fracture behaviour of controlled-rheology ethylene–propylene block copolymers

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Research ArticleReceived: 15 July 2010 Revised: 27 September 2010 Accepted: 27 September 2010 Published online in Wiley Online Library: 6 January 2011

(wileyonlinelibrary.com) DOI 10.1002/pi.3012

Fracture behaviour of controlled-rheologyethylene–propylene block copolymersAlicia Salazar,a∗ Tamara Martın,a Jose M Navarrob and Jesus Rodrıgueza

Abstract

The evolution of the molecular weight distribution and the thermal, mechanical and fracture behaviour of controlled-rheologyethylene-propylene block copolymers (ca 8 wt% ethylene content) has been analysed. Various concentrations of di-tert-butylperoxide were utilized. The melt flow index increased with the peroxide content due to the reduction of the molecularweight and the narrowing of the molecular weight distribution. However, the thermal behaviour and degree of crystallinity werenot improved and some mechanical properties, such as the tensile strength and elongation at break, presented an anomalousbehaviour. This trend can be explained by the presence of the elastomeric phase. The addition of peroxide influenced stronglythe J –R curves obtained via the elastic–plastic fracture mechanics approach. The slope of these curves was markedly reducedwith addition of peroxide to almost being flat for the highest concentration. This loss of ductility and the sudden decrease ofthe fracture toughness values with an increasing amount of peroxide were mainly due to the reduction in the molecular weight.c© 2011 Society of Chemical Industry

Keywords: controlled rheology; ethylene-propylene block copolymers; fracture toughness; J –R curves; normalization method

INTRODUCTIONLow manufacturing costs, ease of recycling and possibility oftailoring properties are some of the reasons why isotacticpolypropylene (PP) has been enjoying the fastest growth inconsumption since its discovery.1,2 However, its impact toughnessneeds to be improved, especially at low temperatures.3,4 Forthat reason, a dispersed rubbery phase is incorporated in thePP matrix inducing the occurrence of toughening mechanismsvia either physical blending1,2 or by copolymerization with otherpolyolefins with lower glass transition temperature than PP, such aspolyethylene (PE). The copolymerization with ethylene is preferredover blend systems due to the strongly incompatible nature ofPEs and PPs.5 The resulting block copolymers are heterophasicmaterials with a two-phase structure where an elastomeric phasein the form of spherical domains, usually ethylene–propylenecopolymer rubber (EPR), is dispersed uniformly within the PPhomopolymer matrix.6,7

Commercial PP and ethylene–propylene block copolymers(EPBCs), also termed heterophasic copolymers, produced inconventional reactors using fourth-generation Ziegler–Nattacatalyst systems are characterized by relatively high weight-average molecular weight (Mw) and broad molecular weightdistribution (MWD = Mw/Mn). These features cause high meltviscosity, which makes these materials unsuitable for commercialend-uses such as fibre spinning, blown film, extrusion and injectionmoulding. In order to improve PP response during processing andto achieve specific grades, a post-reactor procedure that consistsof degradation with organic peroxides is used to adjust and controlMw and MWD. Peroxide-initiated scission reactions efficiently resultin polymers with tailor-made properties. The PPs produced in thisway are termed controlled-rheology polypropylenes (CRPPs) andthey have improved melt flow characteristics due to reduced Mw

and narrow MWD.8 – 22 The advantages of CRPP versus normal PPare less shear sensitivity, high elongation at break and thermal

deformation temperature, surface smoothness and better physicalproperties such as clarity and gloss.8,17

Reactor-made PP and EPBCs are semi-crystalline polymers thathave been extensively investigated. The mechanical and fractureresponse of PPs produced via catalysers is strongly influenced bythe microstructure (the degree of crystallinity, the size and shape ofspherulites, the lamellar thickness and the crystalline orientation),and the ethylene content in the case of EPBCs.1,23 – 39 In the caseof PP, crazes are the main micromechanism of deformation andfailure. They are formed in the weak points of the microstructure,normally near the intercrystalline regions. Several authors haveproved that crack initiation and growth, as well as breakdown,are controlled by the amorphous interconnections among thespherulites, the entanglement density of which improves asthe molecular weight increases.21,26,28,31,32,35,37 Failure in EPBCsis related to the dispersed elastomeric particles. Upon loading,small cavities are nucleated at the weak points of the copolymersuch as the boundary between the EPR particles and the PP matrixor the intercrystalline zones in the PP matrix.28 – 31,39 These voids arestabilized by fibrillar bridges of PP filaments which are plasticallydeformed and orientated. As in the case of PPs, these fibrillarbridges are stronger the greater the molecular weight.27,28,31 – 33

∗ Correspondence to: Alicia Salazar, Departamento de Ciencia e Ingenier ıade Materiales, Escuela Superior de Ciencias Experimentales y Tecnologıa,Universidad Rey Juan Carlos, C/Tulipan, s/n, 28933 Mostoles, Madrid, Spain.E-mail: [email protected]

a Departamento de Ciencia e Ingenier ıa de Materiales, Escuela Superior deCiencias Experimentales y Tecnologıa, Universidad Rey Juan Carlos, C/Tulipan,s/n, 28933 Mostoles, Madrid, Spain

b Direccion de Tecnologıa, Repsol YPF, Carretera de Extremadura, N-V, Km 18,28931 Mostoles, Madrid, Spain

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www.soci.org A Salazar et al.

Many studies have focused on the reactive mechanism, therheological and crystallization behaviour as well as the mechanicalproperties of CRPPs8 – 12,16,17,19 – 22,40 and blends of PP and PE,12 – 15

but not so many are devoted to EPBCs.18 Moreover, the fracturebehaviour of CRPPs has attracted little attention.41,42 Salazaret al.41 analysed the evolution of the fracture toughness underelastic–plastic fracture mechanics (EPFM) of different grades ofPP homopolymer prepared using controlled addition of di-tert-butylperoxide (DTBP) in reactive extrusion. In turn, Sheng et al.42

analysed the effect on the fracture behaviour with the post-yieldfracture mechanics approach through the evaluation of essentialwork of fracture parameters of controlled-rheology EPBC films themolecular weight of which was adjusted by reactive extrusionwith the incorporation of dicumylperoxide. The results revealedthe same tendency as in the case of reactor-made PP or EPBCs withthe same ethylene content but different molecular weight: thefracture toughness decreases as the molecular weight decreasesas a result of the addition of peroxide.

However, much research is needed to understand the fracturebehaviour of bulk controlled-rheology EPBCs. For that reason,the study reported in the present paper mainly focused on theeffect of peroxide content on the fracture parameters of an EPBCdetermined through the J-integral methodology, paying specialattention to the analysis of the fracture micromechanisms viaoptical microscopy and SEM. The effect of peroxide content andstructural parameters on the toughness is discussed in the lightof the test results and fractographic information. An analysisof the rheological, thermal and mechanical response is alsoprovided.

EXPERIMENTALMaterials and sample preparationThe material under study was an EPBC with high isotacticity (ca90%) and an ethylene content of 8–9 wt% supplied by REPSOLQuimica. It was manufactured using a Spheripol process with afourth-generation Ziegler–Natta catalyst. The peroxide used wasDTBP.

The experiments and sample preparation were carried outusing a twin-screw extruder with a length-to-diameter ratio ofthe dies of 25. The peroxide was added to the EPBC to preparemasterbatches with peroxide contents of 0, 101, 332 and 471 ppm.Both components were inserted into the extruder. Experimentswere performed at profile temperatures of 190, 220, 240, 220 and200 ◦C and with a screw rotation speed of 150 rpm. Extrudateswere cooled through a water bath and were granulated. Samplesfor mechanical and fracture properties were injection-moulded.

Melt index measurementsMelt flow index (MFI) was measured following the ISO1133 stan-dard using a Ceast 6932 extrusion plastometer at 230 ◦C/2.16 kg.

Molecular weight characterizationMWDs were determined with gel permeation chromatography(GPC) using Polymer Labs PL220 equipment and taking ISO14014as a guide. The samples were dissolved at 143 ◦C in 1,2,4-trichlorobenzene at a polymer concentration of 1.3 mg mL−1.A phenolic antioxidant, Irganox 1010, was added to the solutionto prevent any degradation.

Thermal analysisThe apparent melting temperature, Tm, the crystallization temper-ature, Tc, and the crystallinity index, α, of all the samples weremeasured via DSC using a Mettler-Toledo (model DSC822) instru-ment. Two scans were carried out at 10 ◦C min−1, from 0 to 200 ◦Cunder nitrogen atmosphere, in aluminium pans with 10 mg ofsample. The values of Tc were obtained from the maxima of thecrystalline peaks, and the values of Tm and the apparent enthalpy,�H, were calculated from the maxima and the area of the meltingpeaks, respectively. The crystallinity index via this technique wasdetermined using

α = �H

�H0 (1)

where the enthalpy of fusion of 100% pure crystalline α-PP, �H0,was taken as 190 J g−1.1

Mechanical characterizationTensile tests were carried out, following ISO527-2 : 1997, in orderto measure the yield strength, the stress at break and the strain atbreak. Specimens of dimensions 10 mm × 115 mm × 4 mm in thenarrow section were tested using an electromechanical testingmachine (MTS Alliance RT/5), under displacement control at acrosshead speed of 50 mm min−1. Strain values were measuredwith a high-strain extensometer (model MTS DX2000) attached tothe sample.

Flexure tests were performed following the guidelines describedin ISO178 : 2003 to determine the Young’s modulus. Samples withdimensions of 10 mm × 80 mm × 4 mm were tested using anelectromechanical testing machine (Instron 4465) under strokecontrol at a crosshead speed of 2 mm min−1.

Fracture characterizationAt room temperature and under low loading rates, EPBCs present apronounced nonlinear mechanical response. The EPFM approachthrough the J-integral methodology was used to find the fracturetoughness at crack initiation, JIC, which is determined from thecrack resistance curve, J –R curve, where J is plotted versus the crackextension, �a. For the construction of J –R curves of polymers,ASTM43 and ESIS44 recommend the multiple specimen method.This methodology, though straightforward and effective, is time-and material-intensive, as a minimum of seven specimens have tobe tested to generate the R-curve. For that reason, indirect methodshave been developed to obtain J –R curves with fewer specimensand, thus, less time requirements. The single-specimen methodsare based on the load separation criterion,45 and offer an easy andeffective alternative approach to obtain J –R curves. Among thesingle-specimen methods, the normalization method46 – 50 and theload separation parameter method have been successfully appliedto polymeric materials.51 – 53

The J –R curves in the present case were obtained via thenormalization method which is focused on determining accuratecrack length predictions using the load (P)–displacement (δ) dataalone. The instructions given by ASTM E1820-0654 were taken as aguide.

Once the J –R curve is constructed, it should be described by apower law, J = C�aN , with N ≤ 1. The crack initiation resistance,JIC, was calculated following the guidelines described by Hale andRamsteiner,44 where this critical value is replaced by a pseudo-initiation value, J0.2, which defines crack resistance at 0.2 mm ofthe total crack growth.

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Fracture behaviour of controlled-rheology EPBCs www.soci.org

Table 1. Ethylene (ET) content, melt flow index (MFI), number- andweight-average molecular weights and polydispersity (Mw/Mn) of thevarious EPBCs

SampleET

(wt%)MFI (g (10

min)−1)Mw (kgmol−1)

Mn (kgmol−1)

MWD(Mw/Mn)

EPBC0 7.78 12 287.0 56.4 5.09

EPBC-CR101 8.19 20 253.0 53.3 4.75

EPBC-CR332 8.36 41 199.4 53.5 3.73

EPBC-CR471 7.91 63 178.2 50.3 3.54

The size requirements for plane strain JIC are given by55

B, a, W − a > 25JIC

σ ys(2)

Fracture toughness tests were carried out using single edgenotch bending (SENB) specimens obtained directly from the mouldwith dimensions of 6 mm × 18 mm × 79 mm and an initial notchlength of 8.1 mm. At the centre of the notch, a razor blade wasinserted to create a sharp crack by tapping into the material. Theresulting notch was ca 9.0 mm in depth.

The tests were conducted at room temperature and underdisplacement control at a crosshead speed of 1 mm min−1

using a three-point bend fixture of 72 mm loading span. Anelectromechanical testing machine (MTS Alliance RF/100) with aload cell of ±5 kN was utilized.

After the fracture tests, the fracture surfaces of the brokenspecimens were examined using optical microscopy (Leyca DMR)and SEM (Hitachi S-3400N) to analyse the extension of thewhite stress region due to plastic deformation as well as themicromechanisms of failure. For SEM analysis, the samples weresputter-coated with platinum.

RESULTS AND DISCUSSIONMFI and MWDTable 1 summarizes the ethylene content, MFI and GPC resultsfor the EPBCs with 0, 101, 332 and 471 ppm of DTBP, termedEPBC0, EPBC-CR101, EPBC-CR332 and EPBC-CR471, respectively.As expected, the MFI increases noticeably with increasing peroxidecontent, also decreasing the average molecular weights of theEPBC-CRs. The polydispersity decreases by as much as 30%.This decrease contrasts with the results obtained for CRPPhomopolymers, which underwent much greater degradation forsimilar peroxide contents, with reported reductions in the MWDup to ca 60%.11,16 – 18 Berzin et al.18 noted the presence of highresidual masses, which indicates the presence of long chains,for the copolymers and even for the more degraded products.PP/PE mixtures are known to show opposite effects duringperoxide-initiated scission reactions.13,14,15,18 While for the PPmatrix the free radicals formed from the thermal decompositionof the organic peroxide lead to β-scissions because of the lowstability of the tertiary hydrogen atoms of macroradicals, forthe PE matrix peroxide attack leads to chain branching and tocrosslinking by macroradical recombination. These two competingmechanisms could be the reason for the much less reduction inMWD for copolymers than for homopolymers during peroxidedegradation.

Table 2. Thermal parameters obtained from DSC measurements

Tm (◦C) Tc (◦C) �H (J g−1) α (%)

EPBC0 164 123 95 53

EPBC-CR101 164 122 94 51

EPBC-CR332 163 122 93 50

EPBC-CR471 162 123 91 49

Table 3. Mechanical properties of the various EPBCs

Sample E (MPa) σ ys (MPa) σ t (MPa) εr (%)

EPBC0 1165 ± 20 25.2 ± 0.5 17.5 ± 0.5 92 ± 50

EPBC-CR101 1061 ± 15 23.9 ± 0.3 17.2 ± 0.3 175 ± 42

EPBC-CR332 1022 ± 31 22.6 ± 0.1 15.1 ± 0.5 49 ± 23

EPBC-CR471 995 ± 30 22.2 ± 0.1 17.5 ± 0.3 58 ± 4

Thermal propertiesThe values of Tm, Tc, �H and α, measured using DSC, of the EPBCswith various DTBP contents are listed in Table 2. Tm and α slightlydecrease with peroxide content while the Tc is almost constant.During the degradation of the copolymer, both chain scissions inthe PP matrix and crosslinking in the elastomeric phase occur. Doet al.13 and Berzin et al.18 have reported that extensive crosslinksshould disturb the crystallinity, reducing not only the size but alsothe amount. This might be the reason for the slight reduction incrystallinity of the EPBC-CRs.

Mechanical characterizationThe average Young’s modulus, E, the yield stress, σ ys, the tensilestrength, σ t, and the elongation at break, εr, together withtheir corresponding standard errors are summarized in Table 3.Regarding the elastic properties, the Young’s modulus decreaseswith the peroxide content, which is not surprising since thecrystallinity is reduced on addition of peroxide (Table 2). As isknown, the elastic modulus is highly dependent on the stiffestpart of the structure, that is, the crystalline region;24,30,35 thus, thelower the latter, the lower the former.

The yield stress decreases with increasing peroxide content andthis tendency is in accordance with the relationships betweenthe mechanical properties and the molecular weight reportedpreviously.23,33,35 However, two exceptions are noted. Firstly, theelongation at break increases with the addition of DTBP up to101 ppm and decreases after that. This same behaviour hasalso been reported by Azizi and Ghasemi21 and Salazar et al.41

who assumed that at low levels of peroxide the high molecularweight tail of the PP matrix is degraded while the low molecularweight remains unchanged and therefore the slippage of polymerchains could be easier and greater. Secondly, the other parameterthat shows an anomalous trend is the tensile strength, whichinitially decreases with the addition of peroxide, but for greateramounts of DTBP a sudden increase occurs. At this degree ofcopolymer degradation, probably the elastomeric phase could beconstituted of either solid crosslinked particles or high-molecular-weight highly branched species and the failure properties aretransferred to the minority phase.18

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0

50

100

150

200

0 0.5 1 1.5 2 2.5 3 3.5 4

EPBC0EPBC-CR101EPBC-CR332EPBC-CR471

P (

N)

(mm)

Figure 1. Load, P, versus displacement, δ, of EPBC0 and the controlled-rheology copolymers EPBC-CR101, EPBC-CR332 and EPBC-CR471.

0

5

10

15

20

25

30

0 0.2 0.4 0.6 0.8

EPBC0

EPBC-CR101

EPBC-CR332

EPBC-CR471

J = 33.94·∆a0.54088 R= 0.99478

J = 7.2571·∆a0.22011 R= 0.99097

J = 3.2017·∆a0.070557 R= 0.98948

J = 2.4639·∆a0.077287 R= 0.96273

J (N

mm

−1)

∆a (mm)

Figure 2. J –R curves obtained using the normalization method of the non-degraded EPBC0 and the controlled-rheology copolymers EPBC-CR101,EPBC-CR332 and EPBC-CR471.

Fracture behaviourTypical load (P)–displacement (δ) records obtained from fracturetests for every copolymer are presented in Fig. 1. The mechanicalresponse for all the copolymers under study is clearly nonlinearwith complete stable fracture. Hence, the appropriate fracturemechanics procedure to characterize the copolymers is the EPFMnormalization method. Although all the materials show not onlythe same geometry and size but also similar initial crack lengths, itis evident that the area under the curves, i.e. the energy absorbedby the specimen, differs from that of the non-peroxide-treatedcopolymer (EPBC0) to that with a small content of peroxide (EPBC-CR101) and even more to those with higher peroxide content(EPBC-CR332 and EPBC-CR471). The load relaxation, related tostable crack growth, occurs at lower load values as the peroxidecontent increases and the degree of fracture stability is also less.

Figure 2 presents the J –R curves constructed for thenon-degraded copolymer (EPBC0) and the peroxide-degraded

copolymers (EPBC-CR101, EPBC-CR332 and EPBC-CR471). Analysisof the curves reveals that the reactor-made copolymer is the tough-est material and the addition of peroxide decreases the toughness.Interestingly, the copolymers with the highest peroxide content,EPBC-CR332 and EPBC-CR471, show almost flat resistance curves,indicating the marked loss of ductility with increasing peroxidecontent. This can be easily observed in Fig. 3, which shows opticalmicrographs of the fracture surfaces of tested specimens brokenafterwards at liquid nitrogen temperature and high loading ratesto reveal a stable crack length. All the copolymers show stablecrack lengths of ca 0.6 mm. Independent of the material, threeregions are distinguishable. The first one, close to the notch tip,is attributed to stable crack propagation. This zone is followedby a stress-whitening area and, ahead of it, the remainder of thefracture surface is characterized by a rough, non-whitened anduniform area related to the virgin material.

Two main features are evident from the analysis of these fracturesurfaces. Firstly, the stable crack length is more and more diffusewith increasing peroxide content. Indeed, to minimize the risk ofincorrect measurements in the crack length, SEM was used forthe peroxide-degraded copolymers instead of optical microscopy.Secondly, the intensity and the extension of the stress-whiteningregion ahead of the stable crack length decrease with the additionof peroxide. These two factors explain the decrease of the slope ofthe R curves with increasing peroxide content.

Figure 4 shows the crack initiation resistance, JIC, of all thecopolymers under study. The fracture toughness of the peroxide-degraded copolymers verifies the size requirement specified inEqn (2), the fulfilment of which guarantees the plane strain state,but the value for the reactor-made copolymer is not in the planestrain state. Even so, the values are comparable as all the fracturespecimens are of the same thickness. As expected, the fracturevalues decrease abruptly with a small addition of peroxide andthis reduction continues gradually with peroxide content. Thisbehaviour is similar to that shown by PP homopolymers alsodegraded with DTBP41 and can be explained in terms of themolecular weight. Avella et al.,24, Sugimoto et al.26 and Fukuhara32

focused their research on understanding the effect of Mw on thefracture toughness of semi-crystalline polymers. They observedthat the fracture toughness increases strongly with increasingmolecular weight and evidenced that the key factor is thelinking molecules that join the crystalline blocks together. Asthe molecular weight decreases, the number of linking moleculesdecreases, the material becomes less interconnected and fractureoccurs at lower stress and strain levels.

With the aim of shedding more light on the fracture behaviourof the controlled-rheology copolymers, the fracture surfaceswere also examined via SEM. Figure 5 shows the morphologyof the stable crack growth region for the copolymers underinvestigation. All the fracture surfaces display a macroductiletearing formed by broken stretched filaments oriented in thedirection perpendicular to the crack propagation. Upon loading,small cavities are nucleated at the weak points of the copolymersuch as the boundary between the elastomeric particles and the PPmatrix or the intercrystalline zones in the PP matrix.28,29,35,36,38,39

These voids are stabilized by fibrillar bridges of PP filamentswhich are plastically deformed and orientated. With a consequentincrease in deformation, excessive plastic flow occurs at these PPfilaments and a stable crack nucleates and propagates throughthe close PP bridges giving rise to the fibrillated morphology withductile pulling of ligaments left behind. It is worth mentioningthat the extension of such ductile tearing is less pronounced with


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1 mm

(a)

Plastic deformation

Notch

1 mm

(b)

1 mm

(c)

Plastic deformation

1 mm

(d)

Plastic deformation

Notch

Notch

Figure 3. Fracture surfaces obtained via optical microscopy: (a) EPBC0; (b) EPBC-CR101; (c) EPBC-CR332; (d) EPBC-CR471. All specimens show a stablecrack length of ca 0.6 mm.

0

5

10

15

EPBC0 EPBC-CR101 EPBC-CR332 EPBC-CR471

J IC (

N m

m−1

)

Figure 4. Evolution of the crack growth initiation energy, JIC, with peroxidecontent.

the addition of DTBP as the strength of these fibrillar PP bridges isreduced as the molecular weight decreases.

The nucleation and growth of this mechanism of failure is clearlyobserved in the stress-whitening region, which occurs ahead ofthe stable crack growth zone, of the post-mortem fracture surfaces(Fig. 6). The micrographs reveal the way the voids grow aroundthe elastomeric phase. Despite the difference in peroxide content,no appreciable differences are evident in this region.

CONCLUSIONSThe effect of the addition of 0, 101, 332 and 471 ppm DTBP toEPBCs on the MWD, the thermal and mechanical properties andthe fracture behaviour under EPFM was evaluated. The peroxidereduced the molecular weight and narrowed the MWD leading toan increase of the MFI. However, this reduction was not as much asexpected because of the presence of competing effects:β-scissionsof the long PP chains and chain branching and crosslinking of the PEphase. This latter phenomenon is the reason for the deteriorationof the thermal properties. Regarding the mechanical response,the yield stress decreased with peroxide content due to the lowermolecular weight but the tensile strength and the elongation atbreak presented an anomalous behaviour also explained in thelight of the presence of the long residual chains of PE which wereprobably branched.

Concerning the fracture behaviour, the loss of ductility withperoxide content was the main characteristic not only evidentfrom the load–displacement records of the fracture tests andthe naked-eye examination of the fracture surfaces but also fromthe slope of the J –R curves. This trend was also reflected in thefracture toughness with a sudden decrease of this parameterwith a small addition of peroxide content and a continuousreduction with further addition of DTBP. The mechanism of failurein all cases involved growth of voids around the elastomericparticles which resulted in a macroductile tearing. The degreeof ductile tearing was less marked as the polymer degradationwas more accentuated. This reduction of the fracture parameterswith the decrease in the molecular weight in semi-crystallinecopolymers can be explained by the reduction in numberand strength of the linking molecules that join the crystallineblocks.


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0


20 µm

(a)

20 µm

(b)

20 µm

(c)

20 µm

(d)

Figure 5. Fracture surfaces associated to the stable crack growth region obtained from SENB fracture toughness tests: (a) EPBC0; (b) EPBC-CR101;(c) EPBC-CR332; (d) EPBC-CR471.

5 µm

(a)

5 µm

(b)

5 µm

(c)

5 µm

(d)

Figure 6. Stress-whitening region ahead of the stable crack growth zone obtained from post-mortem fracture surfaces: (a) EPBC0; (b) EPBC-CR101;(c) EPBC-CR332; (d) EPBC-CR471.

ACKNOWLEDGEMENTSThe authors are indebted to Ministerio de Educacion of Spainfor its financial support through project MAT2009-14294, and toREPSOL for the supply of materials.

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fracture behaviour of controlled-rheology ethylene–propylene block copolymers

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