Impact behaviour of polypropylene filled with multi-walled carbon nanotubes

EUROPEAN

European Polymer Journal 43 (2007) 3197–3207

www.elsevier.com/locate/europolj

POLYMERJOURNAL

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Macromolecular Nanotechnology

Impact behaviour of polypropylene filledwith multi-walled carbon nanotubes

Hui Zhang a,b, Zhong Zhang a,*

a National Center for Nanoscience and Technology, China, 100080 Beijing, Chinab Institute for Composite Materials, University of Kaiserslautern, 67663 Kaiserslautern, Germany

Received 5 December 2006; received in revised form 29 April 2007; accepted 12 May 2007Available online 24 May 2007

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Abstract

Multi-walled carbon nanotubes/polypropylene composites were compounded using a twin-screw extruder. Here, nano-tubes with different lengths, i.e. 1–2 lm and 5–15 lm, respectively, were applied at a constant volume content of 1%.Notched Charpy impact tests showed that toughening effects of nanotubes depended highly on testing temperatures.The impact resistance was notably enhanced at a temperature above the glass transition temperature of matrix. Longernanotubes performed more effective in toughening compared to the shorter ones. The increment of impact resistance ofnanotube-filled polypropylene was considered due to enhanced load-carrying capability and much-increased deformationof matrix. SEM fractography further revealed the toughening mechanisms in a micro-scale. The impact energy wasimproved via nanotube breakage and pullout, which likely led to a series of energy consuming actions. In addition, thesmaller spherulite size induced by nanotubes would be favourable to the impact resistance partially.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Multi-walled carbon nanotubes; Polymer nanocomposites; Impact resistance; Polypropylene

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1. Introduction

Owing to extraordinarily high elastic modulus,strength and resilience [1–3], carbon nanotubesincluding both single-walled (SWNT) and multi-walled (MWNT) ones promise to become the next-generation reinforcements for polymer composites.Numerous efforts have been concentrated on thisdirection in the past decade. As reported in the liter-ature, even a small amount of nanotubes was

0014-3057/$ - see front matter � 2007 Elsevier Ltd. All rights reserved

doi:10.1016/j.eurpolymj.2007.05.010

* Corresponding author. Fax: +86 10 62650450.E-mail address: [email protected] (Z. Zhang).

observed to benefit mechanical performance ofpolymers. However, the reinforcing efficiency wasdependent on physicochemical properties of poly-mer system. Coleman et al. [4] reported that withless than 1 vol.% MWNT, poly(vinyl alcohol) andchlorinated polypropylene achieved approximately300–400% increases in modulus and strength,respectively, which may represent the highest rein-forcing effects up to now. Whereas, for some glassypolymers, e.g. epoxy and poly(methyl methacry-late), moderate enhancement or even slight declinein modulus and strength were usually obtained withthe nanotube incorporation [5–9]. The great discrep-ancy in reinforcing efficiency is ascribed to several

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3198 H. Zhang, Z. Zhang / European Polymer Journal 43 (2007) 3197–3207

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well-known key issues, such as dispersion state, fil-ler-matrix interfacial property and nanotube align-ment in polymers.

Apart from modulus and strength, fracturetoughness is another crucial parameter in polymerapplications, which reflects the ability of a materialto resist a pre-crack under stress applied. There aresome studies that deal with the toughness ofpolymer-nanotube composites [10–13,7,14]. Forthermosets, some authors [5,8] found substantialimprovements in double-notched impact strengthby adding only minute weight fractions of nanotubesinto epoxy resins. By contrast, a slightly decreasedIzod impact strength in fluorinated SWNT/epoxysystems was stated by others [10]. With respect tothermoplastics, a moderate increase in Izod impactstrength of MWNT/polypropylene system wasobserved [11]. MWNTs were found to be able toenhance the resistance to crack propagation com-pared to neat polycarbonate matrix under quasi-sta-tic tensile test via an essential work of fractureapproach [12]. While, obvious negative effects ofnanotubes on impact resistance were reported inPA6/ABS blends [13]. In addition, several investiga-tions [7,14] pointed out that the tension workcharacterized by the area under the static tensilestress–strain curves was enhanced more or less fornanotube-filled polymers, in comparison with thatof the neat matrices. However, it is worth notingthat high tension work does not necessarily meanhigh fracture toughness [15] since the specimensare loaded under different stress conditions, i.e. aplane-stress state normally for the former but aplane-strain state for the latter. However, materialsare more often broken at the plain-strain state, andhence the notched fracture toughness is always con-sidered as a critical parameter in material selections[15]. In summary, the published results of the tough-ening effect of nanotubes are somewhat contradic-tory, a lot of efforts are still necessary to draw anunambiguous conclusion.

Polypropylene (PP) is one of the widely usedsemi-crystalline thermoplastics for general purpose.In practical applications, some components madeof PP and its composites should be applied at highertemperatures, e.g. piping systems for hot water sup-ply, panels for automobiles, as well as containers forfood industries. Therefore, the fracture behaviour ofPP at higher temperatures would be interesting bothacademically and practically. In the present study,polypropylene was reinforced with different lengthMWNTs. The impact behaviour was investigated

in a broad temperature range from �196 to 80 �C.It was found that in term of notched Charpy impactapproach, the toughening effect of nanotubes reliedstrongly on the test temperature applied. Nearly notoughening effect was observed at a temperaturebelow the glass transition temperature of matrix,while notably positive effect occurred at a tempera-ture above the glass transition temperature,although some nanotube agglomerates were stillfound in the matrix. Furthermore, the nanotube-based composites offered moderate improvementsin dynamic tensile modulus. The corresponding frac-ture mechanisms were discussed based on SEM frac-tography and force–time diagrams determined fromCharpy impact tests.

2. Experimental

2.1. Materials and preparation

Commercially available isotactic polypropylenehomopolymer (Moplen� HP501H) were suppliedby Basell Company, Germany. Some key parame-ters of this PP were: melt flow rate = 2.1 g/10 min(230 �C/2.16 kg), density = 0.90 g/cm3, meltingpoint = 168 �C. The multi-walled carbon nanotubes(MWNT) were produced via a chemical vapordeposition approach by Shenzhen NTP Company,China. Two kinds of MWNT were chosen. Bothof them had a similar diameter of 10–30 nm but dif-ferent lengths of 1–2 lm and 5–15 lm, respectively.

Before blending, MWNT powder was dried in avacuum oven at 80 �C for about 2 days to eliminatemoisture. A two-step mixing process was employedto produce PP filled with MWNTs. In the first step,master batches with �5 wt.% MWNTs were pre-pared by using a co-rotating twin-screw extruder(Berstoff ZE 25A · 44D-UTS) at barrel temperatureof 185–200 �C, and a screw speed was 150 rpm.During melt extrusion ventilation was kept on toremove trapped air in blends. In the second step,the resulting composites were obtained by mixingfresh PP granules with the related master batchesin the same twin-screw extruder at a screw speedof 200 rpm. After pelletizing, the blend granuleswere injection-moulded into 4-mm-thick plates forimpact test using injection-moulding machine(Arburg Allrounder 320S). The barrel temperatureranged 215–220 �C and the mould temperaturewas kept at 40 �C. The injection pressure and speedwere 500 bar and 80 ccm/s, respectively. The finalcomposites containing �2 wt.% (�1 vol.%) short

Fig. 1. A representative TEM image of 1MPP (1 vol.% MWNTwith a length of 1–2 lm).

H. Zhang, Z. Zhang / European Polymer Journal 43 (2007) 3197–3207 3199

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or long MWNTs were designated hereafter as1MPP and 1LPP, respectively. Relatively good dis-persions of the nanotubes in the PP matrix wereachieved after mixing, which were proved by trans-mission electron microscopy (FEI Tecnai G 20)inspection, as shown in Fig. 1. As a reference, neatPP was also similarly processed to ensure analogousprocess conditions and thermo-mechanical historyfor all studied samples.

2.2. Property investigations

The thermal analysis of injection-moulded sam-ples were performed on a DSC821 apparatus(METTLER TOLEDO) in nitrogen atmospherewith a sample mass of about 8–10 mg. The scanningrate was 10 �C/min. The sample was held at 220 �Cfor 5 min, and then cooled down to 0 �C at the samescanning rate. The crystallinity of sample (Xc) wascalculated according to equation: X c ¼ DHm

DH0mð1�wtÞ

,

where, DHm is the specific melting heat, DH 0m is

the theoretical specific melting heat of 100% crystal-line isotactic PP, which is taken as 209 J/g [16], andwt is the weight fraction of nanotubes. Here the Xc

value was obtained from DSC first scanning,because we considered that the sample used inDSC had the same micro-structure and thermal his-tory as the sample used in mechanical tests.

A thin section was cut by razor blade from thematerials studied. The section was placed betweenglass slides under slight contact pressure, and then

heated at 230 �C for 10 min, subsequently cooleddown to room temperature with natural coolingrate. The PP spherulite morphology in the glassslides was observed by Leica Diaplan polarizedoptical microscopy (POM).

Dynamic mechanical thermal analysis (DMTA)was performed by use a Gabo Qualimeter Explexor25N under tension mode. The dynamic complexmodulus and loss factor of specimen (55 ·10 · 1 mm3) were determined at a constant fre-quency of 10 Hz in a temperature range of �50 to100 �C at a heating rate of 2 �C/min. The span ofthe specimen supports is about 30 mm.

Rectangular bars (4 · 10 · 80 mm3) were cutfrom 4-mm injection-moulded plates along themould flow direction (MFD) for notched Charpyimpact test. A single-edge V-shape notch of 2 mmdepth and a tip radius of 0.25 mm were milled inthe middle of the rectangular bar. Prior to test thespecimens were conditioned at 65 �C for 8 h so asto eliminate the internal stress caused by processand possible moisture; and then they were kept atroom temperature in desiccator. The impact testwas conducted with an instrumented impact tester(AFS-MKs fractorscope of Ceast, Torino, Italy)according to ISO179-2 standard. A striker energyof 4 J, testing time of 4–8 ms and final velocity of2.9 m/s were applied. Impact tests were performedat �196, 23, 50 and 80 �C, respectively. The lowtemperature test was achieved by immersing speci-mens into liquid nitrogen more than 30 min andthen tested immediately (less than 5 s delay). Forthe tests above room temperature, specimens werebeforehand kept in an oven for 1 h and then quicklytested on the impact tester. Measurement was per-formed on at least six specimens and the averagevalues with standard deviations were reported.After impact tests, the fracture surfaces of speci-mens were coated with gold and observed by SEM(JEOL 5400). Some higher resolution images wereobtained by a SEM HITACHI S5200.

3. Results and discussion

3.1. Crystallization behaviour

It is well accepted that the crystallization charac-teristics such as crystallinity, spherulite size andstructure etc., have a profound influence on finalmechanical properties of semicrystalline thermo-plastics. Fig. 2 illustrates DSC heating and coolingcurves of the materials studied. It is seen that

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Fig. 2. DSC thermo analysis of PP and its composites: (a)heating and (b) cooling curves.


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1 vol.% MWNTs did not alter the melting point andthe degree of crystallinity (42–46%) of PP matrix(Fig. 2a and Table 1), but it increased crystallization

Table 1Thermal properties and impact strength of multi-walled carbon nanotu

Sample Volume content(%)

Tm (�C)a Xc (%)a Tg (�C)b

Neat PP 0 169.0 42.1 12.9

1MPP 1 169.5 46.0 11.5

1LPP 1 167.9 43.2 11.5

a Measured by DSC.b Measured by DMTA.

temperature by about 13 �C (Fig. 2b). This evi-dences that the nanotube may act as a nucleatingagent, which allows the crystallization process tooccur easier when melted PP is cooling down. Dueto the very similar crystallinity and numerous nucle-ating sites, the spherulite size in nanotube-basedcomposites should be much smaller than that ofthe neat PP. Indeed, Fig. 3 presents the typicalPOM micrographs of the neat PP and nanotube-based composites. Very large spherulites, rangingfrom c.a. 60 to 163 lm in diameter, were observedin the neat matrix, while the spherulites in the filledpolymer were too tiny to be detected by POM. Asimilar phenomenon was reported in PP/nano-CaCO3 systems by Zhang et al. [17].

3.2. Dynamic mechanical thermal analysis

Fig. 4 illustrates DMTA traces of the materials.The complex moduli were moderately enhanced inthe whole temperature range after addition ofMWNTs. Long tubes seemed to be more efficientin reinforcing matrix than short ones. The case isanalogous with fibre reinforced polymer systemsthat long fibre favours the load transfer betweentwo phases, and consequently leading to better stiff-ness of the composites. The stiffening effect wasmore remarkable at lower temperature, which wasconsistent with our previous finding in the nanosil-ica/epoxy systems [18]. This phenomenon wasexplained by the mismatch in coefficient of thermalexpansion between matrix and inorganic fillers,which might allow better stress transfer betweenmatrices and fillers at low temperatures. Besides,

be/polypropylene composites

Max. lossfactorb

Test temperature(�C)

Impact strength(kJ/m2)

0.074 �196 2.31 ± 0.2023 3.57 ± 0.0850 5.88 ± 0.2680 9.52 ± 0.51

0.073 �196 2.47 ± 0.1223 4.09 ± 0.2150 11.19 ± 0.4680 24.35 ± 0.49

0.071 �196 2.4 ± 0.0923 4.47 ± 0.1350 14.82 ± 0.7580 25.41 ± 1.73

Fig. 3. Typical polarized optical micrographs of (a) neat PP and(b) 1MPP.

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Fig. 4. Dynamic mechanical thermal analysis curves of lossfactor and dynamic complex modulus of PP and its composites.


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the glass transition temperatures and loss factorshad no substantial alternations after incorporationof nanotubes (summarized in Table 1 additionally).

3.3. Impact properties

Fig. 5 gives the impact force (and correspondingfracture energy) vs. test time curves of neat PP and1LPP. As expected, the impact behaviour washighly temperature-dependent. At �196 �C and23 �C, all materials performed a brittle manner,which was characterised with an abrupt breakageafter reaching peak values of impact load (Fig. 5aand b). Comparatively, obvious crack propagationtook place at 50 �C and 80 �C, therefore the testtime (or fracture deflection) was dramatically pro-longed (Fig. 5c and d). Actually, all specimens werenot completely broken at a temperature higher than50 �C. Fig. 6 shows that all materials performedbrittle-to-ductile transitions (BDT) near 23 �C,which approximated to the Tg of PP (around12 �C, measured by DMTA as given in Table 1).Obviously, when T P Tg, the increased mobility ofmacromolecular segments was of help to dissipateimpact energy, and consequently enhance theimpact resistance. Carbon nanotubes provided someadditional toughening effects, only when the testtemperature was higher than Tg. As given in Table1, the impact energy of 1LPP increased from 1.04to 2.67 times of that of neat PP, when the test tem-perature changed from �196 �C to 80 �C. Addition-ally, long nanotubes exhibited more contribution tofracture energy than short ones.

Furthermore, it is deduced from Fig. 5 that thefracture energy is practically dominated by two fac-tors: impact force and test time (or fracture deflec-tion). To analyse these factors separately wouldprovide insight into the toughening mechanisms ofnanotubes. Fig. 7 shows the variations of the peakimpact force, Fmax, and the maximum fracturedeflection, dmax, in the test temperature range. TheFmax is nearly linearly decreased with increasingtemperature. From �196 �C to 80 �C, the Fmax

reduces by 58% and 40% for neat PP and 1LPP,respectively (Fig. 7a). For nanocomposites, the rela-tively lower loss in Fmax is probably due to the rein-forcing effect of nanotubes. The dmax is, however,non-linearly increased with increasing temperature.All materials present similar turning points at tem-perature of 23 �C in dmax-temperature curves(Fig. 7b). From �196 �C to 80 �C, neat PP and1LPP obtained increases in dmax by 6.5 and 27.2times, respectively. On the basis of analysis, it couldbe concluded that for the nanotube-filled compos-ites, the improved impact resistance lies in two com-bining factors, i.e. a relatively higher Fmax and a

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Fig. 5. Curves of impact force (and fracture energy) as a function of test time for PP and 1LPP at various temperatures: (a) �196 �C, (b)23 �C, (c) 50 �C and (d) 80 �C.


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much superiorly increased dmax. The latter appearedmore important. Since the dmax is directly related tothe deformation of matrix, it is reasonable to inferthat at higher temperatures, the larger deformabilityof the nanotube-based composites is responsible forthe major improvement of impact resistance. Thisconclusion is further supported by SEM observa-tions, which will be discussed in the next section.

3.4. Fractography and fracture mechanisms

The typical SEM fractographs, as shown inFig. 8, can supply some useful information abouttoughening mechanisms of nanotubes in microscale.No significant difference of fracture surfacesbetween neat PP and 1LPP is observed at �196 �Cand 23 �C (Fig. 8a–d), while the difference tends to

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Fig. 6. Temperature dependence of notched Charpy impactstrength of PP and its composites.

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Fig. 7. Temperature dependence of (a) maximum impact loadand (b) maximum deflection of PP and its composites.


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become obvious at 80 �C (Fig. 8e and f). All samplessoftened at 80 �C, and therefore the fracture sur-

faces look more even, as compared to the veryrough fracture surfaces at lower temperature(Fig. 8a and b). There are numerous fibril-like orbulge-like structures on the fracture surfaces at80 �C, which are indicated by arrows in Fig. 8g.By contrast such structures are rarely observed atlower temperatures. We believed such a featuresuggested the enhanced deformability of the sam-ples. As compared to neat matrix, 1LPP shows rel-atively higher level of plastic deformation, becauseit has higher density of fibril-like or bulge-likestructures (cf. Fig. 8e and f). Due to the enhanceddeformability, the higher impact energy of nano-tube-filled PP appears reasonable. In addition, someagglomerates of nanotubes can still be found. Oneof them is highlighted by a black arrow in Fig. 8f,for example. Evidently, to completely break upthese agglomerates by conventional mechanicalextrusion is not easy. It is well agreed that agglom-erate the size of which is above a critical value isadverse to the impact resistance of the composites[19,20]. But, from the other angle, there is a greatpotential to enhance the polymer toughness by car-bon nanotubes if only a better dispersion can beachieved.

It was well understood [21] that for fibre-rein-forced polymer composites, fibres could toughenpolymer matrix through several energy-consumingevents, such as fibre fracture, fibre pullout and fibrebridging. In the present work, similar features ofcarbon nanotubes were also observed. Fig. 9 illus-trates a high resolution SEM image of cryogenicallyfractured surface of 1LPP, where nanotube fracture(A) and pullout (B) could be recognized. To discernthe nanotube fracture from the nanotube pulloutmainly lay on the lengths of left nanotubes, whichprotrude from the fracture surface. As seen inFig. 9, a majority of the left nanotubes have thelengths of several hundred nanometres. This valueseems to be close to the critical fracture length ofnanotubes, ranging from several tens to several hun-dreds nanometres, as reported in single nanotubefragmentation tests [22]. Some authors [4] alsofound if nanotubes were pulled out of matrices(chlorinated PP or PVA), the pullout length wasroughly equal to a quarter of the total nanotubelength. If the similar process (pullout) took placein our case, the length of the most left nanotubesshould be within micrometer scale. Based on abovediscussions, we considered temporarily that thenanotube breakage would be the dominant failuremode in the present system.

Fig. 8. Typical low magnification SEM fractographs after impact tests: (a) neat PP at �196 �C, (b) 1LPP at �196 �C, (c) neat PP at 23 �C,(d) 1LPP at 23 �C, (e) neat PP at 80 �C and (f) 1LPP at 80 �C, (g) a magnified image of the rectangular part in (f). An agglomerate ofnanotubes was highlighted by a black arrow in (f), as an example.


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Nanotube breakage and pullout can consumeadditional energy [23] (e.g. fracture energy of nano-tubes and nanotube-matrix frictional work). Fur-ther, the large aspect ratio of nanotubes wouldcause complex matrix-filler interaction during nano-tube breakage and pullout, which probably pro-

motes the locally plastic deformation of matrix.We surmised the nanotube-matrix interaction wouldbe a reason of the enhanced deformability of matrixat higher temperatures. Another reason is consid-ered to be the spherulite size, which will be discussedlater on. It is also observed that long nanotubes

Fig. 9. A high magnification SEM image of cryogenically fractured surface of 1LPP (1 vol.% CNT with a length of 5–15 lm).


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toughen the matrix more notably than the shortones (Fig. 6).

In addition to nanotube breakage and pullout,the spherulite size (cf. Section 3.1) seemed to be theother possible toughening factor, which allows thematrix to deform easier. Fig. 3 illustrates the spheru-lites of the nanotube-filled composites are muchsmaller than that of the neat PP. For a given degreeof crystallinity, larger spherulites are normallydetrimental to polymer toughness. As reported inthe previous studies [24,25], polymers with largerspherulites tend to be more brittle than a finely gran-ular structure. This is because the larger spherulitescan result in higher level of stress concentration,which induces premature fracture. Moreover, crackspropagate easily at the interfaces between largerspherulites. Therefore, it is reasonable to considerthat in the present work the tiny spherulites in nano-tube-based composites benefited the impact resis-tance of PP matrix partially.

3.5. Temperature-dependent toughening

It can be seen from Fig. 6 that both long andshort nanotubes hardly affected the impact behav-iour of the polymer at �196 �C; whereas, they cantoughen the matrix significantly at higher tempera-

tures. We may qualitatively explain this phenome-non by a balance of two competitive effects: (1)toughening effect due to nanotube breakage, pull-out, as well as smaller spherulite; (2) embrittlingeffect of nanotube aggregates. It is known that theaggregates larger than a critical size can act as struc-tural flaws that initiate brittle response. At a tem-perature below Tg, polymer segments are in a‘‘frozen’’ state, and the polymer matrix has very lim-ited deformability. With limited stress relaxation,even smaller aggregates can trigger brittle fractureand becomes dominant effect. On the contrary, withrise in the temperature (T P Tg), the embrittlingeffect of aggregates tends to be ineffective becauseof the easier relaxation of polymer segments (Inother words, the critical size of aggregates that trig-ger brittle response becomes larger at higher tem-peratures). Therefore the influence of nanotubebreakage, pullout as well as spherulite size becomesnotable. Based on above discussions, it could beinferred that the proper matrix deformability is aprecondition for the nanotube toughening. Ourexperimental phenomenon is also agreement withthe results obtained by Shah et al. [26]. In their casenanoclay can greatly toughen semicrystalline andamorphous polymers through nanoclay orientationunder applied stress field, if the polymer possessed


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sufficient mobility (T > Tg). However, if the mobil-ity of polymer was restricted (T < Tg), the nanoclaycan initiate crack formation and ultimately lead topoor toughness.

4. Conclusions

Based on experimental investigations of impactbehaviour of PP filled with multi-walled carbonnanotubes of different lengths, the following conclu-sions can be drawn:

1. Both shorter and longer nanotubes can greatlyimprove the impact energy of PP matrix at tem-peratures above Tg. The longer nanotubes exhib-ited higher toughening efficiency than the shorterones at a given temperature (above Tg).

2. The toughening effect was ascribed to theenhanced load-carrying ability and the much-increased deformability of nanotube-filled com-posites. The microcosmic toughening mechanismswould originate from nanotube pullout andbreakage, which accompanied by the consump-tion of fracture energy. In addition, the reducedspherulite size induced by nanotubes wouldfavour the impact resistance to some extent.

3. Nearly no toughening effect was obtained belowTg. This is due to the fact that the embrittlingeffect of the aggregates became active at lowertemperatures and balanced the aforementionedpositive toughening effects.

Acknowledgements

The authors appreciate the Key Item of theKnowledge Innovation Project of Chinese Academyof Sciences (Grant No. KJCX2-YW-M01) for finan-cial support. Z. Zhang is grateful to the Alexandervon Humboldt Foundation for his Sofja Kov-alevskaja Award, financed by the German FederalMinistry of Education and Research (BMBF) with-in the German Government’s ‘‘ZIP’’ program forinvestment in the future. Thanks to Dr. M. He(NCNST) and Dr. L. Song (Institute of Physics,CAS) for their kind assistance on TEM and SEMinspections.

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Impact behaviour of polypropylene filled with multi-walled carbon nanotubes