Tarea 9 Rheological Behavior

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Polymer43 (2002) 3247-3255

polymerwww.elsevier.com/locate/polymer

Rheologicalbehavior ofmultiwalled c rbnnanotube/polycarbonate composites

Petra Potschke*1, T.D.Fornes, D.R. PaulDepartment uf ChemicalEngineering andTexas Material Institute,Cerner for PolymerResearch, University ofTexasat Austin, Austin, TX 78712-1062, USA

Received 29 November 2001; received in revised form1 February2002; accepted 5February2002

AbstractThe rheological behavior of compression molded mixtures of polycarbonate containing between 0.5 and 15 wt c rbnnanotubeswas

investigated using oscillatory rheometry at 260 C.Thenanotubes havediameters between 10 and 15 nm and lengths ranging from 1 to10 u.m. The composites were obtained bydilutinga masterbatch containing 15 wt nanotubesusing a twin-screw extruder. Theincreaseinviscosity associated w i t h the addition ofnanotubes is much higher than viscositychanges reported for c rbn nanofibers having largerdiameters and for c rbnblack composites; this can be explained by the higher aspect ratio of the nanotubes. The viscosityincreaseisaccompanied by anincreasein the elastic melt properties, represented by thestoragemodulusG1,whichis much higher than theincreaseinthe loss modulusG .The viscosity curves above 2 wt nanotubesexhibit a largerdecreasewithfrequency thansamplescontaining lowernanotube ioadings. Composites containing more than 2w t nanotubesexhibitnon-Newtonian behavior at lower frequencies. Astepincreaseat approximately 2w t nanotubeswas observed in theviscosity-compositioncurves at low frequencies. Thisstepchangemay be regardedas a rheological threshold.Ultimately,the rheological threshold coincides withthe electricalconductivitypercolation thresholdwhichwasfoundto be between 1 and 2w t nanotubes. 2002 Elsevier Science L t d .AJI rights reserved.Keywords: Carbnnanotube composites; Rheological behavior;Electrical conductivty

1. IntroductionComposites of c rbn nanoiubes (CNTs) in polymeric

matrices have attracted considerable attention in theresearch and industrial communities due to their uniquemechanical and electrical properties. CNT polymer nano-composites possess high stiffness, high strength, and goodelectrical conductivity at relatively low concentrations ofCNT f i l l e r [ 1 - 9 ] .2 These enhancements ultimately stemf r o m the distinct properties o f the CNTs themselves. Forexample, various studies involving singlewallednanotubes(SWNT)and multiwalled nanotubes( M W N T )havedemon-strated that CNT can have m o d i i l iand strength levisin therange200-1000 GPa and 200-900 MPa, respectively [10-12] . Other studies have shown that CNTs have diverse

* Conesponding author. Tel.: +49-351-4658395; fax: +49-351-4658565.

mail addresses: [email protected] (P. Potschke), [email protected]. Paul).

Permanent address: Institute of Polymer Research Dresden, HoheStrasse 6, D-01069 Dresden, Germany. Tel.: +49-351-4658395; fax:+49-351-4658565.

2 www.fibrils.com, www.fibrils.com/grafibs.htm (accessed September2001).

electrical properties, capable of acting as metallic-likecoid uctors or having characteristics of a semiconductordeoending upon the dist orti on or 'chirality' of the graphitelattice [13,14]. In addition, CNTs have very large aspectratios (length to diameter ratio), as high as 100-1000 [ l ] 2Such special properties make CNTs excellentcandidatesforhigh strength and electronically conductive, polymercomposite applications.

Currently, there are two major re s in which CNTs arebeng used, electronics and automotive. In electronic appli-ca ions, particularly in the semiconductor field, CNTs areused to dissipate unwanted static charge build -up. Thisdissipative effect is achieved by thoroughly dispersingCNTsin the polymeric material such that an interconnectingstructure of CNT is formed. This interconnecting networkthus provides a conductive pathway for charge to flow.Conductive compounds need very low fibril Ioadings,typically less than 5 wt to achieve electrostatic d issipa-t i o n , compared to about 8-20 wt for c rbn black-basedcoTtpounds. These different loading levis offer manyadvantages. In the automotive industry, CNTs are used asa conducting agent to aid in electrostatic painting. Suchp sare produced by melt processing. A keyissuein produ-ci r ig superior CNT nanocomposites applications is the

0032-3861/02/ - see front matter 2002Elsevier Science Ltd Allri htsreserved.Pl l : S0032-3861(02)00151-9


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3 48 P Potschke et al./Polymer 43 2002) 3247-3255a b i l i t y to control dispersin of the CNT in polymericmatrices. Thi s, however, hinges on an in-depth understand-ing of CNT- po ly me r nanocomposite rheology, a topic thathas not received much attention.

Presently, three methods are commonly used to i n c o r p -rate nanotubesinto polymers: (i) filmcasting ofsuspensionso f nanotubes in dissolved polymers, (i i) polymerization ofnanotube-polymer monomer mixtures, and (i ii ) meltmixing of nanotubes w i t h polymers. In most fundamentalstudies, film casting was used to investg tethe properties ofpolymers containing CNTsincluding the effects of nanotubedispersin and orientation [8,15,16], deformation mechan-isms [8,17] and interfacial bonding [ 8 , 1 7 ] . Mechanicalproperties [8] , dynamic-mechanical properties [18 ], andelectrical conductivity [18,19]havealso been investigated.Raman spectroscopy has beenused to obtaininformation oninteractions between nanotubes and the polymer matrices[8,16,20]. Orientation and degree of alignment have beenstudied by X-ray diffraction [15].F i l m casting has oftenbeen the only processing option used owing to l i m i t e davailability of CNTs and/or theirhigh cost.

Studies using meltprocessedthermoplastic polymer/CNTnanocomposites have beenquite l i m i t e d .On the other hand,meltprocessing is the preferred method of composite forma-t i o n in many cases. The tendency of nanotubes to f o r maggregates may be minimized by appropriate applicationo fshearduring melt m i x i ng . 2 For example, Haggenmuelleret al. [2] applied a combined solvent casting and meltprocessing procedure to produce f i lms of poly(methylmethacrylate) P M M A ) containing SWNT. They pressedsmall pieces of cast f i lms between heated plates, thenbroke the resulting film again into small pieces, andrepeated the procedure many times. The f i lms obtained bythis melt processing technique had a more homogeneousnanotube dist ribution than the cast film and led to muchbetter mechanical properties. Jin et al. [5] used a miniaturemixer-molder A T L A S ) to produce small quantities (ca.0.4 g) of well-dispersed mixtures of M W N T in P M M A .The well-dispersed mixtures were then compressed intot h i n f i l m s to investg tetheir dynamic-mechanical behavior;a significantincreasein the storage modulus was observed.Lozano et al. [3,4] used a Haake miniature laboratory mixer(14-20 g) todispersevapor grownc rbnfibers in polypro-pylene (PP).Aftercompression molding intothin f i lms theyinvestigated the d i s p e r s i n , melt rheology, conductivi ty,mechanical and dynamic-mechanical properties, and theinfluence of the nanotube fibrils on the PP crystall izationbehavior. Ferguson et al. [7 ] reported onkilogramquantitieso f polycarbonate-based nanotube formulations produced ina Buss Kneader. They compared formulations obtained bymixing of a diluted masterbatch w i t h different amounts ofreprocessed compounds. The reprocessing in a BussKneader led to better dispersin of the fibrils resulting inincreased conductivity.

To our knowledge, there are no reports on the meltrheological behavior of M W N T filled polymers. On the

other hand, Lozano and Barrera [3]have studied the meltrheological properties of c rbn fiber filled thermoplasticpolymers. The vapor grown c rbn fibers used in theseinvestigations are treated as a model system for SWNTand M W N T . These fibers were similar in length as theM W N T , but their diameters were about 10 times largerthan the M W N T used in the present study. Therefore, theresults may not be directly comparable to the rheologicalbehavior of SWNT or M W N T composites.

The purpose of this paper is to examine the rheologicalproperties of M W N T filled polycarbonate nanocompositeslormed by melt e x t r u s i n . The influence of nanotubecontent on complex viscosity, storage modulus and lossmodulus are examined. Rheological behavior at highfrequencies is used to estm te the effect of the filler onprocessing properties; however, the normal Cox-Merzrule relating complex viscosity to steady shear viscositycan break down in complex systems l i k e composites [21].Low frequency behavior is sensitive to the structure of thecomposites and can be used to obtaininformation about thepercolation state of the M W N T w i t hi n the composite.The materials are also characterized by scanning electrnmicroscopy (SEM) and electrical resistivitymeasurements.A n attempt is made to connect the morphological andresistivity information w i t h rheological observations.

2.Experimental2 Materials and characterization

A masterbatch o f 15 wt c rbn M W N T in polycarbonate(PC/CNT) was obtained f r o mHyperion Catalysis International, Cambridge, MA . The nanotubes are vapor grownand typically consist of 8-15 graphitic layers wrappedaround ahollow5 nm core.2 They are produced as agglom-erates and exist as curved intertwined entanglements[22,23]. Typical diameters range f r o m 10 to 15 nm, whilelengths are between 1 and 10 u.m. The density is approxi-mately 1.75 g/cm3 [18] ; a surface re of 250 m 2 /g wasdetermined by the BET method. 2 The masterbatch wascompounded using a M D K 46 Buss Kneader LID =

1/1), according to reports f r o m Hyperion [7 ], and wasdeliveredin pellet f o r m . The masterbatch was diluted w i t ha polycarbonate supplied by Mitsubishi Engineering Plas-tics w i t h the commercial designation of I u p i l o n E-2000. Inaddition, a masterbatch of 20 wt M W N Tin polyamide-6(PA/CNT), also obtained f r o m Hyperion, was used forcomparative purposesin the SEM investigations.2.2. Nanocomposite preparation

The materials were dried for amn imumof 16 h at 80 Ci na vacuum oven. One-kilogram mixtures of polycarbonatew i t h the masterbatch were extmded using a Haake co-rotat-i n g , intermeshing twin-screw extruder (lengthD = 30 mm,j = 10) to obtain concentrations of 0.5, 1, 2, and 5 wt


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P Potschke et al. /Polymer 43 2002) 3247-3255 3249nanotubes i n polycarbonate. According to the materialdensities (1.20 g/ cm 3 for PC2 and 1.75 g/ cm3 for CNTL18J), the corresponding volume concentrations are 0.34,0.68, 1.37, and 3.40 v o l . The screw design has beendescribed previously [24] . Compounding was carried outusing a barrel temperature o f 240 C , a screw speed of280 rpm, and a feed rate of 980 g/h. Polycarbonatecompounds were compression molded into65 X 13 X 3 mm 3 bars.2.3. Electrical resistivity measurements

The volumeresistivityof compression moldedbarsweredetermined by measuring the DC resistance along the lengthdirection through bars approximately 40 mm in length. AKeithley electrometer Model 6517 w i t h a 8002A H i g hResistance Test Fixture was used to measure the highresistance samples. Special clamps were designed forholding the injection molded bars. Instrument control, dataacquisition and evaluation were performed bymeansof anapplication software, which was self-developed using the'Testpoint' software ki t of Keithley Instruments. Thisequipment allows resistivity measurements up to 101 7 fi.Different applied voltages were used on different samples,depending on thelevelofresistivityof the specimen. H i g h l yconductive samples caused short-circuiting of the equipment when the applied voltage was too high. Thus, thevoltage was adapted to the resistivity and was 500 V forPC and composites w i t h up to 1 wt CNT and 1 V for2wt and greater CNT. The samples w i t h 5 and 15 wtwere measured using a Keithley Model 2000 electrometer,whichis more sensitive to lowerresistivity l e v i s .Toensurethat volume resistivity did not reflect surface resistivityeffects, t h i n sheets approximately 1 mm i n thickness ofthe 2 wt CN T composite were compression molded andtested for surface and volume resistance using a 8009 Resist i v i t y Test Fixture equipped w i t h ring electrodes. Thesurface resistivity results showed similar vales as thevolume results, thusindicating no signif icant surface resist i v i t y effect. According to A S T M D4496 and D257, theresistivity was converted to volume resistivity, pv usingthe formulap v = WDRJL (1)where W is the w i d t h , D the thickness, L the length of thesample, and Rv is the measured resistance. The reportedvalesrepresent the mean of 7-10 samples, where the standard deviation rela tive to the mean was lessthan 10 .2.4. Rheological measurements

Prior to rheological measurements, compression moldedbars were cut into 13 X 13 x3 m m3 squares and dried at80 C for aminimumof 24 h under vacuum. Dynamic rheological measurements were performed using an advancedrheometric expansin system (ARES) rheometer fro mRheometrics, Inc. The measurements were carried out in

an oscillatoryshear mode using a parallel pate geometry(25 mm diameter) at 260 C under nitrogen atmosphere.Frequency sweeps between 0.1 and 100rad/s were carriedoutat low strains (0. 1-10 )whichwere shown to be w i t h i nthe linear elastic range forthesematerials. The upper l imi t so f the viscoelastic range, determined in strain sweeps at10 rad/s, were found to decrease w i t h nanotube c o n t e n .Repeated sweeps w i t h increasing and decreasing frequencies showed that the material is stable under the measure-ment conditions. Specimens were placed between thepreheated plates and were allowed to equilbrate forapproximately 10 minprior to each frequency sweeprun.The obtained vales were corrected to the true volumebetween the plates.2.5. Morphological characterization

A L EO 1530 scanning electrn microscope was used tocharacterize composite morphology. Samples takendirectlyf r o m the masterbatch were investigated by cutting pelletsw i t h a razor blade at room temperature which causedfracture; SE M images were made of the surface of thefractured r e g i n .SEM images were also made for cryofrac-tured-compression molded bars formed fro m dilutedmixtures. The samples were investigated w i t h and withouta sputtered coating.

I n addition to SEM analysis, the nanocomposites weredisolved in tetrahydrofuran (THF) in an effort to assessthe state of dispersin of the nanotubes. A small pellet ofthe masterbatch (8 mg, corresponding to 1.2 mg nanotubes)and corresponding weights of the diluted materials wereimmersed in approximately 50 mi of THF for about 2weeks at room temperature. The solutions were formulatedto have an equal amount of nanotubes in each v i a l . Underthesecondit ions, polycarbonate is dissolved thereby leadingto a suspensinof the nanotubes in the solvent-PC solution.

3 Results

3 J Electrical resistivity

F ig . 1 shows the effect of adding CNTs on volume resist i v i t y . At very low concentrations of CNT, the resistivitygradually decreases w i t h increasing nanotube content.However, at 2 w t , a sizable reduction in resistivity, inthe order of 1 1 i cm, is observed. This stepwise changei n resistivityis a result of theformationof an interconnectedstructure of CNTs and can be regarded as an electricalpexolationthreshold. Thissimplymeans that at concentrationsbetween 1 and 2 wt CNT, a veryhigh percentage ofele ctrons are permitted to flowthrough the sample due to thecreation of an interconnecting conductive pathway. Atco icentrations above 2 wt CNT , the volume resistivitiesare low and decrease marginally w i t h increasing CNTcoitent. It should be noted that the measured resistivityvaiues in Fig. 1 were obtained using different applied


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Potschke et al. /Polymer 43 '2002) 3247-3255 32511 0 '

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Fig 4. Storage modulus G of nanotube filled polycarbonate at 260 C

I t is known f r o m the literature that interconnectedstructures of anisometric fillers result in an apparent y ie ldstress which is visible in dynamic measurements by aplateau of G1 or G versus frequency at low frequencies[21,28,29,38]. This effect is more pronounced in G1 thani n G [ 2 8 ] .As the nanotube comentincreasesin this composite system, nanotube-nanotube interactions begin to d o m nate, eventually lead to percolation and the formationof aninterconnected structure of nanotubes. Starting at about2wt% nanotubes, G seems to reach such a plateau at lowfrequencies. Therefore, an interconnected structure isassumed to f o r m . This critical composi tion is regarded asa rheological percolation composition. Athigh concentrations of nanotubes, conn ectiv ity is more pronounced, asseen in the enhanced elasticity.

F ig . 6 shows aplot of the storage modulus G' versus theloss modulus G w i t h frequency as a parameter; analogoustoCole-Cole plots used in dielectric spectroscopy [30,31].Such plots were used by Han et al. [32-34] to investg tetemperature inducedchangesin the microstructure of homo-polymers, block copolymers and blends. It was proposed

es

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1 0 6

1 0 5

1 0 4

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w t n a n o t u b e s n PC

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Fig 5.LossmodulusG of nanotubi filled polycarbonate at 260 C

Fig 6. Storage modulus G1 as function of loss modulusG of nanotubefilled polycarbonate at 260 C

that i f the microstructuredoesnotchange w i t h temperature,curves of logG1 versus logG at different temperaturesshould coincide, as is the case in the single phase melt.Wnen the microstructurechanges w i t h temperature, differ-em curves of logG1 versus logG are expected. Such plotscan also be used to elucd testructure differences at a fixedtemperature. For example, Harrell and Nakayama [35,36]used logG versus logG1plotswhichthey called 'modifiedCole-Coleplo ts' t o explore the influence of branching andthe broadening of the molecular weightdistributionofpoly-ethylene on the microstructure. It was shown that at a givenG', G1 increases as the degree of long chain branchingincreased. I n multiphase systems, such plots can be usedi n a similar way to indc te structural differences betweenthe matrix and filled systems at a given temperature. Forexample, Han and K i m [37] show that increasing the rubbercontent increases the elastic properties of an acrylonitrile-butadiene-styrene material.

For the nanocomposites of interest here, the storagemodulus, G' (for a given loss modulus, G ), increasessignificantly w i t h increasing content of nanotubes (Fig . 6).A t contents 5 wt% and above, G' is higher than G . Theslope of G' versus G decreases w i t h increasing nanotubecontent. Kitano et al. [27]found a similar response of thef irs t normal-stress difference versus shear stress for glassfiber filled polyethylene melts under steady-state shearconditions.Accordingto Han and Lem [ 3 2 ] there is aquali-tative similarity between plots of the first normal-stressdilference versus shear stress and plots of G1 versus G .Theshiftand thechangein slope of theG1 versus G curvesindc tethat the microstructure ofthesecomposites changessignificantly w i t h addi tion of nanotubes.

Figs. 7 and 8 show plots o f the storage modulus G' andthe loss modulus G versus nanotube content at differentfrequencies. These plots again show that the increasei n G' w i t h the nanotube content is much higher thanthat of G . The increase w i t h composition is non-linearand is more prominen t at concentrat ions below 5 wt %


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Fig. 10. SEM micrographs of PC + 5 wt nanotubes, fracture surface,without sputtering.

material but does not necessarily destroy the nanotubesthemselves. As in the masterbatch, the nanotubes s t i l lappear to be arranged randomly in space. The nanotubesseem to bridge the crack i n the mat rix. This may be a waytoenhancethe strength of the composite. Such an effectiveload transfer mechanism has been described by Jin et al.[15] and Qian et al. [8] for M W N T in thermoplastics andby Ajayan et al. [39] for SWNT in epoxy composites.

F i g . 11 shows a photomicrog-aph ofsamplesdissolved inTHF. Each sample contains thesame amount of nanotubes.The solutions made f r o m the lowest concentrated composites, 0.5 and 1 wt composite;- lef t twovials),havea verydarkappearance and are believ;d to consist predominantlyo f finely dispersed individual nanotubes. Dissolution of the2wt nanocomposite i n TH F (middle v i a l ) reveis anoticeable change in appearance. The solution containsfinely dispersed nanotubes, corresponding to the overallgray color, and precipitated blarJc particles. The black sedi-ment is believed to be the remains of an interconnectedstructure of nanotubes. At the percolat ion threshold, thenanotubes f o r m an interconnecting structure, exhibiting ahigh degree of part icle-par tic le interactions and/or entan-glements. This increased nanotube interaction inhibi tscomplete dispersin of the nanotubes in THF, as seen inthe 2 wt composite v i a l .Beyond the percolation threshold,nanotube connectivit y is more pronounced. This effect isevident in the inhomogeneous iolut ions pertaining to the 5and 15 wt composites (right two vials in Fig. 11). Bothvialscontain a considerable amount ofsolidblack particles,

Fig. 11. Dissolving experiments on nai.otube filled polycarbonates after 2weeks in T HF from left to right: 0.5, 2, 5, and 15 wt composites.

while the l i q u i dphase appearsto be nearly transparent. Thel i q u i d phases have only a small fraction of free floatingindividual nanotubes. Instead, the bulk of the nanotubesreside w i t h i n the black particles, in the f o r m of a highlyinterconnected structure. In general, the percolation effectseenheresupports both electrical and rheological findings.

4. Discussion

4.1. Electrical and rheological percolation thresholds

The results shown above strongly suggest an electricalpercolation threshold between 1 and 2 wt nanotubes inpolycarbonate. This threshold is much lower than the9-18 wt range reported by Lozano et al. [3,4] for vaporgrown carbn nanofibers used as a model for SWNT andM W N T i n polypropylene. The reasonfor this difference isprimarilydue to the differentaspect ratios which are about100-1000 for thenanotubesused here versusabout 10-100for the fibers used by Lozano et al. A simple relationshipbetween geometrical percolation threshold andaspectratiowas given by Balberg [40]. For randomly distributed c y l i n -ders, the critical volumefraction for percolat ion is propor-tional to the inverse of the aspect ratio; this suggests thepercolation threshold for the thinner nanotubes used hereshould occur at a concentration about one-tenth of that forthe fibers used by Lozano et al. The experimental findingsare inreasonableagreement w i t h this calculation.

Inaddition, the electrical percolation threshold seemstobedependenton the matrix material orprocessused. Shafferand Windle [18] reported that p o l y v i n y l alcohol) compo-sitis formed f ro m the same k i n d of Hyperion nanotubesha^e a percolation threshold between 5 and 10 wt nanotube content. Composites based on epoxy resins producedby curing of nanotube dispersions i n the l i q u i d precursorwere found to exhibit a percolation threshold below0.04 wt as described by Sandler et al . [23]. In addition,thesecomposites havea much higher electrical conductivitycompared to composites w i t h thermoplastic polymers.Shaffer and Windle [18] attributedthese differences to anadiorbed polymer layer around thenanotubeswhich reducestht; qualit y and quantity of electrical contacts between thenaiotubes. On the other hand, such a layer may also be arequirement forhighphaseadhesinand enhanced mechan-i c d properties.

Interestingly,the observed resistivity threshold occurs inthe same concentration range as the increase in meltviscosity found at low frequencies. Starting at 2 wt nanotubes the frequency dependence of the viscosity curveschanged significantly and astep increasein the viscosityco uposit ion rela tion was observed at low frequencies. Thissu*gests that the rheological behavior can be a tool foridentifying the percola tion threshold forthese composites.This notation agrees w i t h findings by other authors.Lczano et al. [4] reported a melt viscosity threshold for


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3254 Potschke et al./Polymer 43 2002) 3247-3255lo w frequencies at around 10 wt nanofibers in PP corresponding to the conductivi ty threshold. Compositionscontaining less than 10 wt nanofibers showed only asmall increasein viscosity and followed closely the viscos-ity-frequency behavior of the unf i l led PP. Higher fillercontents up to 30 w t showed a significantincreasei n viscosity, especially at low frequencies.

4.2. Rheological properties and expected processingbehavior

Compared topriorresults f r o mthe literature, the increaseinviscosityw it h filler content reportedhereis much higher.The largeincreaseis believed to becausedby the very highaspectratio of the nanotube b r i l s . The comparison of theabove results w i t hthe results of Lozano et al. [4] indicatesthat the increase in viscosity at a given fiber content isstrongly dependent on the aspectratio of the filler. Similarresults were found by Kitano et al. [27] for polyethylenemelts filled w i t h glass fibers. The viscosity increase at agiven fiber concentration was higher the larger the aspectratio; this was quite pronounced at low shear rates.However, the increase in viscosity at the threshold compositionrelative to thepurmatrix in thepresentsystemseemsto be in about the same range as the relative increase inviscosity shown by Lozano et al. [4] at their respectivethreshold composition.

A t high frequencies, characteristic of processingbehavior, the viscosity is only slightly changed by addingup to 5 wt nanotubes. Thus, composites around the percolationthreshold are expected tohaveabout thesameprocessing behavior as the neatpolymer in practical applications.However,wehaveto consider that the Cox-Merz rule mayf a i l in filled systems. For highly filled wollastonite-poly-propylene systems [21] , it was fo und that the steady-statedata measured at low shear rates, e.g. below 1 s , wereconsistently higher than the dynamic data. The differenceincreased w i t h increasing loading (up to 60 phr) and wasconsidered to be causedby predominantly higher particle-particle interactions under steady-state conditions. On theother hand, at higher shear ratesorientation of anisometricplate-like fillers may lead to lo wer viscosities in capil laryflow regimes as mentioned by Fornes et al. [41] for organo-clay nanocomposites based on polyamide 6. Whether suchsignificant differences also exist in composites w i t h lownanotube contents sho uld be explored in future investigations. Hagerstrom and Greene [ 6]2reported that low nanotube Ioadings facilitate higher melt flow in composites ascompared to other types of conductive fillers which requiremuch higher concentrat ion to achieve the conduct ivitythreshold (e.g. carbn fibers). Hagerstrom et al. point outthat this is a significant advantage for t h i n w a l l moldingapplications.

I t should be emphasized that the composites investigatedhere were obtained by compression molding, a techniquethat typicallyresults in a relatively random distribution of

fibers. I n injection molded samples, which are morecommonly encountered in practical applications, somedegreeof orientation of the nanotubesin the flowdirectionispossible. W i t hthis inmind,the percolation threshold maybe somewhat different from that observed herefor compression molded samples.

Summary and conclusionsThe rheological behavior of compression molded

mixtures of polycarbonate and CNTs were investigatedusing osci llatory rheometry at a constant temperature of260 C . The nanotubes have a diameter o f about 10-_5nm and lengths in the range 1-10 \xm. The viscosityincreases significantly w i t h increasing nanotube concentrat i o n . The viscosity ofthese composites was shown to besignificantly dependent upon testfrequency. ur polycarbonate and composites containing less than 2 wt CNTshow similar frequency dependencies and reach a Newtonian plateau at lo w frequencies. Above 2 w t nanotubes,the viscosi ty curves exhibit a much greater decrease w i t hfrequency and exhibit non-Newtonian behavior to muchlower frequencies. Therefore, 2 w t may be regarded as aheological threshold compositio n. The viscosityincreaseis

accompanied by an increase in the elastic melt properties,tepresented by the storage modulus G', which is muchhigher than the increase of the loss modulus G .

Ultimately, the rheological threshold coincidences w i t hthe co nductivity percolation threshold which was foundto be between 1 and 2 w t . This indicates that the rheological response is sensitive to the interconnectivity ofthe nanotubes, which is also directly related to electricalconductivity.

The increase in viscosity w i t h composition is muchhigher than reported for nanofibers having higher diametersand forcarbn black composites. This difference iscausedby the higher aspect ratio of the nanotubes used. Interest-i n g l y ,the relative viscosityincreaseat the threshold compositionis about thesamerangeas reported for nanofibers w i t hlower aspectratio.

Acknowledgements

The authors are grateful for the financial support grantedbythe Max-Kade-Foundation (NewY o r k )to P.P. We wouldalso l i k e to thank Hyperion Catalysis International forproviding the masterbatch materials and Prof. PeterF. Green for use of the ARES rheometer.

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Tarea 9 Rheological Behavior

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