Macromol.RapidCommun. 21,57–61(2000) 57

Communication

Morphologyandrheologyof polystyrenenanocompositesbaseduponorganoclay

BothoHoffmann,ChristophDietricha, Ralf Thomann,Christian Friedrich*, Rolf Mulhaupt

FreiburgerMaterialforschungszentrumundInstitut fur MakromolekulareChemie,Stefan-Meier-Str. 21,D-79104Freiburg i. Br., Germanychf@fmf.uni-freiburg.de

(Received:August18,1999;revised:October04,1999)

Intr oductionIt is well establishedthateffectivedispersionof anisotro-pic particleswith highaspect ratiosuchasshortfibers,pla-teletsandwhiskerswithin thecontinuouspolymermatrixin combination with adequate interfacial adhesionbetween filler andpolymer canaccount for substantiallyimprovedreinforcementof thepolymermatrix1). Pioneer-ing advancesat Toyota research2) during the early 1990’shasstimulatedthe developmentof various polymer/orga-noclay nanocompositeswith attractive property profilessuchasimprovedstiffnesscombined with elevated dimen-sional stability, barrierresistance, improvedthermal stabi-lity and inherent flame retardency3–8). Severalsyntheticrouteswereexaminedto producepolystyrene/organoclaynanocomposites,aiming at improvingexfoliation of orga-

noclay. Moetandcoworkers5,9–11) reportedin-situ bulk andsolution polymerization of styreneusingcoreactiveorga-nophilic montmorillonite, obtained via ion exchange ofsodium montmorillonite with vinylbenzyltrimethylammonium chloride, in orderto achieve interfacial graft-ing of polystyrene onto clay and to promote swelling ofclay in styreneandvarioussolvents.In-situbulk andsolu-tion styrene polymerization as well as compounding oforganoclay in polystyrenemelts12,13), including the useofpoly(styrene-co-vinyloxazoline)14) andpolystyrene-block-poly(ethylene oxide)15) ascompatibilizers, affordedinter-calationbut failed to achievecompleteexfoliation. In aqu-eousemulsionpolymerizationprocesseswaterswelling ofclays was combined with intergallery polymerization toimprove exfoliation16,17). In a recent advanceSogahandcoworkers18) converted clay into initiators for TEMPO-mediatedcontrolled radical polymerization,thusimprov-ing controlof polystyrenegrafting ontosilicates andsub-stantially improving exfoliation. In the past incomplete

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A correlation between morphology development andrheologyof polystyrenenanocompositesbaseduponorga-nophilic layeredsilicates(organoclay)suchasfluoromicaswasfoundasa functionof thesilicatemodification.Orga-noclay was obtainedby meansof ion exchangeof claywith protonated amine-terminated polystyrenes withmolar mass of M

—n = 121 and 5800 g/mol. Only when

applying shearforcesduring melt compoundingof orga-noclay modified with high molar masspolystyrene(PS),individual silicateplateletsof 1 nm diameterand600 nmlength were obtained.Dispersionsof suchin-situ formednanoparticleswith aspect ratio of 600 accountedforunique elastic propertiesobservedin the low frequencyrangeof thedynamicmodulus,whereasorganoclaymodi-fied with low molecularPSdid not exfoliateandexhibitedrheologicalbehaviorvery similar to that of conventionalfillers.

a Current address: Freiburger Materialforschungszentrum undInstitut fur Mikrosystemtechnik,Stefan-Meier-Str. 21; D-79104Freiburg i. Br., Germany.

TEM micrograph of an exfoliated PS nanocomposite.Silicateloading:5 wt.-%

58 B. Hoffmann,Ch.Dietrich, R. Thomann,Chr. Friedrich,R. Mulhaupt

exfoliation and formation of complexnanoparticle mix-tureshashampered investigationof rheologicalbehavioralthoughis waswell recognizedthatnanocompositesexhi-bit unusualrheologicalproperties.

Therheological properties of in-situ polymerizednano-compositeswith end-tetheredpolymerchainswereat firstdescribed by Giannelis et al.19,20) The flow behavior ofpoly(e-caprolactone) and polyamide-6 nanocompositesdifferedextremely from thatof theneatmatrices,whereasthe thermorheological properties (Arrhenius activationenergy of flow) of the composites were entirely deter-minedby that behavior of the matrix20). The slope of thestorage modulus G9 and the loss modulus G99 versusthefrequency x in the terminal region was smaller than 2and 1, respectively. Values of 2 and 1 are expectedformelts of linear monodispersepolymers and large devia-tions, especiallyfor small amountsof silicate loading inthepercentagerangemaybeindicativeof networkforma-tion. However, suchnanocompositesbased on the in-situpolymerization techniqueexhibit fairly broadmolarmassdistribution of the polymer matrix which hides the struc-ture relevantinformation and impedes the interpretationof the results. Therefore, quantitative correlationsbetween the viscoelasticity and the morphology of thecomposites cannot be established. For a better under-standing of the relationshipof viscoelasticity and struc-turewell defined model materialsare necessary.

Fromearlyresearchby Weiss21) andVansant22) it is wellknownthattheinterlayerdistanceof organoclaysincreaseswith increasingchain lengthof alkyl groupsusedassubsti-tuentsof alkyl ammoniumcations,whichrepresenttypicalmodifiers of organoclays. Similar to observations byOkada23), whousedx-amino acidsasmodifiers,interlayerdistancecanincreasefrom 0.95 nmupto 2 nmwhenusingsuchsmallmolecularweight modifiers.Also highmolecu-larweight modifiershavebeenused.Forexample,anindus-trial gradedifunctional amine-terminatedpoly(butadiene-co-acrylonitrile) with number-average molecular weight(M

—n) of 1090g/molandM

—w/M

—n = 6.75wasappliedasmodi-

fier5). Althoughintercalationwasobserved,all thesemodi-fiersfailedtoaffordcompleteexfoliation.

In our researchwe haveusedpolystyreneswith narrowmolecular weight distributions as continuous matrix andasammonium-functional modifier of organoclayin orderto establish correlations betweenmorphology develop-ment andrheological behavior of polystyrene/organoclaynanocomposites.

Experimental part

Materials

The synthetic layered silicate SOMASIFTM ME100(ME100),which representsa fluromicaandwaspreparedbyheatingtalcum in the presenceof Na2SiF6, wassuppliedbyCO-OP Ltd., Japan.The negativecharge of the layers iscompensatedby Na+-ions in the interlayer galleries. Thecation exchangecapacity (CEC) was in a rangeof 0.7 to0.8meq/g for ME100. The interlayer spacing calculatedfrom thewide angleX-ray scattering(WAXS) d001-reflectionwas0.95nm. The cationexchangewasperformedwith twodifferent swelling agents: 2-phenylethylamine (PEA,M—

n = 121g/mol) andamine-terminatedpolystyrene(AT-PS8)with M

—n = 5800 g/mol and degreeof polymerizationof 56,

which was preparedby meansof anionic polymerization24).Polystyrene(PS100),preparedby meansof anionicpolymer-ization,wasusedascontinuousmatrix. Thepropertiesof thepolystyrenesarelistedin Tab.1.

Swellingof thelayeredsilicate

A mixture of 5.6 mmol/l swelling agent(AT-PS8or PEA,respectively)and 6.7mmol/l hydrochloric acid were sus-pendedin a THF/water(4 vol.-%/1 vol.-%) mixtureat 408C.Then8 g/l of layeredsilicate wasaddedto the hot solutionandwhite precipitateswereobtained.The precipitateswerefiltered after30 min andwashedtwice: first with a hot THF/water mixture and thereafterwith hot water. The modifiedlayeredsilicatewasdriedin vacuumat 458C for 72 h. Fluor-omicamodifiedwith AT-PS8will bedenotedasorganoclay-PS, and fluoromica modified with 2-phenylethylaminewillbedenotedorganoclay-PEA.

Compounding

The compositeswereobtainedby compoundingthe organo-clays with PS100at 2008C in a DACA Microcompounderfor 5 min. The compoundof PS100with organoclay-PSiscalled C-PS,the compoundof PS100with organoclay-PEAis calledC-PEA.Theoverallsilicateloadingwas5 wt.-% forthecomposites.

Morphologicalcharacterization

Ultrathin sectionsof the compositeswith a thicknessofapproximately50 nm werepreparedwith an ultramicrotome(Ultracut E, Reichert & Jung) equippedwith a diamondknife. Transmissionelectronmicroscopy(TEM) wascarriedout with a ZeissCEM 912transmissionelectronmicroscopeusing an accelerationvoltage of 120 keV. Due to the highelectron density differencesbetweensilicate and polymerstainingof thesampleswasnot necessary.

Tab.1. Propertiesof usedmaterials

Material Abbreviation Function Mn

g=molM—

w /M—

n Tg

8C

polystyrene PS100 matrix 106000 1.07 100amine-terminatedpolystyrene AT-PS8 modifier 5800 1.33 78

2-phenylethylamine PEA modifier 121 1 –

Morphology andrheology of polystyrenenanocompositesbaseduponorganoclay 59

WAXSmeasurements

The interlayer distanceof the organoclayswas studiedbymeansof wide angleX-ray scattering(WAXS) usinga Sie-mens D500 apparatuswith CuKa radiation (k = 0.154nm)anda scanningrateof 0.38/min.

Rheologicalcharacterization

The neatPS100and the compositesC-PSand C-PEAweredried for 48h at 608C in vacuumand molten for 20 minundervacuumat a temperatureof 2008C in a Collin PCS2vacuumpress.Thesampleswerepressedfor 10min andsub-sequentlyquenchedto ambienttemperature.Theplateshadadiameterof 25mm anda thicknessof approximately1 mm.For the rheological measurements the RheometricsRMS800, a strain controlledrheometer, with parallel plategeometry was used. All measurementswere carried outundernitrogen.The strain region in which the materialcanbe regardedaslinear viscoelasticwasdeterminedby ampli-tudesweepexperimentsandis in the orderof 10%. Isother-mal frequencysweepswere takenat different temperaturesranging from 1408C to 2208C. The master curves wereshifted with the program LSSHIFT18). The temperatureof1808C waschosenasreferencetemperature.

Resultsand discussionThe synthetic fluoromica (ME100), which represents ahectorite obtainedby heating talcum with Na2SiF6, wasrenderedorganophilic by means of ion exchange of thesodium intergallery cationsfor protonatedamine-termi-

nated polystyrene(AT-PS8,Tab.1) with number averagemolecularweightof 5800,which correspondsto a degreeof polymerizationof 56, and2-phenylethylamine (PEA),which correspondsto apolystyrenedegreeof polymeriza-tion of 1. As illustratedin Scheme1, the ammonium-ter-minatedpolystyrene modifier wasprepared by meansofanionic polymerization followed by chain terminationwith dimethylchlorosilane,silylationwith allyl amineandprotonation of the resulting amine-terminated polystyr-ene.

Fig. 1 shows the WAXS tracesof neatlayeredsilicate(ME100), ME100 modified with PEA (organoclay-PEA)andME100 modified with AT-PS8 (organoclay-PS).The

Scheme 1: Syntheticroutefor thepreparation of AT-PS8andorganoclay-PS

Fig. 1. WAXS traces of neat ME100 (a), organoclay-PEA(b),andorganoclay-PS

60 B. Hoffmann,Ch.Dietrich, R. Thomann,Chr. Friedrich,R. Mulhaupt

interlayer distancesof the organoclays were obtainedfrom the peakposition (d001-reflection) of WAXS traces.The d001-reflection for neat ME100 was found at a 2Hvalue of 9.38, which correspondsto aninterlayerdistanceof 0.95nm (Fig. 1, trace a). Organoclay modificationaffordedsubstantially increased interlayer distances.Thereflection of organoclay-PEA (trace b in Fig. 1) wasfoundat 2H of 68, correspondingto aninterlayerdistanceof 1.4nm. In contrast, organoclay-PS(trace c in Fig. 1)exhibited no d001-reflection in the relevant angle region,thusindicatingthe existenceof interlayer layer distanceslargerthan4 nm!

Transmission electron microscopy (TEM) was per-formed to examinemorphology developmentof compo-sites, abbreviatedasC, preparedby meansof melt com-

pounding polystyrene with M—

n = 106000 in the presenceof organoclay-PEA(compoundC-PEA)or organoclay-PS(compound C-PS). The overall silicate loading was 5wt.-% for both composites.The TEM micrographsaredisplayedin Fig. 2 where thedark linesrepresent thesili-cate layers in the polystyrene matrix (bright). FromFig. 2(a) it is apparentthat in the caseof compound C-PEA thePEA modification causedintercalationbut failedto exfoliate the intercalatedparticles.In sharpcontrast,compoundC-PS exhibiteda very fine dispersionof indi-vidual platelets being composed of individual silicatelayers. Typically suchlayerswere600nm long and1 nmthick. From TEM tilting experiments the layers widthwas estimated to be approximately 100 nm. Obviously,the largeinterlayerdistancein organoclay-PS(largerthan4 nm) was accompanied by weakeningof the silicateinterlayer interactions and afforded complete exfoliationwhen applying shear forces during melt compoundingwith matrix polystyrene(Fig. 2(b)).

Rheologicalpropertiesweredeterminedto examine thepresenceof particle network formation via interparticleinteractionsand self-assembly. We found that time tem-perature superposition (TTS) holds for all investigatedsamples.Therewasno influence of a silicateloadingof 5wt.-% on TTS becauseof thehigh compatibility betweenorganophilic modified ME100 and PS100.Fig. 3 showsthestoragemodulus(G9) versusthereducedfrequency (xaT) for PS100(black line), C-PS (F), and C-PEA (9).PS100 showedthe normal materialresponseof a narrowdistributed thermoplastic polymer. The curve for C-PEAwasslightly shifted to highermoduli values,asexpectedfor the filler effect. The slopein the terminalregimewasapproximately 2 for both curves. The material responseof C-PSwas found to be very different from that of C-PEA. At the lowest realized frequencies, which corre-spond to the markedregionIII, strongly increasedvalues

Fig. 2. TEM micrographsof the melt compoundsC-PEA (a)andC-PS(b). Silicateloading:5 wt.-%

Fig. 3. G9 versusreducedfrequency for PS100matrix (blackline), C-PS(F), andC-PEA(9). Silicateloading:5 wt.-%

Morphology andrheology of polystyrenenanocompositesbaseduponorganoclay 61

of storage moduluswerefoundandtheslope of thecurveapproachedzero.Sucha behavior is an indicationof net-work formation involving assemblyof individual plate-lets being composed of silicate layers. In the regimeofintermediatefrequencies(region II in Fig. 3) the modulivalues were reducedin comparisonof that of the neatmatrix. This might be a dilution effect caused by theamine-terminatedpolystyrene having a molecular weightlower than the entanglement molecular weight of poly-styrene. A validation of the thinning effect will bedescribed elsewhere25). At high frequencies(region I) anincreasein thestoragemoduluswasdetected with respectto that of the neatmatrix. This modulusincreasein thisfrequency rangewas independentof the stateof disper-sion andrepresentsthe combination of elasticity of bothcomponents,thematrix andthe silicate.

ConclusionsOrganophilic modification of clay with amine-terminatedpolystyrene,obtainedby meansof anionic polymerization,affordscompletely exfoliatedpolystyrenenanocompositescontaining silicate nanoparticles with an aspect ratioexceeding 600, when suchorganoclays were melt com-pounded together with polystyrene. In contrast, smallmolecular weight modifiers, suchas2-phenylethylamine,only promotedintercalation andfailed to exfoliate thesili-cate particlesduring melt compounding. Since both thepolystyrenecontinuousmatrix andthe ammonium-termi-natedpolystyreneattachedto thesilicatenanoparticlesur-faceexhibitedvery narrowmolecular weight distributionsit becamepossibleto examinerheologicalbehavior andtocorrelate superstructureformationwith rheologicalprop-erties.Only whenhigh molecular weightpolystyrene wasgrafted onto the dispersed silicate layers, rheologicalinvestigation revealed the formation of networks viaassembly of in-situformedsilicatenanoparticles.

It shouldbe possibleto control silicate/polymer inter-action of such “hairy” silicate nanoparticles by varyingthechainlengthof polystyreneswhich areattachedto thesilicate nanoparticles surface via their ammonium endgroup. Fully exfoliatednanocompositesbaseduponorga-noclay and polystyrene represent very effective modelsystemsto achieve better understanding of morphologydevelopment, processingand structure/property correla-tionsof nanocompositesbaseduponlayeredsilicates.

Acknowledgement:Theauthorsgratefullyacknowledgefinan-cial support by the Sonderforschungsbereich SFB 428 of theDeutscheForschungsgemeinschaft, andthank K. Cramer for thesynthesisof At-758.

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