evolution of phase morphology of high impact polypropylene particles upon thermal treatment

EUROPEAN

European Polymer Journal 43 (2007) 2999–3008

www.elsevier.com/locate/europolj

POLYMERJOURNAL

Evolution of phase morphology of high impactpolypropylene particles upon thermal treatment

Yong Chen a, Ye Chen a, Wei Chen b, Decai Yang a,*

a State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,

Graduate School of the Chinese Academy of Sciences, Chinese Academy of Sciences, Changchun 130022, PR Chinab Beijing Research Institute of Chemical Industry, China Petroleum and Chemical Corporation, Beijing 100013, PR China

Received 7 February 2007; received in revised form 9 April 2007; accepted 19 April 2007Available online 3 May 2007

Abstract

Solvent fractionation and differential scanning calorimetry (DSC) results show that high impact polypropylene (hiPP)produced by a multistage polymerization process consists of PP homopolymer, amorphous ethylene–propylene randomcopolymer (EPR), and semicrystalline ethylene–propylene copolymer. For the original hiPP particles obtained right afterpolymerization, direct transmission electron microscopy (TEM) observation reveals a fairly homogeneous morphology ofthe ethylene–propylene copolymer (EP) phase regions inside, while the polyethylene-rich interfacial layer observed betweenthe EP region and the iPP matrix supports that EP copolymers form on the subglobule surface of the original iPP particles.Compared with that in original hiPP particles, the dispersed EP domains in pellets have much smaller average size andrelatively uniform size distribution, indicating homogenization of the EP domains in the hiPP by melt-compounding. Uponheat-treatment, phase reorganization occurs in hiPP, and the dispersed EP domains can form a multiple-layered core–shellstructure, comprising a polyethylene-rich core, an EPR intermediate layer and an outer shell formed by EP block copoly-mer, which accounts to some extent for the good toughness-rigidity balance of the material. The results indicate that toestablish the optimum phase structure and desired properties for hiPP, both the architecture of original hiPP particlesand subsequent melt-processing conditions should be carefully modulated.� 2007 Published by Elsevier Ltd.

Keywords: High impact polypropylene; Phase morphology; Evolution; Thermal treatment

1. Introduction

To improve the impact properties of isotacticpolypropylene (iPP), a rubbery ethylene–propylene

0014-3057/$ - see front matter � 2007 Published by Elsevier Ltd.

doi:10.1016/j.eurpolymj.2007.04.026

* Corresponding author. Tel.: +86 431 85262139; fax: +86 43185262126.

E-mail address: [email protected] (D. Yang).

copolymer (EP) phase is usually incorporated intothe iPP matrix, forming heterophase impact poly-propylene blends. It is well known that phase mor-phology has great influence on the performance ofsuch kind of material. Different from the mechanicalblends of iPP/EP, the original phase structure of thereactor-made polypropylene blends, also called highimpact polypropylene (hiPP), is not formed throughmelt-blending; instead it is established directly in

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3000 Y. Chen et al. / European Polymer Journal 43 (2007) 2999–3008

reactor by a multistage polymerization processbased on spherical TiCl4/MgCl2 catalyst [1–4]. Inthe first stage, iPP particles with a tailored pore net-work inside are prepared, while in the second stage,ethylene/propylene comonomers diffuse into the iPPparticles, forming well distributed EP elastomerphase in the pores.

As the hiPP particles produced after copolymer-ization (the original hiPP particles) can be directlyused without post-reactor pelleting, a deeperunderstanding of the phase morphologies of origi-nal hiPP particles and the melt-processed productsis necessary. The architecture of original hiPP par-ticle has been revealed in our previous report [5] asshown schematically in Fig. 1. The precursor of ahiPP particle, i.e., an iPP particle, is an agglomer-ate of many subglobules (ca. several to hundredmicrons in diameter), which in turn are composedof a great deal of primary particles (ca. 100 nm indiameter) of iPP. Within the hiPP particles, thereare relatively large EP phase regions between thesubglobules and finely distributed EP domainswithin the subglobules. The composition of theEP phase formed during copolymerization stageis not simply the amorphous EP random copoly-mers (EPR). As a consequence of the heterogeneityof the active sites in the Ziegler-Natta catalyst

Fig. 1. Schematic illustration of the architecture of an originalhiPP particle.

[6–9], highly non-crystalline EP random copolymerand semicrystalline EP copolymers, which may beethylene-rich or propylene-rich, have been identi-fied in the EP phase [10–12]. Moreover, althoughsynthesis of the true EP block copolymer with bothcrystalline segments based on Ziegler-Natta systemhas been a debate of a long time [12–16], somerecent papers [17–21] reported that EP multiblockcopolymers with PE and PP segments of differentlengths can indeed be produced in hiPP, and inter-chain continuity and intrachain polydispersity arecharacteristics of the hiPP chain structure. Any-way, the complicated composition of the EP phasesuggests the multicomponent and multiphase nat-ure of hiPP.

On the other hand, the phase character of melt-processed hiPP products (e.g. the pellets) was alsostudied. Largely depending on the ethylene contentof the EP copolymers, a core–shell structure of thedispersed phase, comprising a PE core and a singleEPR shell, is usually observed in the matrix of hiPP[22–27].

Unambiguously, the physical properties of end-products of hiPP are closely related to the phasestructure therein, which can be very different fromthat of the original hiPP particles. Interest of thepresent study was thus to explore the evolution ofthe EP phase morphology from the original hiPPparticles to the melt-processed products. Intricatechanges occurred in the EP phase upon thermaltreatment were revealed using differential scanningcalorimetry (DSC), field-emission scanning electronmicroscopy (SEM), optical microscopy, and espe-cially direct transmission electron microscopy(TEM) observation of thin sections of hiPP particleswithout staining.

2. Experimental

2.1. Materials and sample preparation

Two kinds of hiPP materials were used in theexperiment, the ‘‘original particles’’ and the ‘‘pel-lets’’. The original hiPP particles (the same materialused in our previous study [5]) were obtaineddirectly after a two stage sequential polymerizationwith high activity TiCl4/MgCl2 catalyst. The weight-average molecular weight was 2.06 · 105 and thepolydispersity index (Mw/Mn) was 6.1, measuredby GPC. The ethylene content of the hiPP wasca. 14.1 mol% as determined by 13C NMR. The

Fig. 2. DSC melting curves of the original hiPP particles in twoheating runs. The heating and cooling rates are 10 �C/min.

Y. Chen et al. / European Polymer Journal 43 (2007) 2999–3008 3001

particles (ca. 1 mm in diameter) were embedded inEPONTM 812 resin, which was solidified at ca.55 �C for 30 h; then the resin blocks, each withone hiPP particle encapsulated inside, were heat-treated in an oven and then cooled at room temper-ature. The hiPP pellets were prepared throughmelt-compounding of the original hiPP particles at200 �C with a single-screw extruder. Some of thepellets were aged at 200 �C and then cooled at roomtemperature. Thermal treatments performed in hotoven were all under nitrogen gas protection. Sam-ples were sectioned with glass knife in a Leica Ultra-cut R microtome operated at �90 �C and a cuttingspeed of 1 mm/s. Cross-section surfaces of somesamples were extracted with xylene for 30 min atroom temperature.

Three fractions of the hiPP were also obtained bysuccessive solvent extraction. The original hiPP par-ticles were dissolved in xylene at 130 �C, and thenthe solution was slowly cooled to room tempera-ture. The precipitate was separated from the solu-tion by filtration. The solvent in the remainingclear solution was evaporated, and the rubberycomponent was obtained as fraction Fa. The filteredprecipitate was extracted by xylene at 100 �C, andtwo fractions were obtained after solvent evapora-tion: the dissolved part as fraction Fb and theremainder as fraction Fc.

2.2. Measurements

For TEM observation, thin sections of the sam-ples were transferred onto copper grids and subse-quently vacuum coated with a thin layer of carbonto minimize beam damage. A JEOL 1011 TEMoperated at 100 kV was used and bright-field (BF)electron micrographs were obtained.

A Zeiss A1M optical microscope operated inreflection mode was used to observe the cross-sec-tion of the sample.

The cross-section surface after extraction wascoated with Au and then examined with an XL30ESEM FEG scanning electron microscope at anaccelerating voltage of 20 kV.

DSC measurements were carried out using a Per-kin Elmer Diamond differential scanning calorimeterunder a protective nitrogen atmosphere. Sampleswere heated from room temperature to 210 �C andmaintained for 5 min, and then cooled to 50 �C andmaintained for 5 min, and subsequently heated to210 �C. Both the heating and the cooling rates were10 �C/min.

3. Results and discussion

3.1. Thermal behavior of the original hiPP particles

and the fractions

Fig. 2 shows the first and the second heatingcurves of the original hiPP particles. The first heat-ing curve reflects the thermal behavior of the mate-rial right after polymerization, where twotransitional regions, one at ca. 80–125 �C and theother at ca. 125–170 �C, correspond to the meltingof the PE and PP crystals in the hiPP, respectively.Moreover, the weak and broad melting region ofthe crystalline PE not only indicates the presenceof EP copolymer with long PE sequences in the hiPP(suggesting the ethylene-specific active sites of thecatalyst), but also means a broad size distributionof the PE crystallites in the EP phase of the originalhiPP particles. As for the PP, a high degree of iso-tacticity and crystallinity should be formed duringpolymerization according to the narrow meltingpeak and the high melting peak temperature (ca.165 �C). Compared with the first heating curve, inthe second heating scan, the melting peak of thePE crystals becomes narrower and more evident,while the crystallinity of PP also increases from ca.31.4% (first heating) to ca. 40.1% (calculated basedon DH 0

f ¼ 207:1 J=g [28]), indicating the formationof more perfect crystalline phases of PE and PPafter recrystallization, especially PE.

Crystallization and melting behavior of the hiPPand its three fractions are shown in Fig. 3a and b,respectively. In Fig. 3a, the hiPP exhibits two crystal-lization peaks, which are attributed to crystallization

Fig. 3. DSC cooling (a) and reheating (b) curves of the hiPP andits three fractions. The cooling and reheating rates are 10 �C/min.


of PP (the major one at 116.9 �C) and PE crystals(the weaker one at 95.5 �C), respectively, and corre-spond to the second melting curve of the hiPP asshown in Fig. 2. The solvent fractionation for thehiPP was performed according to the crystallizabil-ity of its various components. Since Fa was separatedfrom xylene solution of the hiPP at room tempera-ture, it should mainly be composed of amorphousEP random copolymers. However, from the coolingand reheating curves of Fa, a weak crystallizationpeak at 70.1 �C and an obvious one at 113.8 �Ccan be found, as well as a broad melting peak atca. 160.0 �C. This observation indicates the presenceof crystalline EP copolymers with short PE (and PP)segments and fairly amount of low molecular weightPP molecules in Fa. As for Fb, which is the solublefraction in xylene at 100 �C, in the cooling curve,there exist a main crystallization peak at 112.0 �C

and shoulder peaks at lower temperatures, whichcorrespond to the crystallization of PP and PE(including blocks and homopolymers) respectively,whereas in the reheating curve, multiple meltingpeaks are observed, the one at lower temperature(119.2 �C) corresponding to PE melting and thoseat higher temperatures (from ca. 142.5 �C to158.8 �C) mainly attributed to PP crystals formedby PP blocks of different lengths. For fraction Fc

(the insoluble fraction at 100 �C), the DSC coolingand reheating curves show typical thermal behaviorof PP homopolymer. Apparently, the compositionand chain structure of the hiPP are quite complex,and there is a continuous distribution of interchaincomposition and an intrachain heterogeneity of hiPPchain structure [10–21]. These characters inevitablyimply complex phase morphologies of the hiPP. Inaddition, the weight fractions of Fa, Fb and Fc inthe hiPP were ca. 20.0%, 10.0% and 70.0%, respec-tively, which suggest that in the hiPP, polypropylenehomopolymer (Fc) constitutes the matrix providingrigidity, whereas the rubbery component (Fa) playsthe role of toughening agent with the segmentedEP copolymer (Fb).

3.2. Morphology of the EP phase in the original hiPP

particles

Generally, to observe the phase morphology ofhiPP multiphase polymers with TEM, the elastomerphase is usually stained with heavy atoms to enhancethe contrast in ultrathin sections [3,11,23,27]. How-ever, some information about delicate structureswithin the EP phase is inevitably lost by the staining.In fact, differentiation of the PP, PE and EPR phasesby direct observation under TEM is possible largelybased on their slight differences in density, i.e., PEcrystals exhibit a stronger contrast than that of crys-talline PP, while the amorphous EPR phase has theweakest contrast [29–32]. With careful sample prep-aration, structural details of the EP phase in hiPP arerevealed in this work.

Fig. 4 is a BF electron micrograph of thin sectionof an original hiPP particle. A relatively large EPphase region between the hiPP subglobules is dis-played, which exhibits weaker contrast than thematrix of the iPP subglobules. Some sparsely dis-persed darker patches inside the EP region shouldbe mainly PE crystallites (maybe including somePP crystallites). This is consistent with the DSCresults and confirms the formation of imperfectPE crystallites in the hiPP copolymerization pro-

Fig. 4. BF electron micrograph of a thin section of an originalhiPP particle, showing the EP phase region between hiPPsubglobules. Fig. 5. BF electron micrograph of a thin section of a hiPP

particle after heat-treatment at 130 �C for 120 min, showingmorphological changes in the EP phase region between hiPPsubglobules.


cess. Moreover, it is interesting to note that there isalways an interfacial region (exhibiting darker con-trast) between the EP phase and the subglobules,implying higher accumulation of PE crystallites inthese interfacial areas. This observation providesfurther evidence that EP copolymer is formed onactive sites on the surface of iPP subglobules [3,5].

3.3. Morphological changes of the EP phase in the

hiPP particle induced by thermal treatment

Fig. 5 presents a BF electron micrograph of anEP phase region located between subglobules ofhiPP particle after heat-treated at 130 �C for120 min. Here the matrix of the particle (iPP subglo-bules) seems to have not altered much comparedwith that of the original particle (Fig. 4), while greatchanges have occurred in the EP phase regions.Many dispersed circular domains (see from thecross-section) of tens of nanometers to severalmicrons in diameter, can clearly be observed insidethe EP region. These inclusions probably are mainlycomposed of PE crystals (some iPP crystallites cannot be excluded as well), formed through theagglomeration of PE-rich copolymer in the EPphase in the melting state. This quiescent phasecoarsening may be described as an Ostwald ripeningprocess [33–36], taking place in the current case bydiffusion and accumulation of the crystalline PE-rich copolymer molecules driven by interfacialforces between these copolymers and the amor-

phous EPR copolymer. It is worth noting that thePE crystallites enriched close to the subglobule sur-face (see also Fig. 4) did not disappear or reduce;rather they became more perfect after long-timeannealing, implying these copolymers should becrystalline PE–PP block copolymers rooted on thesubglobule surface. The above observations wellreflect the complicated compositional heterogeneityof the EP copolymers in hiPP (see also the resultsbelow).

Quiescent heat-treatment of the hiPP particleswas also performed at 200 �C. Here the architectureof the original hiPP particle established in reactorwould be destroyed. As shown in Fig. 6a, a typicalphase-separated morphology is observed after theparticle was melted at 200 �C for 20 min. DeformedEP domains of various sizes are dispersed in a uni-form iPP matrix. A closer examination of these EPdomains reveals one or more PE-rich inclusionsinside (Fig. 6b), displaying a multiple-phase mor-phology. Unambiguously, this phase evolution pro-cess by quiescent melting starts from a pre-existingphase-separated architecture of the original hiPPparticles, rather than an intimate mixture of differ-ent molecular species of hiPP as prepared by solventprecipitation [34,35].

With a longer melting time, morphology of theEP phase within the hiPP particle continues toevolve. Fig. 7a and b shows phase morphologiesof the particle after heat-treatment at 200 �C for

Fig. 6. Phase morphology in a hiPP particle after melted at200 �C for 20 min. (a) Optical micrograph of the cross-section ofthe particle and (b) BF electron micrograph of a thin section ofthe particle.

Fig. 7. Phase morphology in a hiPP particle after melted at200 �C for 100 min. (a) Optical micrograph of the cross-section ofthe particle and (b) BF electron micrograph of a thin section ofthe particle.


100 min. Compared with Fig. 6, further agglomera-tion of the EP domains and the PE-rich inclusionscan clearly be observed, indicating that for bothEPR and PE-rich copolymer, phase reorganizationin a larger scale occurs upon aging at such a longtime. It should be noted that even under a long-timecoarsening at 200 �C, many PE-rich inclusionsinside the EP domains still maintain a discrete state,implying that some EP block copolymer may bepresent at the interface between the PE-rich phaseand the EPR phase inside the EP region and stabi-lize the phase structure. That is, the interfacially-active block copolymer will suppress the phasecoarsening process [37–41].

3.4. Phase morphology of the hiPP pellet and its

evolution upon thermal treatment

The hiPP pellets were obtained through melt-compounding of the original hiPP particles. Fig. 8shows a SEM micrograph of the cross-section sur-face of a hiPP pellet after extraction with xyleneto remove the EPR. Compared with the morphol-ogy of the original hiPP particle (Fig. 4), the dis-persed EP domains in the hiPP pellet have muchsmaller average size and relatively uniform size dis-tribution (ca. 0.5–2.0 lm in diameter), indicatinghomogenization of the EP domains in the hiPP bymelt-compounding. It should be noted that evenunder violent conditions (melt-compounding andsubsequent water-cooling), the EP phase still forms

Fig. 8. SEM micrograph of the cross-section surface of a hiPPpellet after being extracted with xylene.

Fig. 9. SEM micrographs of the extracted cross-section surfaces of hiPP(b) 30 min; (c) 100 min; (d) 200 min.


a core–shell structure with a PE core and an EPRshell, implying fast phase separation and recon-struction processes of various components in thehiPP during the melt-compounding and subsequentsolidification.

Melt annealing of the pellets was also performedand more stable phase structure was expected. Fig. 9presents SEM micrographs of the extracted cross-section surfaces of hiPP pellets after storage in themelt at 200 �C for a series of times. With a shortmelting time (10 min), the average size of the dis-persed particles (for both the PE core and theEPR shell) increases rapidly and the size distribu-tion becomes broader (ca. 0.5–4.0 lm). The particlemorphology (including the average size and size dis-tribution) reaches stabilization after 30 min of melt-ing (ca. 0.5–5.0 lm), and most of the particles

pellets after maintaining at 200 �C for a series of time: (a) 10 min;


exhibit a single-core structure and more regularshapes. Subsequent longer times of coarsening(100 min and 200 min) only make the average parti-cle size increase a little. These results indicate thatthe phase morphology of the hiPP particles aftermelt-compounding is not stable. The EPR and thePE-rich copolymer diffuse and accumulate to formlarger agglomerates in the melt state, i.e., to enlargethe PE core and the EPR shell of the core–shell par-ticles. Clearly, this process is relatively easy for theEPR molecules, while for the PE-rich copolymermolecules, they have to pass through the iPP matrixand the EPR layer to reach the core area. On theother hand, the stabilization of the phase morphol-ogy is attributed to the interfacially-active EP blockcopolymer, which resides in the interfacial areabetween the dispersed domains and the iPP matrixand retards the Ostwald ripening process of theEP copolymer. As a result, there is a balancebetween the interfacial forces, which propel phasegrowth, and the stabilization effect brought by theEP block copolymer in the hiPP.

The interfacial layer formed by the EP blockcopolymer can indeed be observed from the TEMimage of a thin section of a hiPP pellet as shownin Fig. 10. The overall phase morphology is consis-tent with the corresponding SEM observation(Fig. 9c), while the interfacial layer between the dis-persed particles and the iPP matrix can be observedas a dark ring around each particle, which should beformed by the EP block copolymer with crystalline

Fig. 10. BF electron micrograph of a thin section of a hiPP pelletafter melted at 200 �C for 100 min, showing dispersed EPdomains with multiple-layered core–shell structure.

PE blocks. That is, the dispersed EP phase in thehiPP in fact can form a kind of multiple-layeredcore–shell structure, consisting of a PE-rich core,an EPR intermediate layer, and an outer shellformed by EP block copolymer.

It is well known that iPP/EPR/PE ternary blendswith EPR and PE as minor components usuallyform well defined core–shell structure, consistingof a PE core and an EPR shell [42–44]. Formationof this morphology has been explained by differ-ences in the spreading behavior of the individualblend components, and the dominant morphologyis the one with the lowest interfacial energy or posi-tive spreading coefficient [44–48]. Although the hiPPhas various EP copolymers with complex composi-tions, the primary components are PP, EPR andPE-rich copolymer, so the mechanism of core–shellformation in the hiPP should generally resemblethat of the iPP/EPR/PE ternary blends. However,the EP block copolymer in the hiPP also plays animportant role in the phase formation of the hiPP,as displayed by the formation of the interfacial layer(outer shell) between the EPR and the iPP matrix,which helps to bridge the EP phase with the matrixand stabilize the phase structure. In fact, an interfa-cial layer between the PE-rich core and the EPRlayer may also exist in light of complex species ofthe EP copolymer in the hiPP (Fig. 7).

As a special kind of impact modifier, multiple-layered core–shell particles with tailored structureand composition, e.g. PMMA/acrylate-rubber/PMMA core–shell particles, have proved to be effi-cient for achieving an optimum balance betweentoughness and stiffness of the matrix material(PMMA, PSAN et al.) [49–52]. The EP particles inthe hiPP exhibit similar morphological features,where PE crystals form rigid core and EPR formselastomeric intermediate layer, while various EPblock copolymers form interfacial layer bondingthe EP phase and the iPP matrix. It is reasonableto believe that this kind of phase structure of thehiPP explains, to some extent, why the materialcan keep a good balance between rigidity andtoughness.

4. Conclusion

Evolution of phase morphology of the hiPP par-ticles under different thermal conditions indicatesthat hiPP is a complex multicomponent and multi-phase thermoplastic material. The EP phase formedduring polymerization consists of amorphous EP


copolymers, various crystalline EP block copoly-mers with different lengths of PE, PP and EP blocks.The EP region in the original hiPP particle exhibitsa relatively homogeneous morphology with sparselydispersed PE crystallites, while the PE-rich interfa-cial layer observed between the EP region and theiPP matrix supports that EP copolymers form onthe subglobule surface of the original iPP particles.Melt-compounding promotes uniform distributionof the EP regions in the hiPP. Reorganization ofthe phase morphology takes place during ther-mal treatments. The PE-rich copolymers diffuseand accumulate to form PE-rich agglomerateswithin the EP region. The ethylene-propylene blockcopolymers can suppress the phase growth and sta-bilize the phase structure by forming an interfaciallayer between the EP phase and the PP matrix. Amultiple-layered core–shell structure of the dis-persed EP phase is formed, comprising a PE-richcore, an EPR intermediate layer and an outer shellformed by EP block copolymers, which accountsto some extent for the good toughness-rigidity bal-ance of the material. The results indicate that inorder to establish the optimum phase structure ofhiPP for desired performance, both the architectureof the original hiPP particle and the subsequentmelt-processing conditions should be carefullymodulated.

Acknowledgements

Financial support from the National BasicResearch Program of China (2005CB623800) andChinese Academy of Sciences (KJCX2-SW-H07)is gratefully acknowledged. Use of the ZeissA1M optical microscope in Prof. Yongfeng Men’slab at Changchun Institute of Applied Chemistry isalso acknowledged.

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evolution of phase morphology of high impact polypropylene particles upon thermal treatment

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