactedwithpMDIbybothmethods(a)GnP(750)treatedbyexcessmethod;(b)GnP(750)treatedbymini-malmethod..................................95xivxvFigure4.17:FESEMimageofstrutcutwithFIBinPUR/PIRrigidfoamwith8wt%pMDItreatedGnP(750)byminimalmethod.SomeGnPishighlightedwitharrows...................................95Figure4.18:FESEMimageofstrutcutwithFIBinPUR/PIRrigidfoamwith8wt%pMDItreatedGnP(750)byminimalmethod.SomeGnPishighlightedwitharrows...................................96Figure4.19:FESEMofFIBcutofstrutinrigidPUR/PIRfoamwith8wt%minimaltreatedTDIGnP(750).SomeGnPishighlightedwitharrows.......97Figure4.20:RawdataofneatrigidPUR/PIRfoamspecimen'smechanicalperfor-manceduringcompression..........................108Figure4.21:RawdataofrigidPUR/PIRfoamwith5wt%GnP-25specimen'smechanicalperformanceduringcompression................108Figure4.22:RawdataofrigidPUR/PIRfoamwith5wt%pMDItreatedGnP-25specimen'smechanicalperformanceduringcompression..........109Figure4.23:RawdataofanotherrigidPUR/PIRfoamwith5wt%pMDItreatedGnP-25specimen'smechanicalperformanceduringcompression.....109Figure5.1:SEMimageof3MŠiM16Kglassbubblessprinkledwithamorphoussilicaonsurfaceandmeasureddiameters.....................113Figure5.2:SEMimageofHGScoatedwithGnP-5andGnP(750)afterdrying.Thesamplewasnotcoatedpriortoimaging...................115Figure5.3:Realpermeabilityasaratiotothepermeabilityoffreespacefornanocom-positePDMSsamplesfrom8.2to12.4GHz,comparedtotheneatPDMSelastomer....................................117Figure5.4:Realpermittivityasaratiotothepermittivityoffreespacefornanocom-positePDMSsamplesfrom8.2to12.4GHz,comparedtotheneatPDMSelastomer....................................118Figure5.5:Alternatingelectriclosstangent(tana=""="0)ofnanocompositePDMSsamplesfrom8.2to12.4GHz.....................118Figure5.6:Fractionofappliedelectricthattransmitsthroughthenanocom-positePDMSsamplesfrom8.2to12.4GHz.................119Figure5.7:FractionofappliedelectricthatctsofnanocompositePDMSsamplesfrom8.2to12.4GHz.........................119xvxviFigure5.8:FractionofappliedelectricldthatisabsorbedbythenanocompositePDMSsamplesfrom8.2to12.4GHz.....................120Figure5.9:TotalEMIshieldingenessofnanocompositePDMSsamplesfrom8.2to12.4GHz................................120Figure5.10:Realrelativepermittivity("0r)versusalternatinglosstangent(tana)ofnanocompositePDMSsamplesandneatelastomerat11GHz.......125Figure5.11:Agraphicillustratingthethreewaysanelectromagneticplanewavecanbeectedwhenconfrontedwithamaterial.................127Figure5.12:SEMimagesof2wt%GnP-25inPDMSelastomer.ArrowsandcirclesaretohighlightthelocationofsomeofthedistinguishableGnP:(a)LargeagglomerateofGnP-25intoprightcornerandarrowpointstoathinplatelet;(b)AgglomeratesandthinGnParerelativelyclosetogether;(c)DistancebetweensomeGnPagglomerates;and,(d)GnPagglomeratescanbeverycloseordecentlyfarfromeachother.......129Figure5.13:SEMimagesofPDMSwith2.15wt%GnP-5and1.45wt%GnP(750):(a)DarklargeparticlesareagglomeratesofGnP(750);(b)StackofGnP-5.130Figure5.14:SEMimageofdispersionof40vol%HGSinPDMSelastomer......131Figure5.15:SEMimagesof40vol%GnPcoatedHGSinPDMSelastomer.TheamountofGnPrelativetothespheresis2.15wt%GnP-5and1.45wt%GnP(750):(a)Abletodistinguishonly1GnPonHGSinPDMS;(b)ClumpofGnPcoatedHGSinPDMS....................132xvi1CHAPTER1INTRODUCTIONInthepastseveraldecadestechnologyhasfocusedonthepursuitofextremes,astrivingtocreateproductsthataresmaller,faster,lighterandwilltakepeopledeeperandfurtherthaneverbefore.Meetingthesechallengesbeginswithmaterialsandoverthepastdecadestherehasbeenamajorshiftinthetypeofmaterialsthathavebecomeavailablethroughresearchdevelopmentandcommercialization.Historicallytheevolutionofmaterialshasrevolvedaroundstartingwithwhatisreadilyavailableandmodifyingitwithstraightforwardprocedurestomeetnewdemands.Asourunderstandinggrew,thisresultedintheapplicationoffundamentalsciencetothedevelopmentofnewmaterials.Fromchemistrycamethediscoveryofpolymersandnewformabilitymethods,fromceramicscamesemiconductorsandsinglecrystalswhichpavedthemovefromanalyticalenginestothedigitalcomputer[1]andthepastfewdecadeshasseenthebirthofhighstrength,highlightweighterreinforcedcompositesthathasfundamentallychangedthecharacteristicsofmaterials.Astechnologycontinuestogrowandexpandalongwithconsumerexpectationsandinventiveness,theneedforhighlyfunctioningmaterialsbecomesincreasinglyimportant.Thecreationofnewmaterialtechnologiesmustbeweighedagainstthefeasibilityofsuchcreations.Findingcostivewaystosynthesizethesematerialsbecomesincreasinglymoretasthedemandsontheirperformancealsoincreases.Thediscoveryofnanoparticlesandthesciencommunity?ssubsequentinvestigationoftheircorrespondingpropertiesaddedanewdimensionforachievingtechnologies?materialdemands.Muchresearchhasgonetowardsinvestigatingtwaysofproducingandutilizingnanoparticles.Wehavelearnedthatwithnanomaterials,manyoftheproblemsatthemacroscalearegreatlyreducedoreveneliminatedatthenanoscale..Therearenowmanyttypesofnanoparticlesthatvaryinatomicmake-up,sourcesandmorphology.The12morecommononesaremadefrommetalsandmetaloxides,cellulose,aswellasfrompurecarbon.Theirmorphologycanincludespheres,particles,ribbons,solidcylinders,tubes,andplatelets.Thecarbon-basednanoparticleshavemultiplepromisingqualitiesthatmakethemdesirableinpolymersandcomposites.Sincetherediscoveryofcarbonnanotubesin1991interestinttypesofcarbonnanoparticleshasexpanded[2].Alargeamountofresearchhasbeendirectedatsynthesizingttypesofcarbonnanoparticlesandinvestigatingtheircorrespondingproperties.Themostcommontypesofcarbonnanoparticlesarecarbonnanotubes,fullerenetubes,buckyminsterfullereneandgraphene.Theseallconsistentirelyofcarbonatoms,eachwithatmorphologyand/orbondingstructurewithatleastonedimensiononthenanometerscale.Mostoftheseparticlesarechallengingtosynthesizeespeciallywhenabottom-upapproachisused,butsuchmethodsarestillusefulforunderstandingthefundamentalsoftheirformationandproperties,butaretypicallyveryexpensivewhenitcomestocommercialization.Someofthesematerials,however,canstartwithnaturalmineralsandbeproducedwithatop-downapproachtoyieldnanoparticles,whicharegenerallylessexpensive,butmayalsohaveslightlytproperties.1.1Background1.1.1PolymerNancompositesThelasthalfofthepastcenturysawarevolutionarychangeinthematerialsmanufacturingindustrywiththedevelopmentofcomposites.Composites,astheirnameimplies,combinestwoormoretmaterialsinthehopeofcreatinganewmaterialwithsynergisticOneofthemajormaterialgroupsusedinthesynthesisofcompositesarepolymers.Polymerswerequitethenovelmaterialwhencommerciallyintroducedduetotheirhighyandsubsequentlyformability,inadditionthermoplasticsarealsorecyclableandboastofrelativelyinexpensiveproductioncosts.Thishasledtothempervadingall23societyaseverythingfromdisposablecontainers,casingforelectronics,furniture,sounddampenersandeveninclothing.Theirversatilitymakethemanattractivematerialforavarietyofapplications,butpolymersingeneralalsoerfrompoormechanicalproperties,lowtransportproperties,suchasthermalandelectronic,andpoorthermalstabilitytherebylimitingtheirapplications.Polymersareutilizedforcompositemanufacturingasthematrixandcontinuetobeutilizedasthematrixinapplicationswerethecombinationofhighperformanceandlowweightareimportantasintheaerospaceindustry.ttypesofnanoparticlesthathavebeensuccessfullyaddedtoavarietyofpolymersaswellforeverythingfromelectronics,bio-engineering,robotics,gassensing,energyandaerospaceapplications.Theapplicationstronglydictateswhattypeofpolymerandnanopar-ticletouse.Thetypeofnanoparticlesincludettypesofmetals[3,4],metaloxides[5,6],carbon-basednanoparticles[7,8,9,10]andevennanoclays[11].Thetypesofpolymersarejustasvaried;commononesincludeepoxy,polypropylene,polystyrene,vinyls,rubbers[12,13,14]andmorespecializedpolymersincludingelectricallyconducivepolythiophenes[3]andpolyanilines[15].Theadditionofnanoparticlestopolymerfoamshasalsobeengainedattentioninresearch,thefocushasbeenmainlyontheadditionofnanoclays,butalsoincludescarbonnanotubes(CNTs),carbonblack(CB)andevengraphenehasbeenutilized[16,17,18].Theseresearchsuccessfullyshowedthatlowamountsofnanoparticlescouldbeaddedtopolymericfoamsandeverythingfromtheirmechanicalproperties,thermaldegradationtemperature,cellmorphology,andeventheelectricalconductivity.1.1.2GrapheneOneofthenewestadditionstothediversearrayofresearchedcarbonnanomaterialsisgraphene.Grapheneconsistsofasinglelayerofsp2hybridizedcarbonatomsinahexagonalarrangement.Grapheneisthebasisofmanycarbonnanostructuresandisthebasestructureofgraphite[19].GraphiteconsistsofindividuallayersofgraphenebondedtoothergraphenelayersthroughVanderWaalsforces,whichisthereasongraphitemakessuchagoodsolid34lubricant.Althoughthechemistryandstructureofgraphitehasbeenknownforover150years,GeimandNovoselovfromManchesterUniversitywereabletocreateatransistorfromasinglelayerofcarbonatoms(graphene)anditallowedforthetimethecharacterizationofasinglegraphenelayer[20].Graphenehasbeenshowntohavehighthermalconductivityofaround3000W=mK[21]andaYoung'smodulusofaround1TPa[22].Graphenealsohasuniqueelectricalpropertiesowingtoits2Dstructureincludinghighcarriermobilitiesgreaterthan15,000cm2=Vs[19].Inaddition,asinglelayerofgrapheneabsorbsonlyabout2.3%ofwhitelightmakingitopticallytransparent[23].Theuniquemechanical,thermalandelectronicpropertiesofsingle-layergraphenehasresultedinanexplosioninresearchregardingthentwaystosynthesizegraphenesheets.Originallygraphenewasthoughttobeunstableashistorically2Dcrystalswereconsideredthermodynamicallystable[19].Thesuccessfulmethodsresultinginsinglelayergraphenesheetsweredonebyremovingthelayerfromhighlyorientedpyrolyticgraphitewithcellophanetape,whichwhileeforcharacterizingthepropertiesofatomicallythingraphene,isimpracticalbecausetheyieldonsuchaprocessisverylow[20]andthishasledtothedevelopmentofothermethods.Althoughvan-der-waalsforcesareweakcomparedtoothertypesofchemicalbonding,separatingthegraphenelayersonalargescaleprovestobeachallenge.tmethod-ologiesformakinggrapheneincludebothtop-downandbottom-upapproaches.Themostcommontypeoftop-downproceduresuseacombinationofintercalationandexfoliationthrougheithermechanical,thermal,chemicalmeans,orincombination,toseparatethelayers.Suchproceduresfromsuchlimitationsaslowthrough-put,inabilitytoformsinglelayersand/orlowperformancecharacteristicsduetoresidualchemicalgroupsadheringtothegraphenebasalplane.Bottom-upapproachesfocusoncreatinghighquality,singlegraphenelayersmainlyforuseinelectronicdevices.Theytendtobeexpensiveandtime-consuming.Themostpopularbottom-upapproachesarechemicalvapordepositionandepitaxialgrowth,bothofwhichgrowcarbononasubstrate.Somemoreexoticmethodsincludetheuseof45polycyclicaromatichydrocarbons(PAHs)[24]asastartingmaterial,aswellasotherorganiccompounds,toformgraphenenanoribbons[25].Themostcommonbottom-upandtop-downapproachestographenesynthesiswillnowbediscussedinmoredetail.Someofthemostutilizedbottom-upapproachestomakinggrapheneincludechemicalvapordeposition(CVD)andepitaxialgrowth.Bothusesubstratestogrowsingle-layertofewlayergrapheneonthesurface.ForCVD,transitionmetalsubstrates,typicallysinglecrystalline,areexposedtoahydrocarbongasunderlowpressureorultra-highvacuum(UHV)conditions.Carbonatomsdepositonthesurfacecausingnucleationfollowedbygrowthintographene[26,27].Thegrapheneisgenerallyveryorderedbutisalsotypicallysmall-scaleduetoprocessingtimesandratherexpensivetoproduceduetothehighcostsfromthesubstratesortheexperimentalenvironmentthatmustbemaintainedthatallowsforsuchgrowth.Comparably4H-and6H-SiCisahexagonalcrystalstructurewithalternatinglayersofsiliconandcarbon.TreatmentsofSiCthatfocusonexposingcertainfacesofthecrystalstructurehavebeenstudiedformanyyears[28].Exposingthe(0001)siliconfaceand(0001)carbonfaceofthesiliconcarbidecrystalstotemperaturesabove1300°Cinultrahighvacuumresultsintheformationofultrathingraphiticifthearegrowninanenvironmentthathasacontrolledbackgroundgaslessstringentvacuumconditionsareallowed[29].Obviouslybothoftheseapproachesresultinthinhighpuritygraphiticlayersbutduetotheprocessingconditionsresultinhighcostsandlowyield.Whiletheyshowpotentialforhighconductingelectronicsurfacesandlithographicpatterningforcircuits[30],thesemethodsremainimpracticalformakingivelargeamountsofgrapheneforuseinnanocomposites.Asmentionedpreviouslyanotherlesscommonapproachtomakinggrapheneusesaromatichydrocarbonstocreatesegmentsofsp2bondedcarbonatoms.Theseorganicsyntheticmethodsusepolycyclicaromatichydrocarbons(PAHs)tocreatehighlyaromaticstructuresorinsomecasesfree-standinggraphenenanoribbons.AccordingtoWuetal.,PAHsareconsideredtobe"two-dimensionalgraphitesegmentscomposedofallsp2carbons"[24].56Themostcommonmethodforcreatingordered2DgraphenemoleculesusesPAHsthataddaliphaticchainstothearomaticcores.Thiscausesananophaseseparationbetweenthecoresthatstackduetothep-pattractionbetweenthearomaticblocks,thealkylchainsthentheperiphery[24].Thesesupramoleculesarethenalignedintothinforuseintelectronicdevices.Todevelopthegraphenenanoribbonsfromstarttotheprocessresultsina65%yieldanddevelopsindividualgraphenesheetsfrom8to12nminlength[31,32].Thesegraphenesheetstendtoalignthemselvesintoribbonswhencastontoasilicasubstratewith2.5nmbetweeneachribbonproducingobjectswith100nmindiameterandupto5minlength[32].Astheexceptionalpropertiesofgraphenebecamemorewidelyknown,therewasaneedforaemethodforsynthesizinggraphene.Someofthepotentialapplicationsforgrapheneinvolveusingthemasreinforcingagents.Thisrequiredthatthesheetsbefree-standingsothetheycouldbeeasilyaddedtoamaterialmatrix,butmakesallowancesforthepurityandthicknessoftheparticlesoutsideofthestringentrequirementsforelectronicdevices.Themostcommoncommercialmethodstomakegrapheneesincludechemicalreductionofgraphiteoxideandthermalexfoliationofgraphiteintercalatedcompounds(GICs).Thesemethodsproducecontrastingtypesofgrapheneparticles,butbothareabletoproducerobustesthatareontheorderofamormore.Thetop-downapproachstartswithconvertinggraphitetographiteoxide(GO).Thismethodisappealingbecauseofitsabilitytoproducethincarbonsheets,whichismorecommonlyreferredtoasgrapheneoxide.BecauseGOcontainsmultiplesp3bondsthismaterialshowsamuchhigherelectricalresistancedemonstrate4=square)andthereforemustbereducedbacktoamaterialmuchmoresimilartographene[33].NaturalgraphiteisusedasthestartingmaterialandconvertedtoGOusingstrongacidsaccordingtoeithertheHummersorBrodiemethodasthetwomostpopularprocesses[34,35].Uponoxidationthegraphitestillhasaromaticregionswherethecarbonsarenowsp3insteadofsp2becausealargenumberofthecarbonsarenowbondedtooxygenandhydrogengroups.67Thisalsoresultsinawrinklingofcertainareasofthesheetsalongwiththesectionsthatstillhaveaplanarringstructureinadditiontostructuraldefects[36,37].Togetsingleorfewlayersheetsthelayersmustbeseparatedandthetwomostcommonmethodsuseeitherthermalormechanicalmeans.Forthethermalmethod,driedGOesareexposedtorapidheating,generallyinexcessof1050°Cformingalikeappearanceorworms;thisisimportanttonotebecausechangingtheprocessingtemperaturetherateofreductionandallowsfortheoptionoftuningtheelectricalconductivitytodesiredlevels[38,39].Aftertheresultingwormsarebrokenapartintosingletofewlayerwrinkledgraphenesheets[40].ReducedGOusingthethermalmethodhasbeensuccessfullymadeintothatshowareductioninelectricalresistivitydowntolessthan1=square[33].ThesecondmethodthatusesmechanicalenergytakesadvantageofthehydrophillicnatureofGOanddispersesthematerialinwaterusingmechanicalagitationtoproduceverythinsheets.Thesesheetsarethencommonlyreducedusinghazardouschemicalslikehydrazinehydrate.Theresultingwrinkledsheetsareverythinandonceintoorpapershowconductivitiesontheorder102S=m[41,42].Thesecondcommercialtop-downapproachreliesonusinggraphiteintercalatedcompounds(GICs)asthestartingmaterials.GICsaregraphitecompoundsthatcontainacidmoleculesadsorbedbetweenthelayers,whichwhenexposedtohighheattheintercalatedgroupsexpandforcingthelayersapartonceagaincreatingworms[43].JustlikethemethodforGO,thewormsarepulverizedresultingingrapheneesmanylayersthickduetotheinabilityofensuringthatintercalatedgroupsarebetweeneverylayer,butbecausethereisnodisruptionofthebasalplanestructureastheintercalatedmoleculesareadsorbednotbonded,therearelessdefectsandnowrinkling.Ingeneral,theestendtoalsobelargerontheorderofseveralmicronsindiameter.Whentheseexfoliatedgrapheneesarecompactedintoa'paper'usingvacuumtheelectricalconductivitiesareeasily10timesgreaterthanthatreportedusingreducedGO[44,38,33].AschematicapproachoftheprocessusingGICsisshowninFigure1.1.78Figure1.1:SchematicforsynthesisofexfoliatedmultilayeredgrapheneesfromGICsDuetothemultifunctionalpropertiesofgraphene,thismaterial,sometimesincon-junctionwithotherhasbeenusedextensivelyinthesynthesisofhighfunctioningpolymercomposites.Thisincludeseverythingfromhigh-densitypolyethylene,polyurethanes,polyetherimides,poly(p-phenylenepolypropyleneandrubbers,justtonameafew[10].Mostofthepolymernanocompositeshaveusedgraphenepowders,eitherreducedGOorexfoliatedgraphene.Thechallengewithcreatinganeresultingnanocompositereliesontherebeingagoodinteractionanddispersionbetweenthegrapheneesandthematrix.Iftheinteractionispooragglomerationsformwhichectthecomposites'macroscopicproperties,morespthemechanicalperformance.Forthisreasonoftenthegrapheneisfunctionalizedbeforeaddingtothecomposite.ReducedGOhasepoxideandcarboxylategroupsreadilyavailabletofunctionalize,whereas,theexfoliatedgrapheneeshavealimitednumberofhydoxylandcarboxylateedgegroupsavailable.ForreducedGOchemicalfunctionalizationiscommonlyandelyemployedtoimprovethedispersion[45,41,46].Fortheexfoliatedgrapheneesaddingsurfactantswhichinteractwiththeˇˇbondsandthematrixisamorepromisingfunctionalizationrouteasthismethodmaintainsthebasalplanestructureofthegraphene[47,48].Asmentionedpreviously,polymericnanocompositefoamsaregainingattentionduetotheirgreatpotentialforamyriadofapplications.Itsmostobviousadvantageisthatacellularmaterialhasvastlylowerdensityduetoitscellularstructurethatincreasesthevolume.Inadditionusingapolymericmaterialgenerallydecreasesthematerialandprocessingcostsandbycombiningsuchamaterialwithamultifunctionalnanoreinforcingagentsuchasgraphene,89Figure1.2:SEMimageofXGSciences'graphenenanoplatelets(GnP)whichconsistsofmultiplelayersofgraphene.resultsinthesynthesisofamultifunctionallow-cost,light-weightmaterialthatcouldhavemanyusefulapplicationsespeciallyintheaerospaceindustry.AerospacetechnologieswasthemainfocusofthisresearchasitwasfundedbytheNASASpaceTechnologiesResearchFellowship(NSTRF),whichisdesignedtoidentifygraduateresearchprojectsthatalignwithNASA'sgoals,spidenthroughtheir14TechnologyRoadmap[49].ApolymernanocompositefoamwasselectedasthefocusforthisresearchandmettheobjectsofTA(technologyarea)10:NanotechnologyandTA12:Materials,Structures,MechanicalSystemsandManufacturing[50].Forthenanoparticle,acommercialgraphenenanoplatelet(GnP)waschosenfromXGSciences(seeFigure1.2)whichissynthesizedfromGICs.Ithasacostsimilartothatofhighstructuredcarbonblackandconsistsofmultiplelayersofgraphenegivingthemathicknessbetween3-8nm.Theyhavehighpurityandincludeabout5atomicpercentofreactiveedgegroups,asbyx-rayphotoelectronspectroscopy(XPS).9101.2ResearchObjectivesandGoalsThegoalofthisresearchprojectistocreateamultifunctionalmaterialusingaporouspolymerasamatrixandutilizingagraphenenanoplatelet(GnP)asamulti-functionaladditivetomodifythefoam.Usingaporouspolymerasamatrixresultsinfurtherweightsavingsanimportantconsiderationinaerospaceapplications.Thechallengewithworkingwithfoamsisthatmanypropertiesdecreaseasthedensitydoesaswell,thustheneedfornanoreinforcementagenttocombatthatSuchananocompositecouldpotentiallyshowimprovedthermalperformance,electricalpropertiesanddielectricpropertieswhilemaintaininggoodmechanicalperformance.Byadjustingthenanoparticleloadingandtypethereisthepotentialthatthefoampropertiescouldbetailoredtospapplications.Oneobjectiveistogainabetterunderstandingofhowa2Dparticleorientswithina3Dstructure,butalsotodeterminemethodstoimprovetheinteractionbetweentheGnPandthematrix.Thetechniquesarefocusedone,readilyavailablesynthesismethods,andasaresultofthisresearch,theknowledgegainedwouldhelpinthedevelopmentofnanoparticlemopolymericfoamsystemsatarelativelylowcost.Thisresearchinvolvestheinvestigationofamulti-phasesystemandtheknowledgegleanedcouldbeappliedtoothercompositesystems.Thisinvestigationincludesdeterminingthevalidityofttechniquesusedtodispersenanoparticlesinapolymermatrix,observingttypesofdispersionanditsontheoverallproperties,andisasteptowardsfullycharacterizingtheidealdispersionthatneedstobeachieved.Dispersionisimportantfortheformationofthepercolatednetworkthatallowsforelectronconduction.Thoroughlycharacterizingthedispersionisimportantforthedeterminingthenecessaryparametersneededtocreateaconductivenetworkthatdoesnotadverselythemechanicalpropertiesforthemodelingofnanoparticlesystems.Twotpolymermatriceswerechosen,alongwithtwotkindsoffoamandmultipletypesofGnPtodevelopthenanocompositefoam.Theisarigidpolyurethane/polyisocyanurate(PUR/PIR)foamwithachemicalblowingagent.Polyurethane1011(PUR)isreadilyavailableandheavilyutilizedcommercialmaterialanditsfoamcounterparthasbeenextensivelystudied.PURfoamiscommonlyusedineverythingfrombuildinginsu-lationtosoundprotofurniture.Ithasbeenaroundfordecadesandheavilyresearchedforcommercialapplications,whichiswhythematerialhasawellunderstoodprocessing-structure-propertyrelationships.Itwasanidealpolymericfoamtocharacterizethechangestheadditionofnanoparticlesmakesonthemolecular,microscopicandmacroscopicscalesofthefoamwithabreadthofpotentialapplications.Polyisocyanurate(PIR)isthenextgenerationofPUR;ithasbetterthermalpropertiesduetoitshighercross-linkdensityfromtheformationoftrimergroupsandbetterheatandresistance[51].PIRisformedfrompolyurethanebyaddingacatalystthatcausescyclotrimerizationreactionstooccur[51].Thesecondpolymertypeisahighperformancepolymeralreadyutilizedintheaerospaceindustry.Siliconesareusedaseverythingfromcaulkingagents,inelectronicsandkitchenbakeware.Ithasveryuniquethermalpropertiesduetoitsunusualinorganic/organicstructure.Therearemanynttypesofsilicone,butoneofthemostcommonispolydimethylsiloxane(PDMS).Likeitsnameimpliesithasasilicon-oxygenbackbonewithmethylsidegroups.Itisnotaswellunderstoodaspolyurethaneandmuchoftheresearchofsiliconesisprotectedbycompanies.However,itsmanyusesandpropertiesuptolowtemperatures,generallylessthan-50°C,suggestsamaterialthatcouldbfromtheuseofmultifunctionalreinforcingagent.Silicondioxideisalreadyaddedtomanyttypesofsiliconetoimprovetheirmechanicalproperties.AsmentionedearlierduetotheproprietarynatureofsiliconesthisresearchwillfocusontheGnPhasonareadilyavailablecommercialsystem.Therearemanyttypesofcuringapproachesavailable,butroom-temperature-vulcanization(RTV),whichallowsforcureatroomtemperature,iscommonlyutilized.ThePDMSelastomercomesintwo-parts.Onepartcontainstheliquidrubberbaseandtheotherthecuringagent(catalyst).CommercialPDMSfoamsystemscommonlyhavelargeconcentrationsofsilicondioxideparticlesinthemgreaterthan15wt%.SinceoneofthepropertiestheGnPcouldpotentiallyimproveistheelectricalperformance1112havinganinsultingwoulddisrupttheformationoftheconductivenetwork.ForthisreasonPDMSelastomerwithnowaschosenasthematrixandhollowglassspheres(HGS)wereaddedtocreateasyntacticfoam.TheHGSareusedasatemplatetoformapercolatednetworkbycoatingeachspherewithGnPbeforeaddingtothePDMS.EachspherethenactsassingleconducivepointinthenetworkandwhentheHGSarecloseenoughtogetherelectronconductioncanoccur.LastlyforthisresearchttypesofGnPwillbeusedtogaininsightintothethatthesizehasontheresultingpolymericfoam.Ifelectricalcontactmattersthanaspectratioisanimportantfactortokeepinmindwhentryingtoreachthepercolationthresholdatlowloadings[52].ForthisreasonxGnP-M-25wasselected.AllM-gradematerialshaveasurfaceareabetween120-150m=2g,andthicknessesbetween6-8nm,inadditionxGnP-M-25haveanaverageparticlediameterof25m.Theseparticles,though,arerestrictedtoareasinthefoamthataccommodatetheirlargesizesoasmallerparticleisalsoused;xGnP-M-5whichhasanaveragediameterof5m.TheseM-gradeparticlesareverysimilarinappearanceasshowninFigures1.2and1.3(a).Thelargerparticleshavelargerbasalplaneswhicharegoodforconduction,butcanalsocausetheplateletstore-stackduetotheVan-der-Waalsattractionofthelayers,soparticlesthathaveadistinctlylargeredgedensitywereaddedaswell.ThesmallestparticlegradeavailableistheC-grade.Theyarehighsurfaceareaplateletswiththicknessesaroundafewnm.Theirdiametersarealso<2mandcommonlyonthescaleofhundredsofnm,givingthemdimensionssimilartothatofreducedGO,butstillhavebetterbasalplaneintegrityasthetotaladditionalgroupsofoxygenandnitrogenmakeuponlyabout11atomic%accordingtoXPS,partofwhichisduetothehigherconcentrationofedges.OneofthemaingroupsthatformonGnParehydroxyls,whichisoneofthereactiongroupsintheformationofurethane,inadditionitisbelievedthatoneofthereasonssilicadispersessowellinsiliconeisduetotheinteractionwiththehydroxylgroupsonthesilicondioxide[53].ThismeansthattherewasthepotentialthattheGnPcouldinteractwellwithbothmatricesandevenwhentheinteractionispoortheedge-groupsprovideaway1213toimprovethedispersionthroughchemicalmeanswhenthemechanicaldispersionmethodsprovet.(a)(b)Figure1.3:SEMimagesoftgradesofxGnP:(a)XGSciencesM-gradexGnPofaveragediameterof25mandthicknessof6-8nm(b)XGSciencesC-gradexGnPafterultrasonicationinacetone.ThegoalofthisprojectistocreateamultifunctionalnanocompositefoamthroughtheuseofamultifunctionalgrapheneWhatfollowsinthisdissertationisaninvestigationintothethatGnPhasonresultantpropertiesofthefoam.ThisisdonebyobservingtheGnPhasonthelocalmolecularstructure,onthemicroscopicstructureandllythemacroscopicproperties;thenutilizingtmeanstoimprovetheinteractionandtherebythedispersion.Chemicalmeansthroughedgefunctionalizationandcouplingagentsaswellasmorerigorousmechanicaldispersionmethodshavebothbeenemployed.Theofthesemethodswasnotonlyobservedinchangesinmacroscopicproperties,butimagesshowingtheparticledispersioninthefoamwithandwithoutatemplatewerealsoimportanttoolsforcharacterization.tchallengescamefromworkingwithachemicallyblownfoamversusasyntacticfoam,butitwasfoundthatbothwereectedbytheloadingsandaspectratiooftheparticlesandthethesehadonthedispersionwasakeypartandwillbediscussedinthisresearch.1314CHAPTER2MATERIALSANDEXPERIMENTALTECHNIQUES2.1Materials2.1.1PolymerMatricesThisresearchfocusesonmodifyingtwottypesofpolymerfoamswiththeadditionofamultifunctionalreinforcingnanomaterial.Thefoamhasapolyurethane/polyisocyanuratematrixthatutilizesachemicalblowingagenttopolymerizeandevolvegassimultaneously.Thesyntacticfoamuseshollowglassspheresasthecellsandthematrixisapoly-dimethylsiloxane(PDMS).Themultifunctionalisbothfoamsaregraphenenanoplatelets(GnP).2.1.1.1RigidPolyurethane/PolyisocyanurateFoamPolyurethane(PUR)waschosenasthematerialmatrixfortheblownfoambecauseitisareadilyavailablecommercialmaterialusedheavilyinindustry.Ithasbeenaroundfordecades,heavilyresearchedandcharacterizedsothereisawellunderstoodprocess-structure-propertyrelationship.Itisastandardpolymerofarelativelylowcost,whichmeansithasgreatpotentialformanytapplicationsasaealternative.However,itiscurrentlylimitedinitsapplicationsduetotheratherlimitingperformancepropertiesthatarecharacteristicofpolymers,suchasitslowmechanicalanddielectricperformanceaswellaspoorthermalstabilityandhighelectricalresistance.Thismaterialcouldthengreatlybfromtheadditionofmultifunctionalnanoreinforcementagent.Polyurethane(-NCOO)isaresultoftheadditionreactionbetweenpolymericisocyanate(-N=C=O)andhydroxyl(-OH)groups[51].ThereactionforsynthesizingpolyurethaneisshowninFigure2.1alongwithallotherreactionsoffocusinthissystem.Thismaterial1415Figure2.1:ChemicalreactionsforpolymerizationandgasevolutioninPUR/PIRfoam1516(a)(b)Figure2.2:ChemicalstructuresofisocyanatesusedinthesynthesisofPURfoams:(a)Polymericdiphenylmethanediisocyanate(pMDI)(b)2,4'-Toluenediisocyante(TDI)canbefoamedusingeitherphysicalorchemicalblowingagents.Forthephysicalblowingagentthepolyurethaneissynthesizedandcuredpriortoinfusingwithagas.Achemicalblowingagentaddsachemicalcomponenttothepolyolsidepriortopolymerization.Thesysteminthisresearchesutilizesanenvironmentallyfriendlychemicalblowingagent,whichiswater.Isocyanateiscommonlyreactswithwaterandwiththeassistanceofablowingcatalystthisreactionisacceleratedtoreadilyformcarbondioxideandurea.Thisispolyurethane/polyisocyanurate(PUR/PIR)mixture.PIRcontainstrimersthatformfromtheisocyanatewiththeassistanceofacatalysttopromotecross-linkinggivingthepolymerbetterthermalstability[51].Commonisocyanatesusedformakingpolyurethaneincludetoluenediisocyanate(TDI)andmethylenediphenylenediisocyanate(MDI)bothshowninFigure2.2.Becausethefoamisapolyurethane/polyisocyanurateblend,polymericMDI(pMDI)waschosenastheisocyanateduetothefactTDIisanunsuitableisocyanateforPIRfoampreparation[51].PolymericMDIisaliquidmainlycomposedof4,4'-MDIisomersandupto10%of2,4'-MDIandhasthegeneralstructureshowninFigure2.2(a).PolymericMDI(RubinateM)hasbeensuppliedbyHuntsmanandhasaviscosityof190mPas2,aspgravityat25°Cof1.23andanisocyanatecontentof31.1%NCO[54].HuntsmanalsosuppliedtwodifunctionalpolyolsFX-31andG30-650.FX-31isalowviscosityat25°C(250mPas2)polyolwithahydroxylnumberof240%OHcomparedtoG30-650whichhasaviscosityof1617880mPas2andhydroxylnumberof650%OH[54].TokeeptheviscosityaslowaspossibleethyleneglycolreagentfromCCI(#216500)wasutilizedasoneofthehydroxylcomponentsasithasawater-likeviscosity,andhydroxylcontentof181%OH[55].TheseisocyanateandhydroxylvaluesareusedtodeterminetheindexasshowninthesetofequationsinEquation2.1[51].TheindexnumberisusedasawaytodeterminewhethereveryNCOgrouphasacorrespondingOHgroup.Anumberabove100meansthereisanexcessofNCO,forrigidPURfoamsthisvalueisnormallybetween105and125andforrigidPUR-PIRtheindexisgenerallybetween180and135[56].AmineEquivalent=FormulaweightofNCO100%NCO=42:02100%NCO(2.1)OHEquivalent=FormulaweightofOH100%OH=56:11100%OH(2.2)IsocyanateIndex=NumberofAmineEquivalentNumberofOHEquivalent100(2.3)ForthePURfoamtheadditionalcomponentsarethechemicalblowingagent,poly-merizationandblowingcatalyst,andthesurfactanttopromoteuniformbubbleformation.Asmentionedbeforethechemicalblowingagentisdistilledwaterwithanapproximatedhydroxylvalueof7%OH.ThesurfactantandcatalystswereallsuppliedbyAirProducts.Thesurfactantisapolysiloxane(DabcoDC193)whichhelpsincellformationandhasnohydroxyls[57].Theblowingcatalystisacommonlyusedcommercialproduct(DabcoBL-11)with200%OH[57].Thepolymerization/gellingcatalyst(DabcoTMR-3)aidsintheformationoftrimersthatareakeycharacteristicofPIRs,andhasalargehydroxylvalueof2244%OH[57].BecauseitwasdesiredthatthefoammatrixbeaPUR,PIRmixtureavaluewaschosenandthecorrespondingratiosoftheindividualcomponentswasthenback-calculatedfromtheretakingintoaccountthatthedesireddensityisaround0.16g=m3.Lastly,2,4-TDIfromTCIChemicals(#T0264)wasusedtotreattheedgesoftheGnP[58].TheconcentrationofeachcomponentinthesystemareshowninTables2.1and2.2.1718Table2.1:RatioofconcentrationsofhydroxylliquidsusedinPUR/PIRrigidfoamformulation.PolyolComponentsConcentration(parts)FX31-24070G30-65015Ethyleneglycol15Table2.2:Concentrationoftcomponentsusedforfoamformulationnormalizedtopartsperhundredofpolyol(pphp).PUR/PIRComponentsConcentration(pphp)Polyols100Polysiloxanesurfactant0.8Blowingcatalyst0.05Cyclotrimerizationcatalyst0.6Distilledwater0.4pMDI1582.1.1.2FlexiblePolydimethylsiloxaneThesecondpolymermatrixusedinthisresearchisapolydimethylsiloxane(PDMS).Itisauniquepolymerconsistingofaninorganicbackbone(Si-O)andorganicmethylsidegroups.Thisstructurecausesittobebelowroomtemperatureasitsglasstransitiontemperaturesisgenerallylessthan-50°C.Thishigh-performancepolymerisusedheavilyintheaerospaceindustriesandwasidenasanotherpolymermatrixthatcouldgreatlybfromtheadditionofnanoparticlesforthisreason.ThePDMSusedinthisexperimentisfromMomentiveproductRTV615[59].Itisaconsistsoftwo-parts,aliquidrubberandcatalystthatuponmixingcuretoformaPDMSelastomer.1819Figure2.3:SEMimageof3MŠiM16Kglassbubblessprinkledwithamorphoussilicaonsurfaceandmeasureddiameters2.1.2HollowGlassSpheresTheglassbubblesiM16K(#98021327964)weredonatedby3MŠ.Thewhitepowdercontainssodalimeborosilicateglasssphereswithanaveragediameterof20mandcontainlessthan3%ofasyntheticamorphoussilicathatisnecessarytoensuretheglassspheresw[60].Thehollowglassspheres(HGS)thatcontaintheamorphoussilicagranulescanbeseeninFigure2.3Theseglassbubbleswerechosenfortheirhighcrushstrengthofgreaterthan110MPathatmakesthemabletowithstandthethermalexpansioncotofthePDMS[60].Lastly,astheglassspheresarehollowtheyhaveadensityof0.46g=cm2[60].AsilanecouplingagentwasusedtoadheretheGnPtotheglassbubbles.Thebinderisatrimethoxysilylpropylmopolyletheniminesilane(tPEI)fromGelest(#SSP-060),aschematicofwhichisshowninFigure2.4[61].1920Figure2.4:Chemicalstructureoftrimethoxysilylpropylmopolylethenimine,thesilanecouplingagentusedtoadhereGnPtoHGS.2.1.3TheusedinthisfoamisacostematerialsourcedfromXGSciences,graphenenanoplatelets(GnP).Grapheneconsistsofasinglelayerofhexagonallyarrangedofsp2carbonatoms.Thisuniquebondingandstructuregivessinglelayergrapheneveryhighin-planeproperties.Hightensilestrengthsof1TPa[22],highelectricalconductivityof104S=m[62]andhighthermalconductivityaround3000W=mK[21].Itismadethroughtheexfoliationofintercalatedacidmoleculesbetweenlayersgraphite,whichuponheatingforcethelayersapart,theprocessisdescribedinFigure1.1onpage8.BecauseitistointercalateacidgroupsbetweeneachlayerGnPgenerallyconsistsofstacksofgraphenesheetsmakingitamuchmorerobustnanoparticle,buthaspropertysimilartosinglegraphenesheets.Itcomesinavarietyoftypesandsizes.InthisresearchbothaM-gradeproductandC-gradeproductwereutilized.Thebetweenthetwoaretheirsurfaceareasandthicknesses.M-gradematerialshavesurfaceareasofaround120m2=gandthicknessesof6-8nm,inadditiontwosizeswereusedthathadaveragelateraldimensionsof25(GnP-25)or5(GnP-5)m[63].TheotherC-gradetypeisahighsurfaceareamaterialof750m2=gwithaveragediametersoflessthan2m(GnP-750),buttheas-receivedmaterialappearsassub-micronaggregates[63].Allas-receivedGnPisheatedat450°Cpriortoaddingtothenanocompositefoam.20212.2ExperimentalProcedure2.2.1PUR/PIRNancompositeFoamSynthesisThefoamsynthesisisdoneaccordingtothestepsbelow:1.Thetwopolyolswithethyleneglycolarecombinedandstirredfor2haccordingtotheratioslistedinTable2.1.2.Thesurfactant,catalystsandblowingagentareaddedtothepolyolblendandstirredforonehouraccordingtheratiosinTable2.2.3.IfGnPisrequiredintheformulationitisthenaddedtothepolyolblendatthecorrectweightratio.‹3wt%ofthe5wt%totalGnP-25.‹6wt%ofthe8wt%totalGnP-5andGnP(750).4.GnPishigh-speedshear-mixedfor2minat1600rpmand1minat2400rpmfollowedbyultrasonicationat100Wwitha2.54cmprobefrom5to20mindependingontheconcentrationandtypeofGnP.5.PolymericMDIisaddedintoaseparatecontainer.6.IfGnPisrequiredtheremainingwt%isaddedintothepMDIsothattheviscositiesofthetwocomponentsiskeptaslowaspossible.7.PolymericMDIblendfollowsthesamehigh-speedshear-mixingprotocolandisthenultrasonicatedwiththesameprobeforthesameconditionsuntilblendedgenerallyfor5to15min.8.Thetwoblendsarecombinedwithanimmersionblenderfor45s.9.Thematerialispouredintoamoldtofree-riseandallowedtocureovernightatroomtemperature.2122ThedispersionofGnPinpolyolblendischeckedusingantanceopticalmicroscopetoensurethattheparticlesarewelldispersed.Sampleswerealsosynthesizedthatcontainednoblowingcatalyst,agentorsurfactant.Theprocedureandconcentrationofcomponentsstaysthesameexceptsteptwoismoditoonlystirfor30minwithonlytheadditionofthepolymerizationcatalyst.Themoldisastainlesssteelpancoveredinnon-porouspaperandhasthedimensions:5cmx15cmx25cm.2.2.2Edge-FunctionalizationofGnPAs-receivedGnPcontainshydroxyl-edgegroups,whichfromthedetailsofthechemistrystatedinsectiononereadilyreactwithpolymericisocyanatetoforformpolyurethane.PolymericMDIwasaddedtotheGnPinoneoftwoways.Forthemethod,GnPareheatedtoabove100°Cforanhourtoremovedadsorbedwatermolecules,enoughpMDIisaddedtocompletelycovertheGnP.Thismixtureisthenheatedto140°CeitherovernightorforonehourthencooledpriortoremovingtheexcesspMDI.TheGnPiswashedwithacetone,centrifugedtocollecttheparticlesandtheexcessliquidisdecantedandtheprocessisrepeatedmultipletimes.Theotherprocedureismuchmoreinvolvedandfocusesoncreatingamonolayerofurethanetypegroupsontheparticlesurfaceaccordingtothefollowingsteps:1.GnPisdispersedinacetoneusinga2.54cmultrasonicationprobeat100Wataconcentrationofabout6gofGnPpereveryLofacetone.2.Thesolutionisultrasonicatedforatleast1h.Thesolutionisdropcastontoaglassslidetoobservethedispersionwithanopticalmicroscope.3.AsmallamountofpMDIisaddedtothesolutionataratioof64Lforevery6gofGnP.4.Ultrasonicationwiththesameprobeandpowerisdoneforanadditionalhour.22235.Theedge-functionalizedGnPisthenretainedusingthesamewashing-centrifuging-decantproceduredetailedearlierinthechapter.6.Afterthelastdecantthematerialisdriedovernightatslightlyelevatedtemperature.ThissameprocedurewasalsodoneusedtotreattheGnPwithTDIinplaceofpMDI.2.2.3NanocompositePDMSTheneatPDMSelastomerismadebypouringpartAthenpartBintoamixingvesselataratioof10:1byweight.Thematerialisthenhandmixedfor30sfollowedbyhighspeedshearmixingat3000rpmfor2min.Aftermixingitispouredintoasecondarycontainerfordegassingthencastintomolds.Thecastspecimensarethenbakedat100°Cfor1h.ForthesamplesthatcontainGnP,itisaddedafterthehandmixingofpartAandBandpriortothehighspeedshearmixing.Theweightadditionsaredeterminedrelativetotheneatsample.TheGnP/PDMSisthenmixedat3000rpmfor2minfollowedbyanadditionalminuteafterthematerialcoolsdowntolimithowhotthematerialgetsduringmixingtoavoidcuring.2.2.3.1CoatingGlassMicrosphereswithGnPTheglassbubblesarecoatedwithGnP-5andGnP(750)priortoaddingtothePDMSelastomer.Thetotalweightpercent(Wt%)ofGnPneededtocoatthesphereswith5layersisgivenbyequation2.4below:(Wt%=6tˆGnPDiM16KˆiM16K)5(2.4)Inthisequation,tisthethickness,ˆGnPandˆiM16KistherelativegravityoftheGnPandglassbubbles,respectively,andDiM16Kistheaveragediameteroftheglassbubbles.Thetotalamountismultipliedby5as5layerswaschosentoensureaconductivecoatingformsonthesurface.TheresultingamountofGnP-5andGnP(750)wasdividedbytwosincebothtypesofGnPweresimultaneouslycoatedonthesurface.Theprocedureisasfollows:23241.GnP(750)isaddedtoreverseosmosis(RO)waterataratioofabout90mgto500ml.2.Thesolutionisultrasonicatedwitha2.54cmprobeat100Wfor1h.3.TheGnP-5isthenaddedfollowedbyadditionalultrasonicationfor1hwiththesameparameters.4.AddthetPEIbindertothesolutionataratioof1:1wt%toGnP.5.Ultrasonicatefor4minwitha2.54cmprobeat50W.6.SlowlyaddtheHGSwhilestirring7.Continuestirringovernight8.TheGnPcoatedglassbubblesarethencollectedbybeforedrying.9.AfterdryingtheGnPcoatediM16Kishighspeedshearmixedat1200rpmfor30stobreakuptheclumps.TheresultingglassbubblesareshowninFigure2.5.ComparingtheneatandGnPcoatediM16KthereismoreplateletlikeappearanceonthesurfaceduetotheGnPandespeciallytheverythinGnP(750).2.2.3.2PDMSSyntacticFoamProcedureTheinitialstepsarethesamefortheneatPDMSsamplewherethetwopartsoftheelastomerarecombinedbyhandmixingfor30s.TheglassbubblesareaddedbyvolumepercenttothePDMSelastomerpriortobeinghighspeedshearmixedat3000rpmfor2min.TheGnPcoatedglassbubblesaremixedforanadditionalminat3000rpmaftercooldowntoensureadequatemixingandtopreventthematerialfromgettinghotenoughtostartsolidfying.Thematerialisthenpouredintoasecondarycontainerfordegassing,followedbycastingintomolds.Thematerialisthenheatedat100°Cfor1h.2425Figure2.5:SEMimageofHGScoatedwithGnP-5andGnP(750)afterdrying.Thesamplewasnotcoatedpriortoimaging.2.3TestingProceduresTheexothermicbehaviorofthefoamwasobservedwithameteredprobe.Afterthefoamwasmixedandpouredintothemoldtheprobewasplacedinthecontainer,heldinplaceatananglewithabinderclip.Thetemperaturewasthenrecordedfor500s,whichisthemaximumnumberofdatapointsthemetercouldhold.2.3.1X-rayPhotoelectronSpectroscopyDuetothesmallamountoffunctionalgroupsexpectedtobeonthesurfaceGnP,x-rayphotoelectronspectroscopy(XPS)waschosentocharacterizethechemicalgroups.InadditiontoXPSbeingasurfacetechnique,withapenetrationdepthofupto1m,XPSmeasuresthebindingenergiesofthesurfacegroups,whichalsocangiveinsightthentothechemicalgroupsaswellastheelementsonthesurface.BecauseM-gradexGnPhasnonitrogengroupspriortofunctionalizationthenitrogenspectrawasfocusedontodeterminewhattypeoffunctional2526groupsactuallyformonthesurfaceafterchemicaltreatment.FirstascanwasdoneonaneatPUR/PIRsamplethatcontainednoGnP.Theonlypeakthatappearedonthenitrogenspectrawasabroadpeakat400eV,whichisthenthoughttobedotourethane-typebonds.Thenitrogenspectraoftheedge-functionalizedGnPshowsthreepeaks:theiscenteredataround399eVandiscommonlyattributedtoaminegroupsandprobablyrepresentsurea[64],thesecondistheurethanepeakandthethirdiscenteredataround401.3eV,thehighestbindingenergyisprobablyduetopMDIgroups.ThepMDIcouldbefromunreactedpMDIorsimplytounreactedsegmentsofthepolymerchain.2.3.2ThermalAnalysisThermogravimetricanalysis(TGA)wasusedtodeterminehowthematerialdegradeswithtemperatureatarateof20°C=mininanitrogenenvironmentataresolutionof4.tialscanningcalorimetry(DSC)wasusedtodeterminetheglasstransitiontemperature(Tg).Thereferencewasanemptypan.Thesampleswererunat10°C=minto140°Cbeforebeingallowedtocoolto50°Catarateof20°C=min.EachsamplewasruntwiceandtheTgwasdeterminedfromthecoolingramp.2.3.3MicroscopyAnopticalcemicroscopewasusedtocheckthedispersionofGnPinthesolutionbydroppingadropofthesolutionontoaglassslidewithadisposablepipette.Inaddition,scanningelectronmicroscopy(SEM)wasutilizedtoimagethecellstructureofthefoamaswellasthedispersionoftheGnP.Thesamplesarealwaysadheredtothestubwithconductivecarbonpaste.FortherigidPUR/PIRthesamplesarecutforimaging,whereasthePDMSsamplestheimagesweretakenofthetornsurfaceafterasmallcutonthesurface.FieldemissionSEM,ahighresolutionSEM,incombinationwithfocusedionbeam(FIB)wasabletocutsectionsofthePUR/PIRrigidfoamtodeterminehowwelltheplateletsdispersed.FIBusesastreamofgalliumionstocutthematerialbyrasteringthroughthematerial.2627ImagesofthecellstructureweretakenontheSEM.SixtycellsweremeasuredwithImageProsoftwaretodetermineeachcell'smaximum,minimumandaveragediameter.2.3.4MechanicalandElectricalFourspecimenswerecutfromthePUR/PIRfoamsampleswithdimensions25.8c2mby2.54cmthick.Thespecimensarethentestedonauniversaltestingsystem(UTS)placedbetweentwoparallelplatenscompressedto10%atarateof2.5mm=min.Electricalresistivity(AC)wasdoneonthreespecimenseithercuttothedimensionsof3mmthickby10mmwideby40mmlongorcastthosedimensionsforthePDMS.Atwo-pointprobewithadistanceof1cmbetweentheprobeswasincontactwiththespecimensurfacewiththeassistanceofconductivesilverpaint,measurementwastakenontheGamry.2.3.5ElectromagneticPerformanceThedielectricpropertiesweredeterminedusingatransmissionlinesystem,whichincludesavectornetworkanalyzer(VNA)connectedtoawaveguide.Thewaveguidedimensionswereasfollows:10.5mmby22.9mmby7.4mmthick.ThreespecimenswerecuttothisdimensionfortherigidPUR/PIRfoamorcastfortheblePDMS.TheNRW(NicholsonandRoss[65]andWeir[66])algorithmwasusedtodeterminethecomplexrelativepermittivityandpermeabilityaccordingtotherelationshipsbelowassumingthesamplesareisotropic.Relativerefersthefactthatthepermittivityandpermeabilityarearatiorelativetothepermittivity2728andpermeabilityoffreespace.Alltestingwasdoneatroomtemperature.r=2ˇqk20k2c1+1(2.5)"r=1rk20(4ˇ2+k2c)(2.6)where;X=S211S221+12S11(2.7)=XpX21(2.8)T=S11+S211(S11+S21(2.9)12=12ˇLln(T)2(2.10)andkc=2ˇ(2.11)k0=2ˇ0(2.12)k0isthewavenumberinair.Italsousedinapplyingaphasecorrectionfactor,e0L,forthetransmissionscatteringparameter,S12=S21,toaccountforthetransmissionlinelengthdisplacedbythesampleoflengthL,where0=jk0.Inaddition,S12inthisset-upisameasureoftherespectivevoltageswhenamaterialispresentandwhenitisnotpresent,andisusedintheoftheshieldingeenessaccordingtotheequation2.13,SET=20logV1V2=20logjS12j(dB)(2.13)ThetotalshieldingenessisthesumofthelossesfromonSER,absorptionSEAandmultipleSEMasthewavepropagatesthroughthesample.ThescatteringparametersateachoftheportscanbeusedtodeterminethefractionoftheEMwavethatistransmitted,andabsorbed.Thetransmittance(T),(R),whichincludescontributionfrommultipleandabsorbance(A)withrespecttotheincidentwave2829isgivenbythefollowingequations[67]T=jS12j2(2.14)R=jS11j2(2.15)A=1RT(2.16)Thisconcludesthedetailsregardingthematerials,experimentalproceduresandtestingmethodsperformedontherigidPUR/PIR,rigidPUR/PIRfoamandlePDMSsamples.Themechanical,electrical,thermalanddielectricpropertiesoftherigidPUR/PIRfoamweretested.TheonlyPDMSpropertiescharacterizedwerethedielectricandEMIshieldingperformance.BothnanocompositefoamswereimagedonaSEMtodeterminecellstructureandthedispersionofGnP.2930CHAPTER3INTERACTIONSOFGNPINRIGIDPUR/PIRFOAM3.1IntroductionTechnologicaladvancementsarealwaysgoingtobelimitedbythematerialsthatareavailable.Andastechnologyprogressesthereareincreasinglyhigherdemandsonthematerialsemployed.Itisnolongerenoughtohavematerialsthataresimplystrong,tough,orCertainapplicationsrequirethattheyalsobeopticallytransparent,light,showhighelectricalconductivity,lowheattransferorhighpermittivity.Suchmultifunctionalmaterialscannotbesynthesizedsimplyofpolymers,ceramicsormetalsandrequireamixtureofthetpropertiesofthesematerialsthusleadingtothedevelopmentofengineeredmaterials.Thesematerialsarevastlymorecomplexandoftencombinetclassesandaswellasformsofmaterialsandsynthesistechniquestoachievethedesiredproperties.Onerecenttrendintheofengineeredmaterialsiscompositesandwiththediscoveryofnanoparticles,theevolutionofnanocomposites.Polymernanocompositesarearelativelynewbutsuchmaterialsshowpromisingresultsaseverythingfromthermalshielding[8]toenergyapplications[10],toelectromagneticinterference(EMI)shieldingdevices[68,69,70,71]topackaging[7]andbeyondasnewmultifunctionalpolymernanocompositesarebeingproducedallthetime.Carbonbasednanoparticlesshowagreatdealofpromiseasacomponentinthesynthesisofmultifunctionalmaterials.Buttherearemanychallengesassociatedwiththeapplicationofthesematerialsfromtheirmanufacture,processingtocharacterization;asoftenthesematerialsneedtobetightlyengineeredatthemolecularleveltocreatematerialsthathavethedesiredmacroscopicproperties.Oneofthemorepromisingcarbonnanomaterialsthatiscurrentlyverypopularin3031nanocompositeresearchisgraphenedueitsmultifunctionalproperties.Graphenedescribesasinglelayerofsp2hybridizedcarbonatomsinahexagonalarrangement.SinglelayergraphenehasbeenshowntohavegoodmechanicalpropertieswithahighYoung'smodulusofaround1TPa[22],highthermalconductivityofabout3000W=mK[21]andgoodelectricalpropertieswithcarriermobilitiesgreaterthan15,000cm2=Vs[19]duetoitsunique2Dstructure.Therearecurrentlymanymethodstosynthesizegraphene,ingeneralthepurityandabilitytocreatesinglesheetsscaleswithcost.Consideringthathighloadingscouldbeneededinabulknanocompositetoimprovetheoverallproperties,onecommoncostegraphenesynthesismethodreliesonexfoliatinggraphiteintothinlayerstocreatemultilayeredgrapheneparticlesthathavesimilarpropertiestosinglelayergrapheneinamorerobustform.Onedesirablepropertyofpolymersistheirrelativelylightweightthatcanbedecreasedevenfurtherasafoam.Ofcoursepolymericfoamserfrommanyundesirablepropertieswhichlimittheirapplicationsandthustheadditionofanantothepolymericmatrixcouldbeagreatwaytoimproveitsproperties.Thischapterdetailstheinvestigationsdonetocharacterizethethataddingananoplateletmaterialhasonthepolymericpropertiesofachemicallyblownfoam.Sincepolymerizationandgasevolutionoccursimultaneouslyinachemicallyblownsystemtherearemanywaysthataddingnanoparticlescouldtheresultantpolymericfoamproperties.Understandingthepositiveandnegativeaddingnanoplateletshaveonsuchasystemwillhelpinunderstandingtherelationshipbetweenthegas,polymerandnano-reinforcementmaterial;therebymakingiteasiertoidentifywaystoimprovetheinteraction.Focuswasonthethenanomaterialhasonthemolecularformation,stabilizationofthefoamandonthebehaviorofthegasevolution.Thisresearchwillhelpinunderstandingsuchthreephasesystemsandhowbesttooptimizetheirinteractionstogetthedesiredresultantproperties.Thisexperimentalstudyfocusesonapolyurethane/polyisocyanurate(PUR/PIR)matrix.Thismaterialmatrixwaschosenforitswellunderstoodprocess-property-structurerelationshipduetoitsheavyuseinindustryaseverythingfrombuildingmaterials,furniture,andsound3132dampeners.Thereinforcingaidisamultifunctionalgraphenematerialthatconsistsofstacksofgraphenesheets.Becausethisproductreliesheavilyongraphiticmaterialtoimprovesppropertiesthereisthepotentialforthisnanomaterialtobeadaptedtootherchemicallyblownpolymericsystemdependingontheapplication.Inaddition,graphenenanoplatelets(GnP)arearelativelyinexpensivenanoreinforcingagentsothereisgreatpotentialthatanyresultantfoamcouldberelativelyhavealowdensity,andmultifunctionality,makingforbetter,lessexpensiveproducts.3.2ExperimentalProcedure3.2.1MaterialsThepolymericmatrixisaPUR/PIRfree-riseclosed-cellrigidfoamandhasadensityof0.16g=cm3.Itutilizeswaterasthechemicalblowingagent,whereanadditionreactionbetweenwaterandisocyanateformsureaandevolvescarbondioxide.HuntsmansuppliedFX31-240,G30-650andRubinateM.JeFX31-240andG30-650arediolpolyols,withhydroxylnumbersof240and650mgKOH,respectively.G30-650hasamolecularweightof260andaviscosityatroomtemperatureof880cP-s.FX31-240hasamolecularweightof700androomtemperatureviscosityof250cP-s.ThesetwopolyolsaremixedwithethyleneglycolfromJTBakeraccordingtotheratioshowninTable2.1tocreateadifunctionalpolyolsystemthatstillmaintainsalowviscosity.RubinateMisapolymericdiphenylmethanediisocyanate(pMDI)thathasanNCOvalueof31.2androomtemperatureviscosityof190cP-s.ThestructureofpMDIisshowninFigure2.2(a).ThesurfactantandcatalystswerereceivedfromAirProducts;DabcoDC193isapolydimethylsiloxanesurfactant,DabcoBL-11istheblowingcatalystandDabcoTMR-3isapolymerizationcatalystthatformsthetrimergroupsfoundinthePIR.DistilledwaterisusedastheblowingagentandallthecomponentsaremixedaccordingtotheratioinTable2.2.Toluenediisocyanate(2,4-TDI),anisocyanatealsocommonlyusedinthesynthesisofpolyurethane,isfromTCI3233ChemicalsanditsstructureisshowninFigure2.2(b).GnPareamultifunctionalsourcedfromXGSciences.Itisacostethingraphiteparticleconsistingofmultiplelayersofgraphene,butwithcomparablepropertiestosinglegraphenesheetsinamorerobustform.ForthisstudythreetypeswereusedfromXGSciences:thetwohave120m2=gsurfaceareawithlateraldimensionsofeither5m,designatedGnP-5,or25mdesignationGnP-25;thelastisahighsurfaceareamaterialof750m2=gwithanaveragediameteroflessthan1m,designatedGnP(750).AllGnPisbakedat450°Cfor2hourspriortouse.AnSEMimageofas-receivedM-gradeGnPisshowninFigure1.2onpage9.3.2.2ExperimentalProcedure3.2.2.1RigidPUR/PIRSynthesisTwotypesofrigidPUR/PIRmaterialweresynthesized;onealowdensityfoamandtheotheramonolithicpolymerthatdidnotcontainthesurfactant,blowingcatalystoragent.Theprocedureisasfollows:thepolyolswerecombinedwithethyleneglycolinatypicallabenvironment.Thentherestoftheexperimentiscarriedoutinanenvironmentalgloveboxtoreduceexposureofthechemicalstomoisturewhichcanabsorbincertainchemicalsdecreasingthedensity.Thepolyolsaremixedwithapaddle-stirrerfor2hours.Forthefoamthesurfactant,catalystsandblowingagentareaddedintothesystemthenmixedforonehour,whereas,forthemonolithicpolymeronlythepolymerizationcatalystisadded,followedbymixingfor30minutes.Asmentionedpreviously,Tables2.1and2.2onpage18detailstheratiosusedforthesynthesisofboththemonolithicandfoam,ifthecomponentswerenotneededtheywereremovedfromthechemistrybuttheratioswerekeptconstant.IfGnPisrequireditisaddedtothepolyolblendupto6wt%forGnP-5andGnP(750)orupto3wt%forGnP-25,theremainingamountofGnPrequiredisaddedtothepMDI.Eachblendisthenhigh-speedshearmixedat1600for1min,then2400rpmfor2min.Thenproceeds3334tobeultrasonicatedwithaone-inchprobeat100Wuntilwelldispersed;thedispersionoftheGnPinthepolyolblendisusinganopticalmicroscopebuttakesanywherefrom5-20minofultrasonicationdependingontheloadingandtypeofGnP.Thetwocomponentsarethenallowedtocoolbeforebeingcombined,stirredwithanimmersionblenderfor45sandthenpouredintoamoldandcuredovernight.Themoldisa0.175mLstainlesspanlinedwithpaperandsamplesarecutfromthecuredsample.3.2.2.2Edge-functionalizationofGnPForthepMDItreatedGnP,thebakedGnPwaspouredintoabeakerandheatedtoover100°CtoremoveanyadsorbedwatermoleculesontheGnP.ThenenoughpMDIisaddedtocompletelycovertheGnPandisthenreactedat130°Cfor1horovernight.Thematerialisthencooledanddispersedinacetonebeforecentrifugingtocollectthesolidswhiletheexcessliquidisdecanted.ThewashingprocedurewithacetoneisrepeatedatleastsixtimessothatthemajorityoftheexcesspMDIisremoved,followedbydrying.AnothermethodfocusedoncreatingathinlayerofedgegroupsbyreducingtheamountofpMDIavailableforreaction,designatedas"M".First,theGnPisdispersedinacetonebyultrasonicationatlowconcentrations,about6gofGnPperevery1Lofacetone.Thisisfollowedbytheadditionofabout64LofpMDI.Thesolutionisultrasonicatedwitha2.54cmprobeat100Wfor1hourtoreact.Themajorityoftheacetoneisthenboiledbeforethematerialgetswashedmultipletimesusingthesameprocedurewithcentrifugationdetailedbeforetoremoveanyunreactedpolymer.Thissameprocedurewasrepeatedbutwithtoluenediisocyanate(TDI)asthereactantinstead.BothofthesemoleculargroupsareshowninFigure2.2onpage16.3.2.3TestingProceduresThefunctionalgroupsonthepMDItreatedGnPischaracterizedusingax-rayphotoelectronspectrometer(XPS)asthebasalplaneoftheGnPisveryinertandreactionwouldbelimited3435totheedges.XPSmeasuresthebindingenergyoftheatomsandthereforeisabletogetidentifyatomsandthemolecularstructureonthesurfaceasthepenetrationdepthfortheinstrumentislessthan1m.ThenitrogenspectrawasusedtoidentifythechangeintheedgecharacteristicsoftheGnPafterreactionwiththepMDIandTDIasM-gradexGnPcontainsnonitrogenpriortothetreatmentandC-gradecontainsonlyabout1atomicpercent.ThethermaldegradationwastestedonaTAInstrumentsthermograviometricanalysis(TGA)systematarampof20°C=minataresolutionof4underinertnitrogengasto750°C.TheglasstemperaturewasdeterminedusingaTAtialscanningcalorimetry(DSC).Threespecimensweretakenfromeachsample.Eachspecimenwasrampedto140°Cat10°C=minandcooledto30°Catarateof20°C=minandthecoolingcurveisusedtodeterminetheTg.Inaddition,theexothermicbehavioroftwofoamsampleswasmeasuredbyinsertingameterprobeintothematerialrightafteritwaspouredintoapanfor500s,themaximumcountforthemeter.ImagesofthedispersionofGnPinthestrutsofthefoamweretakenonaemissionSEM,ahighresolutionSEM,incombinationwithfocusedionbeam(FIB)wasabletocutsectionsofthePUR/PIRrigidfoamtodeterminehowwelltheplateletsdispersed.FIBusesastreamofgalliumionstocutthematerialbyrasteringthroughthematerial.Lastly,thecellssizesweredeterminedfromimagestakenonascanningelectronmicroscope(SEM).Thediametersof60cellsweredeterminedwiththeassistanceofImageProAnalysissoftwarethatmeasuresthecell'smeandiameter.AcellalongwiththeotherpartsofthefoamstructurearehighlightedinFigure3.1.Sinceallofthehistogramshaveatleasttwo"peaks"duetothecellscausedbythegasevolutionandthosecausedbyairbubblesduetothemixingthemediancellsizeisgiveninadditiontotheaveragediameterandsampledeviation.3.3ResultsTables3.1and3.2showtheatomicpercentsofthenitrogenspectraonthetreatedGnPanexampleofwhichisshowninFigure3.2.XPSwasalsoperformedontheneatPUR/PIR3536Figure3.1:SEMimageofPUR/PIRrigidfoamhighlightingthetpartsinthecellularstructure.rigidfoamandtheonlypeakinthenitrogenspectrawasabroadpeak300eVsothisbindingenergyisattributedtourethane-typegroups(PUR,polyurea).Thepeakat398eViscommonlyattributedtoamines[64].Thepeakat401.3eVisbelievedtobeduetounreactedsegmentsofpMDIorpossiblyexcessthatwasnotremovedduringthewashingprocess.Table3.1:Atomic%ofchemicalgroupsonGnPafterreactionovernightwithpMDIasdeterminedbyXPSBindingEnergy(eV)pMDIGnP-25pMDIGnP-5401.30.6(12%)0.6(17%)4002.9(57%)2.5(71%)3981.6(31%)0.4(12%)Total5.13.53637Figure3.2:AnexampleofthenitrogenspectraofpMDItreatedGnP.AbroadnitrogenpeaksuggeststhatthebindingenergyisnotthesameforallthenitrogenatomsandusingFTTsoftwarethebroadspectracanbebrokendownintotheirindividualnitrogengroups.3738Table3.2:Atomicpercent(at%)ofchemicalgroupsonGnPafterreactionwithpMDIandTDIasdeterminedbyXPS.Mdesignatesthemonolayermethodofreaction,forallothersthereactantwasinexcess.BindingEnergy(eV)pMDIGnP-25pMDIGnP-5TDIMGnP-5pMDIGnP(750)pMDIMGnP(750)TDIMGnP(750)401.50.3(9%)1.4(22%)0.2(29%)0.9(15%)0.6(35%)0.4(22%)400.12.7(84%)4.3(69%)0.3(42%)4.2(69%)0.9(53%)0.9(50%)398.90.2(7%)0.5(8%)0.2(29%)1.0(16%)0.2(12%)0.5(28%)Total3.26.20.76.11.71.839Figure3.3:ThermaldegradationofPUR/PIRrigidfoamandthemonolithicpolymer.ThesolidlinesrepresentstherigidPUR/PIRfoamandthedashedlineistheforthemonolithicrigidPUR/PIR.Figure3.3isacomparisonofthethermaldegradationfortheneatrigidmonolithicandfoamedPUR/PIRsampleswithnoGnP.Italsoshowshowthederivativeplotsrelatetotheplotsofweightloss.Figures3.4and3.5areoverlaysofthemonolithicrigidPUR/PIRmatrixwithvaryingtypesofGnP.Figures3.6-3.10covertheTGAandderivativefunctionoftherigidPUR/PIRfoamsamples.Thederivativeplotsoftheweightlosshighlightthefactthattherearesharpchangesintherateofdecompositionatcertaintemperaturesthatmatchthedegradationtemperatureofcertainmoleculargroups.Thepeaksthatformthencouldbeattributedtothedecompositionordissociationoftmoleculargroupsinthepolymer.IsocyanateisaveryreactivemoleculeandFigure3.11showssomeofthereactionsthatareoccurringduringtheformationwater-blownPUR/PIRfoamsomeofwhicharereversiblelikethebiuretsandallophanatesandsomeofwhicharepromptedtoformbycertaincatalystssuchasthecyclotrimerizationreactionthatformsisocyanurate[51].Ureacanformfromthereactionofisocyanatewith3940Figure3.4:GraphoverlayofthethermaldegradationofthemonolithicrigidPUR/PIRwithandwithoutGnP("standard").Figure3.5:GraphoverlayofthederivativethermaldegradationofthemonolithicrigidPUR/PIRwithandwithoutGnP("standard").4041Figure3.6:ThermaldegradationofPUR/PIRrigidfoambefore("standard")andaftertheadditionofttypesandloadingsofGnP.primaryandsecondaryaminesandisalsooneofthebyproductsfromthereactionofisocyanateandwater.Theotherproductiscarbondioxide,andisthereforethemainreactionresponsibleforthematerialexpanding,theotherisfromacondensationreaction[51].Previousresearch[72,73,74,75,76,77]hasinvestigatedhowthemoleculargroupsinPURdecomposeontheirownandinPURfoams.Biuretsandallophanatesstarttodissociateafter100°C,butthesereactionsaresomewhatreversibleandsodissociatebacktothegroupstheyformfrom:polyurea,PURandpMDI[74]andsodonotnecessarilyappearinthedecompositionandinfact,thederivateplotsofthedegradationshownoweightlossbelow200°C.ThismatchesotherworkdonebyGrassieandMendoza[73]that4142Figure3.7:GraphoverlayofthederivativeweightofthethermaldegradationofPUR/PIRrigidfoambefore("standard")andaftertheadditionofttypesandloadingsofGnP.Figure3.8:ComparisonofthederivativeweightthermaldegradationpofrigidPUR/PIRfoamoftheneat("standard")versusaftertheadditionof5wt%bakedGnP-25andedge-functionalizedpMDItreatedGnP-25.4243Figure3.9:ComparisonofthederivativeweightthermaldegradationpofrigidPUR/PIRfoamoftheneat("standard")versusaftertheadditionof8wt%bakedGnP-5andedge-functionalizedGnP-5withpMDIinexcessorTDIusingthemonolayermethod(M).structuralchangesinthepolymerbelow250°C,butdidnotresultindegradationuntil270°C.Theyconcludedthatthesharppeakabove250°Cisdueprimarilytothedepolymerizationofpolyurethane[73,74].Thesecondandgenerallylargestpeakincludesthebreakdownofthemorethermallystableurea,aswellasthedecompositionofthenowurethanemonomer.UrethaneinPURfoamdecomposesataround300°Candureahasadecompositiontemperatureabout20°Chigherthanthatofurethaneandformsfromatleast2reactions[78,51].DecompositionofpolyureacouldalsobehavesimilarlytoPURwherepolyureabreaksdowntomonomerspriortothedecomposition.Atabout400°Ctheisocyanurate-urethanelinkagebeginstodecompose[78,51]atamuchslowerrateasshownbythepeakcenteredaround400°ConthederivativeweightplotsofthethermaldegradationThereisanotherpeakcenteredat350°Cthatcouldbeduetothedecompositionofproductsthatformedfromthedecompositionofurea[79].GrassieandMendozafoundthatundervacuumprimarydecompositionproductscan4344Figure3.10:ComparisonofthederivativeweightthermaldegradationofrigidPUR/PIRfoamoftheneat("standard")versusaftertheadditionof8wt%bakedGnP(750)andedge-functionalizedGnP-5withpMDIinexcess,pMDIusingthemonolayermethodorTDIusingthemonolayermethod(M).readilyescapefromthedegradingpolymer,butforhigherpressuresthatescapeiscontrolledandifnotfastenoughallowsforsecondaryreactionstooccur,suchascondensationreactionsresultingincarbodiimideandamidesthatalsoreleasecarbondioxide[80,73].Othervolatileproductssuchascyanicacid,butadiene,tetrahydrofurananddihydrofuranandwaterarealsobeingformedfromsecondaryreactionstakingplacewithinthehotpolymer[80,73,75].Sothesubsequentmoleculargroupsthataredegradingcouldalsocomefromsecondaryreactionsthathavetakenplaceafterthepolymerstartstobreakdown.Withinallthedecompositionthereisacontinuedbreakdownofpreviouslyunaccessiblemoleculargroupsasthetemperatureincreasesandthepolymerbecomesmoreresultingincascadingsecondaryreactionsaswell[80].Asthesemoleculescontinuetodissociateandescape,lessofthedecomposedproductsareinvolvedinsecondaryreactionsbutthoseproductsthatdidnotescapereacttoformnewmoleculesthatbecomethecontributingmonomerstothe4445Figure3.11:SomechemicalreactionsthatoccurduringthefoamingofaPUR/PIRwater-blownfoam.4546Table3.3:GlasstransitiontemperatureofmonolithicPUR/PIRbefore("standard")andaftertheadditionofGnPfrom3specimens.RigidPUR/PIRTg(°C)SampledeviationStandard105.90.35wt%GnP-25100.52.35wt%pMDIGnP-25103.52.08wt%GnP-593.77.28wt%pMDIGnP-592.85.0polymerstructureandbegintobreakdownaswellbecomingthecontributorstothechemicaldegradationabove450°C.TheTGAplotsalsoshowthatthereisstillaresidueatabout730°C,whichiscommoninpolyurethanefoamsthatdonothavetheassistanceofremovalofdecomposedproductsfromthehotpolymerandisfoundtoincludealargeamountofcarbodiimide[73].Thisiswhythedegradationisvirtuallynonexistentabove500°Castherategoesbacktozero.Theremainingdecomposedproductsmustbecontinuallydecomposingandreactingwitheachother.Theglasstemperature(Tg)asdeterminedbyDSCisshowninTables3.3and3.4.InadditiontheexothermicbehaviorofthefoamstandardsamplewithoutGnPcomparedtothefoamsamplewith5wt%GnP-25isshowninFigure3.12.Lastly,sincethissystemusesachemicalblowingagentthecellsizecanbedrasticallybytheadditionofthenanoparticles.TheaveragecellsizeshownwiththesampledeviationisdetailedinTable3.5.Itisimportanttonotethatthemixingmethodinputslargeairbubblesintothesystem,thisisfromthemonolithicsamplesthatstillhaveasmallamountoflargecells.Therefore,cellslargerthan300marenotbelievedtobefromthenucleationandgrowthofthechemicalblowingagent,butarestillincludedinthecellcountsothemediancellsizeisalsogivenasmoreaccuraterepresentationoftheblowncells.4647Table3.4:Theaverageglasstemperature(Tg)andsampledeviationofrigidPUR/PIRfoamwithandwithout("standard")GnPandwithtreatedGnPasmeasuredbyDSCtakenfrom3specimens.RigidPUR/PIRfoamsampleTg(°C)SampledeviationStandard107.41.05wt%GnP-25100.74.55wt%pMDIGnP-25103.63.98wt%GnP-5102.17.08wt%pMDIGnP-597.84.58wt%TDIMGnP-590.010.48wt%GnP(750)107.64.28wt%pMDIGnP(750)102.24.58wt%pMDIMGnP(750)105.31.28wt%TDIMGnP(750)106.41.6Figure3.12:TheamountofheatincreaseasthefoamcureswithoutGnP(solidline)andwith5wt%GnP-25(dashedline).4748Table3.5:CellsizeofthemeanandmediandiameterofPUR/PIRrigidfoamwithandwithoutttypesandloadingsofGnPversustheneatrigidPUR/PIRfoam("standard".)RigidPUR/PIRfoamsampleMeanCellSize(m)SampledeviationMedianCellSize(m)Standard119.246.2114.55wt%GnP-2582.741.576.05wt%pMDIGnP-25179.8213.5112.88wt%GnP-5168.5148.895.78wt%pMDIGnP-5170.7149.1115.58wt%TDIMGnP-5213.9113.4218.58wt%GnP(750)177.8142.1127.28wt%pMDIGnP(750)231.7113.4218.58wt%pMDIMGnP(750)184.4111.2174.58wt%TDIMGnP(750)147.957.0150.6493.4Discussion3.4.1ComparisonofMonolithicRigidPUR/PIRandRigidPUR/PIRFoamwithnoGnP.Rememberthatthemonolithicpolymerwassynthesizedwithouttheblowingagent(water),surfactantorblowingcatalystthatareusedinthefoamingprocess,butstillinadryenvironment,sooneofthereactionmechanismsforformingpolyureawasremovedwithoutaccesstowater.ThisdoesnotseemtonotthemolecularstructureastheTgbetweenthemonolithicandfoamPUR/PIRarecloselyrelated.Wherethereisbetweenthetwoisinthethermaldegradationperformance.Figure3.3showsthatmoreweightislostinitiallyfromtherigidPUR/PIRfoambelow300°Cspecfrompeaks1and2,althoughpeak2isthemoresigtshowingaofaround15wt%.Asmentionedpreviouslythetemperaturefrom275toabout325°Cisforthedecompositionofurethanemonomersfollowedbyureaatabout20°Chigher.ThemoregradualweightlossatthistemperatureforthemonolithicPUR/PIRcouldbeduetoamoretamountofureathanthefoam.However,thesimilarglasstemperaturebetweenthetwosuggestsanothermechanismaspolyureahasahigherTgthanPUR.Itcouldalsobethatthebroadnessoftheurethane/ureapeakisduetothelesstremovaloftheprimaryreactionproductsthatformfromthedecompositionduetolowersurfaceareacomparedtothefoam.Thisismorelikelybecausetheweightlossat325°Cmatchesbetweenthetwosamples.Theamountofpolymerretainedevenat500°Csuggeststhatthestructureofthefoamplaysatpartintheremovalofthedecomposedproductspreventingtheoccurrenceofsecondaryreactions.ThePUR/PIRfoamretainsabout50%moreweightthanthemonolithicpolymerat750°Ceventhoughtheweightlossisinitiallyhigherforthefoam.Thecrossoverpointoccursabove400°Cwhichincludesthedecompositionofproductsthatformedfromthedecompositionoftheprimarymolecules.Itcouldbethatinitiallythehighersurfaceareacellstructureincreasetheremovalconcentration,butthissamestructurecanalsoaidin4950retentionofanydecomposedproductsfromtheinternalsurfaceandthissamehighsurfaceareaallowsforthepossibilityofmoresecondaryreactionstooccurastheyescape.3.4.2CharacterizationofMonolithicRigidPUR/PIRwithNeatGnPForthemonolithicrigidPUR/PIRtheadditionofGnPresultsinachangeintheamountofweightlossovercertaintemperatureranges.TheonsetofdegradationseemstobesimilarbetweentheneatpolymerandthosesampleswiththeadditionofthebakedGnP.Therateofdecompositionbetweenthesamples,however,varies.Startingattheendofthethengoingbackwardsat750°Cthedecompositionofthethreesamplesareallparalleltoeachother.SinceGnPcanbestableupto750°CthevariationinweightlossattheendpointcouldbesolelyduetotheadditionoftheGnP.Thereisabout4.5wt%betweenthetwosamplesofGnPanddoesnotmatchtheexpected3wt%,however,whencomparedtothestandard,thesamplethatissupposedtocontain5wt%ofGnP-25isonlyshowingaof3.3wt%at750°Cwhichislowerthanexpected.Comparisonoftheneatpolymertothatwhichcontains8wt%GnP-5thereis7.8wt%at750°CtheexpectedamountofGnP.ThissthatinthepolymertheGnPisprobablystableuptoabout750°CsothedegradationplotsarepurelyofthePUR/PIR.TherealsoappearstobevariabilityinthePUR/PIRsamplewithGnP-25astheloadingislowerthanexpected.Itcouldbeduethelargerparticlessettlingoutoftheliquidpolymer.Asitchangeinthethermaldegradationoccursatabout350°CwherethestandardPUR/PIRstartstoexperienceamuchmoretweightlossandseparatesfromthenanocompositesampleswithbakedGnPasseeninFigure3.4.ThisisbyFigure3.5wherethestandardalsohasahigherrateofdegradationthatismaintainedbetween325and375°Cthetemperaturerangethatlikelyrepresentsthedecompositionoftheproductsthatformedfromthedecompositionofurethaneandurea.ItispossiblethattheneatGnPismakingthethermallydecomposedgroupsthatformedfromureaandurethanemorethermallystable,shownbythehigherratesofdegradationabove400°Cfortheneat5051GnP-25andGnP-5.ThisisinthethermaldegradationwhichshowsatdropintheabsoluteweightlossatthissametemperaturewhencomparedtothestandardTheglasstemperatureiscommonlyrelatedtocross-linkdensity,thehighertheTgthemorecross-linkedthestructure.However,thereisanotherfeaturethatcancauseanincreaseinTgandthatisanimportantfeatureinPURsynthesisandthatisthephaseseparation.PURhasbeenshowntophaseseparateintohardandsoftsegmentseitherphaseorbothcanbecrystalline.Inwater-blownfoamsthehardsegmentisgenerallyattributedtourea,whichtendstohaveahighglasstemperature[81].TheadditionofnanoparticleshasbeenshowntohaveagreatonthekineticsandtherebythehardandsoftdomainformationinPURwhethertheirsizeorseparation[82].TheadditionoftheGnPcouldbedisruptingthemicrophasestructure,afactthatwouldbemoreprominentwithahigherloadingofparticles.Inaddition,thispolymerisPUR/PIRmixtureandwhattheadditionofisocyanuratehasonthephase-separatedmorphologyisunclear.AccordingtoTable3.3,inbothcasestheadditionoftheneatGnPresultedinadecreaseinTgthatwasmoretwiththehigherloadingofGnP-5.Whentheinweightlossbetween375and425°Cismeasured,thereisadecreaseinabsoluteweightlosswhencomparedtothestandardof0.7%and1.7%forthe5wt%GnP-25and8wt%GnP-5,respectively.ItcouldbeGnPisdecreasingtheconcentrationoftrimerlinkagesresultinginadropweightthatislostatthedecompositiontemperaturesuggestingthatthePIRhelpswithcrosslinking,nowwhethertheyarelocatedinthehardorsoftdomainofthepolymerisunclear.OtherwisethethermalofthenanocompositePUR/PIRcloselymatch,whichiswhychangesinthemicrophasestructureislikelyresponsible.3.4.3CharacterizationofFoamSampleswithNeatGnPFigure3.6showsthatthefoamedPUR/PIRismorectedbytheloadingsofthebakedGnPasdemonstratedbytheshiftsintheonsetofdegradationofthesubsequentnanocomposite5152Figure3.13:DegradationprofrigidPUR/PIRfoamwithnoGnPcomparedtofoamwithneatGnP.foamsamplesinFigures3.8-3.10.TheadditionofGnPseemstoimprovethethermalstabilityofthePUR/PIRmoleculargroups,thisislikelyduetothehighspheatcapacityofthatiscloselythatofgraphitethatwoulddecreasetheheatconductiontothepolymer[83].TherateofdecompositionofrigidPUR/PIRwith5wt%GnP-25ismuchbroaderforpeak2asshowninFigure3.8,whichresultsinalowerweightretentionbetween300to350°CinFigure3.13suggestingthattherearemoreureagroupsthataredecomposingatthehighertemperature.ThehigherpolyureaformationascomparedtothestandardareprobablytocompensatefortheadditionofthelargerGnP.Anunexpectedfeatureistheamountofweightretainedatcomparedbetweenthesamplesat750°C.ThesamplewithGnP-25haslostmoreweightthanthestandardandthiscrossinseemstohappenatabout450°C.Asimilarprisseeninthefoamsampleloadedwith8wt%GnP-5andshowsalweightlossthatmatchesthestandard.Eventhesamplewith8wt%GnP(750)onlyshowsainweightlossofabout3%fromthestandard.Thedatasuggests5253thatthefoamsampleswithGnP-5andGnP-25haveimprovedremovalofthedecompositionproductsandthiscouldbeduetothetlyreducedmediancellsizeresultinginanincreaseincelldensityforahighersurfacearea,butthecellsizeforthefoamwithGnP(750)issimilartothestandardsothelossmustbeduetoanothermechanismforthissample.ThefoamsamplethatcontainsGnP(750)isuniqueincomparisontotheothertypesofneatGnP,butstillshowsedecomposition.FortheanalysisitiseasiertosimplycomparetheGnP-5toGnP(750)asbothhavethesameloadingofGnP.ItappearsthattheadditionofGnP(750)doesresultinasimilarincreaseinthethermalstabilityofthetmoleculargroups,butwhatistintheweightlossAbove300°Cthereisamuchsharperdecreaseinweightlossuntilabout350°C.Thiscorrespondstotherateofdecompositionforpeak2,whichismuchbroaderandactuallysuggeststwopeakswhichwouldcorrespondtothethermaldegradationofurethanefollowedbyurea.ThisistthanthesampleswithGnP-5orGnP-25wherethisfeatureistoresolveandsuggeststhattheGnP(750)iscantlytheformationofurea.ImagestakencomparingthedispersionoftheGnPinFigure3.14showthatsmallersizeoftheGnP(750)isbetterforgoingintothinareasthanthelargerGnPresultinginmoreinteractionbetweenthepolymerandthenanoreinforcingagent.Thismeansthatmoremoleculargroups,inthiscasepolyurea,wouldformbondswiththeGnPresultinginanincreaseofthesestablegroups,whereasthelimiteddispersionofGnP-5andGnP-25duetoagglomerationincreasingthepolyureacontentonlymarginally.Theofagglomerationswillbediscussedmoresoon.TheGnP(750)isalsosmallersotheconcentrationofparticlesishigher,butareapparentlytoosmalltopreventbubblesfromcoalescingresultinginalargermediancellsize(seeTable3.5andalesstdecompositionat450°C.LookingatthethattheGnPadditionshaveontheglasstemperatureofthefoamshowninTable3.4.FirstdatapointofnoteisthattheofGnP-25doesnotchangebetweenthemonolithicpolymerandthesample.Thiscouldbeduetotheheavyagglomerationoflargeparticlesthatlimititsinteractionwiththepolymer(seeFigure3.15.5354(a)(b)Figure3.14:SEMimagesofGnPdispersioninstrutsinrigidPUR/PIRfoam:(a)FIBcutoffoamwith8wt%GnP(750);(b)FIBcutoffoamwith8wt%GnP-5GnP-5showsalowerTgthantheneatfoam,buthasahigherTgthanitscounterpartinthemonolithicpolymersuggestingthatthefoamingallowsthepolymertobetteradjusttotheinvasionoftheGnP-5tomaintainmoreofitscross-linkorpossiblyevenhardsegmentstructure.ThisseemstobethecaseastheevensmallerGnP(750)particleshaveasimilarglasstemperaturetothestandard.IngeneralitappearsthatthesmallersizeoftheGnPisbetterforthemolecularstructure,butrememberGnP-25isbetterfordecreasingthemedianbubblesizeinTable3.5.Stericallythismakessenseasitwouldbeharderforthebubblestomovearoundandcoalescewiththelargerplateletsespeciallyastheviscosityincreasesandtheyagglomerate.FurtherevidenceoftheGnPhasonthemolecularstructureanditsfoamingbehaviorisseenbythechangeintheexothermicinFigure3.12.ThisplotdemonstratesthethateventhelargerplateletscanhaveonthefoamasthereisamoregradualrateofheatevolutionandalikelydecreaseinthemaximumtemperatureandgivesfurtherproofthatGnPisinteractingwiththepolymeronamolecularscale.5455Figure3.15:SEMimageofGnPdispersionattheapexofstrutsinrigidPUR/PIRfoamwith5wt%GnP-25.3.4.4CharacterizationoftreatedGnPTreatmentoftheGnPwasdonetoimprovetheinteractionoftheGnPwiththematrixandtheresultantedgegroupswerecharacterizedbyXPSandtheresultsareshowninTables3.1and3.2.TheedgesaretheonlypointsavailableforreactionasthebasalplaneofGnPisverychemicallyinert.ThenitrogenplotwasfocusedonfordeterminingtheamountandconcentrationsoffunctionalizedgroupssinceGnPismadeupofmostlycarbonandasmallamountofoxygen,andonlyGnP(750)showsasmallamountofnitrogenatall.Mostoftheoxygengroupsarelikelyhydroxyls,whichreadilybondtoisocyanatetoformurethanemolecules.XPSresultsthatbyallowingisocyanatetointeractwithGnPatelevatedtemperaturescausesreactiontooccurformingurethanegroups.Theheatwasappliedeitherexternallyorbythehotspotscavitationcausesduringultrasonication[84].TheprocedureswherepMDIisaddedinexcessresultsisdesignedtoformthemaximumamountofofpolymericgroupsformingontheedges,whereas,thereducedmethodseeksto5556formonlyasinglelayeramountofpolymergroupsonthesurfacewhichisinthedecreaseinconcentrationofreactedgroupsshowninTable3.2.Thepeakat400eVwasassumedtobefromurethane-likegroupsasaanXPSscanofPUR/PIRrigidfoamwasdoneandtheonlypeakthatappearedwasabroadpeakat400eVsuggestingthatitrepresentsurethanebonds.Thepeakat399eViscommonlyattributedtoamines[64]aswell.Thehighestbindingenergyisprobablyfromun-reactedpMDIwhetherthatisfromexcesspMDIorunreactedsegmentsinthepMDIisunknown.Withoutacatalystitisunlikelyisocyanurateformed.Sinceallthefunctionalgroupsincludinghydroxylsareonlyontheedges,theGnPwiththehigheredgedensityshouldhavemorefunctionalgroups,sptheconcentrationshouldgoGnP(750)>GnP-5>GnP-25.TheresultoftheovernighttreatmentwithpMDIdoesnotfollowthispatternasshowninTable3.1.Sincetheisocyanateispolymericitcouldbethatthelargerbasalplaneallowsmorepolymericchainstowraparoundtheplateletsgivenacientamountoftimeresultinginahighercountofaminetypegroupsforming.CompareFigure3.16whichistheGnP-25thathasbeenreactedwithpMDIfor1hatabout130°CtoFigure1.3thatclearlyshowsthatevenatareactiontimeof1handreactiononlyoccurringatedges,thepolymerstillmanagestowraparoundtheparticleisolatingit.ComparetotheresultsoftheedgegroupconcentrationthatformedafteranhourofreactiontimeandthereisadecreaseintheconcentrationofreactedgroupsontheGnP-25comparedtotheGnP-5,accordingtoTable3.2.Howevertheat%ofreactedgroupsonGnP-5issimilartothatonGnP(750).Thisisduetothefactthatas-receivedGnP(750)isaggregatedintosub-micronparticlesalthoughtheplateletsaregenerallylessthan1mindiametersomanyoftheedgesareinternalandunavailableforreaction.CorrelatetothesamplesthathavebeentreatedwithasmallamountofTDI,butareultrasonicatedpriortocoatingtobreakuptheparticlestoexposeasmuchoftheedgesaspossible.NowthereistheexpectedhigherconcentrationofedgegroupsontheGnP(750)comparedtotheGnP-5.FunctionalizingtheedgesoftheGnPalterstheithasonthemolecularstructure5657(a)(b)Figure3.16:SEMimagesofGnP-25reactedwithpMDIfor1hatelevatedtemperatureasshownbythechangesintheglasstemperatureinTables3.3and3.4.ThegreatestthoughinTg,stillseemstocomefromthesizeandloadingoftheparticles.WhencomparingthechangeinTginthenanocompositeswhenneatGnPisusedcomparedtotreatedGnPtheglasstemperaturemayonlychangeafewdegrees,butrepresentstchangeinthecross-linkdensity.TheexceptionisthebetweentheTginthenanocompositefoamwithGnP-5treatedwithexcesspMDIandthattreatedwithasmallamountofTDI.However,thesampledeviationissolargeitsuggestsinconsistencyinthelocalstructure.ThesampleswithGnP-25showsomeuniquebehaviorintheTgaswell.First,theglasstemperatureofthePUR/PIRsolidwithpMDItreatedGnP-25matcheswiththatofthefoamthesamesimilaritywasobservedintheneatGnP-25betweensolidandfoam.SecondthepMDItreatmentofGnP-25resultsinanincreaseinglasstemperaturewhencomparedtotheneatwhereallothernanocompositesamplesshowitremainsconstantoradecrease.EventhoughtheconcentrationofatomicgroupsisntbetweentheGnP-25usedinthemonolithicpolymerandthatinthefoam,thepolymerthatnowsurroundstheagglomeratedparticlesmustallowforcross-linkingwhateverphasethisoccursin.ThenanocompositefoamwithGnP(750)andaminimalamountoftreatment,demon-5758Figure3.17:FESEMimageofGnPdispersioninrigidPUR/PIRfoamwith8wt%pMDItreatedGnP(750).StrutwaspreparedwiththeFIBandaggregatesareoutlinedinblack.stratedasimilarglasstemperaturetothenon-treatedGnP(750).SincepMDItreatedGnP(750)consistsofmuchlargeragglomerationsasseeninFigure4.16,whereastheneatandminimaltreatedGnP(750)donot;itappearsthatthesmallparticlesdoamuchbetterjobatallowingthepolymertocross-linkinthefoam.ThesmallerGnPappeartotendnottodisruptthelocalmolecularstructureevenwhenthereisaedge-functionalization,whichisslightlysurprisingduetoitshigheredgedensity,butprobablyhastodowiththesizesoftheparticlesbeingmoreonthescaleofthemicrophasestructure.ForthemonolithicPUR/PIRaddingthetreatedGnPcausessometchangesinthedegradationofcertainmoleculargroupsaccordingtoFigures3.4and3.5.PUR/PIRwithpMDItreatedGnP-5andGnP-25showearlyonsetforthedepolymerizationofurethane(peak1)theoppositeoftheexpectedimprovementduetheGnPspheatcapacity.PMDItreatedGnP-5showsnoimprovementovertheneatGnP-5andaslightdecreaseintheamountofurethaneandurea.Inaddition,themonolithicpolymerwith5859pMDItreatedGnP-25alsoshowsearlieronsetforpeak2representingthebreakdownofurethanemonomers.ThisgivesfurtherevidencethattheGnPiscoatedwithpolymerasitpreventstheGnPfromabsorbingtheheatpreventingitfromtransferringtotheparticleandcausesalowerheatcapacity.Ureainthesamesampleappearstohaveaboutthesamethermalstabilityasthereappearstobeasecondpeakinthethathasformedabove300°C.TheincreasedamountpolyureasuggestedbythetincreaseinweightlossofthesolidwithneatGnP-25isprobablyresponsibleforthehigherTg.ThereisatconcentrationofGnP-5whichseemstobethedecidingfactoralongwiththesizethatisthecross-linkdensityandlikelythemicrophasemorphology.OnethingthepMDItreatedGnP-5doeshelpisinthethermalstabilityandsinceitisonlyreallyattheendofpeak2thatsuggeststhatitmightbemakingpolyureamorethermallystable.ComparedtothemonolithicpolymertherearetchangesinthethermalpropertiesoftherigidPUR/PIRfoamwiththeadditionoftheedge-functionalizedGnP.ThesizehoweverstillseemstobethetfactorastheoftheneatandtreatedGnP-25overlapforthemostpart.LookingatFigures3.6-3.10thedegradationstillshowanincreaseinthethermalstabilityofthemoleculargroupsinthefoamsampleswithtreatedGnPwhencomparedtothefoamssamplewithnoGnP;demonstratedbytheshiftinonsettemperature,althoughthechangeisnotassigntsuggestingagainthatcoatingtheGnPwithpolymerisdecreasingitsspheatcapacity.Thepeakstartingataround300°CforthenanocompositefoamwithpMDItreatedGnP-5followsthepatternofthemonolithcpolymerandclearlyresolvestwopeaks,suggestingamoretcontributionfrompolyurea.FoamsampleswithpMDItreatedGnP(750),bothminimalandexcess,donotshowthetwopeaksandinfactthepMDItreatedGnP(750)showadecreaseinthethermalstabilityofthemoleculargroupssuggestingachangeinmolecularstructurethatlowersthepolymersheatcapacity.TheminimaltreatedpMDItreatedGnPexperiencesconstantweightlossfrom300to400°C,thatisprobablyduetoadecreaseinPURthathasbeencompensatedforwithincreasedpolyureaorPIRandexplainswhytheTgissimilartothatoftheneat5960Figure3.18:ThermaldegradationofedgetreatedGnP-5comparedtotheneatfoam.foam.FunctionalizingtheGnP(750)withtheshortTDImoleculeinsteadofthepolymericisocyanateseemstonotpreventtheheatabsorptionoftheparticles.ThisisnotthecaseforthefoamthatcontainsTDItreatedGnP-5thathasaloweronsettemperaturewhencomparedtotheGnPtreatedwiththepolymericisocyanateandinfactcausesaslightlyearlierdegradationonsetofthedepolymerizationofPURcomparedtoeventhestandard.InadditionthetreatmentontheedgesoftheGnP-5seemstopreventtheformationofthePIRasseenasseeninthelowerslopeoftheweightlossinFigure3.18andderivativeweightlossprat400°CandthisisbythecantlylowerTg,whichisnotcompensatedforwithanincreaseinpolyureaaswasseeninothersamples.Onceagaintheloadingandsizeoftheparticleseemtobethedominatingfactor.ThereareechangesinthecelldiametersduetothechangesinthereinforcementsexceptforthepMDItreatedGnP-5andGnP-25whichshowsimilarcellsizetotheneatrigidPUR/PIRfoamsample.Itappears,however,thatedgefunctionalizationbetteraccommodatespolymerchainformationduringfoamingshownbytheincreaseinTgalongwiththecellsizeforthetreatedGnP-25overtheneatGnP-25.AddingthetreatedGnP(750)resultina6061largercellsizecomparedtotheneatGnP(750)nomatterwhattypeoftreatedGnPisused.Generallyadecreaseinglasstemperaturethatcorrelatestoachangeincross-linkdensitywouldallowformoremovablelinearpolymerchainstofacilitatethecoalesceofthebubblescausinglargercellsizes.ButthesampleswithminimalamountoftreatedGnP(750)havesimilarTgtothestandardandtlylargercells.Soitappearsthatcross-linkinginthiscasedoesnotthecellsize.ItcouldbeatphysicalphenomenonasevidencedinthefoamwithtreatedGnP(750)andGnP-5whereatlyhigherconcentrationofparticlescouldbeprolongingthegelationofthefoamallowingthebubblestocoalesceandstabilizationofthelargerairbubblesduetomixing[82].Thesechangesinviscosityandmobilitythekinetics,whichinturnthephaseseparationandcouldbeadecidingfactorinthecellstructure.Ingeneralthesamplesthathavealargermediancellsizealsohaveahighercountofbubblessizesgreaterthan300m(seeAppendix).Onceagain,thelargercellscouldberesponsibleforthecantdecreaseinweightlossthatreducestheamountsecondaryreactionsthatoccurasthedecomposedproductsescape.3.5ConclusionTheadditionofananomaterialtoachemicallyblownfoamappearstodisruptthepolymericsystemonaphysicalandchemicallevelchangingtheresultantpolymer'smolecularandmicroscopicproperties.Thegasexpansionprocessalonecauseschangesinthethermaldegradationbutnotinthecrosslinkdensity,astheexpandedstructureresultsinalargersurfacearea,butsimilarTg.Alargesurfaceareaiscommonlymorebinacontrolledprocess.Ithelpstoassistintheremovalofthedecomposedproductsonthesurface,whichisedbythemuchnarrowerratepeaks.Thisstructurecanalsobedisadvantageousasitseemstopreventtheremovalofthedecomposedproductsoftheinternalstructureresultinginallthefoamsampleshavingahigherrateretentionthanthemonolithicpolymer.Ingeneraltherateofdecompositionismuchbroaderformonolithicpolymerstructureduetothelimitationsinthesolidpolymer.6162Theadditionofthenanoparticle'shighspheatcapacityincreasesthefoam'sthermaldegradation.TheadditionofneatGnP-5andGnP-25totherigidPUR/PIRdemonstratedasimilardegradationleexceptfrom325to380°C,whichcouldcomefrommorethermallystablehardsegments.TheadditionofcomparativelylargerGnP-25,bothneatandtreated,resultedinasimilarcross-linkdensitybetweenthemonolithicpolymerandthefoam,whichwastlylower.Thelargeparticlesthentendtointeractthesamewiththepolymerregardlessofthestructure.Although,theadditionoftheneatGnP-25inthefoamalsoresultedinalowercellsizesothelargerparticleshelpedtostabilizethebubblesandpreventcoalesce.TheGnP-5helpsinthisregardaswell,butnottoasquiteassmallacellsize,andthesmallestparticles(GnP(750))donothingforthecellstructureresultingintlylargercellsizes,butcouldalsobethatthemicrophasestructureenablesthecoalesce.Itdoesappear,however,thatthefoamisbetterabletoadaptitsmolecularstructureastheTgoftheneatGnP-5andpMDItreatedGnP-5wastlylargerinthefoamascomparedtothesolid.Inaddition,thefoamsampleswithneatandtreatedGnP(750)showasimilarTgtothestandardtheonlyexceptionisthepMDItreatedGnP(750),andimagesshowtheyhavelargemaggregatesandcouldbereasonitshowsasimilarTgasthepMDItreatedGnP-25.Theseagglomerationscouldbetheresultantpropertiesevenmorethantheparticlesizesandmovingforwardmethodstoreducetheaggregateswillhavetobeemployedasfunctionalizationoftheedgesisnottenoughespeciallyforparticlesthathavealowedgedensity.Thesmallestparticleswerethemosteatimprovingthermalstabilitywhilemaintainingasimilarcross-linkdensityandweremostdbytheedgetreatmentduetotheirhigheredgedensity.Thefoamingactionalsocompensatedbetteronamolecularlevelwiththeadditionofnanoplateletstomaintainthecross-linkdensity.IngeneraltheadditionofGnPtendstopreventtheformationofisocyanurate,whichthepolymercompensatesforbyincreasingtheconcentrationofpolyureatomaintainthecross-linkdensity.Addingananoplateletreinforcingmaterialtoachemicallyblownpolymerfoamthe6263resultingchemicalbondingandmolecularandcellstructureandtheresultcanbeimprovementofthethermalstabilityofcertainmoleculargroups.Whenchangingtheresultantpropertiesofthepolymer,plateletsizeistalongwithloadinganditisimportantthattheinteractionbetweentheindividualparticlesandthematrixismaximized.Anyaddedtosuchasysteminthefuturewillhavetotakeintoaccountthetheyhaveonthechemicalreactionsandstructuretoimprovetheresultantpropertiesoftheoverallnanocompositefoam.6364Appendix6465(a)(b)Figure3.19:Maxandmincelldiametersof60cellsinrigidPUR/PIRfoam:(a)Histogramofmaximumcellsize(m);(b)Histogramofminimumcellsize(m)6566(a)(b)Figure3.20:Maxandmincelldiametersof60cellsinrigidPUR/PIRfoamwith5wt%GnP-25:(a)Histogramofmaximumcellsize(m);(b)Histogramofminimumcellsize(m)6667(a)(b)Figure3.21:Maxandmincelldiametersof60cellsinrigidPUR/PIRfoamwith5wt%pMDIGnP-25:(a)Histogramofmaximumcellsize(m);(b)Histogramofminimumcellsize(m)6768(a)(b)Figure3.22:Maxandmincelldiametersof60cellsinrigidPUR/PIRfoamwith8wt%GnP-5:(a)Histogramofmaximumcellsize(m);(b)Histogramofminimumcellsize(m)6869(a)(b)Figure3.23:Maxandmincelldiametersof60cellsinrigidPUR/PIRfoamwith8wt%pMDIGnP-5:(a)Histogramofmaximumcellsize(m);(b)Histogramofminimumcellsize(m)6970(a)(b)Figure3.24:Maxandmincelldiametersof60cellsinrigidPUR/PIRfoamwith8wt%TDIMGnP-5:(a)Histogramofmaximumcellsize(m);(b)Histogramofminimumcellsize(m)7071(a)(b)Figure3.25:Maxandmincelldiametersof60cellsinrigidPUR/PIRfoamwith8wt%GnP(750):(a)Histogramofmaximumcellsize(m);(b)Histogramofminimumcellsize(m)7172(a)(b)Figure3.26:Maxandmincelldiametersof60cellsinrigidPUR/PIRfoamwith8wt%pMDIGnP(750):(a)Histogramofmaximumcellsize(m);(b)Histogramofminimumcellsize(m)7273(a)(b)Figure3.27:Maxandmincelldiametersof60cellsinrigidPUR/PIRfoamwith8wt%pMDIMGnP(750):(a)Histogramofmaximumcellsize(m);(b)Histogramofminimumcellsize(m)7374(a)(b)Figure3.28:Maxandmincelldiametersof60cellsinrigidPUR/PIRfoamwith8wt%TDIMGnP(750):(a)Histogramofmaximumcellsize(m);(b)Histogramofminimumcellsize(m)7475CHAPTER4MULTIFUNCTIONALPERFORMANCEOFRIGIDPUR/PIRFOAM4.1IntroductionAstechnologygoesfurtherintospacethereisagreaterneedfortivematerialswithhigh-functionality.Thenecessityfromtheaerospaceaswellasothershasledtothedevelopmentofmultifunctionalmaterials.Thechallengeisstructuringthesematerialswhilemaintainingcertainpropertiesthathistoricallyhavebeenconsideredopposite,likehighsurfaceareaandlowweight,ortheybeboththinandstrong.Thishasmotivatedamovebeyondclassicmaterialclassestodevelopingnewones.Suchasthedevelopmentofcompositematerials.Withtheircommercialsuccessandthediscoveryofnanomaterialstherehasbeenanuptickintheexperimentationandimplementationofnanocompositematerials.Researchhasalreadyfoundthatfewweight%additionofhigh-functioningnanoparticlescangreatlyincreaseamaterialsperformance,especiallyinpolymers[8,6].Polymersaregenerallyalow-costmaterialthatcomeinavarietyofproperties.Inaddition,theyareamultitudeofwaystosynthesizethemandarereadilyformable.Thevariousmethodsemployedtomakepolymerssuggestitcouldberelativelyeasytoaddnanoparticlestothepolymermatrix.Smallweight%additionsofnanoparticleshavebeenfoundtoimprovetheirpropertiesbroadeningtheirpotentialapplicationsineverythingfromfromrobotics[14]toenergyapplications[10],toelectromagneticinterference(EMI)shieldingdevices[69,70,71]topackaging[7]andmoreasusesarebeingresearchedallthetime.Inaddition,gascanbeaddedtothesystemtomakelight-weightcellularstructures.Suchnanocompositefoammaterialshaveshownimprovedmechanical,dielectricandelectricalperformance[68,16,85,86].Whenitcomestodeterminingwhatnanoparticlestoputinsidepolymernanocomposites7576itmostlydependsontheapplication.Carbonbasednanoparticlesaresomeofthemoreheavilyutilizedones,whichincludesgraphene,asduetocarbonbondingstructurethisnanoparticlehasmultifunctionalproperties.SinglelayergraphenehasbeendemonstratedtohaveahighYoung'smodulusofaround1TPa[22],highthermalconductivityofabout3000W=mK[21]andexcellentcarriermobilitiesgreaterthan15,000cm2=Vs[19].Currentmethodstosynthesizesinglelayergrapheneareveryparticular.Highpuritygraphenecomeswithahighcostandlowyieldbymethodssuchaschemicalvapordeposition[26,27]orexpitaxialgrowth[29],andlowpuritygraphenehasalowcostandhighyieldsuchasseenwithreducedgraphiteoxide[33,43].Anotherpopularmethodforciblyseparatethegraphenesheetsfromitsnaturallyoccurringparent,graphite[43].Thismethodhighpurity,highyieldthin,robustparticlesconsistingofmultiplelayersofgraphenesheetsmakingitapotentialforbulkpolymernanocomposites.4.2ExperimentalMethods4.2.1MaterialsThefoamisachemicallyblownpolymer.Thismeansthatuponthemixtureoftwocom-ponentsthematerialpolymerizesandevolvesgassimultaneously.Thepolymermatrixispolyurethane/polyisocyanurate(PUR/PIR)mixedfoam,acommoncommercialmaterial.Ithasbeenaroundfordecadeswithawell-understoodprocess-structure-propertiesrelationship.Urethaneinthiscaseisformedfromtheadditionreactionofisocyanteandhydroxyls[51].Isocyanuratedescribesthetrimersthatformfromthereactionofthreeisocyanategroupswiththeassistanceofacatalyst.Theadditionofthecyclotrimerizationreactionduringthepolymerizationofthefoamresultsinamorethermallystablematerial[51].Achemicallyblownfoamalsocontainsablowingcatalystandblowingagent.Theblowingagentchosenforthisexperimentationwasenvironmentallyfriendlywater.Lastly,asurfactantisaddedtoassistincellformation,whichinPURfoamsiscommonlyapolysiloxane.7677Huntsmangenerouslysuppliedthepolymericmethylenebisphenyldiisocyanate(pMDI)andthepolyols.ThepMDI(RubinateM),astandardpMDIwithaviscosityof190cPsaspgravityat25°Cof1.23andanisocyanatecontentof31.1%NCO[54].ThepolyolsareFX31-240andG30-650bothdifunctional.FX31-240isalowviscositypolyolat250cP-scomparedtoG30-650,whichhastripletheviscosityat880cP-s[54].EthyleneglycolreagentfromCCI(#216500),whichhasawater-likeviscosity,wasalsousedasahydroxylcomponenttolowertheviscosityofthesystem[55].ThecatalystsweresuppliedbyAirProducts:DabcoBL-11isastandardblowingcatalystandDabcoTMR-3isastandardcyclotrimerizationcatalystforwater-blownfoams[57].ThepolysiloxanesurfactantisalsofromAirProducts,DabcoDC193[57].Lastly,theblowingagentisdistilledwater.TheratiosforthesecomponentsareaslistedinTables2.1and2.2onpage18.Thesourceofthe2,4'-toluenediisocyanate(TDI)thatwasusedtofunctionalizetheGnPwasTCIChemicals(#T0264)[58].AllthenanoparticlesusedinthisresearcharesourcedfromXGSciences.ttypesofgraphenenanoplatelets(GnP)areutilizedasaereplacementforsingle-layergrapheneandconsistsofmultiplestacksofgraphenesheets.Table4.1showsthettypesofGnPusedinthisresearchandtheirphysicalcharacteristics.TheGnPbasalplaneisnotdisruptedbyanychemicalgroups.Allchemicalgroupsarefocusedontheedges,themajorityofwhicharehydroxyls.Allas-receivedGnPisheattreatedto450°Cfor2hpriortouse.Table4.1:Propertiesofenttypesofas-receivedGnPusedinnanocompositefoams.PropertyGnP-25GnP-5GnP(750)Surfacearea(m2=g)120120750AverageDiameter(m)255<1Thickness(nm)6-86-83-477784.2.2SynthesisofPUR/PIRNanocompositeRigidFoamThePUR/PIRrigidfoamwassynthesizedbycombiningbothdifunctionalpolyolswithethyleneglycol.Thissolutionwasthenmixedwithapaddlestirrerfor2hinanenvironmentalgloveboxkeptatbelow40%humidity.Thesurfactant,catalystsandblowingagentwerethenaddedandstirredforanadditionalh.IfGnPwasrequiredthenitwasaddedtothepolyolsolutionateither6wt%forGnP-5andGnP(750)or3wt%forGnP-25.TheremainingGnPwasaddedtothepMDI.EachGnPblendwasthenindividuallyhigh-speedshear-mixedat1600rpmfor2minfollowedby2400rpmfor1min.Followedbyultrasonicationwitha2.54cmprobeat100Wuntilwellblended,whichwastypicallybetween5-10min.Adequatedispersioninthepolyolblendwascheckedusinganopticalmicroscope.AfterofthedispersionthepolyolblendispouredintothepMDIcomponent,andstirredwithanimmersionblenderfor45s,priortopouringintoamold.Thefoamwasthenallowedtofree-riseandcureovernight.Theneatfoamhadanaveragedensityof0.16g=c3m.4.2.3Edge-functionalizationofGnPToimprovetheinteractionofGnPwiththepolymermatrixtheparticleedgeswerefunction-alized.AsmentionedpreviouslymanyoftheedgegroupsofGnParehydroxylswhichasalreadymentionreactwithisocyanatestoformurethanes.ThesetofexperimentstofunctionalizetheGnPsimplyaddedpMDItotheGnPandreliedonheattoforceareactiontooccurbetweenthepMDIandhydroxylsformingpolyurethanes.1.Heat-treatedGnPisaddedtobeakerandheatedto>100°Conahotplatetoremoveanyadsorbedwatermolecules2.EnoughpMDIispouredoverGnPtocover3.Solutionreactsat140°Cforonehour4.Solutionisremovedfromheatandcooled78795.Materialiswashedwithacetoneandsolidsarecollectedbycentrifugeandexcessisdecanted.6.Processisrepeatedsixtimes7.GnPisdriedtoremoveacetoneataslightlyelevatedtemperatureThesecondsetofexperimentsattemptedtocreateaminimal(M)amountofreactedgroupsontheedgesoftheparticles.ThiswasdonewithboththepolymericMDIandthemolecularTDI.ThedetailoftheirstructuresisinFigure2.2onpage16.1.GnPisdispersedinacetoneusinga2.54cmultrasonicationprobeat100Wataconcentrationofabout6gofGnPpereveryLofacetone.2.Thesolutionisultrasonicatedforatleast1h.Thedispersionisobservedwithanopticalmicroscope.3.AsmallamountofpMDIorTDIisaddedtothesolutionataratioof64Lforevery6gofGnP.4.Ultrasonicationwiththesameprobeandpowerisdoneforanadditionalh.5.Theedge-functionalizedGnPisthenretainedusingthesamewashing-centrifuging-decantprocedurefrombefore6.GnPisdriedtoremoveacetoneataslightlyelevatedtemperature4.2.4TestingProcedures4.2.4.1XPSX-rayphotoelectronspectroscopy(XPS)wasusedtocharacterizetheamountandtypeofreactedgroupsontheGnPaftertreatmentwithpMDIandTDI.XPSisasurfacecharacteri-zationtechniquewithapenetrationdepthupto1mmakingitthebestspectrometerto7980measurethesmallamountofreactedgroupsontheGnPedges.Inaddition,thespectrometercanmeasureenergyshiftsintheatomicgroupsgivinginformationaboutthemolecularstructureaswell.GnP-5andGnP-25initiallyhavenonitrogenandGnP(750)hasonlyabout1atomicpercent(at%)sothefocuswasonthenitrogenspectratoidentifytheleveloffunctionalization.First,ascanwasdoneonaneatPUR/PIRsamplethatcontainednoGnP.Theonlypeakthatappearedonthenitrogenspectrawasabroadpeakat400eV,whichmeansallthegroupsformedinthefoamcenteraroundthispeak.Thenitrogenspectraoftheedge-functionalizedGnPshowsthreepeaksresolvedfromtheoverallpeakat400eV:theiscenteredataround399eVandiscommonlyattributedtoaminegroups[64]includingurea[64],thesecondisthehighestpeakcenteredover400eVforurethanesandthethirdiscenteredataround401.3eV,likelyduetopMDI.ThepMDIcouldbefromexcessunreactedpMDIorsimplytoanyunreactedsegmentsofthepolymerchain.4.2.4.2MechanicalandElectricalThetestingcompletedontherigidPUR/PIRfoamsamplesincludestheirmechanicalperformance,electricalperformanceanddielectricproperties.ForthemechanicalpropertiesfourspecimenswerecutfromthePUR/PIRfoamsamplestothesizeof25.8cm2by2.54cmthick.Thespecimenswerecompressedatarateof2.5mm=mintoa3.8mmonaUniversalTestingSystem(UTS).Compressiontestingisthecommonmechanicaltestingmethodforrigidfoams,inadditioncompressiontestsarenotasdependentondefectsinthesample.Electricalresistivitywastestedonthreespecimenswiththedimensions:40mmlongby10mmwideby3mmthick.TwoprobeswereincontactwiththesurfacebyconductivesilverpasteandtestedontheGamryusingACat1Hz.4.2.4.3DielectricandEMISEThedielectricperformancewasmeasuredusingthetransmissionlinetechniqueonavectornetworkanalyzer(VNA)between8.2GHzto12.0GHz.Threespecimenswerecutto10.058081mmwideby22.4mmlongandamaximumthicknessof7.4mm.TheVNAsuppliestheelectromagnetic(EM)wavetothesampleandthenmeasureshowmuchofthatiscted(S11)ortransmitted(S21)throughthesample.TheNRWalgorithmdevelopedbyNicolsonandRoss[65]andWeir[66]isthenappliedtothesevaluestodeterminethecomplexrelativepermittivity("r)assuminganisotropicmaterial.Theequationsdescribingtherelativepermittivityarecomplexnumbersmadeupofrealandimaginaryparts,showninEquation2.5onpage28."0rdescribesthematerialsabilitytostoreelectronicenergywhenencounteredbyanelectromagnetic""risthefactorthatshowshowmuchofthatenergyislostinstead.Therealrelativepermittivityvalueisalsoknownasthedielectricconstant.Inaddition,S21,whichisameasureoftheratioofpowerorvoltagewhenamaterialispresent(V1)versuswhenitisnot(V2)isusedtodeterminetheelectromagneticinterference(EMI)shieldingeness(SE)accordingtoequation2.13onpage28.TheSEofanymaterialisthecombinationofthreeterms:theamountofwavethatis(SER),absorbed(SEA)ormultipletimes(SEM).Thefractionofappliedthatistransmitted(T),ed(R)andabsorbed(A)isgivenbythescatteringparametersasshownintheequationsbelow[67]T=jS12j2(4.1)R=jS11j2(4.2)A=1RT(4.3)4.2.4.4MicroscopyAspreviouslystatedaopticalmicroscopewasusedtoobservethedispersionoftheGnPintheacetoneandpolyolblendbydrop-castingthesolutionontoaglassslide.Ahighresolutionemissionsecondaryelectronmicroscope(FESEM)wasusedinconjunctionwithafocusedionbeam(FIB)toobservethedispersionoftheGnPinthefoammatrix.AFIBusesabeamofgalliumionstocutthroughaspecimenwhileundervacuumintheSEM.8182Figure4.1:CompressivestrengthoffoamwithnoGnP("standard")comparedtothenanocompositefoamwithsampledeviationsat10%ormaximumstrength.Thisisagoodmethodforshowingmicrometerarchitecturalfeaturesinamaterialasthereislessofachanceofartifactsappearingonthesampleduetothecuttingandpreppingmethodscommonlyemployedelsewhere.Thesurfacewascoatedwith6nmofgoldandgroundedtothestubwithconductivecarbonpastepriortoimaging.AlltheFIBcutsaredonethroughthestrutsofthefoam,whichisthetermusedtodescribethepointinthematrixbetweentwogascellsasinFigure3.1onpage36.4.3Results4.3.1MechanicalPropertiesThecompressivemechanicalpropertiesofthestandardfoamcomparedtothenanocompositefoamswithbothneatandtreatedGnPareshowninFigures4.1.-4.2.8283Figure4.2:CompressionelasticmodulusoffoamwithnoGnP("standard")comparedtothenanocompositefoamwithsampledeviation.Figure4.3:TheresistivityofthefoamspecimensshownonalogarithmicscalecomparingthestandardwithnoGnPtothenanocompositefoamwithsampledeviation.8384Figure4.4:Theratiooftherealpermittivityofthefoamsamplesrelativetofreespacefrom8.2to12.0GHz.4.3.2ElectricalPropertiesFigure4.3showtheresultsoftheelectricalresistivityforthePUR/PIRrigidfoamsampleswithheat-treatedandfunctionalizedGnPcomparedtothestandardthatcontainsnoGnPusingatwo-pointprobemethod.4.3.3ElectromagneticPropertiesFigures4.4-4.8aretheelectronicbehavioroftheneatandvariousnanocompositefoamsampleswhenexposedtoanelectromagnetic(EM)wave.Realpermittivity,alsocalledthedielectricconstantisshownFigure4.4.ThefractionofthewavethatistransmittedandtheapproximateamountofthatwavethatisandabsorbedbythesamplesareplottedinFigures4.5-4.7overthefrequency.ThetotalEMIshieldingenessisgiveninFigure4.8.8485Figure4.5:ThefractionofthetotalEMwavethatistransmittedthroughthetfoamsamplesfrom8.2to12.0GHz.Figure4.6:ThefractionofthetotalEMwavethatistedbackfromtheentfoamsamplesfrom8.2to12.0GHz.8586Figure4.7:ThefractionofthetotalEMwavethatisabsorbedbythetfoamsamplesfrom8.2to12.0GHz.Figure4.8:ThetotalEMISEofthestandardrigidPUR/PIRfoamascomparedtothenanocompositefoamfrom8.2to12.0GHz.86874.4DiscussionAddingnanoparticlestoapolymerseemsprettysimplisticintheory,buttherealityturnsouttobefarmorecomplicated.Asdiscussedinthepreviouschaptertheadditionofthenanoparticlesisnotaninertprocessandthereisamolecularinteractionthattakesplacenotonlybetweenthepolymerandtheparticles,butalsobetweentheparticlesthemselvesthathasaprofoundimpactonthelocalmolecularpropertiesthatinturnthemacroscopicones.4.4.0.1MechanicalPropertiesofNancompositeRigidPUR/PIRfoamThemechanicalpropertiesofanyfoamispartlydependentonthesolidpropertiesofthematrixandpartlyonthestructure,includingthedegreeofopen-toclosed-cells.Whenafoamhasahighdegreeofclosed-cellthecompressivestrengthincludesanaddedrestoringforcefromcompressingtheentrappedliquidalongwiththemechanicalcontributionfromthecellwallsastheyarestretchedduringcompressiontoincrease[87].Foropen-cellfoamsonlythestrutscontributetothemechanicalperformance.Figure4.9showsthatwhilethisisaclosed-cellfoamthecellwallsareconsistsofverythinmembranes.Suchthincellmembranesdonotcontributetothemechanicalandinfacteasilybendandrupturereleasingtheentrappedgas.SuchthincellwallswouldcertainlybfromthereinforcingofGnP,butduringtheexpansionprocessthePURsurfacetensioncausestheliquidtoformattheedges[87]pullingthemajorityofGnPintothestrutsalthoughsomeamountscanstillbeseeneveninthethinmembranesinFigure4.9.Theopticalimagesofthecellstructuremakeithardtodistinguishhowthickthecellwallsareandtodetermineifanyhavetherequiredtoactuallycontributetothemechanicalperformance.Theimagesdoshow,however,thatGnPdisplaysakindofclumpingbehaviorinthefoamwhichwouldbetoaccommodateintheverythincellwalls.Ingeneral,cellwallsareprobablynotthemaincontributorsto8788(a)(b)Figure4.9:OpticalimagesofcellwallsinrigidPUR/PIRnancompositefoam:(a)Cellwallsaresothintheyarewrinkledfromgasexpansion;(b)Easytoidentifythincellwalls,butthesmallamountcouldbedotoahighernumberofthickercellwallsmechanicalbutamorein-depthanalysisofeachtypeofGnPwillfollow.Whentalkingaboutfoamsthecompressivestress-straincurvescanalsogiveinsightintoitsproperties.Thereexistthreetypesofcompressivecurvesforfoamsthatdescribeanelastomericfoam,anelasto-plasticfoamandlastly,abrittlefoam.PURfoamisnotabrittlefoamincompressionand,infact,followsthestress-strainbehaviorofeithertheelastomericfoamoranelasto-plasticfoam,bothtypesareshowninFigure4.10.Themechanicaldatareportedinthisresearchfocusesontheelasticregionofthefoams,whichshowstwotbehaviorsincompression.Linearelasticityiscontrolledbythebendingofthestrutsandpossiblythestretchingofthecellwallsalongwithinternalpressure,whichcontinuesuntileitherelasticbucklingortheformationofplastichinges,anon-recoverableprocessgivingthepeakseeninthestress-straincurveoftheelasto-plasticfoam.Theneatfoamandthefoamloadedwith8wt%GnP(750)demonstratepurelyelasticbehaviorintheelasticregion,butsomeofthenanocompositefoamsamplesshowtheformationofamaximumplasticstress.Itisinconsistentwithineachsample,exceptthesamplewithminimaltreatedTDItreatedGnP-5thatdisploysonlyelasto-plasticresponse,suggestingthatforthatforthealltheothernanocompositefoamstheadditionofthenanoplatelets8889Figure4.10:Standardstress-straincurvesofelasto-plastic(left)andelastomeric(right)foams.createsvariationsinthelocalresponsethroughoutthefoam.Therawdataofsomesamplesdemonstratingthedtbehaviorintheelasticregionisgivenintheappendix.ThiscouldleadbacktothechangesinmicrophasethattheadditionofthenanoplateletsarelikelycausingshowninthethermaldegradationandinthelowTg.Asdiscussedinchapter3,PURfoamhasbeenfoundtobeaphase-segmentedstructureasitconsistsofhardsegments,commonlyattributedtopolyureainwater-blownfoamsandsoftsegmentsofpolyurethanechains.Analysisofthethermaldegradationinchapter3suggeststhatoneofthemodesforcompensatingtheadditionofthenanoplateletsincludeanincreaseinpolyureaandpossiblytherebyanincreaseinsizeofhardsegmentsorseparation.Thesehardsegmentscouldformplastichingeswithinthefoamcausingthefoamtodisplayanelasto-plasticresponseincompressionatthemacroscopiclevel.AsseenwiththethermalpropertiestheadditionofGnPdoesnotalwaysresultinapositiveoutcome.GnPhashighmechanicalpropertiesthattheexpectationwasthataddingthemtothepolymerfoamwouldresultinanincreaseinmechanicalperformanceintheelasticregion,butasseeninFigure4.1theadditionofneatGnP-5andGnP-258990(a)(b)Figure4.11:FESEMimageofFIBcutinrigidPUR/PIRfoamwith5wt%GnP-25:(a)FIBcutofstrut;(b)DispersionofGnP-25instrutisactuallydetrimentaltothecompressivestrength,onlythehighsurfaceareaGnP(750)causesanincrease.Theelasticmodulus,however,appearsasthereisstatisticallytchangesintheYoung'smodulus(seeFigure4.2)forthevariousloadingsofGnP.Thenanocompositefoamwith8wt%GnP(750)isalsotheonlyneatsampletomaintainthesameglasstemperatureandhaveacellsizesimilartotheneatsample.Theimprovementinmechanicalpropertiesisduetoalackofaggregatesandpossiblyfromhavingasizeonasimilarscaletothemicrophasestructuresothemicrostructureismaintained.Inordertoseeatchangeinthemechanicalpropertiesforthelargerparticlestheywouldhavetoovercometheirownattractionbyimprovementintheinterfacialcontactbetweenthefoamandtheparticles.ImagesofthedispersionoftheparticlesshowthatthelargeparticlestendtoagglomeratepreferentiallybondingtoeachotherratherthanthefoamasseeninFigure4.11.ThebasalplaneisdominatingthenanoparticlebehaviorandtheonlywaythatcanbeminimizedisbyeithershrinkingtheparticleaswiththeGnP(750)orfunctionalizingthebasalinwaythatitispreferentialtothematrixbutthatdoesnotdisrupttheelectroniccharacter.Otherwise,theseaggregateswillactasstressconcentratorsreducingthemechanical9091performanceofthenanocompositefoam.Inaddition,highconcentrationsoftheparticlesinonepartofthefoammeanthatotherareashavealowconcentration,forexample,inthincellwallsandstruts.ThisappearstobethecaseaseventhesmallerGnP-5particlesshowthinpolymerareaslikeinFigure4.12thatcontainnoGnPatall.ThebetweenFigure4.11and4.12isthesizeofthestrut.ThestrutthatcontainsGnP-25isontheorderof100ms,whereastheonewithnoGnP-5hasasizeofaround6m.GnP(750)hasplateletssizesunder1m,sotheexpectationwasthattheseparticlescoulddisperseinthesethinareas,whichisinFigure4.13.However,noticethateventhesesmallparticlesdonotorientalongthethinningdirection;anthatwouldbenecessaryforthelargerGnPtostayintheseregions.Inorderforreorientationtooccuratshearstressmustbeappliedortheremustbeastrongbondwiththematrix.Thelackorreorientationsuggeststhatinterfacialadhesionisweaksincetheparticlesdonotreorientinthethinningdirection.Anothertoinvestigateiswheretheseparticlesorientwithinthemicrophasestructure.Iftheyaredispersedinthehardsegmenttothattendstoformspherelites[82],therewouldbenowaytoalignthesesegmentsinthethinningdirection.Eventhoughtheparticlesdonotreorienttheirbetterdispersionstillresultsinanincreaseinmechanicalstrength.Theseimagescombinedwiththemechanicalresultsthatchangesinthelocalmicroscopicconcentrationhasaresultingtontheoverallmacroscopicmechanicsincompositesandtheweaklinksdominateperformance.BecausethereseemedtobesuchpoorinterfacialpropertiesbetweentheGnPandthepolymer,edge-functionalizationwasemployedtotryandimprovetheinteraction.Func-tionalizationwasrestrictedtotheedgesasawaytopreventdisruptionofthebasalplane.TheparticleswerereactedwithpMDIandTDItoformurethane-typegroupsontheedgesastheparticlesareknowntohavehydroxyledgegroups.Table3.2onpage38showstheamountandtypeofmoleculargroupsontheparticlesafterreaction.Onceagain,thepeakat399eVisaminegroupsincludingurea,thepeakat400eVisurethanetypegroupsandthepeakat401.5eVisunreactedpMDI.Acoupleofthingstonote,theisthat9192Figure4.12:FESEMimageofFIBcutofstrutinrigidPUR/PIRfoamwith8wt%GnP-5.Figure4.13:FESEMimageofFIBcutstrutinPUR/PIRrigidfoamwith8wt%GnP(750).9293Figure4.14:FESEMimageofstrutcutwithFIBinPUR/PIRrigidfoamwith8wt%pMDItreatedGnP(750).ThepMDItreatedGnP(750)agglomeratesarehighlighted.theminimaltreatmentdidresultinareductionoffunctionalization.Secondlysincethefunctionalizationislimitedtotheedgesastheedgedensityincreasesthereshouldbeanaresultingincreaseintheconcentrationoftotalreactedgroups.ThisistrueforGnP-5andGnP-25,butGnP(750)showsasimilarconcentrationtoGnP-5.As-receivedGnP(750)comesassub-micronaggregates,iftheaggregatesarenotbrokenuppriorthepMDIjustcoatsthesurfaceofaggregatesunabletopenetratetheinternaledges.AndaswasseenwiththesamplewithGnP-25theseaggregatesactasstressconcentratorsreducingthemechanicalstrengthwhichisthecasefornancompostiefoampMDItreatedGnP(750)(seeFigure4.1).ThisalsomeansthatultrasonicationdoneonthepMDItreatedGnP(750)afteradditiontothepMDIorpolyolblendisunabletobreakaparttheparticlesandthisisbyimagesintheSEM.Figures4.14-4.18clearlyshowslargesub-micronaggregatesTheminimaltreatmentmethodbreaksaparttheparticlespriortofunctionalizationresultingindecreaseofconcentratedgroupsontheedges.However,thismethodnowfollowstheexpectedpatterninconcentrationwheretheamountofreactedgroupsonTDItreated9394Figure4.15:FESEMimageofGnPdispersioninrigidPUR/PIRfoamwith8wt%pMDItreatedGnP(750).StrutwaspreparedwiththeFIBandaggregatesareoutlinedinblack.GnP(750)isgreaterthanthatontheTDItreatedGnP-5accordingtoTable3.1onpage36.ThisimprovementisalsobythemechanicalstrengththatshowsthattheminimalpMDItreatedGnP(750)hasahighercompressivestrengththantheregulartreatedpMDIGnP(750)andshowsimprovementoverthestandard.TheimagesoftheGnPdispersionsupportthisaswell,wheremanyparticlesinthepolymerappearliketheGnPshowninFigure4.16(b)demonstratingamorphologymoresimilartoplatelets.ThisisincontrasttoFigure4.16(a),whichshowsamuchlargeraggregateparticlecoatedinpolymer.AndtheparticlesinthefoamcontinuetoshowthistrendcomparingFigures4.17and4.18toFigure4.13theparticlemicrostructureinthepolymernowresemblesthatoftheneatGnP(750)inthepolymer.Figures4.1and4.2showthatedge-functionalizationdegradesthemechanicalperformanceofthesamplesthatcontainpMDItreatedGnP-25andGnP(750).GnP-25doesnothaveatedgedensityasalreadydiscussedduetotheVan-der-Waalattractionbetweengraphenelayerscausingthemtore-stack.Thedominantfactoristheparticlediameter9495(a)(b)Figure4.16:SEMimageofGnP(750)reactedwithpMDIbybothmethods(a)GnP(750)treatedbyexcessmethod;(b)GnP(750)treatedbyminimalmethodFigure4.17:FESEMimageofstrutcutwithFIBinPUR/PIRrigidfoamwith8wt%pMDItreatedGnP(750)byminimalmethod.SomeGnPishighlightedwitharrows.9596Figure4.18:FESEMimageofstrutcutwithFIBinPUR/PIRrigidfoamwith8wt%pMDItreatedGnP(750)byminimalmethod.SomeGnPishighlightedwitharrows.andiswhythemechanicalperformancedoesnotchangefromneat.TheproblemswithpMDItreatedGnP(750)isaspecialcaseasdiscussedinthepreviousparagraph.ThepMDItreatedGnP-5,however,showsmarginalimprovementinthemechanicalstrength.Thissuggeststhatthesmallerparticleswiththehigheredgedensityareabletobebytheedge-functionalizationtreatmentanddbythetchangesinthemechanicalpropertiesofthetreatedGnP(750)overthatoftheneat.GnP-5doesappeartosmallenough,however,alongwithGnP(750)although,thepMDItreatedGnP-5specimenssuggesttheparticledispersionisstillnotuniformduetoagglomerationscausingthelargesampledeviation,butcouldstillbeabletoincreasetheinterfacialstrength.AlsothelongpolymerchainsontheedgesappeartoimprovetheinteractionwiththematrixbetterthantheshortmoleculargroupsasboththeminimalTDItreatmentoftheGnP-5andGnP(750)causedonotresultinastatisticallytchangeinthemechanicalstrengthoverthestandard.Inadditionthemechanicalstrengthbetweenthetwoisalmostthesame.Figure4.19showadispersionthatisnotasgoodastheneatGnP(750)oreventheminimaltreatedpMDIGnP(750)causingadecreaseinmechanicalresponse.ItraisesthequestionofwhethertheTg9697Figure4.19:FESEMofFIBcutofstrutinrigidPUR/PIRfoamwith8wt%minimaltreatedTDIGnP(750).SomeGnPishighlightedwitharrows.wouldmatchbetweenthetwoifthedispersionwassimilarwithnoagglomerationsorifthereisanotherfactor.AnotherinterestingfeaturesisthatthereissimilarityinpatternbetweenthecompressivestrengthbehaviorandTgfromchapter3.AstheTglowersthemechanicalstrengthalsolowersincomparisontothestandardthoughnotbythesamemargin.AlsoallthenanocompositefoamwithtreatedGnP(750)andTDItreatedGnP-5showtlylargermediancellsizesascomparedtothestandard(seeChapter3)anditiscommonlyacceptedthatsmallercellsizesarebetterformechanicalproperties.Boththemechanicalpropertiesandcellsize,suggestanotherfactor,namelythemicrophasestructure.4.4.0.2ElectricalresistivityofNancompositePUR/PIRRigidFoamWithfoams,resistivityisfoundtoincreaseastherelativedensitytothesoliddecreases[88,87].GibsonandAshbyproposedthatthisisduetothetortuouspathacellularstructureforcestheelectronstofollow.Asdensityincreasescircularcellswithtamountofpolymersurroundingthem,getthinnerasthecellsappearasmanysidedconvexpolygons.Thisis9798anadditiontothelowconductivitythepolymeralreadyhas,thewholeoftheconductivitythenrestscompletelyontheabilityoftheconductivetoformaconductivenetwork.Thismeansthatthechargecantransfereasilyfromparticletoparticle,thatisphysicallypossiblewhentheparticlesthemselvesaretouchingorcloseenoughtoallowfortunneling.Whenthispathisformedthroughthepolymertherewillbeasharpchangeintheelectricalresistancecalledthepercolationthreshold.Percolationisamathematicsconceptthatdethephenomenonwherea"simpleprobabilisticmodel...exhibitsaphasetransition,"[89],orinthiscase,electricalphasetransitionwherethemacroscopicnanocompositefoamnolongeractsasaninsulatorbutexhibitstheelectricalbehaviorofasemiconductororconductor.Havingparticleswithalargeaspectratioareidealforthispurposeastheyhavelesspointsofcontactresistanceandareabletoachievepercolationatlowerloadings.Thelimitingconcentrationmodel[90]takesthisintoaccountforrod-likeparticleswithstronginteractionanddescribestheelectricalphasetransitionoccurringwhenthecriticalconcentration,fcisreached:fcˇ3d22L2WheredandLarethediameterandlengthoftheparticles.TheofaspectratioasakeyfactorappearstoholdtruefortheGnP-5andGnP-25,astheelectricalresistivityscaleswithaspectratioinFigure4.3.TheaspectratioforeachnanoplateletislistedinTable4.2.Aspectratioisimportantbecauseitreducesthenumberoftimesanelectronmusttransfertoanotherparticle,themovetimesthishappensthehigherthetotalcontactresistance.TheelectricalresponseofthemuchsmallerGnP(750),whichshouldhaveatlyhighertotalcontactresistance,isaboutthesameastheneatGnP-5.However,thelargesampledeviationsuggeststhatcertainareasofthefoamareexperiencingahigherconcentrationofparticleswithoutagglomerating,meaningareductioninparticledistance,improvingparticletransfer.TheofaspectratiodoesscalewiththepMDItreatedGnP,althoughthecoatingontheedgeselectricallyisolatestheparticlesasPURanditsrelatedpolymershavelowelectricalconductance.9899Table4.2:AspectratioforGnPaccordingtotheproductsheetsforlargestthickness.GnPAspectRatioGnP-253125:1GnP-5625:1GnP(750)<250:1Becausehavingparticlesthatareclosetoeachotherisnecessaryforelectrontransferagglomerationsarenolongeraconcernaslongasapathcanform.Figure4.3showsthehighresistivityoftheneatfoamandwith5wt%GnP-25whichformsheavyagglomerationsinthefoam,theresultisadecreaseofabout5ordersofmagnitude.Percolationtheoryonlyrequirestheretobeatleastonepathoflongrangeconnectivity,thatallowsforsomeofthosepointsonthepathtobelargerthanothersduetoagglomerations.Aspreviouslymentioned,densityisalsoafactor,butfortphysicalreasonsfortheLowerdensitycausesparticlestopullapartastheamountofpolymerperunitvolumesimultaneouslydecreases.Yan,etal.studiedhowfoamdensityectspercolationwithMWNTsinpolyurethanefoamthatshowedaincreasingelectricalconductivitywithincreasingdensityandconcludedacriticaldensityatabout0.16g=cm3forelectricalcontact.Sinceelectricalconductivitywasalwaysagoalforthispolymericfoamthisdensitywaschosenforobservation.Inadditiontoparticlesbeingpulledaparttheycanalsoberadiallycompressedcreatingaggregatedpointsthatarealsobeingfurtherseparatedfromeachother.Aggregatesaremorelikelytoformwiththelargerparticles,decreasingthelocalconcentrationinespeciallythethinareaswheretheywouldbemostneededforconnectivity.So,thereisthepotentialthatthelargerplateletscouldbeevenmorebythechangingmicrostructureduetotheirmorphology.Forthesereasonsthepatternofimprovementinelectricalpropertiesdoesnotmatchthatofthemechanicalproperties.However,theretwooutliersfortheelectricalperformance.Thesamplewith8wt%minimalpMDItreatedGnP(750)hasthesameresistivityasthatoftheneatGnP-5.Possiblybecausethefoamwith8wt%neatGnP-5tendstoagglomerate,99100therefore,theresistivitycouldbemoreanofthedispersion,asthecompressivestrengthfortheminimalpMDItreatedGnP(750)demonstratesalikelyslightincreaseinthemechanicalperformanceoverthatofthestandardfoamsuggestinglessagglomeration.Inaddition,becausetheGnP-5particlesaresomuchsmallerthantheGnP-25theycouldbemorebyaggregationdecreasingthelocalconcentrationanddisruptingtheconductivepathespeciallyasFigure4.12hasshownthatbecauseofthelackofreorientationtheytendnottogointoareasofreallysmallvolume.AnotheroutlierexistsbetweentheminimalTDItreatedGnP-5andGnP(750),wheretheparticleswiththesmallestaspectratiotlyoutperformthelargerones.BecausetheelectricalresistivityoftheTDItreatedGnP(750)issolowthemuchshortermoleculesontheedgesarenotisolatingtheparticleslikethepolymercoatingdid.Thesimilaritiesincompressivestrengthsuggestasimilardispersion.Butthereisachangeinonekeyvalueandthatisthecellsize.TheTDItreatedGnP-5hasamuchlargercellstructureaofabout70msbetweenthetwo.Toaccommodatelargercellstherearethinnerstrutsreducingtheconcentrationofparticles.ThesizeoftheGnP-5couldbeinbetweentwophysicalphenomenons,theyaresmallenoughtobemorebyagglomerations,butnotsmallenoughtolocatewithinthinareasofthefoambecauseeventheaggregatesoftheTDItreatedGnP(750)aresmaller.Alsothechangeinmolecularandlikelyphasestructurethatcausesthe10°CinTgcouldalsoberesponsiblepossiblyahowtheparticlesandsmalleraggregatesorganizeinthestructure.4.4.0.3DielectricPerformanceWhenitcomestopolymericfoamsthemajorityhavealowdielectricconstant(realrelativepermittivity)andalowdissipationfactor(loss).Thisrelatestohowmuchenergyisstoredinthematerialwhenexposedtoanexternalalternatingelectricandhowmuchofthatenergyislost.Thelowdielectricconstantisduetothenon-polarnatureofthepolymerwhichisthetrueforPUR/PIR.Onlythepolymercontributestothedielectricpropertiesofthefoam,ascarbondioxidehasaverylowrealpermittivity[91,92].Aslessspaceisor100101thedensitydecreasesthepermittivityexperiencesalineardecrease[87].Thisisthereasoningbehindaddingconductivenanoparticlestothepolymerasconductivityisanotheravenuefordielectricresponsebeyondpolarizationcausedwhenanexternalelectromagnetic(EM)waveisapplied.Importantfactorsforelectricalconductivityhavealreadybeendiscussedandassuchallthenanocompositefoamsampleshaveanincreaseddielectricconstantoverthestandard.Tobetterunderstandthisitisnecessarytogointoamorein-depthdiscussionofpermittivity.Permittivityisslightlydtthanconductivityasitanbebythesmallscaleresponseofboundchargestoanappliedelectricinadditiontomobilecharges.Eachboundchargecanbethoughtofasbeingsurroundedbyanelectricdipole,eithernaturalorinduced,byanexternalAsthedipolesalignandstretchthisrepresentsstoredenergyinthematerial(real),butenergycanalsobelostrepresentedbytheimaginarypart.Togointomoredetailtherearetmodesofdielectricresponsethataremoreprominentatcertainfrequencies.Theelectronic/atomicresponsewhichfocusesontheelectroncloudsurroundingthenucleusofatomsisaminimalcontributioninthisfrequencyrange(8.2-12.0GHz)andalsoconstantandsoitcanbeignored.Themaincontributorstothedielectriccharacterinthetestedrangearetheionicandpolarizationmodes.Sincetherenoionicbondinginthesystemthismodecanalsobeignored.Polarizationcaneitherbeinducedorpermanentandthelowrealpermittivityofthestandardsuggestsanydipolesareprobablyinduced,butmostlyPUR/PIRisnonpolar.GnPhasaninterestingpolarduetoitsbonding.Thecoreismadeupofthesp2carbonatomswhichleavesanextraˇelectronpercarbonatomcreatingaˇ-cloudovertheplatelet,whichisresponsiblefortheelectronicproperties.Thisˇ-cloudisthenreadilyavailabletoorientitselfwhenanelectricisappliedseparatingandcreatinganinterfacebetweenthemobileelectronsandthepositivecoregivingabetterdielectricresponse;aphenomenoncommonlyreferredtoastheinterface/spacechargedielectricmode.AddingtheparticlesshouldthenincreasethedielectricresponseduetotheabilityoftheGnP'topolarize.Figure4.4101102theimprovementinenergystorageofthematerial.Unliketheelectricalresistivity,however,thedielectricresponseseemstoscaleforaspectratioforallneatandtreatedGnPinsteadoffollowingthepatternseenwithelectricalresistivity.ThetreatmentoftheGnPseemstodecreasethisresponse,butunlikefortheelectricalperformanceminimaltreatedGnP(750)hasalowerrealrelativepermittivitythantheneatandTDItreatedGnP-5.Infact,theenergystorageseemstobeevenmoresensitivetoaspectratioastheindividualplotsGnPtendtoclumptogetheraccordingtosizeexceptfornanocompositefoamwithneatGnP(750).NowitisnotunusualforthedielectricresponseofamaterialtochangeoverafrequencyrangewhatististhatitisonlyobservedinthesamplewithneatGnP(750).However,thedielectricresponsedealswithbothconductingchargesandboundchargesandthepreviouschapterdemonstratedthattheadditionofGnPchangesthemicrostructure.ItcouldbethatthisresultisduethethatGnP(750)ishavingeitheronmolecularstructureand/orphaseseparation.ThesamecouldbetrueforwhytheminimaltreatedGnP(750),whichhadasimilarelectricalresponsetotheneat,showsatlylessrealpermittivity.Electromagneticinterference(EMI)shieldingeness(SE)isofgreatimportancewhentryingtomaintainoptimalperformanceofsomanyelectronicsthatareincloseproximitytoeachother.TheEMIshieldingcharacterisnotonlyaboutbeingabletopreventEMwaves,butalsofocusesonhowthisisdone.CurrentlymanyEMIshieldingdevicesarehighelectricallyconductingmetals,butthesematerialstendtotheEMwavesbackwhichcouldstilldisrupttheinternalelectronics[93].Findingmaterialsthatabsorbinsteadofthen,isalsoimportant.Figures4.5-4.7showwhathappenstothewavewhenencounteredbytherigidPUR/PIRfoam.TheneatsamplewithnoGnPlimitedshieldingsoanyshieldinginthematerialisduetheGnP,dueeithertotheconductingnatureGnPitselforpossiblyfromchangesinthemicrostructureofthepolymer.ItappearsthatlikewiththeelectricalpropertiesthebestperformanceiswiththeneatGnP,whichchangeswithfrequencyandbecausefrequencyissuchanimportantfactor,thetransmittancedoesnotscalewithaspectratio.Atthelowerendoftherangethelargeparticlesaremore102103eatloweringtransmittanceandatthehigherfrequencythenanocompositefoamwithGnP(750)isatoutlier.ThesamplewithGnP(750)isthesamesamplethatshowedstrongfrequencydependencefortherealpermittivityinthisrange(seeFigure4.5.ForthisnanocompositefoamitappearsthatelectricalconductivityisnottheonlymethodresponsiblefortheEMIresponse.The(Figure4.6)andabsorbance(Figure4.7)plotsgivemoreofaninsightintothis.TheisdecreasingwithincreasingfrequencyforallnanocompositefoamsamplesalthoughthesamplewithGnP-25isdoingsoatafasterratecomparedtothetreatedGnP(750)atamuchslowerrate.TheincreaseintransmittanceforthefoamwithGnP-25isacombinationfromthefastdecreaseinthatisonlypartiallycompensatedforbyaslowerincreaseinabsorbance.ThedecreaseintransmittancefortheneatGnP(750)isduetothecorrespondingincreaseinabsorbancethatistlybetterthanthefoamwithGnP-25.Thesmallparticlesappeartobemorectiveatabsorbance,butconsideringtheoutlierthatthefoamwithneatGnP(750)isacrossnotonlythedielectricplots,butalsointhemechanicalandelectricalpropertiesthiseabsorbanceislikelyduetothebetterdispersionthattrapstheEMwavesandpossiblychangesinpolymermicrostructurethatcouldbemoreectiveatabsorbance.Theidealnanocompositefoamwouldbeonewherethenanoparticlesinsteadpreventedthewavesfromtransmittingandthenthepolymerabsorbedthem,butthelowabsorbanceofthestandardshowsthatmicrostructureofthePUR/PIRmatrixisnotthepolymertodothisasthechartinFigure4.7Butitispossiblethatchangesinthemicrostructureincludingthemicrophasecouldcorrespondingimprovementsintheshieldingproperties.TreatmentoftheedgesoftheGnPseemstoerlittlebandingeneraldecreasestheSE.Theonlyexceptionistheedge-functionalizedGnP-5thatshowslittleovertheneatGnP-5.Figure4.8alsoshowsthattheedge-treatedGnP(750)isnotonlyectiveatstoppingtheEMwavesitappearstobeonlyminimallybythefrequencywhichiscompletelytthanalltheothernanocompositesamples.Whileitisnotsurprisingthatthesmallparticleswiththehigheredgedensityaremorebytheedgetreatment,103104theyhavenearlythesameEMISEanditappearsthatallthatmattersforthesmallparticleswhethertheyaretreatedoruntreated.ThisalsosaysthatedgecharactercanbeanimportantforSE,whenitistanethathasbeenseeninalltheproperties.TheGnP-5whichdemonstratedthatchangesintheedgecharactercouldthemacroscopicmechanicalandelectricalpropertiesshowedlittlechangeintheSEexcepttheneatGnP-5hadaslowerslopelossinSE.ThechangesinalltheSEplotsuggestthatthenanocompositefoammightalsobemoreeinanotherrangewhichisnotuncommonasmanyttypesofshieldingmaterialshaveoptimalfrequencyranges.4.5ConclusionsTherearemanychallengesthatneedtobeovercomewhentryingtocreateamultifunctionalmaterial.Ingeneralthereisadecreaseinmanypolymericpropertiesthatscalewithdensity,soworkingwithacellularmaterialhasadditionalchallenges.Nanoparticleadditionauniquewaytoovercomethosechallenges,butcanalsocreatenewones.Forexample,theinteractionbetweentheparticlesandthematrixcanhaveasignitonthelevelofsuccess.Morespforpolymerfoamsitisimportantthatparticlesmatchwellwiththematrixsoagoodinterfaceiscreatedbetweentheandthematrixformechanicalpropertiesanddispersionotherwiseaggregatesform.AtproblemwiththelargestGnPasseeninthedecreaseinmechanicalperformanceofabout40%andthatwaswiththeSEMimages.Theseaggregatesthenactasstressconcentratorsdecreasingthemechanicalstrength.Aggregationdoesnotpreventtheformationofapercolatednetworkbutmaythecriticalconcentration.At5wt%GnP-25therewasdecreaseinelectricalresistivityofover5magnitudes.AmuchhigherloadingoftheGnP-5andGnP(750)thathadainaspectratioofabout2.5timesshowedsimilarelectricalresistivityandimagesofthedispersionshowthattheagglomerationoftheGnP-5likelyincreasesthecriticalconcentrationforpercolation.Whiletheseaggregationsmaynottheelectricalpropertiestherestillhavetobeenoughofthemtoformapercolatednetwork,thisbecomes104105abiggerconcernastheparticlesgetsmaller.Caremustbetakenwhentryingtoimprovetheinteractionbetweentheparticlesandthematrix.Edge-treatmentisonlyttoreduceagglomerationwhentheedgesdominateotherwisethebasalplanecharacterdoesasdemonstratedbythenochangeinthemechanicalperformancefortheneatandedge-treatedGnP-25.Itisnotjustaboutthecreatingagoodadhesionbetweenthetwo,butalsobeingcarefulinregardstothemolecularstructureformedintheseregions.TreatingtheGnPwithchemicalgroupstoimproveinteractionbetweentheparticlesandthepolymerresultedintheparticlesbeingelectricallyisolatedduetothecoatingofpolymerthatwasformedontheedgesanderedminimalimprovementintthemechanicalstrength.TheelectricalresistivityincreasedaboutthreeordersofmagnitudewiththetreatedGnP-25overtheneat.Ingeneralakeyfactorwastheaspectratio,aselectricalresistivitywiththetreatedGnPwassimilartotheneatforallothersamples,butthemechanicalperformancedidnotchangefromthefoamwithnon-treatedGnPortheedgetreatmentresultedinadecreaseofanywherefrom16to35%thatwasobservedwiththettreatmentsoftheGnP(750).Theonlyexceptionwasthesamplewith8wt%GnP-5thatshowedaverageimprovementofabout30%.Althoughthesampledeviationwasalsothelargestoutofanyofthesamplesandimagesshowedtheparticleswerestillagglomerated.Thedielectricpropertiesimprovedbyafactorof2.2,1.8,0.7forthenanocompositefoamwithneatGnP-25,GnP-5andGnP(750),respectively.Therealpermittivityimprovedwithelectricalconductance,butnotatthesamedegreebecausepermittivityisaresponseofmobileandboundchargesandthestrongerresponseofthelargerparticlestotheappliedelectricisatfactoraresponsethatdidnotchangebymorethan20%whenedge-treated.ThelargeparticlesgoodforelectricalcontactwereeattheEMwavesastheabout8-9timeshigheroverthestandardfoamforGnP-25andGnP-5at8.2GHzandtheGnP(750)wasonlyabout6timeshigher.ButthesmallerparticlesweregoodforabsorbingthemasatthesamefrequencytheGnP(750)wasabout5.5timeshigherforabsorbancecomparedtotheneatGnP-5andGnP(750)thatonlydemonstrated105106tripletheabsorbanceoverthestandardfoam.TheEMISEimprovedforallsampleswithbothneatandtreatedGnP.Theedge-treatedGnP(750)abouttripledtheSE,GnP-5andGnP(750)improvedtheSEbyabout10and9times,respectivelyandGnP-25increasedtheSEabout12timesat8.2GHz,amountsthatdecreasedbyabout20%whentheedgesofthelargerGnPweretreated.AlsothisimprovementtendedtodecreaseathigherfrequencieswithGnP(750)outperformingallofthemduetothemoretfrequencydependencedemonstratedbythefoamsampleswithneatandtreatedGnP-25andGnP-5.ItcouldbethattheEMISEwouldimproveifthesamplesweretestedatotherfrequencies.Uniquesolutionsmustbefoundtoimprovethenanocompositeassimplyoverloadingthefoamwithmoreisnotassolutionasoneofthegoalsistokeeptheweightaslowaspossible,whichwouldalsohelptokeepcostsdown,animportantfactorforanycommercialapplication.Thekeythenissmarterparticleloading.Keepinginmindtochoosereinforcingmaterialthatmatchesthematrixandidentifywhichgroupscreatetheideallocalmolecularstructuresothatthelocalpropertiesstayconsistentthroughoutthefoamandallowselectrontransfertooccureasilythroughoutthematerial.Thelargertheparticlesthemorethebasalplaneoftheplateletmorphologybecomesandit'simportanttoidentifyawaytofunctionalizethebasalplanethatdoesnotincludechemicalbondsthatwoulddestroyitscharacterifelectricalconductanceisimportantasevenedge-treatmentwhichdoesnotthebasalplanestillincreasedtheelectricalresistivity.Thechangesarealsoedinthemicrostructureandcouldbetheresultantproperties.TogainabetterunderstandingofthephysicalinteractionofGnPwithapolymerastudyofthemicrophaseneedstobedone.DependingonwheretheGnPispreferentiallylocatinginthephasestructurecouldtheabilitytoformapercolatednetworkespeciallyastheparticlesizedecreases.ItcouldalsobethatchangingthemicrophaseofthepolymerwithGnPcouldimprovetheEMISEormorespitsabsorbance.Inconclusionthedevelopmentofthesemultifunctionalhybridmaterialsreliesinengineeringthestructurefromthemolecularlevelonup.106107Appendix107108Figure4.20:RawdataofneatrigidPUR/PIRfoamspecimen'smechanicalperformanceduringcompressionFigure4.21:RawdataofrigidPUR/PIRfoamwith5wt%GnP-25specimen'smechanicalperformanceduringcompression108109Figure4.22:RawdataofrigidPUR/PIRfoamwith5wt%pMDItreatedGnP-25specimen'smechanicalperformanceduringcompressionFigure4.23:RawdataofanotherrigidPUR/PIRfoamwith5wt%pMDItreatedGnP-25specimen'smechanicalperformanceduringcompression109110CHAPTER5DIELECTRICANDEMISHIELDINGPROPERTIESOFPDMSANDPDMSSYNTACTICFOAMNANOCOMPOSITES5.1IntroductionAmultifunctionalmaterialisarelativelynewconceptthatrequiresthedevelopmentofanewmaterialusuallythroughnanostructuringtomakeacomposite.Nanocompositesarepromisingbecauselowweightpercentadditionsofnanoparticlescanresultinimprovedpropertiesofthematrixandevennewfunctionalitiesthatdonotexistintheneatmatrix.Polymersareonetypeofmatrixmaterialthathasbeenshowntogreatlybfromtheadditionofnanoparticles.Thesematerialsespeciallyshowpromiseinaerospaceapplicationsduetotheirlowerweightandprocessingvariabilityandcost.Carbonbasednanoparticlesarecommoninpolymericsystems[8,94,95]andgraphenespllyhasbeenusedinavarietyofpolymersaswellaspolymerfoams[96,97,98,7,71],regardlessofthechallengesworkingwiththematerialpresents.Grapheneissuchapromisingmaterialdueitsuniquebondingstructurethatgivesitadiversearrayofdesirableproperties.Itisasinglelayerofsp2bondedcarbonatomsthatstacktogethertomakeupgraphite.Thiscovalentbondingresultsinamaterialthatisverywithahightensilemodulusgreaterthan1TPathatleavesanextraelectron/atomcreatingaˇcloudthatgivesthematerialmobilitiesgreaterthan15,000cm2=Vs[45,19].Themultifunctionalnatureofgraphenemakesitverypromisingforaerospaceapplicationsasusingoneatlowconcentrationscouldmakelight-weightcompositesystems.Therearebothtop-upandbottom-downapproachestomakinggraphene.Someresultinhigh-puritysingletofew-layergraphenesheets[26,27,29,30],butareveryexpensivetosynthesize.Sometop-downinexpensivemethodsstartwithgraphiticstructuresthenseparatethesheetstomakethegraphene.Reducedgraphiteoxideproducesverythinsheetsbutthe110111oxidationstepcausesmoredamagetothegraphiticstructureresultinginpoorerproperties[38,39].Exfoliatedgraphitereliesonintercalatedgroupstoseparatethelayersandproducesmultilayersheetswithcomparablepropertiestosinglelayergrapheneinamorerobustform[43,44,38,33].Siliconeisahighperformancepolymerthatcouldgreatlybfromtheadditionofgraphene.Itisusedinaerospaceapplicationsduetotheiryatlowtemperaturesandenvironmentalresistanceduetotheiruniqueinorganic-organicstructure,butlikemanypolymershaslowelectricalproperties.Therearemanyttypesofsiliconematerials,buttheelastomersarecommonlypolydimethylsiloxane,anopticallyclearpolymer,butcanalsoappearwhitefromtheadditionofsilicon-dioxideItisaninsulatorwithalowdielectricconstant,butifthepropertiescouldbeimproveditshowspromisesasamultifunctionalsealandgasketforaerospaceapplications.Inaddition,utilizingaPDMSfoamwouldalsoweightsavingsanimportantfactorinspacecrafts.Therearemanytroutestomakefoamsincludingeitherchemical[71,16,99]orphysical[100,85]blowingagents.Anothermethodistocreateasyntacticfoamwherethecellsarereplacedwithsometypeofsphericalparticlesuchasahollowglasssphere[101].Syntacticfoamstheboverothermethodsinthatthecellsizeisrelativelyconsistentandglassspherecanpotentiallyimprovethemechanicalpropertiesoftheoverallcomposite[102,103].InthischapteranancompositePDMSsyntacticfoamisinvestigatedtoseethethattheglassbubblesandagraphenehaveontheontheelectromagneticpropertiesofthefoam.1111125.2Materials/Synthesis5.2.1Materials5.2.1.1PDMSRTV615fromMomentivePerformanceIncwasusedasthematrix[59].ItisaPDMSelastomerwithnollersfromMomentiveandcomesina2-partkitwherepart1isavinyl-terminatedPDMSandpart2containsthecuringagent.Thematerialhasarelativelylowviscosityof4Pasandaspgravityof1.02.5.2.1.2GnPThegraphenematerialwasdonatedbyXGSciences.Thegraphenenanoplatelets(GnP)containmultiplelayersofgrapheneforamorerobustparticle.Threetgradeswereselected.ThearexGnP-M-25andxGnP-M-5,bothgradeMwithsurfaceareasbetween120-150m2=gandbetween6-8layersofgraphenethick[63].GnP-25willdesignatexGnP-M-25whichhasanaverageparticlediameterof25mandGnP-5designatesxGnP-M-5whichhasaveragelateraldimensionsof5m[63].ThelastgradeisxGnP-C-750,theC-gradeisahighsurfaceareamaterialandinthiscasehasasurfaceareaof750m2=g[63].Itcontainsplateletslessthan2mindiameterandthicknessesofonlyafewnmthataregenerallyaggregatedintosub-micronparticles[63].TodistinguishthesehighsurfaceareaparticlesfromthelowsurfaceareaplateletsxGnP-C-750willbelabeledGnP(750).5.2.1.3HollowGlassSpheresThehollowglassspheres(HGS),iM16K,weredonatedby3MŠ(#98021327964)[60].Thewhitepowdercontainssodalimeborosilicateglasssphereswithanaveragediameterof20mandcontainlessthan3%ofasyntheticamorphoussilicathatisnecessarytoensuretheglassspheresw.TheglassbubbleswiththeamorphoussilicagranulescanbeseeninFigure112113Figure5.1:SEMimageof3MŠiM16Kglassbubblessprinkledwithamorphoussilicaonsurfaceandmeasureddiameters5.1.The3MŠiM16Kglassbubbleswerechosenfortheirhighcrushstrengthofgreaterthan110MPathatmakesthemabletowithstandthelargethermalexpansioncotofthePDMS.Lastly,duetotheirrelativelysmalldiameterthehollowglasssphereshaveadensityof0.46g=cm2.AsilanecouplingagentwasusedtoadheretheGnPtotheglassbubbles.ThebinderisatrimethoxysilylpropylmoPEIsilane(tPEI)fromGelest(#SSP-060)[61].5.2.2ExperimentalProcedure5.2.2.1GnP/PDMSTheneatPDMSelastomerismadebypouringpartAthenpartBintoamixingvesselataratioof10:1byweight.Thematerialisthenhandmixedfor30sfollowedbyhighspeedshearmixingat3000rpmfor2min.Aftermixingitispouredintoasecondarycontainerfordegassingthencastintomolds.Thecastspecimensarethenbakedat100°Cfor1h.ForthesamplesthatcontainGnP,itisaddedafterthehandmixingofpartAandBandpriortothehighspeedshearmixing.Theweightadditionsaredeterminedrelativetotheneat113114sample.TheGnP/PDMSisthenhighspeedshearmixedat3000rpmfor2minfollowedbyanadditionalminuteafterthematerialcoolsdowntopreventthematerialfromheatingupandkickstartingthecure.5.2.2.2CoatingGlassBubblesTheglassbubblesarecoatedwithGnP-5andGnP(750)priortoaddingtothePDMSelastomer.Thetotalweightpercent(Wt%)ofGnPneededtocoatthesphereswith5layersisgivenbyequation2.4onpage23.Inthisequation,tisthethickness,ˆGnPandˆHGSistherelativegravityoftheGnPandhollowglassspheres,respectively,andDiM16Kistheaveragediameteroftheglassbubbles.Thetotalamountismultipliedby5because5layersshouldbeenoughtoensureaconductivecoating.TheresultingamountofGnP-5andGnP(750)wasdividedbytwosincebothtypesofGnPweresimultaneouslycoatedonthesurface.Theprocedureisasfollows:1.GnP(750)isaddedtoreverseosmosis(RO)waterataratioofabout90mgto500ml.2.Thesolutionisultrasonicatedwitha2.54cmprobeat100Wfor1h.3.TheGnP-5isthenaddedfollowedbyadditionalultrasonicationfor1hwiththesameparameters.4.AddthetPEIbindertothesolutionataratioof1:1wt%toGnP.5.Ultrasonicatefor4minwitha2.54cmprobeat50W.6.SlowlyaddtheHGSwhilestirring7.Continuestirringovernight8.TheGnPcoatedHGSarethencollectedbybeforedrying.9.AfterdryingtheGnPcoatediM16Kishighspeedshearmixedat1200rpmfor30stobreakuptheclumps.114115Figure5.2:SEMimageofHGScoatedwithGnP-5andGnP(750)afterdrying.Thesamplewasnotcoatedpriortoimaging.TheresultingglassbubblesareshowninFigure5.2.ComparingtheneatandGnPcoatediM16KthereismoreplateletlikeappearanceonthesurfaceduetotheGnP.BecausetheGnPonthesurfaceisverythinandsmallitappearsthattheGnP(750)ismoreeatcoatingthesphericalsurface.5.2.2.3PDMSsyntacticfoamTheinitialstepsarethesamefortheneatPDMSsamplewherethetwopartsoftheelastomerarecombinedbyhandmixingfor30s.ThehollowglassspheresareaddedbyvolumepercenttothePDMSelastomerpriortobeinghighspeedshearmixedat3000rpmfor2min.TheGnPcoatedglassbubblesaremixedforanadditionalminat3000rpmaftercooldowntoensureadequatemixingandtoensurecuredoesnotstartprematurely.Thematerialisthenpouredintoasecondarycontainerfordegassing,followedbycastingintomolds.Thematerialisthenheatedat100°Cfor1h.1151165.2.3Testing5.2.3.1VectorNetworkAnalyzerThedielectricpropertiesweredeterminedusingatransmissionlinesystemwhichincludesavectornetworkanalyzer(VNA)connectedtoawaveguide.Theaveragingfactorwassetat64,anIFbandwidthof5kHzwasselectedandtheimpedancewassetto1Thewaveguidedimensionswereasfollows:10.5mmby22.9mmby7.4mmthickforX-bandtesting.Fourspecimenswerecasttothesedimensionsand3weretested.TheNRW(NicholsonandRoss[65]andWeir[66])algorithmwasusedtodeterminetherelativepermittivityandpermeabilityasaratiotovacuumaccordingtotherelationshipsgiveninSection2.3.5.Inaddition,S12inthisset-upisameasureoftherespectivevoltageswhenamaterialispresentandwhenitisnotpresent,andisusedintheoftheshieldingenessaccordingtoequation2.13,shownagainhere.Alltestingwasdoneatroomtemperature.SET=20logV1V2=20logjS12j(dB)ThetotalshieldingenessisthesumofthelossesfromonSER,absorptionSEAandmultipleSEMasthewavepropagatesthroughthesample.Thetransmittance(T),(R),andabsorbance(A)withrespecttotheincidentwaveshowninchapter2isalsogivenagainhere,T=jS12j2(5.1)R=jS11j2(5.2)A=1RT(5.3)5.2.3.2MicroscopyAllimagesweretakenonavariablepressurescanningelectronmicroscope(SEM).Thesamplesurfaceforimagingwaspreparedbytearingthespecimensafteraslightcuttobegintheseparation.Thespecimenswereadheredtothesampleholderusingconductivecarbonpaste116117Figure5.3:RealpermeabilityasaratiotothepermeabilityoffreespacefornanocompositePDMSsamplesfrom8.2to12.4GHz,comparedtotheneatPDMSelastomer.andallthecuredPDMSsampleswerecoatedwithabout3nmoftungstenpriortoimaging.Theglassbubbleswerenotcoated,butadheredtothestubwiththesameconductivecarbonpastebeforegoingintotheSEMchamberundervariablepressuremodeforimaging.5.3ResultsThefollowingchartsshowtherealpermeabilityandpermittivityofthesamplesrelativetothepermeabilityandpermittivityoffreespaceaswellasthealternatingelectriclosstangentandlastlythetEMshieldingcharacteristicscalculatedfromtheS11andS12scatteringparameters.5.4DiscussionPDMSisanon-magneticpolymer,whichisbythelowrelativepermeabilitygiveninFigure5.3.Graphenealongwithgraphitehasalsobeenfoundtobelowparamagneticmaterials[104].Thepermeabilityresultssubstantiatethisclaimasthemagneticcapabilitiesof117118Figure5.4:RealpermittivityasaratiotothepermittivityoffreespacefornanocompositePDMSsamplesfrom8.2to12.4GHz,comparedtotheneatPDMSelastomer.Figure5.5:Alternatingelectriclosstangent(tana=""="0)ofnanocompositePDMSsamplesfrom8.2to12.4GHz.118119Figure5.6:FractionofappliedelectricthattransmitsthroughthenanocompositePDMSsamplesfrom8.2to12.4GHz.Figure5.7:FractionofappliedelectricthatectsofnanocompositePDMSsamplesfrom8.2to12.4GHz.119120Figure5.8:FractionofappliedelectricthatisabsorbedbythenanocompositePDMSsamplesfrom8.2to12.4GHz.Figure5.9:TotalEMIshieldingenessofnanocompositePDMSsamplesfrom8.2to12.4GHz.120121thenanocompositePDMSisminimalasallthegraphsofthesamplesshowrealpermeabilitiesofaround1H=m.Boththeneatandnanocompositesamplesareconsideredtobenon-magneticsotherestofthediscussionwillfocusontheelectricproperties.5.4.1ModesofDielectricResponseWhenanelectromagneticisappliedtoanymaterialtherewillbeadielectricresponse.Thestrengthoftheresponsewilldependonthedegreeandeaseofalignmentinthelocalareasofthematerial.lnanymaterialtherearewhatcouldbecalledelectricdipoles,theareasofpositiveandnegativecharges.Thisiscommonlyassociatedwithatoms,buttherecanalsobepositiveandnegativeareasinamaterialonalargerscale.Thismeansthatwhenanelectricisappliedthesepositiveandnegativepartsinlocalareasthatwillalignaccordingtoanappliedastheelectricisavectorthathasaforceanddirection.Themorepowerfultheappliedthegreaterresultingseparationbetweenthesecharges.Foranymaterialthemajorityoftheelectricdipolesareboundchargeswheretheirfunctionisnotconduction,buttheycanshiftwhenexposedtoanelectric.Usuallythesedipolesareorientedrandomlyinamaterial;thedielectricresponsethenofanymaterialdescribestheabilityofthesedipolestoalignaccordingtoanexternalelectricIftheeldsaligneasilythereismoreenergystoredinthesystem,themoreultthemoreenergythatislostinthesystem.TherearefourtareasinamaterialthatarebyanappliedelectricIfionicatomsarepresenttheycanbebytheelectricaseachpairofionsisbasicallyanelectricdipole.Becauseionsarerelativelylargetheseparticlesareonlyuptocertainfrequenciesbeforetherateofthealternatingistoofastofamotionfortheionstokeepupwith.Anothermodethatoperatesonlyatthosesamefrequenciesisthedielectricresponsetheorientationalorpolarect.Thesepolaritiesareduetoeitherbepermanentdipolesorifthelocalareaisnon{polar,thoseinducedbytheappliedthatcausesashiftingofthecentersofthepositiveandnegativepartssotheyarenolonger121122concentric.Toinducedipoleswheretherearenonerequiresaforcestrongenoughtoovercometheattractionofthepositiveandnegativecharges.Onceagaintheresponseofthesedipolesislimitedtolowerfrequenciesduetheabilityoftheselargerdipolestokeepupwiththefrequency.Anotherlowfrequencydielectricmechanismistheinterface/spacechargeresponsewherethefocusinnowonmaterialsthathavemetallic-likeconduction,wheretheconductingelectronsinthematerialhavenobandgapandcouldeasilyalignandseparateinthematerialwhenaisappliedcreatinganinterfacebetweentheelectronsandthenowpositivecore.Forhigherfrequenciesthedielectricresponseisdominatedbytheelectronicandatomicresponsesofthedielectric.Atlowfrequenciesthesefactorsareconstantandsmallrelativetotheothercontributions,butatfrequenciesgreaterthanabout1012Hzthesemechanismsstarttodominateastheothersnolongerfunction.BothmodeshavetoduewiththeinducedelectricdipolethatanatomformswhenexposedtoanexternalbutbecausethesemeasurementsaretakenintheX-bandfrequencytheyarenotcontributingmodestothedielectricresponse.5.4.2ProbablemodesofdielectricresponseinnanocompositePDMSSincethenanocompositePDMSisbeingtestedintheX-bandfrequencyfrom8.4-12.2GHztheelectronicandatomiccontributionstothedielectricresponsearebothconstantandsmallsotheywillbeignoredfortheremainderofthediscussion.PDMSisauniquepolymerduetoitspropertiesatlowtemperaturesfromitsunconventionalinorganic-organicstructurethatconsistsofasilicon-oxygenbackbonewithorganicmethylsidegroups.Aswithanyfoamthedielectricpropertiesaredependentonlyonthepolymerandchangeswithdensity[87].Thelowerthedensitythelowertherealpermittivityastheadditionofgasinthehollowglassspherecontributesminimaldielectricresponse[92].PDMSisbothnon-ionicandnon-polarsotherewillbealimitednumberofweakpermanentdipolesanyotherswillhavetobeinduced.Thisisaninsulatingmaterialsoanyfreeelectronsthatappearonthesurfacescreeningtheinteriorfromtheofanexternalalternating[87].Freeelectronsarealsobial122123forelectromagnetic(EM)wavesforelectromagneticinterference(EMI)shielding,ofwhichinsulatorshaveasmallamount.ThebehaviorasaninsulatorisbythelowrealpermittivityandshowstlossforamaterialaccordingtoFigures5.4and5.5.Ageneralguideistoconsideralossydielectriconethathasanalternatinglosstangent,a>0:1andalow-losshasaa<0:01,thePDMSisshowinganalternatinglosstangentrightbetweenthesetwoComparethematrixtothesodalimeborosilicateglassspheres.Thestructureofanyglassisamorphousandinthisglassismadeupoftceramicoxidemoleculesthathavelocalordertoformsilicondioxide,calciumoxide("lime"),borontrioxide(B2O3)andsodiumoxide("soda"-Na2O)indecreasingorderofconcentration.Thisstructurehasionicbondingandtherebycanbesusceptibletoanelectricbutthesesspheresarehollownotsolidandhaveadielectricconstantof1.2to1.7F=m@100MHzaccordingtothe3MŠproductsheet.ThisvalueisnotwithintheX-bandfrequencybutresultsfromreplacingsomeofthePDMSmaterialwiththehollowglassspheresdoesresultinadecreaseintherealrelativepermittivitybutnottheloss(seeFigure5.4).ThiscouldsimplybeduethefactthereissimplylessmaterialtobebytheappliedasthePDMSsyntacticfoamhasa20%lessascomparedtotheneat.ThelastfactortotakeintoaccountisthecontributionoftheGnPtothenanocomposite.Asstatedpreviouslythesp2bondingcreatesaˇcloudmadeupoftheextraˇelectronsthatareavailableforconduction.Whenanelectricisapplieditiseasytoimaginetheseˇelectronsaligningtotheseparatingfromthepositivehoneycomblatticecorecreatinganinterface/spacechargeregionsimilartometals.Alltheothermodesofdielectricresponsewouldbeminimalcontributorscomparedtotheofhavingtheconductingcharges.SothenaddingtheseparticlestoPDMSresultsinanincreaseindielectricresponseevenatlowloadingsasseenfromtheFigure5.4wherethereispositiveincreaseontherealrelativepermittivityduetotheadditionofthenanoplatelets.1231245.4.3ofconductivityonpermittivityFigure5.4isaplotoftherealrelativepermittivityofallsynthesizedPDMSspecimenswithGnPandwithhollowglassspheresrelativetotheneatPDMSsample.Someofthesamplesshowasharpincreaseinthedielectricresponseatcertainfrequencies.Thisduetotheresonancefrequencyofthesample,whichonlyoccursinthisfrequencyrangeforthesampleswithGnP.Inallmaterialsatalltimesatomsarenotstatic,butconstantlyvibratingattemperaturesaboveabsolutezero.Inanalternatingelectrictheseboundatomsareoscillatingmuchlikeanydampingharmonicoscillatorbetweenthepositiveandnegativeionicorelectronicparts.Resonancedescribesthephenomenonwhenthefrequencyoftheappliedmatchesthefrequencyoftheionicorelectronicpolarizationmechanism.Thiscanresultinimprovementinthedielectricperformancewhenthereisanincreaseinthedielectricresponsefromtheincreaseinamplitudeofoscillationsresultinginalargerpolarizationorseparationbetweentheoppositelychargedpartscreatingalargerdipole.BecausethisonlyoccursinthesampleswithadditionofGnPonly,thisdipoledescribestheinterfacebetweenelectronthecloudandpositivecoreoftheplatelets.Alldipoleshaveadipolemoment,whichisrelatedtothemagnitudeofthecharge.Pisthedipolemomentperunitvolume,whichislinearlyrelatedtotheelectricsusceptibility,˜,adimensionlessquantitythatdescribeshoweasilyamaterialpolarizesdueistoanappliedelectricwhichinturnrelatestotherelativepermittivityaccordingtoequations5.4and5.5.P=˜e"0E(5.4)"r=1+˜e(5.5)Wherethisresonancewavelengthoccursisdependentonthedielectricpropertiesofthematerial.WhilePDMSseemstobearelativelylossydielectricitalsocontainsgoodconductingparticles.Aplanewavepropagatinginagoodconductor=2ˇistfromhowitwouldpropagateinfreespace0=c=f.Thefactoristhepenetration/skindepthandisthepointatwhichthewaveenergydecreasesto1=exp.Itisrelatedtothe124125Figure5.10:Realrelativepermittivity("0r)versusalternatinglosstangent(tana)ofnanocom-positePDMSsamplesandneatelastomerat11GHz.conductivity,˙,orconverselytheresistivity,ˆ,accordingtoequation5.6[105].=1pfˇ=rˆˇf(5.6)Sincethematerialisnon-magneticcanbetakenas1andfisthefrequency,forthismaterialtheskindepth,/˙1=2.Astheconductivityincreases,thepenetrationdepthdecreasesandcausingthewavelengththroughthematerialtobeshorterthanifthewavewerepropagatingthroughfreespace.Forthesameresonancewavelengththefrequencyatwhichthiswavelengthisreachedwillbetduetothethematerial.ThefactthattheresonantwavelengthischangingsuggeststhematerialnowhassomecharacteristicsofagoodconductorwhichcanonlybeduetotheadditionoftheGnP.ItsuggeststhantheGnPhasatleastpercolatedthroughthesamplecreatingaconductivepathway.Theofwhetherthematerialhaspercolatedornotseemstobehavinganonthebehavioronthematerialwhichisshownclearlyintheperformancebetweenthesamples.ThisishighlightedinFigure5.10whereallthesamplesbasicallyplotout125126intwohorizontallineswhencomparingtherealrelativepermittivitytothealternatinglosstangent.ItshowsthetimprovementinrealpermittivitybetweenthesampleswiththehighestloadingofGnPascomparedtotheothers.Thismarkedimprovementonceagainsuggeststhatmaterialisbehavinglikeaconductoratthesefrequencieswhichistruewhenthepercolationthresholdisreachedwhichseemstobethecaseforthesampleswith2wt%GnP-25and3.6wt%GnP(2.15wt%GnP-5,1.45wt%GnP(750)).Thesamplewith2wt%GnP-25showsthebestpermittivityatlowerloadingbecauseofthemuchlargeraspectratiooftheparticlewhichisknowntobeacontributingfactortothepercolationthreshold[52].5.4.4ModesoflossindielectricsConductivitycanhavebothpositiveandnegativeeonthedielectricresponseofmaterialwhenexposedtoanelectromagneticwave.ItcanalsocontributetothelossifItproducescollisionsbetweenthemanyfreeelectronsinthematerial.Thatishowevernottheonlymodeoflossinadielectric.Anotherfactorrelatesdirectlytothelossofenergyoftheappliedelectricduetoheateitherfromfrictionand/ortheyinacceleration/deaccelerationofthedipoles[106].Theconditionisdescribedbythestaticlosstangentandthelatterbythealternatingelectriclosstangent.Formanymaterialsoneofthesetermsdominatesmorethantheother,butsincethenanocompositePDMScontainsconductiveparticlesinaninsulatingmatrixbothtermscontributetothetotalloss.tane=tans+tana(5.7)where,tans=˙!"0andtana="""0(5.8)Formostofthesamplesthecontributinglosswillbethealternatinglosstangent,butforthesampleswithtloadingsofGnPthestaticlosstangentwillbecomemoret.Overallthemateriallossdoesnotchangeatamountthroughallthesamplessoforthemostpartthematrixpropertiesstillseemtobedominatingfactorindeterminingthepermittivitylosses.126127Figure5.11:Agraphicillustratingthethreewaysanelectromagneticplanewavecanbewhenconfrontedwithamaterial.5.4.5EMIShieldingElectromagneticinterference(EMI)shieldingeness(SE)quanhoweamaterialisatpreventingelectromagneticpowerfromtransferringthroughit.FortransmissionlineexperimentsthisfactorisrelatedtothescatteringparameterS21bytheequation:SEtotal=20logS21,wherethescatteringparameterisarelationbetweenvoltageswhenthematerialispresentversuswhenitisnot.Therearethreewaysinwhichanelectromagneticwaveinteractswithamaterial,itcanbeabsorbedand/orexperiencemultiplethetotalSEisthesumofallthesefactors.ThethreemodesareshowninFigure5.11anddemonstratewhathappenstoaplanewavewhenitencountersamaterial.Outofallofthesemodesthemostdesiredisabsorbanceasionisonlyabletooutsideelectronics,butallowstheelectromagneticwavestobebackincausingdisruptionintheinternalsystemandisthecurrentdrawbackwithtraditionalmetallicshieldingmaterials.Figure5.7givesfurtherevidencethatPDMSwith2wt%GnP-25and3.6wt%GnPhasmobilechargecarriersdemonstratedbytheincreaseinTheyarealsotheonlytwosamplesthatshowimprovementinabsorbance,itistodistinguishintherestofthem.Theonlyexceptiontothisisthesampleswiththeglassbubbleswhilethe127128absorbanceisstilllowthenoisinessoftheplotshastlydecreased.Absorbancedependsonthicknessthatbecomesmoreprominentwhenthethicknessisgreaterthantheskindepth,whichcorrespondinglyiswhenmultiplecanalsobeignored.Accordingtoequation5.6skindepthdecreasesastheconductivityorpermeabilityincreases.Sincethismaterialisessentiallynon-magneticchangestheincreaseinabsorbanceareduetotheincreaseinconductance.ConductivityisofcoursealsoanimportantfactorinimprovingtheceofthesamplesandthesamenanocompositesamplesshowoverallamuchbettertransmittanceandForthenanocompositePDMStheandtransmittanceseemtobeinverselyrelatedforthemostpart,asonedecreasestheotherincreasesproportionallythiscanbeseeninthechartsthatshowthefractionofeachmodetheincidentelectromagneticwaveisby.Thechangesintransmittance,andabsorbancecanbesolelyexplainedbytheadditionoftheGnPastheadditionofthehollowglassspheressimplyreducestheamountofsolidandglassisaninsulator.ThestrongfrequencydependenceofallthesamplesthoughsuggestamaterialisstillbehavingmorelikeaninsulatorthanaconductingmediumasconductorslikecopperandpolyanilineshowamuchmoreconstantSEovervaryingfrequencies[67].5.4.6ImprovementsThenanocompositePDMSsampleswith2wt%GnP-25and3.6wt%GnPshowedimprovedconnectivityregardlessofthepoordistributionoftheGnPthroughoutthesample.Figures5.12and5.13clearlyshowmultipleagglomerationsthatappearintheimagesasreallythickstackedparticlesor,inthecaseofGnP(750),asamorphousparticles.High-speedshearmixingdoesnotseemtobeavigorousenoughmethodtobreakdownthelargerGnP(750)sub-micronagglomeratesorseparatethelargerGnP-25particles.ThearrowsinFigures5.12(b)and(d)pointstoGnPthatappearstonotbeagglomerated,butalltheseparticlesarealsoofasmallerdiameter.ThisisnotsurprisingsincetheagglomerationoftheGnPisduetotheattractionbetweenthebasalplanesoftheparticles;smallerparticlesdonothaveas128129(a)(b)(c)(d)Figure5.12:SEMimagesof2wt%GnP-25inPDMSelastomer.ArrowsandcirclesaretohighlightthelocationofsomeofthedistinguishableGnP:(a)LargeagglomerateofGnP-25intoprightcornerandarrowpointstoathinplatelet;(b)AgglomeratesandthinGnParerelativelyclosetogether;(c)DistancebetweensomeGnPagglomerates;and,(d)GnPagglomeratescanbeverycloseordecentlyfarfromeachother.129130(a)(b)Figure5.13:SEMimagesofPDMSwith2.15wt%GnP-5and1.45wt%GnP(750):(a)DarklargeparticlesareagglomeratesofGnP(750);(b)StackofGnP-5.largeofbasalplanemakingthemeasiertoseparate.ThewiththeGnP(750)isthattheparticlesarenotsimplystacked,butoverlappedmanytdirectionscontainingamuchlargeramountofplateletsthatneedtobeseparated.Agglomerationisgenerallytobeavoidedasitincreasestheloadingnecessarytoachievepercolation,inaddition,theyactasstressconcentratorsreducingthemechanicalproperties.However,thestackedparticlesallowforeasierlocalconductionresultinginalargerlocaldielectricresponsepotentiallycontributingtoahigherpolarization/unitvolume.IftheparticlesareseparatedandorientedinmanytdirectionsthatcouldreducethelocalpolarizationandeventhoughtherewouldbemoreareasofsmallerlocalpolarizationthatcouldstillbelessthanthepolarizationthatamountswhenGnPstackedtogetherinthesamedirection.Figure5.14demonstrateshowthemicrostructurevariesfromtheneatelastomertothatwith40vol%loadingofHGSthatareresponsibleforthelossintherealpermittivitywiththeadditionoftheempty"cells"inthesyntacticfoam.ComparetheimagesofthenanocompositePDMStothenancompositePDMSsyntacticfoamwhereitappearsthatthereisnoGnPinthesyntacticfoam,buttheimprovementindielectricperformanceinthesyntacticfoamwithGnPoverthatwithoutsuggestingthattheGnPcancombatthelossofdensity.Thisalso130131Figure5.14:SEMimageofdispersionof40vol%HGSinPDMSelastomerthatthereisGnPalthoughitistodistinguishinFigure5.15(a).WhatisalsomissingarethelargeagglomeratesofGnPthatappearedinthenanocompositeGnPsamples.ItappearsthattheadditionofultrasonicationandthenadheringtheparticlestotheHGSseemstodoarelativelygoodjobofbreakingdownandmaintainingtheseparationoftheGnPandespeciallytheGnP(750),whichistoseeatthesensduetotheirsmalldiameter(<2m).However,theagitationofthelowerspeedofthehighspeedshearmixerwasapparentlynotabletobreakdownalltheclumpsofGnPcoatedHGSasnowtheagglomeratesinthesystemareofGnPcoatedHGSthatappeartohavebeenminimallycoatedwiththepolymerasshowninFigure5.15(b).Forthedielectricperformance,however,thehollowglassspheresseempromisingasgenerallyborosilicateglassisalowlossmaterialandoutofallthespecimensinFigure5.10theonewith40vol%hasthelowestalternatinglosstangentat11GHz.CoatingthespheresinGnPimprovestherealpermittivityrelativethesamplewithnon-coatedHGSandcomparableshieldingperformancetotheneatsampledemonstratingcomparableabsorbancewhilebeing20%lighter.131132(a)(b)Figure5.15:SEMimagesof40vol%GnPcoatedHGSinPDMSelastomer.TheamountofGnPrelativetothespheresis2.15wt%GnP-5and1.45wt%GnP(750):(a)Abletodistinguishonly1GnPonHGSinPDMS;(b)ClumpofGnPcoatedHGSinPDMS.5.5ConclusionFoamsareratherpromisingmaterialsduetotheirlowdensitymakingthemidealmaterialsforapplicationsinwhichweightisaconcern.However,bothpermittivityandEMISEdecreasewithfoamdensity,GnPwasaddedasawaytoovercomethatbytheGnPformingapercolatednetwork.GnPisamultifunctionalnanoparticlethatincludeshighelectricalconductivityaswellasapositivecentralcoresurroundedbyap-cloudofelectronspropertiesthatisbforimprovingboththedielectricandEMIshieldingproperties.AddingGnPtoPDMSresultedinimprovementofthedielectricpropertiesandshieldingectivenessofthesamplesincludingincreasingtheabilityofthepolymertoabsorbelectromagneticwaveslikelybydecreasingtheskindepth.Alloftheseimprovementscanbesolelyexplainedbytheadditionoftheconductingparticles.ThegreatestimprovementwasseeninthesamplesthathadthehighestloadingofGnPinthesolid.Alargeaspectratiohasbeenfoundtobeanimportantpropertyforachievingelectricalpercolationatlowerloadings.ThePDMSwith2wt%GnP-25increasedtherealpermittivityabout2.5timesovertheneatmaterialandimprovedtheSEenessabout9132133timesatitshighestwhileincreasingtheabsorbanceandectance.ComparedtothePDMSsamplewith1.6%moreGnPthatismixtureoftwosizesofsigntlysmalleraspectratiowereonlyabletodoublethedielectricconstantandwasabletoimprovetheEMISE,butisunclearbyhowmuchduetothestrongdependenceonfrequency.However,theearliershiftoftheresonancefrequencybyabout1GHzsuggeststhatthesamplewith2wt%GnP-25ismoreconductive.Whetherapercolatednetworkformedinthesystemisunclearasthestrongfrequencydependencesuggestsamaterialthatisstillnotbehavingasametallicconductor.IftheGnPdispersionweremoreuniformandlessaggregatedthatcouldresultinacorrespondingimprovementinelectricalperformancebycreatingmuchmoreconnectedpaths.ThechallengewithincreasingthediameterontheGnPcreatesastrongerattractionduetheVanderWaalsforcesonthebasalplanethatpreferentiallyaggregateGnPratherthaninteractwiththepolymerandanyfuturemethodsemployedtoimprovetheinteractionbetweentheGnPandthematrixmustdosowithoutdisruptingthebasalplanecharacterresponsibleforitsstrongelectricalperformance.PDMSalreadyhasapplicationsinaerospaceduetotheirtyatlowtemperaturesandaddingGnPseemstoshowpromiseasamultifunctionalforthismaterial.CoatingtheHGSpriortodispersinginthepolymerimprovedthebehaviorofthenanocompositesyntacticfoam.IthadthesamerealpermittivityastheneatPDMSandimprovedtherealpermittivitybyabout12%atonly0.64wt%GnP.ItdemonstratedthesameEMISEastheneatandPDMSwith0.64wt%GnPwhilestillbeing20%lighter.However,theresultantimagesofthefoammadeitunclearwhethertheGnPmanagedtostayadheredtothespheresduringprocessing.Alsotheoccasionalclumpsofcoatedspheresreducestheconnectivityproducingareasofhighandlowlocalconductance,thecoatedHGScouldbfromamorevigorousmixingpriortoaddingtothepolymertobreakuptheclumps,butcarefullysothattheGnPstaysadhered.Thematerialsandmethodsemployedinthisresearcharerelativelylow-cost,whichmakesnanostructuringapromisingmethodtoimprovethepropertiesofpolymers.Iftheinteraction133134betweentheGnPandthematrixwereimprovedinsuchawaythattheconductivityofthebasalplanewasnotdisruptedthepercolationcouldbedoneatevenlowerloadings.AsynergisticapproachalsoshowspromisesuchasaddingasmallconcentrationoflargeGnPthatwouldactasabridgebetweentheconductivecontactpointsthataretheGnPcoatedHGS.Theversatilityofthisnanoparticleiswhatmakesitsopromisinginnanocompositeapplications.134135CHAPTER6SUMMARYANDFUTUREWORKFormanydecadesnowtheaerospaceindustryhasbeenaleaderindevelopingnewtechnologies.Thisispartiallyduetotheextremeandinmanywaysunfamiliarenvironmentthatthetechnologymustoperatein.Thisindustryisuniqueinthatitsfocusisnotonlytheshortterm,buttriestoanticipateandmeetchallengesseveraldecadesorevenacenturyinadvance.Buttomakesuchtechnologypossiblerequiresthatthematerialsalreadybeinplace.Onesuchpromisingmaterialisananocompositewhichallowsfornanostructuring,anewwayofcombiningpolymerswithmultifunctionalmaterialsthatcoulddomultiplejobsthusreplacingmanyindividualcomponents.Thelesscomponentsthathavetobemadecouldtranslatetolessweightandalowercost,importantfactorsforviableaerospacetechnologies.Therearemanyareasintechnologythatcouldbfromthenanostructuringofmaterials,everythingfromenergyapplicationstosensorstobiologicalapplicationstospacet.Polymersareapromisingmaterialmatrixduetotheireaseofformabilityandprocessing,relativelylowdensityaswellastheirrelativelylowsynthesizingcosts.Extensiveworkhasalreadybeendoneonformingpolymernanocomposites,butitbecameclearearlyonthatwiththisnewmaterialcomesnewchallenges.Whiletherearemanybtoaddingnanoparticles,theextentofthesebdependsonhowtheinteractswiththematerialmatrixonmolecularandmicroscopiclevel,whichthenthemacroscopicproperties.Theresultsofthisinvestigationhighlightsomeofthechallengesthatmustbedealtwithandovercometomakeabpolymernanocompositeusinglow-costmaterialsandmethods,andhighlightssomemethodsemployedtoovercomethem.Inaddition,thisdissertationexposessomeuniqueobstaclesthatcomefromworkingwithamicrocellularstructureandthethatcanhaveontheoverallpropertiesofthenanocompositematerial.1351366.1RigidPUR/PIRnanocompositefoamTheadditionofananomaterialtoachemicallyblownfoamappearstodisruptthepolymericsystemonaphysicalandchemicallevelchangingtheresultantpolymer'smolecularandmicroscopicproperties.Thegasexpansionprocessalonecauseschangesinthethermaldegradationbehavior,butnotinthecrosslinkdensity,astheexpandedstructureresultsinalargersurfacearea,butsimilarglasstemperature(Tg).Suggestingthattheremovalofdecomposedproducts,whichisacontrolledprocess,isbythechangeinmicrostructurebetweenthesolidandthefoam.Alargesurfaceareaiscommonlymorebwhenisafactor.Acellularstructureallowsformorevolumeatornearthesurface,whichiscobythemuchhigherratesofdegradationexperiencedbythefoam.Thisstructurecanalsobedisadvantageousasitseemstopreventtheremovalofthedecomposedproductsoftheinternalstructureowingtothetortuosityandresultinginallthefoamsampleshavingahigherrateretentionthanthemonolithicpolymerat750°C.Ingeneraltherateofdecompositionismuchbroaderforthemonolithicpolymerstructureduetothelimitationsinthesolidpolymer.TheadditionoftheGnPtothefoamresultsinanincreaseinthermalstabilityowingtothehighspeciheatcapacityofthenanoparticle.However,thiswasnotobservedinthesolidwiththeadditionofneatGnP.Infact,thethermaldegradationonsetsoverlapforrigidPUR/PIRnanocompositeswithsomeexceptionsforthepMDItreatedGnP.AstowhytheGnPisnothelpingtomitigatetheheat,itcouldbetheresultoftheweightlosswithincreasingtemperaturebeingdependentonsothatanyboftheGnPismitigatedbythefactthattherateofremovalofthedecomposedproductsiscontrolledbythebarrierstructuremakingittodetermineanyimprovementsbetweentheneatandnaonocompositesamples.TheneatGnP-25inboththesolidandthefoamshowedthesamecross-linkdensitysuggestingthatthelargeplateletsinteractthesamewayinbothwhichnoneoftheotherssamplesdemonstrated.Thisislargelyduetotheagglomeratednatureoftheparticlesthatlimitsitsinteractionwiththematrix.Although,theadditionof136137theneatGnP-25inthefoamalsoresultedinthecellsbeingalmost40msmallerthanthestandardsothelargerparticleshelpedtostabilizethebubblesandpreventcoalesce.TheGnP-5helpsinthisregard,butnotastlyasthecellsareonlyabout20msmaller.Thesmallestparticles(GnP(750))donothingforthecellstructureresultinginthemediancellsbeing12.7mlargerthanthestandard.Thelargercellsizecouldbeduetoachangeinthemicrophasethatallowsforbubblecoalesceforwhichthemicrostructureofthestandardismoreeatlimiting.Itdoesappear,however,thatthefoamisbetterabletoadaptitsmolecularstructureastheTgoftheneatGnP-5isonaverageabout3°Chigherthanwheninthesolid,butbothshowlargevariationinTglikelybroughtonmythenon-uniformdispersion.Inadditionitappearsthatthesmallerparticlesdobetteratmaintainingthecross-linkstructureastheTgofthefoamwithneatGnP(750)matchesthatofthestandards.Thereisalsominimalaggregationsothelocalconcentrationisuniformthroughoutthefoam.Thismatchincross-linkdensitycouldbebecausethesmallsizeoftheparticlesisonthescaleofthemicrophasestructure.Polyurethaneconsistsofhardandsoftsegmentseitherofwhichcanbecross-linkedandonthescaletenstohundredsofnminlengthandGnP(750)haveparticlesizesonthescaleofhundredsofnm.Treatmentoftheedgeswithisocyanatewasconductedtopromotetheformationofurethane-typegroupsthatwouldbondbetterwiththefoam.Ingeneral,thewasnegativeornoimprovement.TheexceptionisfortheGnP(750),whichdemonstratedchangesinmicrostructureaswellasmacroscopicpropertiesduetothehigheredgedensityofthesesmallparticles.ThemonolithicpolymerwithtreatedGnPshowedearlyonsetfortheseparationofpolyurethaneintoitsmonomersandpMDItreatedGnP-25showedearlydegradationoftheurethane.TheTgshowsthattherearevastchangesinthemicrostructure.TheincreaseinTgof3°CforpMDItreatedGnP-25overtheneatGnP-25isprobablyduetolocalchangesaroundtheaggregatesastheTgforthefoamandsolidstillmatch.ThechangesinTgofcausedbytheadditionofthepMDItreatedGnP-5demonstratedthatthehigherconcentrationofthemoderatelysizedparticlesdisruptsthenetworkinboththemonolithand137138thefoamalthoughlargevariationinsampledeviationresultsfromanon-uniformdispersion.Inaddition,thefoamsampleswithtreatedGnP(750)haveasimilarTgtothestandard.TheonlyexceptionisthepMDItreatedGnP(750),andimagesshowtheyhavelargemicronaggregatesandcouldbereasonitshowsasimilarTgasthepMDItreatedGnP-25.Edge-treatmentofthesmallestparticlesresultedinasimilarcross-linkdensityandweremostbytheedgetreatmentduetotheirhigheredgedensityanddidnotinterferewiththeabilityoftheGnPtoabsorbheat,althoughthedegradationsothattheshiftsinonsettemperatureareabout50%less.ThesesamesuggestthattheadditionofGnPtendstopreventtheformationofisocyanurate,whichthepolymercompensatesforbyincreasingtheconcentrationofpolyureatomaintainthecross-linkdensity.Whenitcomestocellularmaterialsmanypropertiesdecreaseasthedensityandnanopar-ticleadditionauniquewaytoovercomethatbutcanalsocreatenewchallenges.ThepoorinteractionoftheGnPwiththematrixresultedinthelargestGnPshowingadecreaseinmechanicalstrengthofabout40%overthestandardduetotheagglomerationsbyimaging.Theseaggregatesactasstressconcentratorsdecreasingthemechanicalstrength.Aggregationdoesnotpreventtheformationofapercolatednetworkbutmaythecriticalconcentration.At5wt%GnP-25therewasdecreaseinelectricalresistivityofover5magnitudes.AmuchhigherloadingoftheGnP-5andGnP(750)thathadainaspectratioofabout2.5timesshowedsimilarelectricalresistivityatthesameconcentrationandimagesofthedispersionshowthattheagglomerationoftheGnP-5likelyincreasesthecriticalconcentrationforpercolation.Whiletheseaggregationsmaynottheelectricalpropertiestherestillhavetobeenoughofthemtoformapercolatednetwork,thisbecomesabiggerconcernastheparticlesgetsmaller.Edge-treatmentwasfoundonlytobettoreduceagglomerationwhentheedgedensityishighotherwisethebasalplanecharacterdoes,asdemonstratedbythenochangeinthemechanicalperformancefortheneatandedge-treatedGnP-25.Itisnotjustaboutthecreatingagoodadhesionbetweenthetwo,butalsobeingcarefulinregardstothe138139molecularstructureformedintheseregions.TreatingtheGnPwithchemicalgroupstoimproveinteractionbetweentheparticlesandthepolymerresultedintheparticlesbeingelectricallyisolatedduetothecoatingofpolymerthatwasformedontheedgesandminimalimprovementintthemechanicalstrength.TheelectricalresistivityincreasedaboutthreeordersofmagnitudewiththetreatedGnP-25overtheneat.Usingaminimalamountofedge-functionalizationimprovedtheelectricalresistivityoverthepMDItreatedGnP(750)byabout3magnitudes,however,ingeneralthebiggestfactorwasstilltheaspectratioforGnP-5andGnP(750)astheneatandtreatedGnPshowsimilarelectricalcontact.Themechanicalperformanceeitherdidnotchangefromthefoamwithnon-treatedGnPortheedgetreatmentresultedinadecreaseofanywherefrom16to35%thatwasobservedwiththettreatmentsoftheGnP(750).Theonlyexceptionwasthesamplewith8wt%pMDIGnP-5thatshowedanaverageimprovementofabout30%overthefoamwith8wt%GnP-5,although,thesampledeviationwasalsothelargestoutofanyofthesamplesandimagesshowedtheparticleswerestillagglomerated.GnPisamultifunctionalnanoparticlethatincludeshighelectricalconductivityaswellasapositivecentralcoresurroundedbyap-cloudofelectronspropertiesthatisbforimprovingboththedielectricandEMIshieldingproperties.Thedielectricpropertiesimprovedbyafactorof2.2,1.8,and0.7forthenanocompositefoamwithneatGnP-25,GnP-5andGnP(750),respectively.Therealpermittivityimprovedwithelectricalconductance,butnotatthesamedegreebecausepermittivityisaresponseofmobileandboundchargesandlargerparticlesseemedtodemonstrateastrongerresponse;atfactorthatdidnotchangebymorethan20%whenedge-treated.ThelargeparticlesgoodforelectricalcontactwereeattheEMwavesasthewasabout8-9timeshigheroverthestandardfoamforGnP-25andGnP-5at8.2GHzandtheGnP(750)wasonlyabout6timeshigher.ButthesmallerparticlesweregoodforabsorbingthemasatthesamefrequencytheGnP(750)wasabout5.5timeshigherforabsorbancecomparedtotheneatGnP-5andGnP-25thatonlydemonstratedtripletheabsorbanceoverthestandardfoam.139140TheEMISEimprovedforallsampleswithbothneatandtreatedGnP.Theedge-treatedGnP(750)abouttripledtheSE,GnP-5andGnP(750)improvedtheSEbyabout10and9times,respectivelyandGnP-25increasedtheSEabout12timesat8.2GHz,amountsthatdecreasedbyabout20%whentheedgesofthelargerGnPweretreated.ThisimprovementtendedtodecreaseathigherfrequencieswithGnP(750)outperformingallofthemduetothemoretfrequencydependencedemonstratedbythefoamsampleswithneatandtreatedGnP-25andGnP-5.ItcouldbethattheEMISEwouldshowimprovementifthesamplesweretestedatotherfrequencies.6.2FlexibleNancompositePDMSSyntacticFoamAddingGnPtoPDMSresultedinimprovementofthedielectricpropertiesandshieldingenessofthesamplesincludingincreasingtheabilityofthepolymertoabsorbelectro-magneticwaveslikelybydecreasingtheskindepth.Alloftheseimprovementscanbesolelyexplainedbytheadditionoftheconductingparticles.ThegreatestimprovementwasseeninthesamplesthathadthehighestloadingofGnPinthesolid.Alargeaspectratiohasbeenfoundtobeanimportantpropertyforachievingelectricalpercolationatlowerloadings.ThePDMSwith2wt%GnP-25increasedtherealpermittivityabout2.5timesovertheneatmaterialandimprovedtheSEenessabout9timesatitshighestwhileincreasingtheabsorbanceandComparedtothePDMSsamplewith1.6%moreGnPthatismixtureoftwosizesoftlysmalleraspectratiothatwereonlyabletodoublethedielectricconstant.AsforEMISEthereappearstobeimprovement,butisunclearbyhowmuchduetothestrongdependenceonfrequency.However,theearliershiftoftheresonancefrequencybyabout1GHzsuggeststhatthesamplewith2wt%GnP-25ismoreconductive.WhetherapercolatednetworkformedinthesystemisunclearasthestrongfrequencydependenceintheEMIshieldingsuggestsamaterialthatisstillnotbehavingasametallicconductor.IftheGnPdispersionweremoreuniformandlessaggregatedthatcouldresultinacorrespondingimprovementinelectricalperformancebycreatingmanymore140141connectedpaths.ThechallengewithincreasingthediameterontheGnPcreatesastrongerattractionduetheVanderWaalsforcesonthebasalplanethatpreferentiallyaggregateratherthaninteractwiththepolymer.PDMSalreadyhasapplicationsinaerospaceduetotheirtyatlowtemperaturesandaddingGnPseemstoshowpromiseasamultifunctionalforthismaterial.CoatingtheHGSpriortodispersinginthepolymerimprovedthebehaviorofthenanocompositesyntacticfoam.IthadthesamerealpermittivityastheneatPDMSandimprovedtherealpermittivitybyabout12%atonly0.64wt%GnP.ItdemonstratedthesameEMISEastheneatandPDMSwith0.64wt%GnPwhilestillbeing20%lighter.However,theresultantimagesofthefoamareunclearonwhethertheGnPmanagedtostayadheredtothespheresduringprocessing.Alsotheoccasionalclumpsofcoatedspheresreducestheconnectivityproducingareasofhighandlowlocalconductance,thecoatedHGScouldbfromamorevigorousmixingpriortoaddingtothepolymertobreakuptheclumps,butcarefullysothattheGnPstaysadhered.6.3FutureworkformakingnanocompositeswithGnPItappearsthatthekeytoimprovingtheperformanceofnanocompositesisaboutsmarterparticleloading.Keepinginmindtochoosereinforcingmaterialthatmatchesthematrixintermsofmoleculargroupsandidentifywhichgroupscreatetheideallocalmolecularstructuresothatthelocalpropertiesstayconsistentthroughoutthefoamandallowselectrontransfertooccureasilythroughoutthematerial.ForparticleswithaplateletmorphologysuchasGnP,choosingbindersorsurfactantsthatinteractwiththebasalplanewithnaphthaleneorpyrenecompoundsthatwouldbefunctionalizedtointeractwiththepolymercouldbedoneasawaytoimprovethetheinteraction.Thelargerparticlesproducedagreaterofthebasalplaneoftheplateletanditisimportanttoidentifyawaytofunctionalizethebasalplanethatdoesnotincludechemicalbondsthatwoulddestroyitscharacter.Sinceelectricalconductanceisimportant,edge-treatment,whichdoesnotthebasalplane,141142stillincreasedtheelectricalresistivity.ForPURfoamthathasamicrophasestructureamoreindepthinvestigationonhowGnPssizeanddistancebetweenthehardandsoftsegmentsneedstobedonetogainabetterunderstandingofthephysicalinteractionofGnPespeciallywithsizewhentheparticlesarewelldispersed.DependingonwheretheGnPispreferentiallylocatedinthephasestructure,itcouldecttheabilitytoformapercolatednetworkespeciallyastheparticlesizedecreases.ItcouldalsobethatchangingthemicrophaseofthepolymerwithGnPcouldimprovetheEMISEormorespitsabsorbancecapabilities.InadditionsomeofthesechallengescouldbemetbyusingacombinationofparticlesizestocreateasynergisticThisconceptcouldtimprovementinthePDMSwith40vol%GnPcoatedHGSbycreatingbridgesbetweentheconductivespheres.Itcouldalsohelpreducetheagglomerationsinthefoam,byallowingforlowerloadingsofthelargeGnPdecreasingtheamountorsizeoftheaggregatesthatform.142143REFERENCES143144REFERENCES[1]JohnFuegiandJoFrancis.Lovelaceandbabbageandthecreationofthe1843'notes'.IEEEAnnalsoftheHistoryofComputing,pages16{26,2003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