European Polymer Journal 46 (2010) 719–730

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European Polymer Journal

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Morphology and properties of isotactic polypropylene/poly(ethyleneterephthalate) in situ microfibrillar reinforced blends: Influenceof viscosity ratio

Xin Yi a, Ling Xu a, Yu-Ling Wang a, Gan-Ji Zhong a, Xu Ji b,*, Zhong-Ming Li a,*

a College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065,Sichuan, People’s Republic of Chinab School of Chemical Engineering, Sichuan University, Chengdu 610065, People’s Republic of China

a r t i c l e i n f o

Article history:Received 24 September 2009Received in revised form 14 December 2009Accepted 20 December 2009Available online 11 January 2010

Keywords:Isotactic polypropylenePoly(ethylene terephthalate)Microfibrillar blendViscosity ratioCompartibilization

0014-3057/$ - see front matter � 2010 Elsevier Ltddoi:10.1016/j.eurpolymj.2009.12.027

* Corresponding authors.E-mail addresses: jxhhpb@163.com (X. Ji), zmli@

a b s t r a c t

In situ microfibrillar reinforced blends based on blends of isotactic polypropylene (iPP) andpoly(ethylene terephthalate) (PET) were successfully prepared by a ‘‘slit extrusion–hotstretching–quenching” process. Four types of iPP with different apparent viscosity wereutilized to investigate the effect of viscosity ratio on the morphology and mechanical prop-erties of PET/iPP microfibrillar blend. The morphological observation shows that the viscos-ity ratio is closely associated to the size of dispersed phase droplets in the original blends,and accordingly greatly affects the microfibrillation of PET. Lower viscosity ratio is favor-able to formation of smaller and more uniform dispersed phase particles, thus leading tofiner microfibrils with narrower diameter distribution. Addition of a compatibilizer, polypropylene-grafted-glycidyl methacrylate (PP-g-GMA), can increase the viscosity ratio anddecrease the interfacial tension between PET and iPP, which tends to decrease the size ofPET phase in the unstretched blends. After stretched, the aspect ratio of PET microfibrilsin the compatibilized blends is considerably reduced compared to the uncompatibilizedones. The lower viscosity ratio brought out higher mechanical properties of the microfibril-lar blends. Compared to the uncompatibilized microfibrillar blends, the tensile, flexuralstrength and impact toughness of the compatibilized ones are all improved.

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1. Introduction

One type of the polymer blends named ‘microfibrilarreinforced composite or blend, which consists of a thermo-plastic polymer as matrix and in situ thermoplasticpolymer microfibrils as reinforcement, are increasinglyattracting interest [1–14], because they provide a poten-tial, economical way of tailoring the properties of the poly-mer blends’ product according to the desired application[15–20]. The microfibrils in the blends are of miraculousrole, such as making products of conductive polymer mate-rials [21–23]: the conductive particles assemble within themicrofibrils and then the conductive microfibrils construct

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scu.edu.cn (Z.-M. Li).

a 3D conductive network with low loading of conductivefiller and good mechanical properties; application of gasbarrier properties [24–26]: the microfibrils increase thechannel length of gas or vapor passing through the materi-als; the enhancement of mechanical properties in whichthe transcrystallites [27] are formed and the ‘skin-core’structure of injection molded polymer products can bewell suppressed by the microfibrils’ network [28].

The specific process of the microfibrillar reinforcedblends in our previous studies based on general engineer-ing plastics (such as nylon (PA), poly(ethylene terephthal-ate) (PET), polycarbonate (PC), etc.) as the microfibrilcandidate and polyolefin (such as isotactic polypropylene(iPP), polyethylene (PE)) as the matrix can be describedas follows: in the first step, to form microfibrils throughsingle-screw extruder extrusion and subsequent hot

720 X. Yi et al. / European Polymer Journal 46 (2010) 719–730

stretch at the processing temperature of higher meltingpoint component as soon as the extrudate flows out ofthe die; in the second step, to maintain the microfibrils’morphology in the compression or injection molding atthe processing temperature of the lower melting pointcomponent [29,30]. The microfibrillar blend is thus com-prised of an isotropic polymer matrix and reinforcementphase of high aspect ratio microfibrils. From the prepara-tion point of view, the microfibrillar blend is superior tothe common composites reinforced by discontinuous fi-bers, such as glass fiber and carbon fiber, which have theinevitable drawbacks of complex production techniques,high materials and labor costs, environmental impact andinhomogeneous mixture. It is also unlike the commonblends of immiscible polymer pairs, whose mechanicalproperties for most polymer pairs are undesirable becauseof their weak interfacial adhesion [31]. For the microfibril-lar blends, these demerits can be avoided conveniently, be-cause in the process of fabricating microfibrillar blend,blending happens in two melt polymer and the system isorganic–organic phase, the dispersed phase can performmore uniformly and higher interfacial tension, comparedto blending of solid-melt, organic–inorganic systems, suchas glass fiber reinforced polyolefins. Another distinctadvantage for microfibrillar blends is the high efficiencyand energy saving since the required equipment is simpleand the manipulation is also convenient.

Since the morphology of dispersed phase in the blendsgreatly affects the physical and mechanical properties, forexample, the well-dispersed and fine spheres in the matrixcan increase the impact strength of materials, while thesheet-like dispersed morphology has the ability of enhanc-ing the barrier properties of a film, and the microfibrils inthe blends can increase the uniaxial strength greatly [32],it is of great importance to know the formation of morphol-ogy during processing, and then control them to meet ourneeds. There are many factors affecting the morphology ofmicrofibrils in the blend, which include viscosity ratio,stretch ratio, composition, and compatibility of compo-nents. Among these factors, viscosity ratio is one of the mostimportant parameters governing the morphology of micro-fibrils. However, there are little work focusing on the effectof viscosity ratio on microfibrils’ morphologies. Min and hiscoworkers [33] have investigated the relationship betweenviscosity ratio and morphology of dispersed phase in thePC/PE, PA/PE blending systems, and found that the higherviscosity ratio is, the more difficult to form microfibrils be-cause of the harder deformation of dispersed phase. Li andhis coworkers [34] studied the effect of viscosity ratio onthe morphology change of PET/PP and PET/PP/PP-GMAblends, and concluded that the addition of titanium dioxide(TiO2) decreased the viscosity ratio and then resulted in a fi-ner morphology. However, the appropriate viscosity ratiofor formation of finer microfibrils with high aspect ratioand well distribution is still unclear, so the effect of specificviscosity ratio between different materials on morphologyshould be clarified further, which is quite important inindustry to select the raw materials and equipment and tomake products with excellent performance.

In addition, it is well known that the formation ofmicrofibrils is based on the degree of incompatibility be-

tween matrix and dispersed phase. And the mechanicalperformance of polymer composites depends on two fac-tors (1) the aspect ratio of reinforcement phase and (2)the adhesion quality of interface between matrix and rein-forcement. Friedrich et al. [35] found that in situ microfi-brillar blends of iPP and PET with a compatibilizerexhibited lower tensile strength and modulus. They as-cribed the reduction to the compatibilizer as a softeningagent and the shorter length of microfibrils caused by theuse of compatibilizer which covers the PET droplets duringmelt mixing and prevents their coalescence during stretch-ing. Pracella and his coworkers [36] studied the blends ofPET and iPP compatibilized by random copolymer of ethyl-ene and glycidyl methacrylate (E-GMA), and proposed thatthe high interfacial tension and melt viscosity of thecopolymer component facilitate to coalesce into large par-ticles, and the decrease in domain size in reactive blends isattributed to a decrease in the interfacial tension becauseof the chemical interaction between the epoxy groupsand the chain ends of PET, reducing the tendency towardcoalescence. As we know, both viscosity ratio and interfa-cial tension are influenced by compatibilizer, the combinedeffect of viscosity ratio and compatibilizer on the morphol-ogies of microfibrils still needs to be investigated in moredetails. Therefore, in this study, these two most importantfactors for the formation of microfibrils, viscosity ratio andamount of compatibilizer, are investigated. We also at-tempt to illuminate the relationship between morphologyand the mechanical properties of the obtained microfibril-lar blends.

2. Experimental

2.1. Materials

Four types of iPP with different molecular weight werechosen for the matrix phase of microfibrillar blends, andthe PET as the microfibrils candidate is a commercial bottlegrade polyester, with the intrinsic viscosity [g] = 0.658 dl/g, supplied by Yi Zhen, China. The information and physicalparameters of these materials are given in Table 1. PP-g-GMA was obtained in our laboratory using the technolo-gies reported in the literature [37,38], i.e., melting radicalgrafting reaction of iPP with GMA monomer and radicalinitiator of dicumyl peroxide (DCP, commercial grade, Ciba,German). In order to increase both grafting rate and yieldof PP-g-GMA and reduce iPP chain scission, styrene mono-mer (St) was added to the PP/GMA/DCP reacting system.

2.2. Sample preparation

To avoid hydrolysis, PET was dried in a vacuum oven at120 �C for at least 10 h before processing. The dried PETslices were mixed with iPP pellets in a constant mass ratioof 15/85 (PET/iPP). The extrusion of the mixture was per-formed on a single-screw extruder with the ratio of screwlength to diameter, L/D, of 25. The die for the extruder wasa slit die with 20 mm width and 1 mm thickness. The tem-perature profile from hopper to die of the extruder was160, 270, 275 and 270 �C, and the screw rotation was

Table 1Physical parameters of iPP, weight average molecular weight, Mw; molecular weight distribution, Mw/Mn.

Sample iPP1 iPP2 iPP3 iPP4

Material T30S HJ500 K1011 Z30SMelt index 3 10 15 25Mw (�104) 52.9 66.2 39.8 33.5Mw/Mn 3.89 8.3 3.95 4.18Manufacturer Maoming Petroleum

Chemical Corporation,China

Samsung Chemicalcorporation, Korea

Taiwan ChemicalFiber corporation,China

Maoming PetroleumChemical Corporation,China

Tensile strength (MPa) 33.4 32.6 31.5 31.8Flexural strength (MPa) 42.8 42.2 41.7 40.6Flexural modulus (MPa) 1793 1878 1844 1759

X. Yi et al. / European Polymer Journal 46 (2010) 719–730 721

maintained at 80 rpm. The extrudate was hot stretched bya take-up device with three pinching rolls to form themicrofibrils, and the roll temperature was kept at about40 �C by adjusting the volume flow rate of tap water inthe cooling pipe. The speed of the rolls can be changed toobtain different hot stretching ratios (HSR, the area of thetransverse section of the die to the area of the transversesection of the extrudate). Subsequently, the stretchedextrudate was immediately quenched in a cold water bathand then pelletized. Before injection molding the pelletswere also dried in a vacuum oven at 120 �C for 5 h. Thetemperature was set to 165, 205, 210, 205 �C from hopperto nozzle, respectively, far below the melting point of PET(about 265 �C). The blend composition and abbreviationare given in Table 2. In the form of abbreviation for B49,‘B’ represents the blend of PET and iPP, and ‘49’ corre-sponding its viscosity ratio (the ratio of apparent viscosityof dispersed phase to the apparent viscosity of matrix atthe same temperature) of 0.049, the others, B83, 94 and121, have the same meaning, i.e., with their viscosity ratioof 0.083, 0.094 and 0.121, respectively.

2.3. Rheological properties

In order to detect the rheological properties of rawmaterial, we measured the apparent viscosity by AdvancedCapillary Rheometer (RH7, Germany) with an aspect ratioof die, 16:1. The whole process was under a constant tem-perature of 275 �C which matches the processingconditions.

Table 2The designation and composition of materials.

Sample Material Composition by weight

B49 PET/iPP1 15/85B83 PET/iPP2 15/85B94 PET/iPP3 15/85B121 PET/iPP4 15/85CB2 PET/iPP2/PP-g-GMA 15/85/2CB4 PET/iPP2/PP-g-GMA 15/85/4CB6 PET/iPP2/PP-g-GMA 15/85/6C1 iPP2/PP-g-GMA 98/2C2 iPP2/PP-g-GMA 96/4C3 iPP2/PP-g-GMA 94/6C4 PET/PP-g-GMA 98/2C5 PET/PP-g-GMA 96/4C6 PET/PP-g-GMA 94/6

2.4. Scanning electron microscopy (SEM)

For morphological structure observation of PET microfi-ber in the compatibilized and uncompatibilized blends, thematrix iPP was etched away by hot xylene. After the sol-vent volatilized completely, the surfaces were coated witha layer of gold, and observed in a JEOL JSM-5900LV scanningelectron microscope (SEM). In order to observe the originalmorphology of PET droplets in the extrudate, the un-stretched blend, which was just extruded at the exit of theextruder, was quenched in cold water to maintain the mor-phology of the PET phase. The extrudate was subsequentlyheld in liquid nitrogen for 15 min, then quickly broken up,the facture surface was used for SEM observation.

2.5. Mechanical properties

Measurement of tensile and flexural strength was per-formed for dog-bone specimens according to ASTM D-638 at room temperature of 25 �C. During tensile testing,the crosshead speed was maintained at 20 mm/min, andthe tensile modulus was obtained by line’s slope in thestress–strain curve. Flexural properties were measuredthrough three point bending test at room temperature,the loading speed of the apparatus was keep at 2 mm/min, and the span of supporting points was 67 mm. Thenotched impact strength was measured by pendulum im-pact testing machine in accordance with the ASTM D-256. At least five measurements were conducted for eachsample and then the average value was reported.

2.6. FT-IR spectroscopy

FT-IR spectra were obtained using a Nicolet-560 FT-IRspectrometer (Nicolet Corp., USA). The samples for infraredanalysis were thin films prepared by hot pressing, and theresolution for spectrum was 2 cm�1.

3. Results and discussion

3.1. Apparent viscosity of PET/iPP blends

The apparent viscosity measurement was done at a con-stant temperature (275 �C) identical to the temperature ofdie during extrusion over a wide range of shear rate. Theplots of shear viscosity vs. shear rate for various kinds of

1.0 1.5 2.0 2.5 3.0 3.50.000.050.100.150.200.250.300.350.400.450.50

B49 B83 B94 B121

Vsi

cosi

ty r

atio

lg γ. s -1

Fig. 2. Viscosity ratio at different shear rate for four kinds of blends.

1.0 1.5 2.0 2.5 3.0 3.51.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0iPP C1 C2 C3

lgη

(Pa.

s)

lg γ. s-1

PET C4 C5 C6

Fig. 3. Log g vs. log c curves for pure iPP and PET and their blends withPP-g-GMA under 275 �C.

722 X. Yi et al. / European Polymer Journal 46 (2010) 719–730

iPP and PET are shown in Fig. 1. The typical shear thinningappears in all four grades of iPP [39], and the microfibrillarcandidate polymer, PET, is insensitive to the shear rate be-cause of the rigid benzene ring structure of its molecularchains.

The curves of viscosity ratio vs. shear rate for the PET/iPP blends are shown in Fig. 2. One can see clearly thatB121 has the highest viscosity ratio over all shear rates,B49 has the lowest value, and B83 and B94 fall into be-tween these two extremes. The apparent shear rates inthe die were estimated from the Eq. (1):

c�¼ 6q

Wh2 ð1Þ

where c�is the apparent shear rate in the die, q representsthe volumetric output of the extruder, R and h are thewidth and thickness of the die, respectively [40]. Sincethe die used in our processing is a slit die with the sizeof 20 mm � 1 mm, q is calculated from the mass velocity(the mass of extrudate under unit time from die) and thedensity of iPP. The mass velocity is about 1.69 kg/h andthe density is about 0.92 g/cm3, the q is calculated about564 mm3/s. So the apparent shear rate c�is about 153 s�1

which will be chosen in the following work to study the ef-fect of viscosity ratio on the morphologies and mechanicalproperties because it is close to processing condition ofmelt extrusion.

Fig. 3 shows viscosity variations of iPP and PET meltwith different shear rates after addition of compatibilizerat the temperature of 275 �C. For simplification, only B83was chosen to study the effect of compatibilizer contenton the viscosity ratio and the blend morphology. The com-patibilizer, PP-g-GMA, may prefer to locate in the interfacebetween iPP and PET because one part of constituents ofcompatibilizer is iPP chain and the other constituent isGMA with the ability to react with PET during meltingblending. This scenario makes it hard to define the viscos-ity ratio for compatibilized blends since it is difficult toevaluate accurately how much the effect of compatibilizeron iPP or PET. Nevertheless, the tendency of apparent vis-cosity vs. shear is profound after addition of compatibiliz-er. It can be seen that the compatibilizer reduces theapparent viscosity of iPP, but increases the shear viscosityof PET. Now a hypothesis concerning two kinds of extremesituation is brought forward (1) in a compatibilized blend

1.0 1.5 2.0 2.5 3.0 3.51.01.21.41.61.82.02.22.42.62.83.03.23.4

lg η

Pa.

s

lg γ. s -1

iPP1 iPP2iPP3 iPP4 iPET

Fig. 1. Shear viscosity for different grades of iPP as well as PET underdifferent shear rate at 275 �C.

the compatibilizer completely localizes in iPP, so the shearviscosity of iPP/PP-g-GMA decreases (shown in Fig. 3) andthe apparent viscosity of dispersed phase (PET) maintainsconstant, the viscosity ratio for this case increases as thecompatibilizer content increases and (2) all the compatibi-lizer is located in the dispersed phase, PET, the apparentviscosity of PET/PP-g-GMA melt increases with the in-crease of compatibilizer, so the viscosity ratio also rises.In fact, the compatibilizer can stay in both matrix and dis-persed phase during melt mixing, hence, the real viscosityratio must be between these two cases, in other words, itwould increases with increasing the compatibilizer con-tent. PP-g-GMA has a reactive group, epoxy group, in itschains, which may react with PET chains during meltblending and the reaction equation is shown in Fig. 4.Fig. 5 shows FT-IR spectra of B83 and blend compatibilizedwith PP-g-GMA. The absorption bands of epoxy group(C–O–C) in PP-GMA are located at 1221 and 1166 cm�1

[41], it can be seen that the characteristic peak of1221 cm�1 disperses and the peak intensity at 1166 cm�1

decreases sharply, indicating that the chemical reactionoccurs between epoxy group in GMA and hydroxyl (–OH)or carboxyl (–COOH) in PET [42–45]. One can now under-stand why the shear viscosity of PET increases with thePP-g-GMA content. The major reason is that the chemicalreaction between epoxy group in GMA and hydroxyl(–OH) or carboxyl (–COOH) in PET forms the supramolecu-lar chains of iPP-PET, which hinder the chain movementand lead to the increase of the shear viscosity.

Fig. 4. Reaction between PP-g-GMA and PET.

1400 1300 1200 1100 1000 900 800 700

Tra

nsm

itta

nce

Wavenumber cm-1

PET/iPP 85/15

PET/iPP/PP-GM A 85/15/4

PP-GM A1221 1166

Fig. 5. FT-IR spectra of PET/iPP2 and blend compatibilized with PP-g-GMA.

Fig. 6. SEM micrographs of unstretched PET/iPP blends with different viscosity

X. Yi et al. / European Polymer Journal 46 (2010) 719–730 723

Furthermore, the shear viscosity of iPP itself is higher thanPET at the same shear rate, and the unreacted iPP in graftpolymer also increases the apparent viscosity of PET.

3.2. Morphological structure of PET/iPP microfibrillar blends

The relationship between viscosity ratio and size of ori-ginal particles of the unstretched blends, i.e., conventionalblends, is reported firstly. Fig. 6 shows the morphology ofthe cryofractured surface of PET/iPP blend. It can be seenthat the blends present a typical incompatible morphologywhich comprised of clear spherical domains of PET dis-

ratios at a fixed weight ratio of 85:15, (a) B49; (b) B83; (c) B94; (d) B121.

Table 3Average and maximum diameter of dispersed particles in uncompatiblizedand compatibilized PET/iPP blends.

724 X. Yi et al. / European Polymer Journal 46 (2010) 719–730

persed phase. The average size of the spherical particles inthe blend was estimated, as shown in Table 3. The averagediameter of PET particles is about 2.60 lm for B49, and2.62 lm for B83. While for the system of the highest vis-cosity ratio, B121, the average size is about 4.22 lm, whichis much higher than the other two systems. The blendswith different viscosity ratios also have various diameterdistribution. As shown in Fig. 6a and b, B49 and B83 havethe average diameter of about 2.6 lm, and their diameterranges from 0.5 to 3.5 lm, which take on a very uniformdistribution. In contrast, for the high viscosity ratio B121shown in Fig. 6d, their diameter has a wide distributionfrom 0.5 up to 14.5 lm with an average diameter of ca.4.2 lm. These data suggest that the blends with a low vis-cosity ratio can form fine particles and narrow size distri-bution. This may result from the two following factors:firstly, the efficiency of shear stress transfer from matrix

Fig. 7. SEM images of unstretched compatibilized blends with vario

to dispersed phase, the higher apparent viscosity of matrixcan accelerate breakup of droplets due to its efficient shearstress transfer toward to the dispersed phase; Secondly,the possibility of coalescence of the broken PET droplets,higher apparent viscosity of matrix can suppress coales-cence. The coalescence of broken particles is restrainedmore efficiently for high apparent viscosity of matrix (suchas iPP1 and iPP2), and the diameter of droplets is smallerand more uniform in those low viscosity ratio system.

The morphology of the compatibilized PET/iPP blend isshown in Fig. 7a, b and c. The addition of PP-g-GMA makesthe dispersed phase smaller, from the average size of2.58 lm for the uncompatibilized blend to 1.55, 1.32, and1.10 lm, as the compatibilizer content is 2, 4, 6 wt%,respectively. The PET particles size depends on the com-bined effect of the decrease of surface tension and increaseof viscosity ratio [34]. The viscosity ratio increase afteraddition of compatibilizer (see Fig. 3 and Table 3) meanslarger particles in the compatibilized blends than uncom-patibilized ones based on above analysis. However, the re-duced interface tension tends to decrease the size of PETdroplets. These results indicate that the reduction of PETparticles size caused by reduction of the interface tensionis more than the increment of PET droplet size caused bylarger viscosity ratio after addition of compatibilizer [46].

Fig. 8 shows the morphology of uncompatibilizedmicrofibrillar blends in which the iPP matrix was etchedaway. Compared to the unstretched blends (Fig. 6), the dis-persed PET particles have undergone large elongation

us contents of PP-g-GMA, (a) 2 wt%, (b) 4 wt% and (c) 6 wt%.

X. Yi et al. / European Polymer Journal 46 (2010) 719–730 725

deformation during hot stretching. For the high viscosityratio system (B121, Fig. 8d), it can be seen that there aresome fractured and fine microfibrils, and the others appearin the form of fibers with comparatively large diameter,indicating that large viscosity ratio leads to PET microfi-brils with a low aspect ratio and wide diameter distribu-tion. The same tendency can be observed for B94 inFig. 8c. However, for B83, the morphology has greatlychanged, in which few fractured microfibrils can be seen,and the microfibrils show a narrow diameter distributionalmost in the range of 2–4 lm. As for B49, the morphologyof microfibrils also exhibits a high aspect ratio and a uni-form diameter distribution. So a preliminary conclusioncan be made that the blends with low viscosity ratio facil-itates formation of finer microfibrils with high aspect ratioand narrow diameter distribution, Although the exactmechanism is unclear, it is widely accepted that in situmicrofibrillation is dependent on two simultaneous pro-cesses: the large deformation of dispersed phase dropletsand the coalescence of deformed droplets during stretch-ing, which has been proved by Scott and Macosko[47,48]. Fig. 8 shows that for all the blends in this studydispersed phase can form in situ microfibrils through slitdie extrusion and hot stretching, however, low viscosityratio blend is more favorable to form the microfibrils withhigh aspect ratio and narrow diameter distribution.According to the process of microfibrils’ formation, it iseasy to know that the sizes of droplets and their distribu-

Fig. 8. SEM images of PET microfibrils in the microfibrillar blends with differentand (d) B121.

tion dominate the corresponding morphology of microfi-brils and their distribution. Comparing the diameter oforiginal droplets with the final microfibrils, the averagediameter difference is not so distinct, for example, inB83, the average diameter of PET droplets is about2.58 lm while the corresponding microfibrils is about2.3 lm. This seems that the dominant factor influencingthe morphology of microfibrils is coalescence of deformeddroplets during stretching. Eq. (2) illuminates the potentialability of droplets’ deformation in melt blends under elon-gational flow conditions [49], which is based on thehypothesis that the volume of the droplets is unchangedand coalescence does not occur between dispersed phaseparticles throughout the morphology development.

LD¼ 2

3D0

D

� �3

ð2Þ

where L/D is the potential aspect ratio of microfibrillarformed by a single droplet, D0 and D are diameter of drop-let before and after deformation, respectively. However,the deformation of droplets is restricted by adjacent ones,and the large droplets in the melting blend may result inlow aspect ratio and large diameter of microfibrils. So, itshould keep in mind that large domains of dispersed phasedo not mean high aspect ratio of microfibrils, but smalldroplets certainly lead to thin microfibrils. In biphasic sys-tems, when the droplets are of narrow diameter distribu-tion before stretching, they form uniform microfibrils

viscosity ratios, in which iPP was removed away. (a) B49, (b) B83, (c) B94

726 X. Yi et al. / European Polymer Journal 46 (2010) 719–730

because the original droplets have the same deformationability and the same probability of coalescence. It is noweasy to understand the changes of morphology with differ-ent viscosity ratios in Figs. 6 and 8. Taking B83 and B121for example, the lower viscosity ratio for B83 has smallerdiameter of droplets and narrow diameter distribution, soits corresponding diameter distribution of microfibrils be-comes more uniform. While for the higher viscosity ratiosystem (such as B121), droplets of PET are less uniform,leading to less uniform microfibrils as shown in Fig. 8d.The effect of viscosity ratio on morphology of microfibrilscomprises other two factors of the rheological properties:firstly, large shear viscosity of matrix (lower viscosity ratiosystem) has large friction in interface between matrix anddispersed phase, and the increase of friction enhances thedeformation of droplets; Secondly the elastic characteristicis stronger for the system with higher viscosity matrix,such as iPP1 and iPP2, and the elastic factor in viscoelasticsystem has the positive effect on stabilizing dispersedphase, leading to form more uniform microfibrils and avoidbreaking up of the formed microfibrils. Therefore, the B49and the B83 show finer microfibrils with higher aspect ra-tio, compared to higher viscosity ratio systems.

Morphology of the compatibilized microfibrillar blendsis shown in Fig. 9a, b and c. The surface of microfibrils be-comes more random, showing many flat belt-like microfi-brils rather than completely cylindrical ones, which ismainly due to improvement of compatibility between ma-trix and dispersed phase. Another apparent phenomenon is

Fig. 9. SEM images of microfibrils in compatibilized blends with var

that there are very low aspect ratio microfibrils in the com-patibilized microfibrillar blends, which is in accordancewith Fakirov’s results in which the peculiar property ofcompatibilizers, ethylene-glycidyl methacrylate (E-GMA)was used [50]. As discussed above, uniform size distribu-tion of particles can lead to uniform microfibrils. However,it is not the case for the compatibilized system, for in-stance, the most uniform PET particles (Fig. 7c) correspondto less uniform distribution of microfibrils (Fig. 9c). This isoriginated from the influence of PP-g-GMA: firstly, the de-crease of interfacial tension caused by compatibilizer leadsto smaller PET droplets and then to form thin microfibrils,and those thin PET microfibrils are easy to broken up; Sec-ondly, PP-g-GMA coats the surface of the PET droplets dur-ing mixing, and prevents their coalescence during drawing,which reduces the possibility of coalescence of PET micro-fibrils, additionally, the reaction of functional groups be-tween PP-g-GMA and PET enhances the contact of theinterfaces, which protects from deformation and slip ofdroplets during melting, thus forming microfibrils withhigh aspect ratio. Moreover, as shown in Fig. 3, the viscos-ity ratio increases after addition of compatibilizer. Eitherdecrease of matrix apparent viscosity or increase of dis-persed phase apparent viscosity would lower friction onthe interface, which is a negative factor for generation ofmicrofibrils.

The morphology of the injection molded samples afteretched by hot xylene is shown in Fig. 10. It can be seen thatthe microfibrils become isotropic distribution and tangled

ious contents of PP-g-GMA (a) 2 wt%, (b) 4 wt% and (c) 6 wt%.

Fig. 10. SEM micrographs of the microfibrils in injection molded blends after etching of matrix, iPP (a) B83; (b) B94; (c) B121; (d) PET/iPP2/PP-g-GMA, withthe amount of compatibilizer of 6 wt%.

0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11 0.12 0.1330

31

32

33

34

35

36

37

2200

2300

2400

2500

2600

2700

2800

2900

3000

Tens

ile s

tren

gth

(MP

a)

Viscosity ratio

You

ng, s

mod

ulus

(M

Pa)

Fig. 11. Tensile strength and modulus as a function of viscosity ratio ofPET/iPP in situ microfibrillar blends.

0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11 0.12 0.1342

44

46

48

50

52

54

56

1600

1700

1800

1900

2000

2100

2200

2300

2400

2500

Fle

xura

l mod

ulus

(M

Pa)

Viscosity ratio

Fle

xura

l str

engt

h (M

Pa)

Fig. 12. Flexural strength and modulus as a function of viscosity ratio ofPET/iPP in situ microfibrillar blends.

X. Yi et al. / European Polymer Journal 46 (2010) 719–730 727

with each other due to the complicated stress, flow andtemperature fields during injection molding. However,the microfibrils are suffered from a certain degree of break-down, especially for the finer ones, which may be due tothe intense shear and elongational flow at high processingtemperature. Though, most of the microfibrils are of thesame characteristic as the comparative ones, that is, theystill have a high aspect ratio since few ends of the microfi-brils can be observed in the SEM images. The difference be-

tween various systems still exists. The microfibrils have along aspect ratio and narrow diameter distribution in theblends with higher viscosity ratio, as shown in Fig. 10a,while in the blends with lower viscosity ratio, there aresome broken microfibrils, as shown in Fig. 10c. The micro-fibrils in the compatibilized blend are destroyed drasticallyafter injection molding process, because there are only afew microfibrils with very low aspect ratio. Generally, thechanging trends of microfibrils for various systems after

0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11 0.12 0.13

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

Impa

ct s

tren

gth

KJ/

m2

Viscosity ratio

Fig. 13. Impact strength as a function of viscosity ratio of PET/iPPmicrofibrillar blends.

0 1 2 3 4 5 644

46

48

50

52

54

56

58

17001800190020002100220023002400250026002700

Fle

xura

l mod

ulus

/MP

a

Fle

xura

l str

engt

h /M

Pa

PP-GMA content /wt%

Fig. 15. Flexural strength and modulus of in situ microfibrillar blend withdifferent concentration of PP-g-GMA based on B83.

728 X. Yi et al. / European Polymer Journal 46 (2010) 719–730

injection molding is almost the same as just stretchedblends.

3.3. Mechanical properties of PET/iPP blends

The tensile strength and modulus of injection moldedmicrofibrillar blends containing 15 wt% PET is plotted asa function of viscosity ratio in Fig. 11. The maximum ten-sile properties appear at a small viscosity ratio, i.e., 0.083(B83). At higher viscosity ratio, the tensile strength andmodulus reduces linearly. A similar tendency is visiblefor the flexural strength and modulus shown in Fig. 12.Fig. 13 shows the effect of viscosity ratio on the notchedimpact strength. The maximum value also appears at theviscosity ratio of 0.083. Meanwhile, it is slightly higher ofsamples in tensile strength, flexural strength and flexuralmodulus than corresponding neat iPP. The systems withhigh viscosity ratio exhibit a distinct reinforcing effect,such as the blend B94, with a viscosity ratio of 0.094. It isinteresting that reinforcing, stiffening and toughening ef-fects of the microfibrils coexist in our microfibrillar blends,while for conventional blends, the enhancement of tensileand flexural strength is usually at the sacrifice of impacttoughness [51,52]. It can be explained with the influenceof PET microfibrils that in B83 blends, the microfibrils havehigher aspect ratio and more uniform diameter distribu-tion than the microfibrils in other blends.

0 1 2 3 4 5 633

34

35

36

37

38

39

40

2320

2400

2480

2560

2640

2720

2800

2880

2960

Tens

ile s

tren

gth

(MP

a)

PP-g-GMA content (wt%)

You

ng, s

mod

ulus

(M

Pa)

Fig. 14. Tensile strength and modulus of in situ microfibrillar blend withdifferent amounts of compatibilizer based on B83.

The compatibilized microfibrillar blends display im-proved mechanical properties, as shown in Fig. 14. The ten-sile strength increases clearly after the addition of thecompatibilizer, and reaches the highest tensile strengthof 38.6 MPa, namely, 4.5 MPa higher than that of theuncompatibilized ones. The young’s modulus, a parameterclosely related to the static stress transfer at the interface,shows a decrease more and more evidently along with thecontent of compatibilizer, PP-g-GMA. As mentioned above,the compatibilizer is mainly located in the interface, butsince the lower stiffness than PET microfibrils, the rela-tively compliant layer at the interface may firstly producesplastic deformation, so the value determined within only asmall strain range reduces with the increase of PP-g-GMA.The flexural modulus of the compatibilized samples isslightly lower than those uncompatibilized ones, whichmay also due to the introduction of relatively compliantcompatibilizer, PP-g-GMA. The flexural strength has alsoa distinct increase after addition of PP-g-GMA., as shownin Fig. 15. The improved compatibility caused by the PP-g-GMA obviously contributes to improved properties,especially for impact strength. As shown in Fig. 16, the im-pact strength is not obviously increased until the contentof PP-g-GMA is up to 4 wt%. Compared with the uncompat-ibilized ones, the impact strength is increased by about30% when 6 wt% PP-g-GMA is used. Essentially, the addi-

0 1 2 3 4 5 6

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

4.2

Im

pact

str

engt

h (K

J/m

)2

PP-g-GMA content (wt%)

Fig. 16. Izod impact strength of in situ microfibrillar of B83 with differentamount of compatibilizer.

X. Yi et al. / European Polymer Journal 46 (2010) 719–730 729

tion of compatibilizer can lead to two effects (1) increasethe compatibility for microfibrillar blends proved appar-ently as in the SEM images in Fig. 7, which leads to highmechanical properties compared to the uncompatibilizedblends, especially for impact strength; (2) the more frac-tured microfibrils appear with the increase of PP-g-GMA,which can reduce the aspect ratio of microfibrils and leadsto decrease of mechanical properties. These two factorshave the opposite effects on the improvement of themechanical properties, and finally the positive effect ofcompatibility between matrix and dispersed phase onmechanical properties is dominated, so both tensilestrength and impact toughness for the compatibilizedblends are superior to the uncompatibilized ones.

4. Conclusion

Four types of PET/iPP microfibrillar blends with differ-ent viscosity ratios were investigated. The viscosity ratioplays an important role in the size of dispersed phase,and subsequently affects the morphology of PET microfi-brils during stretching. Low viscosity ratio facilitates for-mation of smaller and more uniform dispersed phaseparticles, and then leads to narrow diameter distributionof microfibrils after stretching, which results in higherstrength and modulus. The appropriate viscosity ratio forimproved mechanical properties is about 0.083. Additionof compatibilizer, PP-g-GMA, can increase the viscosity ra-tio and decrease the interfacial tension, which has theopposite effect on the size of PET droplets and generallyleads to smaller diameter of particles. After hot stretched,the aspect ratio of PET microfibrils in the compatibilizedblends is lower than that of the uncompatibilized ones.However, the improvement of interfacial adhesion leadsto better mechanical properties than the uncompatibilizedsystem, especially for the tensile strength and impactstrength. The appropriate amount of compatibilizer isabout 2 wt%. An interesting observation is that for B49with the lowest viscosity ration, the morphology of bothparticles and microfibrils of PET has no distinct differencecompared to B83, however, their mechanical propertiesbehave quite different.

Acknowledgements

The authors gratefully acknowledge the financial sup-port of this subject by National Natural Science Foundationof China (Grant No. 20776087) and National Programs forHigh Technology Research and Development of China(Grant No. 2008AA03Z510).

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