ORIGINAL CONTRIBUTION

Rheology of carbon nanofiber-reinforced polypropylene

Simona Ceccia & Dino Ferri & Daniela Tabuani &Pier Luca Maffettone

Received: 5 June 2007 /Revised: 21 December 2007 /Accepted: 28 January 2008 / Published online: 15 April 2008# Springer-Verlag 2008

Abstract The rheological properties of two different nano-composite systems consisting in the dispersion of carbonnanofibers (CNFs) in polypropylene are investigated. Thenanoreinforced systems were identically prepared with twoCNFs that differ only in the length of the fibers beingotherwise identical to analyze the effect of fiber aspectratio. Linear dynamic viscoelasticity and the steady-staterheology of the two different nanocomposites are presented.The system reinforced with CNFs with larger aspect ratioshows several rheological features that resemble peculiar-ities of rodlike polymers in the nematic liquid crystallinephase.

Keywords Carbon nanofiber . Viscosity . Normal stresses .

Linear viscoelasticity

Introduction

Carbon nanofibers (CNFs) represent a viable solution to thepreparation of polymer nanocomposites with improved

mechanical, thermal, and electrical properties (e.g., Yanget al. 2007; Kelarakis et al. 2006; Gao et al. 2006; Choi etal. 2005; Xu et al. 2004). CNFs have multiple concentricwalls, with diameter ranging from 70 to 200 nm and length50–100 μm. They are not continuous tubes as the graphenecylinders, and they are not parallel to the fiber axis. Rather,they show a 20° angle with respect to fiber axis in a Dixiecup arrangement terminating at the wall of the next outertube (Kang et al. 2006). The characteristic nanoscopicdimension together with a relatively low cost and the easyincorporation into polymers make CNFs an obviouscandidate for the production of high performance lightmaterials. Several recent papers show the benefits of theaddition of CNFs in polymer matrices in terms ofmechanical, electrical, and thermal properties. For example,Kumar et al. (2002) showed that fibers from PP/CNFcomposites can be spun using conventional equipment, andpossess superior modulus and compressive strength withrespect to the pure polymer at only 5 wt% of CNFs. Suchan improvement was obtained by dispersing as receivedCNFs into the polymer via melt processing. Sandler et al.(2003) studied nanoreinforced fibers of a semicrystallinehigh-performance poly(etheretherketone) containing up to10 wt% CNFs. The carbon nanofibers were found to bewell aligned with the direction of flow during processing,and, correspondingly, nanocomposite stiffness, yield stress,and fracture strength improved with respect to neatpolymer. Upon addition of nanofibers, a significant increaseof the degree of crystallinity of the matrix was alsoobserved. Gauthier et al. (2005) studied the reinforcementof rubbery matrices by CNFs and showed that themechanical performances revealed a linear increase of themodulus measured above and below the glass transitiontemperature for nanofiber content up to 10 wt%. In the caseof an epoxy matrix, the ultimate stress and strain were also

Rheol Acta (2008) 47:425–433DOI 10.1007/s00397-008-0265-4

S. Ceccia :D. TabuaniCentro Ingegneria Materie Plastiche,Viale T. Michel 5,15100, Alessandria, Italy

D. FerriCentro Ricerche “Claudio Buonerba”, Polimeri Europa,Via Taliercio 14,46100, Mantova, Italy

P. L. Maffettone (*)Dipartimento di Ingegneria Chimica Università Federico II Napoli,Piazzale V. Tecchio 80,80125, Napoli, Italye-mail: pierluca.maffettone@unina.it

largely increased, even for very low-fiber content (1 wt%).Shen et al. (2006) showed that CNFs can be used both asnucleant and reinforcement for polystyrene (PS) foams. Theinclusion of CNFs exhibited a substantial impact on themorphology and properties of PS foams by decreasingthe cell size and increasing the cell density. A substantialstrength enhancement of PS foams due to the incorporationof CNFs was experimentally observed.

The rather common observation of improvements of themechanical properties of CNF-reinforced plastic products isgenerally related to the formation of a microstructure. In themolten state, such microstructure can be easily probed withrheological experiments. Linear and nonlinear viscoelasticproperties provide important information on the dispersionof the filler in the matrix, on the concentration and sizedistribution of the filler, and its wettability. Only few andvery recent papers, however, were devoted to the rheolog-ical characterization of CNF-reinforced polymers (Lozanoet al. 2004; Xu et al. 2005; Wang et al. 2006). Lozano et al.(2004) characterized the linear dynamic viscoelasticity ofCNF nanoreinforced HDPE. They noted a monotonicincrease of G’ and G” moduli both with frequency andconcentration, with the appearance of a solid-like plateau atlow frequencies at loadings ≥20 wt%. Xu et al. (2005)presented a very detailed study on samples of CNFs inglycerol/water to analyze the effects of different purificationtechniques (sonication and acid treatment). Nanocompo-sites from as-received nanofibers showed large aggregates(mm to cm), whereas sonication significantly reducedaggregate dimensions (50 μm), and an acid treatmentimproved the dispersion but at the cost of shortening theCNFs. A solid-like behavior of the moduli was found forloadings larger than 3 wt%. Xu et al. (2005) also modeledthe nanocomposite systems with either elastic or rigiddumbbells in a Newtonian solvent with isotropic oranisotropic hydrodynamic drag, with or without hydrody-namic interaction. They found that the elastic model withanisotropic hydrodynamic drag and negligible hydrody-namic interaction successfully captures the rheologicalbehavior observed in their experiments. Very recently, thesame research group (Wang et al. 2006) presented ananalysis on shear rheology of nanofiber/polystyrene com-posites. The focus was on the effect of the preparationtechnique as captured by rheological measurements. It wasshown that melt blending strongly reduces the length of thefibers with respect to solvent-casting preparation. A generalmonotonic increase of both G’ and G” was observed. Themelt phase of solvent-cast composites with higher CNFconcentrations exhibits a plateau of the elastic modulus atlow frequencies, an apparent yield stress, and large firstnormal stress difference, N1, at low strain rates. The authorsascribed such features to contact-based network nano-structure which forms in the presence of longer CNFs.

Also, the experimental results were well described by amodel similar to those commonly adopted to describe fibersuspensions in viscoelastic liquids.

In the present work, we characterize the rheologicalresponse of two different nanocomposite systems consistingin the dispersion of CNFs in polypropylene. The nano-reinforced systems were obtained with two CNFs that differonly for the length of the fibers being otherwise identicaland identically prepared. In such a way, a clear analysis ofthe relevance of the fiber aspect ratio can be addressed. Thelinear dynamic viscoelasticity and the steady state rheologyof the two different nanocomposites are presented. As itwill be shown, the rheological characterization of systemreinforced with CNFs with larger aspect ratio shows severalfeatures that resemble peculiarities of rodlike polymers inthe nematic liquid crystalline phase. This phenomenology issimilar to that recently observed for carbon nanotubes,which are considered as a type of highly conjugated, rigid-rod macromolecules. Carbon nanotubes have shown inter-esting similarities with “rigid-rod” macromolecular systems(Song and Windle 2005). Indeed, it was reported thatcarbon nanotubes can form nematic liquid crystallinephases (Song et al. 2003; Davis et al. 2004; Song andWindle 2005). The existence of a nematic phase could ofcourse lead to interesting final properties as large-scalealignment could be achieved under processing conditions.

Materials and methods

The nanoreinforced polymer was prepared with a homo-polymer commonly used to produce injection-moldedproducts (Moplen HP400R, Basell). The relevant polymerproperties are reported in Table 1.

Two different nanofibers produced by Applied SciencesInc were dispersed in the matrix: PR-24 LHT HD (HighDensity) and PR-24 LHT LD (Low Density). The HD andLD designations do not refer to the density of the individualfiber but to the bulk density of the carbon nanofibers. Thebulk, or aggregate density, of the nanofiber is controlled byaltering the mixing intensity and duration of the debulkingprocess. The debulking process reduces the length of theindividual fibers, with high density fiber having shorterfiber lengths than the low density fiber. The producer states

Table 1 Properties of the polymeric matrix

Property Method Value Unit

Density (23 °C) ISO 1183 0.905 g/cm3

Melt flow rate(230 °C/2.16 kg)

ISO 1133 25 g/10 min

Melt volume flow rate(230 °C/2.16 kg)

ISO 1133 34 cm3/10 min

426 Rheol Acta (2008) 47:425–433

that typically, the fiber length in the low density product is100–200 μm, while the fiber length in the high densityproduct is 50–100 μm (Burton, personal communication).The relevant parameters of the nanofibers are reported inTable 2.

The dispersion of nanofibers in polypropylene wasachieved by melt compounding as-received CNFs with thepolymeric matrix in a Brabender internal mixer. Theinternal mixer consisted of a heated chamber with a volumeof 55 cm3. The chamber was closed between two plates: afixed plate with two counter-rotating mini-screws (maxi-mum rate 200 rpm), and a mobile one with the hopper andthe chamber. Mixing was performed at 180 °C, 60 rpm, for10 min. Processing conditions were chosen to optimizenanofiber dispersion and to minimize matrix degradation.Both nanocomposite systems were prepared following thevery same procedure. The melt compounding preparation isexpected to cause the breakage of nanofibers, thus alteringtheir nominal properties. Transmission electron microscopyimages recorded on CNF extracted by dissolving PP inXylene (not shown here) showed that the ratio betweenaverage LD-CNF and HD-CNF length is practicallypreserved after preparation.

Five different compositions were prepared with HDnanofibers as reported in Table 3; nine compositions wereprepared with LD CNFs as reported in Table 4.

Scanning electron microscopy (SEM) imaging wasobtained bymeans of a LEO 1450 VP instrument on cryogenicfracture surfaces. SEM micrographs showed that a ratheruniform dispersion was generally obtained, but some aggre-gates were still present. Similar state of dispersion character-izes materials prepared with HD-CNF and LD-CNF. Anexample is reported in Fig. 1. The fiber length distributionwas affected by the mixing process, and a reduction of thefiber aspect ratio was observed for both CNFs even thoughno quantitative assessment was carried out.

The preparation of the nanoreinforced materials couldaffect the matrix properties by inducing some thermome-

chanical degradation. Thus, for the sake of comparison, thepure polypropylene matrix MPHP400R was also subjectedto the same thermal and mechanical treatments as thenanocomposites to have a consistent reference matrix.

The samples were rheologically characterized withdifferent instruments. The linear viscoelastic storage moduliwere measured in shear at 25 °C with a RheometricsMechanical Spectrometer (RMS) model 800 at a frequencyof 1 Hz using the torsion rectangular geometry. Theinstrument was equipped with a force rebalance transducerable to detect torques within the range 0.02–20 N. Theactuator was an air bearing motor with strain amplitude of0.05–500 mrad and a rotation angular frequency varyingbetween 10–3 and 102 rad/s. The specimens were compres-sion-molded at about 210 °C and recovered as rectangular60×2×8 mm stripes to fit the RMS tools. The temperaturewas stable within 0.2 °C over the range used in this study.Strain sweeps were performed with strain amplitudesranging from 0.01 to 1.4. The shear storage moduli reportedin the following are those measured in the linear viscoelas-tic response region. All tests were performed in N2

atmosphere.For dynamic linear viscoelasticity of the melt, the

samples were compression-molded at 210 °C under apressure of 20 MPa and recovered as 25 mm diameterdisks to fit the RMS tools. The linear viscoelastic functionswere measured in the RMS using the parallel plategeometry (diameter 25 mm and gaps between 0.6 and 1mm). Isothermal frequency scans were performed in therange between 10−1 and 102 rad/s. The temperature wasstable within 0.2 °C over the range used in this study. Strainsweeps were previously performed to investigate the linearviscoelastic response region, and time sweeps were alsoperformed to test the thermal stability of the samples. Alltests were carried out in N2 atmosphere.

The steady-state shear rheology was investigated at 210 °Cusing the rotational rheometer (RMS800) in the low shear rateregion and a capillary rheometer (Göttfert Rheograph 2002) inthe high shear rate region. Step rate experiments wereperformed under a N2 atmosphere with the cone-plategeometry (cone angle 0.1 rad) in the shear rate range [0.02,20 s−1]. The temperature was stable within 0.2 °C over therange used for the rotational rheometer tests. The measure-ments with the capillary rheometer were performed in theshear rate range [1, 5000 s−1] and corrected for non-

Table 3 Compositions of HD-based nanocomposites

Unit Fiber concentration

wt% 0.50 1.00 3.00 6.00 10.0vol% 0.27 0.53 1.62 3.29 5.58

Table 2 Relevant properties of the CNFs

Name N2 surface area,(m2/gm)

Moisture(%)

Iron content(PPM)

PAH content(mg PAH/g fiber)

Diameter(nm)

Length(μm)

Nominal aspectratio

PR-24 LHT HD 35–45 <5 <14,000 <1 100–200 50–100 250–1,000PR-24 LHT LD 35–45 <5 <14,000 <1 100–200 100–200 500–2000

Rheol Acta (2008) 47:425–433 427

Newtonian effects (Rabinowitsch correction). A round holecapillary was used with L/D=30 (diameter=1 mm, length=30 mm, entry angle=180°). This L/D value was large enoughto allow to neglect the Bagley correction. The rheometer wasequipped with a pressure transducer able to detect pressuresup to 1.4×108 Pa.

Experimental results

Linear viscoelasticity

In the molten state, a strain sweep was performed todetermine the limits of linear viscoelastic regime. At anangular frequency 5=6.28 rad/s and at a temperature T=210 °C, the pure reference matrix showed a linear responseup to 100% strain, which was the largest applied strain.Figure 2 shows a reduction of the linear range for thenanoreinforced material with HD fibers starting from 3 wt%,while for the LD samples starting at 1.5 wt%.

Frequency sweep test—HD carbon nanofibers

Frequency sweep tests at T=210 °C were performed withangular frequencies ranging from 0.1 to 100 rad/s. Thepresence of HD-CNFs produces a small increase in moduliup to 3 wt%, where G’ and G” data follow closely theunfilled reference matrix data (PP) throughout the frequen-cy range as shown in Fig. 3. A “hesitation” is foundbetween 0.5 and 1 wt% as the two data sets nearly

superimpose. It can be noted that G’ increases more thanG” in this concentration range. At low concentrations, thestorage modulus enhancement is usually attributed tostiffness imparted by the solid particles that allow efficientstress transfer, which is mainly controlled by the matrix/fiber–matrix interface. At larger loadings (6 and 10 wt%),the moduli increase steeply. Thus, above a threshold value(in this case above 3 wt%), a pseudo-solid-like behavior isobserved at low frequencies, i.e., G’ and G” data show theappearance of a plateau, suggesting the formation of someinterconnected nanofiber network within the matrix. It iswell known from the literature that the interconnectedstructures of anisometric fillers result in an apparent yieldstress which corresponds, in dynamic measurements, to theplateau of G’ or G” at low frequencies (Shenoy 1999;Utracki 1986; Dealy and Wissbrun 1999). This effect ismore pronounced in G’ than in G”. This is in accordancewith theoretical expectations and experimental observationsfor fiber-reinforced composites (Shaffer et al. 1998;Pötschke et al. 2002; Lozano et al. 2004). Indeed, as thenanofiber content increases, nanofiber–nanofiber interac-tions begin to dominate, eventually leading to percolationand to the formation of some interconnected structure.

The magnitudes of the complex viscosity, η*, of thereference matrix and composites with different HD-CNFcontent are shown in Fig. 4. Of course, consistently withmoduli data, at small loadings (<3 wt%), the presence ofsolid particles perturbs the normal flow of polymer, and, asusually observed in dilute suspensions of rigid particles, the

%wt CNF

1 10

Str

ain

0.001

0.01

0.1

1

10

LD samplesHD samples

Fig. 2 The limiting strain for linear viscoelastic response (T=210 °C,5=6.28 rad/s)Fig. 1 A SEM image of HP400R +3 wt% of HD-CNFs

Table 4 Compositions of LD-based nanocomposites

Unit Fiber concentration

wt% 0.25 0.50 1.00 1.50 2.00 3.00 4.50 6.00 10.0vol% 0.13 0.27 0.53 0.80 1.07 1.62 2.45 3.29 5.58

428 Rheol Acta (2008) 47:425–433

magnitude of the complex viscosity of the filled systemincreases. Up to 3% loading, the norm of the complexviscosity of HD-CNF samples shows a dependence on thefrequency similar to that of pure matrix, revealing aNewtonian plateau at low frequencies. A simple verticalshift is observed as the concentration increases as normallyobserved with conventional microcomposite systems, con-sistently with what is observed for G’ and G”. Above 3 wt%,η* data do not show the Newtonian plateau at lowfrequencies anymore, and yield stress appears.

Frequency sweep test—LD carbon nanofibers

A much richer behavior is observed in the case of fibers withlarger aspect ratio, i.e., with the LD-CNF nanocomposites. Inthis case, the frequency sweep measurements are shown inFig. 5 (for the sake of clarity, only a selection ofconcentrations is plotted). It is apparent that the increase ofboth moduli is much larger than that observed with HD-CNFs at the same loading (see Fig. 3), and the pseudo-solid-like plateau is observed at lower concentrations (it alreadyappears above 2 wt%). At the lower nanofiber concentra-

tions, the magnitude of the complex viscosity increases withrespect to the reference matrix preserving the Newtonianplateau at low frequencies (Fig. 6). Above 2 wt%, η* data donot show the Newtonian plateau anymore, and yield stressappears. This evidence confirms the existence of athreshold concentration value between 2 and 3 wt%, mar-king the onset of a solid-like behavior at low frequencies.

The enhancement of moduli due to the presence of LD-CNF is significantly higher at lower frequencies, and theincrease is more pronounced in the storage modulus. Thus,as it could be expected, the system with lower aspect ratio(HD-CNFs) shows a larger percolation concentration withrespect to the system with larger aspect ratio (LD-CNFs). Asimilar effect of the aspect ratio was found by Kitano et al.(1984) with polyethylene melts filled with glass fibers. Thevery recent data proposed by Wang et al. (2006) also showthe same dependence on the fiber aspect ratio.

The most striking feature, however, is the distinctnonmonotonic dependence of the moduli on the fillerconcentration. Indeed, at a fixed frequency, as the loadingincreases from 0% (pure polymer), both moduli first increase(at 0.5%) then decrease (at 1.5%), and then they increaseagain (at 3%). This feature is even more evident in themagnitude of complex viscosity data plotted in Fig. 6.

Figure 7 shows the variation of the shear storagemodulus of composites at 25 °C. It is apparent that bothinclusions improve the modulus as the filler concentrationincreases, but the effect of the LD-CNFs is substantiallylarger. Again, the difference between the two set of data canbe ascribed to the different aspect ratio of the inclusions.

As already observed in linear viscoelastic data taken inthe molten state, an interesting feature of these data is thepresence of a relative maximum at low composition. Thismaximum is clearly visible in the case of LD-CNFs, whileit is hardly detectable in the HD-CNF samples.

Steady-shear tests

Steady-shear tests were performed only on the LD-CNFnanocomposites in view of their peculiar response in linear

100

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104

|η*|

(P

a s)

ω (rad/s)

PP

HD 0.5%

HD 1%

HD 3%

HD 6%

HD 10%•

Fig. 4 The magnitude of the complex viscosity (T=210 °C) for theHD-CNF nanocomposites

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baFig. 3 Effect of CNFs HD onthe linear viscoelastic response(T=210 °C). a Storage modulus;b Loss modulus

Rheol Acta (2008) 47:425–433 429

viscoelasticity. The experiments were carried out in a strain-controlled rotational rheometer at lower shear rates [0.02–20 s−1] and in a capillary rheometer at higher shear rates[1–5,000 s−1]. The viscosity data obtained with the twoinstruments perfectly overlap at intermediate shear rates.

At low shear rates, the addition of LD-CNF to thepolymer matrix produces a general increase of the viscositywith respect to that of the matrix. At lower concentrations, aNewtonian plateau is found at low shear rates, while athigher fractions, yield stress appears at low shear rates (seeFig. 8b). Nanocomposites with solid fraction above 2 wt%exhibit strong shear thinning throughout the investigatedshear rate range. Again, data suggest the formation of somenetwork structure. Shear thinning behavior of the viscositycan be associated at low shear rates to the breakdown of thenetwork structure and at large shear rates to the possiblealignment of the fibers along the flow direction.

At higher shear rates, the effect of the filler on theviscosity is reduced in magnitude, and the nanocompositeviscosity tends to the matrix viscosity. The significantdecrease in viscosity at higher shear rates shows that thedifficulty of processing nanofiber systems should not be anissue with those under investigation here.

The most peculiar behavior is the nonmonotonic depen-dence of the viscosity on the CNF fraction at low concen-trations, which parallels that observed in the linearviscoelastic functions. At low shear rates, a distinct New-tonian plateau is visible for loading up to 1.5 wt%. Here, thezero shear viscosity of nanocomposites first increases thendecreases, thus confirming the linear viscoelastic features.This feature is apparent in Fig. 9, where the zero shearviscosity for the LD-CNF systems that show a Newtonianplateau at low shear rates is plotted as a function ofnanofiber content.

The trend of the viscosity reported in Fig. 9 is verysimilar to that observed in systems showing anisotropicmesophases, such as lyotropic rodlike polymers showingnematic liquid crystalline phases (see the Discussionsection).

The similarity with such systems appears to be evenstronger if the first normal stress difference is considered.The first normal stress difference, N1, was measured duringstart up of steady-shear tests with the rotational rheometerequipped with cone and plate fixture. Special care duringsample loading was necessary to have reproducible data. A

0 1 2 3 4 5 6 7 8 9 10 11

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PP+LD CNF

PP+HD CNFPP

T=25∞C

shea

r st

ora

ge

modulu

s (G

Pa)

CNF LD (wt %)

Fig. 7 Shear storage modulus at T=25 °C for LD and HD CNFnanocomposites

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LD 0.5%

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|η*

| (P

as)•

Fig. 6 Effect of LD-CNFs on the magnitude of complex viscosity(T=210 °C)

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ω (rad/s)

G"

(Pa)

baFig. 5 Effect of LD-CNFs onthe linear viscoelasticity(T=210 °C). a Storage modulus;b Loss modulus

430 Rheol Acta (2008) 47:425–433

waiting time after loading was used to eliminate the effectsof loading history. During this waiting time, the normalforce generated by sample squeezing was monitored up tofull sample relaxation. The highest CNF concentrations ledto a maximum waiting period of 1 h before testing.

Figure 10 shows the N1 transient for a sample with 6 wt%LD-CNF at two different shear rates (5 and 10 s−1). Theshear rate is stepped up after the first 100 strain units. Forthe sake of comparison, the transients of the unfilled PP atthe same shear rates are also presented in the figure. Theeffect of the filler with respect to the unfilled sample isapparent. A clear initial overshoot followed by an under-shoot and long and oscillating N1 transients were observedfor LD-CNF samples above 1 wt%.

Figure 11 reports the average steady state value of thefirst normal stress difference as function of the shear rate.Normal stress signals become measurable only at highshear rates. At larger LD-CNF composition (6 and 10 wt%),N1 attains negative steady-state values.

Both long oscillating transient and negative first normalstress difference are typical features of lyotropic rodlikepolymers in the nematic phase.

Discussion

The viscosity and normal stress data observed with our LD-CNF nanocomposites at 210 °C share several peculiaritiesof lyotropic nematic polymers. The introduction of fillerwithin a polymeric matrix alters the flow properties of thesystem, and usually the processing becomes more difficult.This aspect is easily verified with rheological characteriza-tion, as a distinct increment of viscosity is usually observed.This is not observed, however, in the case of solutions ofrodlike polymers showing a nematic liquid crystalline state(Doi and Edwards 1986). In that case, in fact, as the rodlikeparticle fraction increases, the zero shear viscosity firstincreases, then, when a critical concentration is reached, adistinct decrement of the viscosity is observed. Thisevidence is related to the transition from an isotropic stateto the liquid crystalline phase characterized by a significantdegree of orientational order. Indeed, in the liquid crystal-line state, the rodlike molecules are more free to diffuse asthe overall alignment reduces the effects of entanglementsthus increasing the rotational diffusivity. As the concentra-tion of rodlike molecules further increases, the zero-shear

Strain

0 50 100 150 200 250

N1 (

Pa

)

0

500

1000

1500

PP 5s-1

PP 10s-1

LD6% 5s-1

LD6% 10s-1

Fig. 10 Transients of the first normal stress difference for unfilled PPand for LD-CNF 6 wt% plotted versus strain (T=210 °C)

Fig. 9 The plateau viscosity at low shear rates versus the LD-CNFcomposition (T=210 °C)

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ty (

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shear stress (Pa)

Vis

cosi

ty (

Pa•

s)

a b

Fig. 8 Viscosity for the refer-ence matrix (PP) and nanocom-posites with different LD-CNFloadings (T=210 °C). a Viscos-ity versus shear rate; b Viscosityversus shear stress

Rheol Acta (2008) 47:425–433 431

viscosity eventually starts to increase again as the orderingbeneficial effects cease the way to the progressive increaseof viscous drag due to the relative motion between solventand rods. A viscosity decrease with increasing fiber fractionwas already seen in the PET/CNF nanocomposites studiedby Ma et al. (2003), but no explanation was suggested forthe peculiar behavior. Similar features have been observedin a system containing multiwalled carbon nanotubes inaqueous solvent (Song et al. 2003; Song and Windle 2005)and in single-wall nanotubes in superacid (Davis et al.2004). In these papers, the fact was explained byconsidering that the phase behavior of carbon nanotubesystems show many parallels with that of lyotropicnematogenic rodlike polymer solutions.

Another peculiarity of liquid crystalline polymers is theappearance of negative first normal stress difference undershear at intermediate shear rates (e.g., Kiss and Porter1978). Such a behavior is rather uncommon and is nowwell understood (Marrucci and Maffettone 1989; Larson1990). Its origin lays in the disordering effect of shearingflow onto the orientational order of a nematic phase.Negative first normal stress differences have been measuredalso on single-wall nanotubes in superacid (Davis et al.2004), thus corroborating the LCP similarity of thatmaterial on attractive emulsions (Montesi et al. 2004), andon nanotube suspensions (Lin-Gibson et al. 2004). In thetwo latter cases, however, the occurrence of negativenormal stresses was related to vorticity alignment ofmacroscopic aggregates.

Finally, as far as the transient behavior of first normalstress difference is concerned, the evidence of longoscillating transients is again very similar to the responseof ordered mesophases. Indeed, oscillations in N1 havebeen found for lyotropic nematic solutions of poly(benzylglutamate) (PBG), hydroxypropylcellulose, and other lyo-

tropic as well as thermotropic liquid crystalline polymers(e.g., Quijada-Garrido et al. 2000 and references therein).

All these nonlinear features were here observed for thecase of a large aspect ratio CNF system, thus suggesting thepossible formation of a mesophase. Although intriguing forits processing implications, this parallel should be carefullyexamined in view of an important difference between thetwo systems. Indeed, it seems now undisputable that thenanofibers are non-Brownian objects (e.g., Wang et al.2006). On the contrary, rodlike polymers are Brownian, andabove a critical concentration under quiescent conditions,they spontaneously form an ordered nematic phase. Noevidence is at the moment available on the possibleformation of nanofiber-ordered phases at equilibrium. Itshould be mentioned, however, that under flow condition,nanofibers can behave as pseudo-Brownian objects, rota-tional diffusion being caused by casual collisions (Folgarand Tucker 1984).

A final comment is in order. We did not observe anypeculiar feature with the HD-CNF sample. Furthermore,Wang et al. (2006) do not mention any of these phenomenafor both polystyrene-CNF nanocomposites they investigat-ed. These three systems (our HD-CNF, and the systemsinvestigated by Wang et al.) are characterized by a lowerfiber aspect ratio with respect to that of our LD-CNFsystem. In our opinion, “nematic-like” response can beobserved if the fiber aspect ratio is large enough to promotethe ordering effects at relatively low compositions. If this isnot the case, the possible formation of a network structurehinders the manifestation of orientational ordering.

Final comments

In this paper, we have studied the rheological behavior oftwo carbon nanofiber nanocomposites. The systems differonly for the fiber aspect ratio, being otherwise equivalent.The nanofibers are dispersed in a polymer matrix via meltcompounding. Linear viscoelasticity shows the formationof a network structure for both systems for compositionsabove a critical value. The nanocomposite characterized byCNF with larger aspect ratios shows a more pronouncedincrease of moduli and a lower critical composition.

Nonlinear rheology showed that the system with largernanofiber aspect ratio has several peculiarities similar topolymers in the nematic phase. This aspect can haveinteresting practical implications in the applications, asordered state is usually characterized by exceptionalmechanical properties.

Acknowledgements This work was supported by FIRB MAPIONANO.

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PP

LD 0.5%

LD 1%

LD 1.5%

LD 2%

LD 3%

LD 6%

LD 10%

Fig. 11 First normal stress difference measured for LD-CNF nano-composites subjected to steady shear flows (T=210 °C)

432 Rheol Acta (2008) 47:425–433

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