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Materials Chemistry and Physics 144 (2014) 98e106

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Materials Chemistry and Physics

journal homepage: www.elsevier .com/locate/matchemphys

Novel polypropylene/inorganic fullerene-like WS2 nanocompositescontaining a b-nucleating agent: Mechanical, tribological andrheological properties

Mohammed Naffakh a,*, Ana M. Díez-Pascual b, Carlos Marco b, Gary Ellis b

aUniversidad Politécnica de Madrid, Departamento de Ingeniería y Ciencia de los Materiales, Escuela Técnica Superior de Ingenieros Industriales,José Gutiérrez Abascal 2, 28006 Madrid, Spainb Instituto de Ciencia y Tecnología de Polímeros, ICTP-CSIC, c/ Juan de la Cierva, 3, 28006 Madrid, Spain

h i g h l i g h t s

* Corresponding author. Tel.: þ34 913363164; fax:E-mail address: mohammed.naffakh@upm.es (M.

0254-0584/$ e see front matter � 2014 Elsevier B.V.http://dx.doi.org/10.1016/j.matchemphys.2013.12.020

g r a p h i c a l a b s t r a c t

� b(NJ)/a(IF-WS2) dual-additive areused to produce advanced iPP NCPs.

� Novel dual strategy improves thethermal and mechanical perfor-mances of iPP.

� IF-WS2 is effective in reducing thefriction coefficient of iPP.

� The benefits of using dual-additivecompared to other nanoscale fillersare highlighted.

a r t i c l e i n f o

Article history:Received 12 June 2013Received in revised form12 December 2013Accepted 18 December 2013

Keywords:Composite materialsFullerenesMechanical testingTribology

a b s t r a c t

Environmentally friendly inorganic fullerene-like tungsten disulphide (IF-WS2) and N,N0-dicyclohexyl-2,6-naphthalene (NJ) as dual-additive mixtures are introduced into an iPP matrix to generate new ma-terials that exhibit variable a and b polymorphism. A traditional melt processing strategy was employedproviding a versatile, economic and scalable route to the development of novel composites withimproved properties. The effects of the relative concentration of NJ on the mechanical, tribological andrheological properties are investigated and the results are compared to those of binary reference systems,iPP/NJ and iPP/IF-WS2. In particular, we have found that the mechanical and tribological properties of thehybrids can be finely tuned by adjusting the b(NJ)/a(IF-WS2) ratio. The combined results of the differenttests demonstrate that the incorporation into iPP of the dual additive system, IF-WS2 (nanoparticles) andNJ (microparticles), provides a good balance between stiffness, strength, toughness and wear resistance,making these multiscale nanocomposites suitable for industrial applications.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Isotactic polypropylene (iPP) is one of the most versatile com-mercial thermoplastic materials available due to its low cost andbalanced properties such as low density, high heat distortion

þ34 913363007.Naffakh).

All rights reserved.

temperature (HDT) and good mechanical performance. It is apolymorphic material with four basic crystalline forms, namely themonoclinic (a), trigonal (b), orthorhombic (g) and mesomorphicsmectic (intermediate state between ordered and amorphousphase) forms, all sharing the same 3/1 helical conformation butwith different spatial arrangement of the polymer chains in thecrystal lattice [1e3]. The a-phase is thermodynamically the moststable predominating under normal processing conditions. It ex-hibits excellent modulus and tensile strength albeit poor fracture

M. Naffakh et al. / Materials Chemistry and Physics 144 (2014) 98e106 99

toughness. In contrast, the b-form exhibits improved ductility andimpact strength, especially at low temperatures, attributed to astress-induced transformation from the less dense b-phase to adenser a-phase that favours the absorption of impact energy [4,5].However, b-form is thermodynamically metastable and can only begenerated under special conditions such as quenching [6], abrupttemperature gradients [7], shearing or elongation deformation [8],or by the addition of specific b-nucleating agents (b-NAs) [9e14].

Several methods have been reported to improve iPP toughness,such as blending with different elastomers [15,16], copolymeriza-tion with other olefin monomers [17] and incorporation of b-NAs[9e14]. In general, these approaches lead to an enhancement in theimpact strength and HDT of the polymer, albeit at the expense ofthe stiffness. Therefore, other strategies that simultaneouslyimprove fracture toughness and stiffness, or at least maintain therigidity of the unmodified polymer are being explored. In this re-gard, the combination of a and b-NAs to enhance the overall me-chanical performance of this thermoplastic is of a great interestfrom the viewpoint of industrial applications, and the addition ofboth micro- and nano-scale particles has been reported [18,19] aswell as the case of nanoparticles with different geometry andcharacteristics [20]. In a similar strategy a direct relationship wasalso found between the synergistic efficiency of a b-NA/elastomercombination on the microstructure of PP [21]. Consequently, themechanical properties of the blends can be easily controlled, andsimultaneous increases in stiffness and fracture resistance can beachieved accordingly by adjusting the mass fraction of com-pounded NAs.

Three types of products have been commonly employed as b-NAs [22]: organic pigments (i.e. g-quinacridone and Indigosol),aromatic amide compounds (i.e. N,N0-dicyclohexyl-2,6-naphthalene dicarboxamide and N,N0-dicyclohexylterephthalamide) andGroup IIA metal salts or their mixtures with dicarboxylic acids (i.e.calcium carboxylate, calcium stearate and pimelic acid). Onthe other hand, the most widely employed a-NAs [23] aresorbitol derivatives and organic phosphate salts such as sodium2,20-methylene-bis(4,6-di-tert-butylphenyl) phosphate. Anothercommonly used strategy for improving the properties of iPP andreducing costs is the addition of other organic or inorganic fillers,particularly nanoparticles such as montmorillonite clays [23],CaCO3 [24], SiO2 [25], carbon nitrides [26], inorganic fullerene-liketungsten disulphide (IF-WS2) [27], etc., since their high surface-area-to-volume ratio results in a large contact area between thefiller and the surrounding polymer. Exceptionally, the incorpora-tion of IF-WS2 has been demonstrated as one of the most effectivestrategies to improve the thermal stability (by up to 60 �C) andstorage modulus of iPP due to the high nucleating efficiency ofthese nanoparticles on the crystallization of a-iPP [27]. In thisinvestigation, we have also provided evidence of the homogeneousdispersion of IF-WS2 in iPP, without the need for modifiers or sur-factants, using a conventional melt-processing method, which hasopened new possibilities to prepare other advanced polymercomposites employing these nanoparticles [28].

In previous studies [29,30], the crystallization and crystallinestructure of novel iPP/IF-WS2 nanocomposites incorporating N,N0-dicyclohexyl-2,6-naphthalene dicarboxamide (NJ) as a b-NA wereinvestigated by X-ray diffraction and differential scanning calo-rimetry. In particular, it was shown that the cooling rate and b(NJ)content are very efficient factors for the control of the dynamiccrystallization behaviour and the final crystalline structures innucleated iPP [30]. The presence of a highly b-nucleating agentplayed a key role during iPP crystallization even at low weight ra-tios, inducing a progressive inhibition of the a-nucleating effect ofIF-WS2 on the matrix. The content of b-crystals could be modulatedby adjusting the IF-WS2 and NJ composition ratio and controlling

the cooling rate. In this case, for a specific a-nucleating agentcontent (i.e. 1.0 wt.%), only a very low concentration of about0.05 wt.% was required to obtain a significant fraction of the crys-talline b-phase (54e67%). Further increase in the b-nucleant con-centration, progressively increased the fraction of b-phase topractically reach limiting values at 0.1 wt.% NJ (72e86%). Similarly,when the relative content of NJ is less than 20%, the nucleationefficiency (NE) value increases rapidly and then remains constantwith further increase in the ratio. This effect was also well corre-lated with the variation of the crystallization peak temperature (Tp)observed during the cooling process. The present paper focuses onthe mechanical, tribological and rheological characterization ofthese ternary hybrid composites. The influence of the relativeconcentration of NJ on the Young’s and storage moduli, tensilestrength, toughness, coefficient of friction, wear rate and viscosityof the nanocomposites is discussed in detail, and the results arecompared with those of a binary iPP/IF-WS2 reference sample.Interestingly, the mechanical and tribological properties of thehybrids can be finely tuned by adjusting the b (NJ)/a (IF-WS2) ratio.

2. Experimental section

2.1. Materials and processing

Isotactic polypropylene (iPP) was supplied by Repsol-YPF(Spain), with 95% isotacticity, a viscosity average molecularweight of 179,000 g mol�1 and a polydispersity of 4.77 [31]. Thetungsten disulfide (IF-WS2) nanoparticles (NanoLub�) were kindlyprovided by Nanomaterials (Israel), with dimensions in the range of40e200 nm (average value of 80 nm) and a mean aspect ratio of 1.4[27]. The b-nucleating agent N,N0-dicyclohexyl-2,6-naphthalenedicarboxamide (NJSTAR NU100), designated as NJ, was kindlyprovided by RIKA International Limited (UK).

Composites with different weight fractions of IF-WS2 (a) and NJ(b) were introduced into the iPP matrix by melt-mixing using amicro-extruder (Thermo-Haake Minilab system) operating at210 �C for 15 min with a rotor speed of 150 rpm. To prevent anymatrix degradation, the processing temperature was selected wellbelow the initial degradation temperature of the matrix [27]. Toimprove the dispersion of both fillers within the matrix, the IF-WS2nanofiller was first incorporated into the polymer and mixed for10 min. Subsequently, NJ was added and the mixing continued foranother 5 min. This strategy yields finer dispersion, with IF-WS2almost fully debundled into individual nanoparticles or smallclusters, which are randomly dispersed in the iPP matrix (see SEMdata in the Results section). Concentrations of NJ of 0.05, 0.25, 0.5and 1.0 wt.% were employed, whilst the IF-WS2 content wasmaintained at 1.0 wt.% (relative NJ content ¼ NJ (wt.%)/[NJ(wt.%) þ IF-WS2 (wt.%)] ¼ 0.05, 0.2, 0.33 and 0.5, respectively). Forclarity, the resultant material concentrations were: 98.95/1.0/0.05,98.75/1.0/0.25, 98.5/1.0/0.50 and 98/1.0/1.0 wt.%. Moreover, neatiPP and two binary reference systems, iPP/IF-WS2 (1.0 wt.%) andiPP/NJ (0.1 wt.%), were processed under the same conditions. Thesamples were subsequently compression molded as films using ahot-press system (Collin) with two pairs of heating/cooling plates.

2.2. Characterization techniques

Dynamic mechanical analysis was performed in the tensilemode using a Mettler DMA861 on rectangular shaped bars of di-mensions 20 � 4 � 0.5 mm3 in the temperature rangebetween �100 and 120 �C at a heating rate of 3 �C min�1 and fre-quencies of 0.1, 1 and 10 Hz. A dynamic force of 6 N oscillating atfixed frequency, and an amplitude of 30 mm was used.

0 10 20 30 40 50 600

10

20

30

40

50

60

(MPa

)

(1) iPP(2) 1.0 wt.% IF-WS2(3) 1.0/0.05(4) 1.0/0.25(5) 1.0/0.5(6) 1.0/1.0(7) 0.1 wt.% NJ

1

2

3

456

7

Fig. 1. Representative stressestrain curves for neat iPP, the binary reference systemsand the ternary nanocomposites with different IF-WS2/NJ wt.% content.

M. Naffakh et al. / Materials Chemistry and Physics 144 (2014) 98e106100

Tensile and flexural properties weremeasured with an INSTRON4204 mechanical tester at room temperature and 50� 5% RH, usinga crosshead speed of 1 mm min�1 and a load cell of 1 kN. Tensilespecimens (Type V) and rectangular flexural coupons wereemployed, according to UNE-EN ISO 527-1 and 178 standards,respectively. All samples were conditioned for 24 h prior to mea-surement. The data reported are the average of the results obtainedfrom 5 specimens.

Scanning electronmicroscopy (SEM) images were obtained witha Philips XL30 ESEM scanning electron microscope to investigatethe morphology of the iPP/IF-WS2/NJ nanocomposites. Cryogeni-cally fractured surfaces from film specimens were coated with aw5 nm Au/Pd overlayer to avoid charging during electronirradiation.

Charpy notched impact strength measurements were carriedout using a CEAST Fractovis dart impact tester. A hammer mass of1.096 kg impacted at a constant velocity of 3.60 m s�1 and an en-ergy of 7.10 J on a notched specimen of 33 � 10 � 3 mm3, with a V-shape notch of tip radius ¼ 0.25 mm and depth ¼ 2 mm, asdescribed in the UNE-EN ISO 179 standard. Measurements wereperformed at 23 � 2 �C and 50 � 5% RH. At least 6 specimens weretested for each type of composite to check reproducibility.

Tribological properties were measured with a Microtest MT400-98 pin-on-disk testing machine, using a 6 mm diameter100Cr6 steel ball like pattern slide. Tests were carried out under aconstant load of 5 N at a rotation speed of 375 rpm. The equipmentwas placed under controlled atmospheric conditions, and the ex-periments performed under air at a temperature of 24 � 2 �C and22 � 2% RH. The wear experienced by the flat substrate was

Table 1Mechanical parameters derived from the room temperature tensile tests of iPP/IF-WS2/NJsb: tensile strength at break, 3b: strain at break, A: area under the curve.

IF-WS2 content(wt.%)

NJ content(wt.%)

RelativeNJ contenta

Tensile tests

E (GPa) sy (M

0.0 0.00 0.00 1.3 � 0.1 41.41.0 0.00 0.00 1.8 � 0.1 57.91.0 0.05 0.05 1.5 � 0.3 50.71.0 0.25 0.20 1.3 � 0.1 43.81.0 0.50 0.33 1.4 � 0.1 45.11.0 1.00 0.50 1.4 � 0.1 46.60.0 0.10 1.00 1.2 � 0.1 39.3

a NJ (wt.%)/[NJ (wt.%) þ IF-WS2 (wt.%)].

determined through the measurement of the wearetrack profile byusing a profilometer with a resolution ofw10 nm. Each tribologicaltest was repeated three times and an average value reported.

The surface free energy of the polymeric films was investigatedvia contact angle measurements using the sessile drop method.Experiments were performed at room temperature with a KrüssG10 contact angle measurement system, using two liquids withdifferent polarity: water (g ¼ 72.8 mN m�1) and diiodomethane(g ¼ 50.8 mN m�1). Contact angles were measured by droppingwith a syringe ten drops (w0.5 ml) of each liquid at different posi-tions on the solid surface. The image of the drop was captured byvideo camera connected to a computer, and the imagewas analysedvia software to calculate the contact angle. Finally, the surface en-ergy of the samples was determined using the OwenseWendteRabeleKaelble equation [32]:

ð1þ cos qÞs1 ¼ 2�sds s

dl

�1=2 þ 2�sps s

pl

�1=2(1)

where q is the measured contact angle, gl is the total surface energyof the liquid in contact with the solid, gl

d and glp are the dispersion

and polar components of the liquid, gsd and gs

p are the dispersionand polar components of the solid surface, respectively. For thecalculations, the surface energies of the liquids used for contactangle tests were used (water: glp¼ 51.0mNm�1; gl

d¼ 21.8 mNm�1;diiodimethane: gl

p ¼ 2.3 mN m�1; gld ¼ 48.5 mN m�1).

Dynamic shear viscosities were obtained with a TA InstrumentsAR 1000 rheometer using a parallel plate system with 25 mmdiameter and 0.75 mm gap. Samples were tested at 200 �C in ni-trogen, after drying under vacuum at 100 �C to completely removemoisture. Frequency sweeps from 0.02 to 500 rad s�1 were per-formed within the linear viscoelastic region of the materials.

3. Results and discussion

3.1. Tensile behaviour

Representative tensile curves for neat iPP and the differentcomposites are presented in Fig. 1. The values of Young’s modulus(E), tensile strength (sy), strain at yield ( 3y), strength at break (sb)and strain at break ( 3b) derived from the stressestrain curves arecollected in Table 1, and plotted as a function of the relative NJcontent in Fig. 2. Neat iPP shows a Young’s modulus of about1.3 GPa. The addition of 1.0 wt.% IF-WS2 leads to a significant in-crease in E (w38%), since these nanoparticles behave as a-NAs thatare reported to improve the stiffness of iPP [22]. In contrast, thebinary iPP/NJ (0.1 wt.%) sample exhibits a slight drop in E comparedwith neat iPP, since b-NAs are known to decrease the modulus ofthis thermoplastic [22]. It can be observed from Fig. 2a that theincorporation of very small amounts of NJ to the binary iPP/IF-WS2(1.0 wt.%) nanocomposite leads to a decrease in E of around 24% for

nanocomposites. E: Young’s modulus, sy: tensile strength at yield, 3y: strain at yield,

Pa) 3y (%) sb (MPa) 3b (%) A (a.u.)

� 1.1 10.4 � 0.3 32.1 � 1.0 24.5 � 0.8 805 � 32� 1.3 6.1 � 0.3 43.1 � 1.2 16.4 � 0.9 730 � 25� 0.6 9.0 � 0.3 37.5 � 0.8 32.9 � 1.2 1270 � 44� 1.5 9.8 � 0.2 31.3 � 0.7 49.6 � 2.2 1650 � 55� 0.9 8.0 � 0.3 33.4 � 0.9 41.8 � 3.4 1504 � 52� 0.6 7.2 � 0.2 35.0 � 1.3 37.9 � 1.6 1423 � 38� 0.8 11.1 � 0.3 27.8 � 1.1 59.5 � 3.1 1812 � 65

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.01.0

1.2

1.4

1.6

1.8

2.0

You

ng´s

mod

ulus

(GPa

)

NJ (wt.%)/[NJ (wt.%)+ IF-WS2 (wt.%)]0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

36

40

44

48

52

56

60

Flex

ural

mod

ulus

(GPa

)

NJ (wt.%)/[NJ (wt.%)+ IF-WS2 (wt.%)]

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.08

16

24

32

40

48

56

64

Stra

in a

t bre

ak (%

)

NJ (wt.%)/[NJ (wt.%)+ IF-WS2 (wt.%)]

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.05

6

7

8

9

10

11

12

Stra

in a

t yie

ld (%

)

NJ (wt.%)/[NJ (wt.%)+ IF-WS2 (wt.%)]

)b()a(

)d()c(

Fig. 2. Mechanical parameters derived from the tensile curves as a function of the relative NJ concentration: (a) Young’s modulus E, (b) tensile strength sy, (c) strain at break 3b and(d) strain at yield 3y.

M. Naffakh et al. / Materials Chemistry and Physics 144 (2014) 98e106 101

the sample with 0.2 NJ relative content. However, furtherincreasing the NJ concentration results in a smaller decrease inmodulus, reaching a value of around 1.4 GPa (w20% decreasecompared to iPP/IF-WS2 (1.0 wt.%)) at 0.5 NJ relative content. Theseresults clearly indicate the existence of a minimum in the Young’smodulus of the ternary system at a critical NJ relative concentrationof 0.2. Qualitatively similar trend is found for the tensile strength,which rises from w41 MPa for the unmodified polymer up to58 MPa for the binary iPP/IF-WS2 (1.0 wt.%) system, whereas thereference iPP/NJ (0.1 wt.%.) shows lower sy than neat iPP. Theincorporation of 0.25 wt.% NJ to iPP/IF-WS2 (1.0 wt.%) decreases syto a minimum value of w44 MPa (Fig. 2b) and then increases againup to w47 MPa at 1.0 wt.% NJ concentration. Moreover, sb valuesdisplay an analogous behaviour, mirroring the sy data, with aminimum among the ternary nanocomposites at the aforemen-tioned NJ critical concentration.

On the other hand, the strain at break and strain at yield show acompletely opposite behaviour, as shown in Fig. 2c and d, respec-tively. The addition of 1.0 wt.% IF-WS2 to neat iPP decreases 3b byabout 33%, in agreement with the fact that a-iPP shows inferiorductility because of the interlocking effect of the radial lamellae bythe tangential crystallites that makes the plastic deformation verydifficult [33]. In contrast, iPP/NJ (0.1 wt.%) exhibits around 2.4 timeshigher 3b than the neat polymer since b-NAs are reported toimprove the matrix ductility [34]. It can be observed in Fig. 2c that3b as a function of the relative NJ concentration passes through adefined maximum. It increases dramatically by around 3 timesfrom the binary iPP/IF-WS2 (1.0 wt.%) sample to the criticallynucleated ternary composite, and then decreases sharply to a valueof 38% strain at break for the hybrid with the highest NJ concen-tration. This enhanced ductility of the nanocomposite with a rela-tive concentration of NJ of 0.2 suggests extensive drawing due tothe presence of the crystalline b-phase [34], which promotes the

initiation and propagation of the plastic deformation. Thus, themaximum of 3b among the ternary nanocomposites coincides withthe maximum of 3y (Fig. 2d), which increases from 6.1% for thebinary iPP/IF-WS2 to 9.8% for the nanocomposite with the critical NJconcentration, and then drops again at higher NJ loadings. Never-theless, the highest 3y (w11%) corresponds to the binary iPP/NJ(0.1 wt.%) composite. Moreover, the area under the tensile curves(Table 1), which is a measure of the toughness of the nano-composites, also rises dramatically (w2.3 fold increase) with theincorporation of 0.25 wt.% NJ to the iPP/IF-WS2 (1.0 wt.%). Beyondthis concentration it decreases slightly and monotonically, beingonly about 1.9 fold higher than neat iPP at a relative NJ content of0.5. As expected, the largest area was found for the referencesample, iPP/NJ (0.1 wt.%.). It should be noted that the minima in Eand sy among the ternary nanocomposites clearly correlates withthe maximum of 3b, 3y and the area under the tensile curve. Asshown in our previous work [29], a relative NJ content of 0.2 rep-resents a critical concentration for obtaining a composite contain-ing predominantly b-crystals (59e79%). Further increments in theNJ loading lead to only slight increases in the b-phase content. It hasbeen reported [35] that the presence of the b-crystals induces abetter continuity of the amorphous phase and more connectingbridges between individual crystallites than the material contain-ing only a-crystals. Thus, higher thickness of the amorphous layerwith many interconnecting chains facilitates plastic deformationthat is reflected in higher ductility and toughness.

SEM analysis on cryogenically fractured samples was under-taken in order to correlate the mechanical properties of the nano-composites with their microstructure. As an example, arepresentative image from the binary iPP/IF-WS2 (1.0 wt.%) nano-composite is compared to that of ternary systems with relative NJcontents of 0.05, 0.2 and 0.33 (Fig. 3). The good and relatively ho-mogeneous distribution of the nanoparticles that was described

Fig. 3. SEM micrographs of (a) iPP/IF-WS2 (1.0 wt.%); (b) iPP/IF-WS2 (1.0 wt.%)/NJ (0.25 wt.%); (c) iPP/IF-WS2 (1.0 wt.%)/NJ (0.05 wt.%); (d) iPP/IF-WS2 (1.0 wt.%)/NJ (0.5 wt.%). Thearrows indicate IF-WS2 nanoparticles dispersed within the iPP matrix.

M. Naffakh et al. / Materials Chemistry and Physics 144 (2014) 98e106102

earlier [27] can be clearly observed. Further, in the case of the bi-nary nanocomposite only a-spherulites with a radial lamellar ar-chitecture were observed, as is also the case for neat iPP [36]. Smalla-spherulites are known to improve the optical and mechanicalproperties of iPP [22]. Very similar morphology is found for thenanocomposite with 0.05 wt.% NJ, where a-spherulites clearlypredominate. In contrast, the nanocomposite containing 0.25 wt.%NJ displays a very uniform structure with well developed small b-spherulites that manifest a flower-like morphology with a curvedlamellar arrangement. The distribution of the amorphous regionswithin these b-spherulites favours plasticity and toughnessenhancement [37]. The nanocomposites with NJ content higherthan 0.5 wt.% are also rich in b-spherulites, although the amor-phous regions inside these spherulites are thinner compared tothose of the sample with a relative NJ content of 0.2. It has beensuggested [37] that the thickest amorphous interlayers are formedat a critical b-NA content, which in our ternary system appears to bew0.25 wt.% NJ. Therefore, it seems that the microstructure of thenanocomposites, and in turn the mechanical properties, can betuned by modifying the NJ concentration.

Table 2Mechanical parameters obtained from the flexural and Charpy impact tests of iPP/IF-WS2Echarpy: impact strength.

IF-WS2 content(wt.%)

NJ content(wt.%)

Relative NJcontenta

Flexur

Ef (GP

0.0 0.00 0.00 1.7 �1.0 0.00 0.00 2.2 �1.0 0.05 0.05 2.0 �1.0 0.25 0.20 1.7 �1.0 0.50 0.33 1.8 �1.0 1.00 0.50 1.8 �0.0 0.10 1.00 1.6 �a NJ (wt.%)/[NJ (wt.%) þ IF-WS2 (wt.%)].

3.2. Flexural properties

From the viewpoint of industrial applications it is important toinvestigate the effects of both nucleating agents on the flexuralproperties of iPP, and the results are collected in Table 2. When1.0 wt.% IF-WS2 was incorporated in iPP the flexural modulus (Ef)and strength (sfm) improved by about 33 and 17%, respectively,confirming that the monoclinic a-form crystals are indeed effectivein enhancing the flexural behaviour of the polymer. However, theaddition of 0.1 wt.% NJ to iPP results in a small drop in both pa-rameters, in agreement with the results reported previously [22].The inclusion of very small amounts of NJ (�0.25 wt.%) to iPP/IF-WS2 (1.0 wt.%) leads to a decrease in Ef and sfm, both reaching aminimum when the relative NJ content is 0.2, and a slight rise isobserved at higher NJ loadings. However, the change in stiffness ismore significant than in strength. At 0.2 NJ relative concentrationthe value of Ef is about 22% lower than that of the reference iPP/IF-WS2 sample, whilst the decrease in sfm is around 15%, withapproximately the same value as neat iPP. The results obtainedfollow similar trends to those described for the Young’s modulus

/NJ nanocomposites. Ef: Flexural modulus, sfm: flexural strength, F: impact force and

al tests Charpy impact tests

a) sfm (MPa) F (N) Echarpy (kJ m�2)

0.1 52.7 � 1.7 420 � 20 3.2 � 0.20.1 61.6 � 1.2 320 � 10 2.8 � 0.20.1 57.4 � 0.9 550 � 20 4.7 � 0.30.1 53.5 � 1.3 570 � 60 6.8 � 0.20.1 54.8 � 0.7 520 � 30 6.3 � 0.30.1 53.9 � 0.6 460 � 20 5.9 � 0.30.1 51.7 � 0.8 580 � 40 7.1 � 0.4

-100 -75 -50 -25 0 25 50 75 100 1250.00

0.02

0.04

0.06

0.08

0.10

0.12 iPP1.0 wt.% IF-WS1.0/0.051.0/0.251.0/0.51.0/1.00.1 wt% NJ

T (ºC)

(a)

(b)

-100 -75 -50 -25 0 25 50 75 100 1250

1

2

3

4

5

Stor

age

mod

ulus

E´ (

GPa

)

T (ºC)

iPP1.0 wt.% IF-WS1.0/0.051.0/0.251.0/0.51.0/1.00.1 wt.% NJ

Fig. 4. (a) Storage modulus E0 and (b) tan d as a function of temperature for neat iPP,the reference systems and the nanocomposites with different IF-WS2/NJ wt.% content.

M. Naffakh et al. / Materials Chemistry and Physics 144 (2014) 98e106 103

and tensile strength, and the opposite tendency to that of the strainat break and toughness, but all these parameters show a singularityat the critical NJ concentration of around 0.25 wt.%. Zhao et al. [22],using a barium salt of hexahydrophthalic acid (HHPA-Ba) as a b-NA,also found a minimum in the flexural properties of iPP for 0.2 wt.%loading that corresponded to a b-phase content of around 76%,indicating that this was a saturated concentration for iPP nucleatedwith HHPA-Ba.

3.3. Charpy impact tests

It is well known that Charpy notched impact strength mea-surements can directly reflect the toughness, and hence these wereundertaken at room temperature and compared with the resultsobtained from tensile tests in Table 2. The impact strength of the a-nucleated binary nanocomposite is about 12% lower than that ofneat iPP (w3.2 kJ m�2), confirming that the IF-WS2 reduce thepolymer toughness. Meanwhile, the b-nucleated iPP (0.1 wt.% NJ)displays about 2.2 fold increase in toughness, attributed to energydissipation phenomena favoured in the presence of the b-crystals[34]. Regarding the ternary nanocomposites, the addition of smallamounts of b-NA leads to a considerable improvement in theimpact strength, by 140% at 0.25 wt.% NJ concentration in com-parison to the reference iPP/IF-WS2 (or 110% increment in relationto neat iPP). With further increases in the NJ concentration theimprovement in impact strength is slightly smaller, being around105% at NJ relative content of 0.5. This dependence of the impactstrength on the b-NA content is in agreement with the analysis ofthe areas under the tensile curves, since both techniques are suit-able for estimating this property. However, the results from theimpact tests show a much lower increase in toughness comparedwith the data derived from the stressestrain curves. These dis-crepancies have been previously reported [36,38], and are mostlikely to be caused by the difference in the failure mechanism be-tween both types of experiments, since the cross-head speed in thetensile test is lower than in the impact tests, and the loading rateplays a key role in the failure mechanism. The combined results ofboth tests demonstrate that the incorporation of the dual additivesystem IF-WS2 (nano) and NJ (micro) in iPP provides a good balancebetween stiffness and toughness, making these hybrid nano-composites suitable for industrial applications. In particular, theternary iPP/IF-WS2 (1.0 wt.%)/NJ (0.25 wt.%) sample shows theoptimum mechanical properties, combining a moderate loss instiffness and strength with an exceptional increase in ductility andtoughness.

3.4. Dynamic mechanical properties

The mechanical properties of these nanocomposites as a func-tion of temperature were investigated using DMA, which providesinformation about the transitions and relaxation behaviour of thematrix. Fig. 4a shows the evolution of the storage modulus E0 vs.temperature for the different samples investigated in this work atthe frequency of 1 Hz, and the values of E0 are displayed as afunction of temperature in Table 3. The storage modulus of all thesamples decreases progressively with increasing temperature,showing a steeper decay in the range �10 to 30 �C correspondingwith the glass transition of the materials. The addition of IF-WS2 toiPP leads to an increase in E0 over the whole temperature range,indicating an enhanced stiffness. In particular, E0 at 25 �C increasesby about 34% at 1.0 wt.% loading, in good agreement with the re-sults reported previously [27]. On the other hand, iPP/NJ (0.1 wt.%)exhibits a E0 about 12% lower than that of the neat polymer,consistent with the modulus drop observed in the tensile andflexural tests. As expected, the addition of small amounts of NJ to

iPP/IF-WS2 results in a fall in E0 over the whole temperature range,being more significant at temperatures above the Tg. Thus,at �100 �C, the ternary nanocomposite with 0.2 relative NJ contentexhibitsw16% lower modulus than the binary iPP/IF-WS2, whereasthe decrease is about 30% at 120 �C. These differences could beexplained considering the changes in the size and lamellararrangement of the b-spherulites that can occur at high tempera-tures [37]. Another interesting feature is that the hybrid with 0.2relative NJ content once again shows at 25 �C the lowest E0 amongthe ternary nanocomposites, consistent with the lowest stiffness ofthis sample as revealed by tensile and flexural tests, whereas attemperatures above Tg the modulus decreases slightly and pro-gressively with increasing NJ concentration, and remains almostconstant for a relative NJ content >0.2.

On the other hand, the loss tangent (tan d) for iPP and thecomposites as a function of temperature is shown in Fig. 4b. Theposition of the tan d peak is slightly affected by the fillers; theincorporation of 1.0 wt.% IF-WS2 increases Tg of the polymer byabout 6 �C (Table 3), consistent with the fact that the nanoparticlesrestrict the mobility of the chain segments in the amorphous re-gion. However, the addition of NJ to this binary system results in aslight decrease in Tg, since the chain segments in the amorphousregion of b-iPP are more mobile than in a-iPP [37]. The binary iPP/NJ composite exhibits the lowest Tg value, consistent with a higherb-iPP content [29].

Another interesting parameter is the variation of the area underthe tan d peak with filler concentration, since it is representative of

Table 3Parameters derived fromDMA tests of iPP/IF-WS2/NJ nanocomposites. E0: Storagemodulus, Tg: glass transition temperature, tan dmax: maximum of tan d peak, Atand: area undertan d peak.

IF-WS2 content(wt.%)

NJ content(wt.%)

RelativeNJ contenta

DMA tests

E0�100 �C (GPa) E025 �C (GPa) E0120 �C (GPa) Tg (�C) tan dmax Atand (a.u.)

0.0 0.00 0.00 3.8 1.4 0.1 4.0 0.05 0.61.0 0.00 0.00 5.1 1.9 0.3 9.9 0.04 0.51.0 0.05 0.05 4.5 1.7 0.3 9.3 0.06 0.81.0 0.25 0.20 4.3 1.5 0.2 7.2 0.06 1.01.0 0.50 0.33 4.1 1.5 0.2 5.9 0.06 0.81.0 1.00 0.50 4.0 1.5 0.3 4.3 0.06 0.70.0 0.10 1.00 3.7 1.2 0.2 2.8 0.07 1.1

a NJ (wt.%)/[NJ (wt.%) þ IF-WS2 (wt.%)].

0.18

0.20

0.22

0.24

0.26

Coe

ffic

ient

of f

rict

ion

(a)

M. Naffakh et al. / Materials Chemistry and Physics 144 (2014) 98e106104

the total energy dissipated in viscoelastic relaxations, and hence isan indication of the impact toughness of the composite. Thesevalues are collected in Table 3. It can be observed that the incor-poration of the IF-WS2 leads to a decrease in this area, in agreementwith the results obtained from Charpy impact tests (Table 2) andtensile tests (Table 1). As expected, the addition of NJ increases thearea under the tan d peak indicating improved toughness, and themaximum value among the ternary nanocomposites once againcorresponds with a relative NJ content of 0.2, the critical nucleatingconcentration in this system. The binary iPP/NJ (0.1 wt.%) samplewith the highest b-phase content exhibits the largest area, againconfirming that the trigonal b-form is favourable and effective fortoughening iPP. Moreover, broadening of the loss tangent peak canalso be observed for the ternary nanocomposites (the full-width athalf maximum of iPP/IF-WS2 (1.0 wt.%)/NJ (0.25 wt.%) is about 40%higher than that of neat iPP), which could be attributed to a moreinhomogeneous amorphous phase in the nanocomposites than inthe neat matrix. Regarding the maximum energy dissipated duringthe viscoelastic relaxation, tan dmax (Table 3), the trend observed isqualitatively similar to that found for the areas, and the compositeswith larger amount of b-crystals display higher tan dmax, reflectingtheir enhanced damping capacity. The results confirm that theincorporation of the b-NA is beneficial for simultaneouslyimproving the impact toughness and the damping capacity.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00.14

0.16

NJ (wt.%)/[NJ (wt.%)+ IF-WS2 (wt.%)]

(b)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.02.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

Wea

r ra

te k

104 (m

m3 / N

m)

NJ (wt.%)/[NJ (wt.%)+ IF-WS2 (wt.%)]

Fig. 5. Tribological properties of iPP/IF-WS2/NJ nanocomposites as a function of therelative NJ content: (a) Coefficient of friction m; (b) specific wear rate k.

3.5. Tribological properties

Within the context of the potential of these nanocomposites forindustrial use, it is interesting to analyse their tribological proper-ties. Fig. 5a shows the mean coefficient of friction m as a function ofthe relative NJ content. The value of m for neat iPP is about 0.23, anddecreases by approximately 26% with the addition of 1.0 wt.% IF-WS2. This behaviour can be explained in terms of a lubricating ef-fect of these nanoparticles that is known to reduce the coefficient offriction of thermoplastic polymers such as Nylon [39], poly-phenylene sulphide (PPS) [40] and polyetheretherketone (PEEK)[40]. The shearing force can be effectively decreased owing to thelower shearing strength of the lubricant particles that, combinedwith the increase in heat conductivity hence lower temperature inthe sliding contact, result in lower m values. In contrast, the refer-ence iPP/NJ (0.1 wt.%) system displays slightly higher m than theneat polymer, probably due to the higher degree of ductility foundfor this composite that facilitates indentation of the counterpartinto the matrix. Regarding the ternary nanocomposites, an increasein m compared to the binary iPP/IF-WS2 sample is observed, andreaches a maximum for the sample with a relative NJ content 0.2,showing a value of m only about 6% lower than that of neat iPP.At higher NJ loadings, a slight drop in m is observed in agreementwith the smaller decrease in modulus and strength in relation toiPP/IF-WS2.

The wear rate, k was also analysed for iPP and the compositesand presented in Fig. 5b. Upon addition of IF-WS2 the wear resis-tance of the polymer was considerably enhanced. The binarycomposite shows a reduction in k of around 65% in comparison tothat of neat iPP (w6.3 � 10�4 mm3 Nm�1). This enhancement canbe attributed to several factors, including the improvement in themechanical properties (modulus and strength) and the aforemen-tioned lubricant effect. Therefore, it is confirmed that IF-WS2nanoparticles are ideal tribological additives for thermoplasticpolymers. The incorporation of the b-NA leads to an increase in k inrelation to iPP/IF-WS2 (1.0 wt.%), which should be related to the

10-2 10-1 100 101 102 103

102

103

iPP1.0 wt.% IF-WS1.0/0.051.0/0.251.0/0.51.0/1.00.1 wt.% NJ

Com

plex

vis

cosi

ty

(Pa.

s)

(rad/s)

Fig. 7. Complex viscosity h as a function of frequency u at 200 �C for neat iPP, thereference systems and the nanocomposites with different IF-WS2/NJ wt.% content.

M. Naffakh et al. / Materials Chemistry and Physics 144 (2014) 98e106 105

decrease in stiffness and strength observed for the hybrid nano-composites, and once again the sample with 0.2 relative NJ contentshows the worst wear resistance among these ternary nano-composites. However, it is important to note that the wear rateobtained for the different iPP/IF-WS2/NJ formulations is in therange of 52e40% lower than that of neat iPP, which is a highlysignificant improvement in the tribological performance of thispolymer.

3.6. Contact angle measurements

Taking into account the lubricating character of the IF-WS2, it isimportant to evaluate the influence of both types of fillers on thesurface energy of the matrix. The total surface energy g and itspolar and dispersion components for neat iPP and the nano-composites were calculated from contact angle measurementsfollowing the OwenseWendteRabeleKaelble equation [32], andthe results and plotted in Fig. 6. For neat iPP g is about 32 mN m�1,and decreases by approximately 10% upon addition of 1.0 wt.% IF-WS2, which is consistent with the lubricating character of thesenanoparticles and the improvement in tribological properties, asdiscussed previously. An analogous drop is found in the dispersivepart, which falls from around 30 to 27 mN m�1, whereas the polarcontribution remains practically constant. The latter is very low forboth neat iPP and iPP/IF-WS2 composite (w2 mN m�1) due to theabsence of polar groups. On the other hand, the reference iPP/NJ(0.1 wt.%) exhibits a value of g similar to that of unmodified iPP.Regarding the ternary nanocomposites, only small changes areobserved (differences<6%) when compared with the binary iPP/IF-WS2 sample, showing slightly higher g and dispersive contributionthan this binary reference system. This could be related to thereduction in the interlocking effect of the radial lamellae due to thedecrease in the a-phase fraction caused by the presence of the b-NA, since changes in mechanical interlocking can modify the sur-face energy of the composite [41]. Moreover, a small rise in thepolar contribution is observed, since NJ contains amide groups thatincrease the degree of hydrophilicity of the nanocomposites.

3.7. Rheological behaviour

Further investigation on the properties of the nanocompositeswas performed by rheological measurements, which are important

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.01.1

2.2

27

28

29

30

31

32

33

NJ (wt.%)/[NJ (wt.%)+ IF-WS2 (wt.%)]

Total surface energy Dispersive contribution Polar contribution

Surf

ace

ener

gy

(mN

/m)

Fig. 6. Surface energy g of iPP/IF-WS2/NJ nanocomposites as a function of the relativeNJ content.

for optimizing processing conditions of polymeric materials. Fig. 7shows the frequency dependence of the complex viscosity h forneat iPP and the different composites at 200 �C. The unmodifiedpolymer and the reference iPP/NJ (0.1 wt.%) present a Newtonianbehaviour, while for all the composites incorporating IF-WS2 theviscosity decreases progressively with increasing frequency, indi-cating non-Newtonian and shear thinning behaviour over thefrequency range studied. The value of h of iPP decreases with theaddition of these nanoparticles, and the degree of reduction ismore significant at low frequency, with around a 3-fold decreasefor iPP/IF-WS2. An analogous behaviour in the reduction of themelt viscosity has been reported for silica-reinforced poly(ethylene2,6-naphthalate) (PEN) [42] and poly(ethylene terephthalate) (PET)[43] as well as nylon-6/nanoclay [44] composites. This behaviourhas been previously observed for PPS and PEEK composites rein-forced with IF-WS2 [40], and was ascribed to slippage between thepolymer and the spherical and/or smooth fillers, which act as ballbearing-like agents reducing the coefficient of friction, as demon-strated by tribological tests, thus facilitating the flow of the poly-mer chains. Other factors that could influence the rheologicalbehaviour of the composites could be changes in the number ofphysical entanglements between the polymer chains or in the freevolume due to the addition of the nanofillers [40]. Nevertheless,since the Tg of the matrix phase in the nanocomposites is higherthan that of neat iPP, as revealed by DMA analysis, the effects ofreduced entanglement and/or increased free volume on the vis-cosity can be excluded. The reference iPP/NJ (0.1 wt.%) displayssimilar viscosity to neat iPP, whereas the ternary nanocompositesexhibit higher h values than iPP/IF-WS2 over the whole rangestudied, particularly at low frequencies, and the differencesbecome larger with increasing NJ loading. Thus, iPP/IF-WS2(1.0 wt.%)/NJ (0.05 wt.%) shows only slightly higher h than the bi-nary nanocomposite, while iPP/IF-WS2 (1.0 wt.%)/NJ (1.0 wt.%)presents about 1.6 fold increase at the lowest frequency. This risecould be related to the polar nature of the b-NA due to the presenceof amide groups, since NJ molecules can interact through hydrogenbonding, leading to stronger interactions between the polymerchains. An analogous viscosity increment phenomenon has beenobserved in PP by the addition of low contents of a b-NA with twohydroxyl groups [45]. The results from the rheological testsconfirm that the incorporation of both fillers reduces the viscosityof the matrix, hence enhances the processability of iPP basedcomposites.

M. Naffakh et al. / Materials Chemistry and Physics 144 (2014) 98e106106

4. Conclusions

The mechanical, tribological and rheological properties ofternary iPP/a (IF-WS2)/b (NJ) nanocomposites have been analysedand compared with those of binary iPP/IF-WS2 and iPP/NJ systems.The addition of the b-NA to iPP/IF-WS2 led to a moderate drop instiffness and strength together with an extraordinary increase inductility and toughness. The tests performed demonstrated that ata critical NJ concentration of 0.25 wt.% the Young’s, storage andflexural moduli, tensile and flexural strength show a minimum thatcoincides with the maximum of the strain at break, strain at yield,impact strength and area under the tensile curve. DMA resultsrevealed a slight decrease in the Tg of iPP/IF-WS2 upon addition ofNJ, since the polymer chains in the amorphous regions in b-iPP aremore mobile than in a-iPP. Furthermore, an increase in the area,width and height of the tan d peak was detected, indicating theenhanced damping capacity of the ternary nanocomposites. Theyalso exhibited higher coefficient of friction and wear rate than iPP/IF-WS2 (1.0 wt.%), and both maximum values corresponded to thecritically nucleated material. The total surface energy of the refer-ence nanocomposite hardly changed with the addition of the b-NA,albeit a small rise in the polar contribution was found due to theamide groups of NJ, which also led to increased interactions be-tween the polymer chains reflected in higher composite viscosity.Nevertheless, all the hybrids displayed lower viscosity and wearrate than unmodified iPP, hence enhanced processability andtribological performance. It was demonstrated that the mechanicaland tribological properties of these ternary nanocomposites can befinely tuned by modifying the b (NJ)/a (IF-WS2) ratio. The simul-taneous addition of small amounts of nano- (IF-WS2) and micro-(NJ) fillers to iPP is an effective strategy to develop multiscalenanocomposites with a good balance between stiffness, strength,toughness and wear resistance, making them highly attractive forindustrial applications.

Acknowledgements

This work was supported by the Spanish Ministry of Science &Innovation in the Project MAT-2010-21070-C02-01. Dr. M. Naffakhwould like to acknowledge the Ministerio de Economía y Com-petitividad (MINECO) for a ‘Ramón y Cajal’ Senior ResearchFellowship and Dr. Ana Diez-Pascual wishes to acknowledge theCSIC for a JAE Postdoctoral Fellowship cofinanced by the EU.

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