at SciVerse ScienceDirect

Polymer 53 (2012) 4861e4870

Contents lists available

Polymer

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

Enhancement of b-nucleated crystallization in polypropylene random copolymervia adding isotactic polypropylene

Feng Luo, Yanling Zhu, Ke Wang*, Hua Deng, Feng Chen, Qin Zhang, Qiang Fu*

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

a r t i c l e i n f o

Article history:Received 17 April 2012Received in revised form15 June 2012Accepted 18 August 2012Available online 30 August 2012

Keywords:Polypropylene random copolymerCrystallizationb-modification

* Corresponding authors. Tel./fax: þ86 28 8540540E-mail addresses: wkestar@scu.edu.cn (K. Wang),

0032-3861/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.polymer.2012.08.037

a b s t r a c t

Polypropylene random copolymer (PPR) is one of important polypropylene types for the applicationfields needing for excellent toughness. Because of the random copolymer chain configuration, thepolymorphic behavior of PPR is difficult to be altered even by adding b-nucleating agent (b-NA). In thisstudy, a promising method was developed by adding isotactic polypropylene (iPP) into PPR/b-NA blend,which has leaded to a surprising enhancement in the b-crystallization capability of PPR. At the optimalcomponent condition, the b-crystal content of PPR can reach the highest level of 92 % and the b-crys-tallization capability is improved by 56%. As a result of high b-crystal contents, a superior mechanicaltoughness has been attained. On the other hand, the fractional crystallization experiment suggests thatthe stereoregular chains of iPP could assist the formation of primary b-nuclei at the very early stage ofcrystallization. This special crystallization event dominates the final polymorphic composition in PPR.Furthermore, it is demonstrated that impact polypropylene copolymer (IPC) can be used to substitute iPPfor the improvement of b-crystal content of PPR. This provides a huge possibility to improve the lowtemperature properties of PPR to enlarge its applications.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Polypropylene is a universal semicrystalline polymerwith severalcrystalline forms, and common crystalline forms are monoclinica-formandhexagonalb-form. It iswell knownthat theb-formusuallyshows excellent toughness and ductility in contrast to a-form [1e9].For a common consideration, to achieve better toughness, a highcontent of b-crystal is desired. Polypropylene random copolymer(PPR) is one of important polypropylene types, and widely used asmatrix components in pipe, automobile parts, furniture, and otherindustrial uses in the past two decades [10e12]. Copolymerization ofpropylene with ethylene or other olefins, instead of adding elasto-mers, is an effective route to produce high-performance poly-propylene resins [13e16]. In such copolymers, the homopolymersequences are crystallizable to form a compound crystalline- amor-phous biphase,while the propylene-ethylene random segmentswithhigh ethylene content coalesce together to form a rubbery phase.Therefore PPR is indeed a multiphase polyolefin system with anexcellent rigidity-toughness balance. As crystallizable copolymersystem, the macroscopic mechanical properties of PPR are sensitiveto its crystallization behavior. However, the ethylene units arerandomly embedded into the long propylene sequences during

2.qiangfu@scu.edu.cn (Q. Fu).

All rights reserved.

copolymerization. This disrupts the crystallization ability ofpropylene sequence [17e20], with the result that the crystallinebehavior of PPR is difficult to be tuned. To the best of our knowledge,there is by far no effective method for tailoring the crystallizationability of propylene sequence to introduce superior mechanicalperformance in PPR materials. It is even more difficult for PPR toobtain high b-crystal content. Thuswe need to exploremore effectiveway to improve the crystallization capability and b-crystal contentof PPR.

The addition of b-nucleating agent (NA) is effective and acces-sible to help PP to obtain a high content of b-crystal [1,3,21,22]. Insome previous reports [23,24], the b nucleation ability of b-NA wasprominent for homo- and/or block-PP species with good chainstereoregularity, whereas it was weak for stereo-irregular PPspecies, such as PPR. In our previous work, a new rare earth b-nucleating agent (trade-marked as WBG) was adopted to yieldpredominant b-form morphology in isotactic polypropylene (iPP)[3]. While we found that PPR was hard to be b-nucleated by WBG,and the b-crystal content of b-nucleated PPR was as low as 35% [2].Considering the similarities in chain architecture and physical/chemical properties between PPR and iPP, it is logical to ask if the b-crystal contentof PPR canbe improvedvia addingboth iPPandWBG.

According to the above thinking about the manipulation of PPRcrystallization behavior, we attempt to employ iPP, in this study, toassist the b-crystallization in PPR/WBG system. By introducing

Table 1Characters of polypropylenes.

Sample Brand Mw(�104 g/mol)

Mw/Mn MFI (g/10min, 230�C, 2.16 kg)

Ethylenecontent(wt %)

Manufacturer

PPR R200P 72.2 4.5 0.23 3.8 Hyosung, KoreaiPP T30S 39.9 4.6 2.9 0 Dushanzi Petrochemical Co. ChinaIPC SP179 17.4 3.96 10 13. 5 Qilu Petrochemical Co. China

Table 2The relative fraction of b-form (Kb) for PPR or iPP with various WBG contents.

0.025 %WBG 0.05 %WBG 0.1 %WBG 0.2 %WBG

Kb for PPR (%) 6 36 36 36Kb for iPP (%) 15 82 92 91

F. Luo et al. / Polymer 53 (2012) 4861e48704862

appropriate amount of stereoregular iPP chains, it is found that theb-crystallization capability of PPR can be improved prominently.Correspondingly, superior impact-toughness has been attained inthe PPR samples with high b-crystal content. The contributions ofiPP on the increased b-crystallinity have been assessed quantita-tively by a detailed analysis of polymorphic composition, and theunderline mechanism that stereoregular iPP chain-assistedb-crystallization of PPR/b-NA was revealed by a fractional crystal-lization procedure. Furthermore, this idea is extended to chooseanother polypropylene derivate, impact polypropylene copolymer(IPC), for assistance of b-crystallization capability of PPR. IPCsynthesized via a two-step polymerization procedure [25,26],shows obvious compositional heterogeneity and heterophasicmorphology [27e29]. IPC with high b-crystal content and a supe-rior toughness at low temperature has been achieved by addingWBG due to the strong b-crystallization capability of PP homo-polymer with high chain stereoregularity in IPC [30]. Therefore, it ispossible to improve the toughness at low temperature of PPR byboth adding b-NA and IPC to enlarge its applications.

2. Experimental section

2.1. Materials and sample preparation

The characters of commercially available polypropylene randomcopolymer (PPR), isotactic polypropylene (iPP), and impact poly-propylene copolymer (IPC) used in this study are listed inTable 1. Therare earth b-nucleating agent, trade-marked as WBG, was kindlysuppliedbyWinner FunctionalMaterials Co. (Guangdong, China). Thecomposition andmorphology ofWBG can be seen elsewhere [31,32].

Blends of PPR with various contents of iPP and/or WBG wereprepared using a corotating twin-screw extruder (TSSJ-25, China)at barrel temperatures ranging from 170 to 200 �C. To achievedesired loading and good dispersion of small amounts of nucleatingagent (below 0.5 wt %) in PPR matrix, a master batch of 5 wt %WBGin PPR was firstly prepared and then melt-blended with appro-priate amounts of PPR and/or iPP. After drying at 80 �C for 12 h, thepelletized granules were compression-molded under the condi-tions of 200 �C and 10 MPa for 5 min. The concentrations of WBGwere varied from 0 to 0.2 wt%. For convenience, the specimenswere abbreviated as PPR/xiPP/yWBG, where x and y indicate theweight percents of iPP and b-NA, respectively. For example, PPR/10iPP/0.05WBGmeans a blendwith 10wt % iPP and 0.05wt %WBG.Moreover, the PPR/IPC/WBG blends were also fabricated bya similar procedure and marked in the same manner.

Fig. 1. The WAXD spectra of polymorphism for various samples.

2.2. Characterizations and measurements

2.2.1. Wide angle X-ray diffraction (WAXD)A Philips X’Pert pro MPD apparatus was adopted to acquire

WAXD spectra. A conventional Cu Ka (l ¼ 0.154 nm, reflectionmode) X-ray tube at a voltage of 40 kV and a filament current of40 mA were used. The analysis method of WAXD spectra was

described by Huo [33]. The overall crystallinity, Xc, was calculatedaccording to the following equation:

Xc ¼P

AcrystPAcryst þ

PAamorp

(1)

where Acryst and Aamorp are the fitted areas of crystal and amor-phous region, respectively. While the relative amount of the b-formcrystal, Kb, is evaluated according to the method by Turner-Joneset al. [34]:

Kb ¼ Hbð300ÞHað110Þ þ Hað040Þ þ Hað130Þ þ Hbð300Þ

(2)

where Hb(300) is the area of the (300) reflection peak of b-form at2q ¼ 16.1�; Ha(110), Ha(040) and Ha(130) are the areas of the (110),(040), and (130) reflection peaks of a-form, corresponding to2q ¼ 14.1�, 16.9� and 18.6�, respectively.

2.2.2. Differential scanning calorimetry (DSC)The non-isothermal crystallization behavior of the specimens

was recorded using a PerkineElmer pyris-1 DSC instrument withnitrogen as purge gas, calibrated by indium. The mass of the testedsample was about 5 mg.

Table 3The total crystallinity (Xc), relative fraction of b-form (Kb),partial fraction of b-formcoming from PPR ((KbRþDK)WR), partial fraction of b-form coming from iPP (KbzWz)and enhancement of b-crystallization ability DK of PPR for various samples.

Sample Xc (%) Kb (%) (KbRþDK)WR KbzWz DK

PPR 43 0 e e e

PPR/10iPP 48 0 e e e

PPR/0.05WBG 45 36 36 0 0PPR/10iPP/0.025WBG 49 50 40.8 9.2 39PPR/10iPP/0.05WBG 50 82 72.8 9.2 45PPR/10iPP/0.1WBG 51 82 72.8 9.2 45PPR/10iPP/0.2WBG 54 52 42.8 9.2 12PPR/5iPP/0.05WBG 50 74 69.4 4.6 37PPR/15iPP/0.05WBG 50 85 71.2 13.8 48PPR/20iPP/0.05WBG 51 91 72.6 18.4 55PPR/30iPP/0.05WBG 52 92 64.4 27.6 56

Fig. 3. The non-isothermal crystallization behavior of various samples at cooling rate10 �C/min.

F. Luo et al. / Polymer 53 (2012) 4861e4870 4863

2.2.3. Polarized light microscopy (PLM)Morphological observations of various specimens were per-

formed on a Leica DMIP polarized light microscope (PLM) equippedwith a Linkam THMS 600 hot stage under crossed polarizers. Theextruded granules were melted at 200 �C, and squeezed to obtainthin films. Then, the films were heated to 200 �C and held for 5 min.Subsequently the morphological photographs of crystallizationwere recorded with aid of digital camera during cooling.

2.2.4. Microscopy analysisThe scanning electron microscopy (SEM) experiments were

performed by using an FEI Inspect F SEM instrument with anacceleration voltage of 20 kV. In order to clearly check crystalstructure, the cryo-fractured surfaces were firstly etched by 1.5%potassium permanganate in a mixture of 70% sulfuric acid and 30%phosphoric acid for 12 h [35], subsequently etched by xylene at60 �C for 30 min to remove the amorphous EPR phase. Atomic forcemicroscopy (AFM) analysis was performed on a NanoScope IIIMultimode AFM instrument (Vecco, USA) in tapping mode. The finecrystalline structures distinguished from the height images, phase-contrast images and 3D-view images were collected.

2.2.5. Mechanical properties measurementsThe notched Izod impact strengths of the specimens were tested

with a VJ-40 Izod machine according to ASTM D256-04 standard.Standard tensile tests were performed using an SANS Universaltensile testing machine according to ASTM D638-03 standard. Thevalues of all the mechanical properties were calculated as averagesfor over five specimens.

Fig. 2. The WAXD spectra of polymorphism for various samples. (a) Samples with di

3. Results and discussion

3.1. b-Crystallization ability and crystallization behavior of blends

In order to estimate the effect ofWBG on b-crystallization abilityof each component in blends, the relative fraction of b-form content(Kb) for PPR or iPP samples with various WBG contents has beenmeasured firstly and listed in Table 2. A weak b-nucleation effect ofWBG on PPR with the highest Kb about 36% is exhibited, but strongb-modifiable ability is obtained in iPP with Kb up to 92%. In addi-tion, there is a criticalWBG concentration around 0.1wt % for PPR oriPP, above which there was no further improvement in the b-formcontent. Then the polymorphism behaviors in various PPR/iPP/WBG blends were inspected by X-ray diffraction analysis, and theresults are presented in Fig. 1. Quantitative estimations of the totalcrystallinity (Xc) and relative fraction of b-form (Kb) have been doneaccording to the methods mentioned in the Experimental Section,and the calculated values are listed in Table 3. Because PPR and iPPcomponents can both yield b-crystallization by adding WBG, thecontributions of adding iPP on the increased b-crystallinity of PPRare not straightforwardly seen. In order to evaluate the enhance-ment of b-crystallization ability of PPR arisen from the introductionof iPP, a new parameter,DK, is proposed as the following equation:

fferent concentrations of WBG, (b) Samples with different concentrations of iPP.

Fig. 4. The SEM photographs of etched samples. (a) PPR, (b) PPR/0.05WBG, (c) PPR/10iPP/0.05WBG; and a’, b’, c’ are the high magnification pictures of a, b, c, respectively.

F. Luo et al. / Polymer 53 (2012) 4861e48704864

Kb ¼ KbR þ DK WR þ KbzWz (3)

� �

where W is the weight percent of component. KbR is the b-formcontent of PPR purely nucleated by WBG. The z represents iPP, andKbz is the b-form content of iPP purely nucleated by WBG. For theconvenience of calculation, Kbz is fixed at 92% for blends.Thus,(KbRþDK)WR and KbzWz should represent the partial fractionof b-form in blends coming from PPR and iPP, respectively. So theparameter of DK can represent the enhancement of b-crystalliza-tion ability of PPR resulting from the introduction of iPP. It showsthat only a-crystals exist in the neat PPR and PPR/10iPP samples,

and a polymorphism behavior consisted of a-form and b-form isfound in other samples. A quite low Kb of PPR is obtained by onlyadding WBG. Interestingly, quite high b-form contents are attainedby adding iPP into b-nucleated PPR. Particularly, the relative frac-tion of b-form (Kb) is up to 82% and the b-crystallization capabilityof PPR is enhanced by 45% (DK). Moreover, when adding iPP intoPPR (b-nucleated PPR), the scattering angles of the various crys-talline peaks shifted to low angles, indicating increased spacebetween reflecting faces of original crystal of PPR. Besides possibleexperimental error, the reasons may be that the improved crys-tallization ability by heterogeneous nucleation of iPP induces a lessdense stack as well as some strain in the crystals of PPR.

Fig. 5. AFM photographs of fine structural observation for samples. (a) PPR, (b) PPR/0.05WBG, (c) PPR/10iPP/0.05WBG, and “H,P,D ” represent height images, phase-contrast imagesand 3D-view images, respectively.

F. Luo et al. / Polymer 53 (2012) 4861e4870 4865

Obviously, the occurrence of b-crystallization of PPR in ourstudy is involved closely with two issues: b-NA (WBG) andstereoregular PP chains (iPP). The effects of b-NA concentrationand amount of stereoregular PP chain on the polymorphicbehavior are subsequently illustrated with the WAXD spectra inFig. 2 and the calculated crystallographic parameters are addedinto Table 3. As shown in Fig. 2(a) for samples with iPP contentfixed at 10wt %, Kb increases with increasing the b-NA concen-tration until 0.1 wt %. The value of Kb is up to 82% for PPR/10iPP/0.1WBG. However, it decreases to 52% when the b-NA concentra-tion is 0.2 wt %. On the other hand, for samples with WBG fixed at0.05 wt %, Kb increases continuously and rapidly by adding iPPuntil the iPP content reaches a high value of 20 wt % (Fig. 2(b)).After that, the values of Kb andDK exhibit no substantial changes.The maximum values of Kb and DK are approximately 92% and56%, respectively. It should be noted that such high Kb value issurprising for PPR matrix, which can match up to the highest levelof b-form content in b-nucleated iPP [1,3]. So the addition of b-NAis necessary for the origination of b-nucleation, while

stereoregular PP chains play more important role on the receivingof dominant b-crystallinity of PPR.

On the other hand, the non-isothermal crystallization behaviorwas probed by calorimetry. The DSC non-isothermal cooling crys-tallization curves for some typical blends are shown in Fig. 3. Ashoulder-peak exothermic manner is observed for the PPR/0.05WBG sample. Because the crystallization temperature of b-crystal by adding b-NA is higher than crystallization temperature ofa-crystal [36], the exothermic peak at high location (106 �C) maybelong to the crystallization peak of b-form resulting from addingWBG and the shoulder at low location (102 �C) may be associatedwith the subsequent crystallization of a-crystal. Another possibleexplanation of this exothermic manner may indicate two unequalcrystallization rates resulting from the weak effect of WBG forcrystallization of PPR. That is, relatively high rate of heterogeneouscrystallization assisted by WBG at primary period differs from thelater crystallization behavior. However, as both of iPP and WBG areadded into PPR, only one strong crystallization peak is detected,which is located at higher crystallization temperature. The DSC

a

b c

Fig. 6. (a) The schematic procedures of thermal processing investigated by DSC, (b) Melt curves of PPR/10iPP/0.05WBG undergoing various Tfs for 10 min,(c) Melt curves of PPR/10iPP/0.05WBG held at T f ¼ 145 �C for different time.

F. Luo et al. / Polymer 53 (2012) 4861e48704866

results further indicate that the crystallization rate increases andthe b-crystallization becomes dominant after adding stereoregularPP chains into the PPR/WBG system.

3.2. Microscopy of fine crystalline structure

Because of the different crystalline structure charactersbetween a-crystal and b-crystal [1,37,38], these two crystal formscan be distinguished through SEM observation. The SEM photo-graphs of some etched samples are shown in Fig. 4. By selectingthe suitable dissolution and etched time, the loose structure ofb-crystal can be etched deeply but the exposed structure of densea-crystal is feint. Distinctly, the b-crystal is dominant in thesample of PPR/10iPP/0.05WBG and its content sharply increasescompared with PPR/0.05WBG. More fine structural observation isperformed by AFM technique, as shown in Fig. 5. Distinctboundary among dense a-crystals is presented for PPR (Fig. 5(a)).Some b-crystals with loose lamellae arrangement are introducedinto PPR by adding 0.05WBG (Fig. 5(b)). The lamellae of dominantb-crystals of PPR/10iPP/0.05WBG interpenetrate into each otherresulting from the enhancement of connection among inter-crystals (Fig. 5(c)).

3.3. Crystallization mechanism

In order to deeply elucidate the underline mechanism ofnucleation and crystallization in the PPR/iPP/b-NA blends, a step-wise crystallization protocol is designed to explore the fractionalcrystallization behavior of PPR/10iPP/0.05WBG. For the first crys-tallization step, the specimen primarily nucleated at relative hightemperatures (Tf), 130e145 �C, which is difficult for PPR crystalli-zation but available for iPP crystallization. Then, the specimen wascooled down to a second crystallization temperature (Ts), 120 �C,which is suitable for both of PPR and iPP crystallization. Finally, theheating curves were recorded at a rate of 10 �C/min. The schematicprocedure of thermal processing is shown in Fig. 6(a).

Fig. 6(b) shows the heating curves of PPR/10iPP/0.05WBG afterexperienced crystallization at various Tfs. The heating traces exhibitthree endothermic peaks. The endothermic peak at low tempera-ture is associated with the melting of the b-form (Tmb), whichcontinuously decreases with the increasing Tf. The middle endo-thermic peak located approximately at 147 �C is character for themelting of the original a-form (Tma) of PPR. While the last endo-thermic peak located at about 152 �C (Tma’) indicates the melting ofmore perfect a-form (a0-form) of PPR deriving from heterogeneousnucleation, which continuously increases with the increasing Tf.

Fig. 7. Some typical PLM photographs represent the crystallization of blends during cooling, cooling rate is 5 �C/min. A, B, C, D represent PPR, PPR/10iPP, PPR/0.05WBG and PPR/10iPP/0.05WBG, respectively. The subscripts “ P ” “ M ” and “ L ” represent the stages of morphology development at preliminary, middle and later stage, respectively. The scale forall pictures is 150 mm.

F. Luo et al. / Polymer 53 (2012) 4861e4870 4867

One should pay attention to the selected temperature ranges. Thetemperature range of 110e130 �C is favorable for the growth ofb-crystal induced by adding b-NA into iPP.Within the range of 130e141 �C, the b-crystallization capability becomes weak but stillstronger than the a-crystallization [1,6,38,39]. Subsequently, theb-to-a transition occurs at about 141 �C [1,6]. The dependence ofcrystallization behavior on crystallization temperature for PPR/10iPP/0.05WBG is similar to the case of b-nucleated iPP. Therefore,we can speculate that the event of iPP chains nucleated by WBGmight play an important role during the entire crystallizationprocess. In addition, considering the molecular weight of iPP ismuch lower than that of PPR, as listed in Table 1, a higher crystal-lization rate for iPP at the primary period is possible [40,41].However, in the blend systems of this study, the proportion of iPP ismuch lower than that of PPR. Since stereoregular chains of iPPpossess higher nucleating capability and crystallization rate thanPPR chains, iPP component should be nucleated by WBG to formb-nuclei (precursors) in advance. And then b-precursors constructa framework for guiding the predominant growth of b-crystals. Inorder to ascertain the effect of b-nuclei amount on the final

b-crystal content, the isothermal crystallization at Tf ¼ 145 �C fordifferent time was implemented (Fig. 6(c)). It is found that the peakarea of b-form decreases with increasing crystallization time. Incontrast, the peak area of a0-form increases with crystallizationtime. Because this temperature (145 �C) is almost up to the melttemperature of PPR crystal, the PPR matrix can not crystallize [2].Therefore, the available chains for supporting the formation ofprimary b-nuclei might come only from iPP component. However,at this high crystallization temperature (145 �C), increasing thehold time means more a-nuclei but less b-nuclei formed from iPPchain. So the final polymorphic composition of PPR is stronglydependent on the amounts of b-nuclei consisted of iPP chains andyielded at the very early stage of crystallization. Above all, byintroducing appropriate amounts of stereoregular PP chains, theb-crystallization ability of PPR can be enhanced effectively.

We have seen above that the ability to induce b-crystallization issignificantly improved by adding iPP. In order to ascertain thecrystallization procedure of blends, comparative observation of thecrystallization morphology development of pure PPR, PPR/10iPP,PPR/0.05WBG, PPR/10iPP/0.05WBG is performed by using PLM, as

Fig. 8. The schematic crystallization behaviors of b-nucleated PPR with and without iPP. (A) b-Nucleated PPR without iPP, (B) b-Nucleated PPR with iPP.

F. Luo et al. / Polymer 53 (2012) 4861e48704868

shown in Fig. 7. The cooling rate of 5 �C/min is used for betterinvestigating the development of crystalline morphology. Thecrystallization progress can be observed from left to right. Insequence, it was distinguished as preliminary, middle and laterstages. In the preliminary stage, both PPR and PPR/10iPP form a-nuclei. However, for PPR/0.05WBG, and PPR/10iPP/0.05WBG,dendritic framework b-nuclei are firstly formed in samples. Thedensity of b-nuclei in samples with iPP is much higher than thatwithout iPP. In the middle stage, the premier nuclei grow biggerand some new nuclei appear. We note that plentiful a-nuclei formin PPR/0.05WBG but a few a-nuclei appear in PPR/10iPP/0.05WBGat this stage. In the later stage, different crystallization morphol-ogies are obtained. Only a crystalline morphology appears in PPRand PPR/10iPP. “Flower”-like agglomerate of b-crystal which showshighly birefringent character is observed in the blends with WBG.Promotion of b-crystal percentage (area percent of highlightedspherulites) is obtained by adding iPP. For PPR/0.05WBG, the b-crystals are dispersed as island in photograph and are surrounded

Fig. 9. (a) The WAXD spectra for various samples and (b) Th

by plentiful a-crystals. In contrast, the photograph is almost occu-pied by b-crystal with a few a-crystals for PPR/10iPP/0.05WBG.This result proves again that both the density of b-nuclei and theb-selected ability of WBG increase by adding iPP into b-nucleated PPR.

At the current research stage, the experimental evidences for thedescription of crystallization mechanism are rough, and importantinformation about molecular-level (chain conformation) andnanometer-level (lamellae, lattice arrangement) size-scale struc-tures are unavailable. Some powerful in-situ techniques, such asFTIR and simultaneous WAXS/SAXS, will be utilized to monitor thecrystallization process of PPR blend system in future. Undoubtedly,addingproperamountof iPP intoPPRwithb-NA is anefficientway toassist the formation of primary b-nuclei and enhance the b-crys-tallizability of PPR. The schematic crystallization behaviors ofb-nucleated PPR with and without iPP are plotted in Fig. 8 For theb-nucleated PPR without iPP (Fig. 8(a)), the irregular distribution ofstereoregularity chain of PPR decreases the ability of b nucleation.

e SEM photograph of etched PPR/10IPC/0.05WB sample.

Table 4The Izod notch impact strengths (IS) for various samples tested at different temperatures.

Samples PPR PPR/10iPP PPR/10IPC PPR/0.05WBG PPR/10iPP/0.05WBG PPR/10IPC/0.05WBG

Kb (%) 0 0 0 36 82 69IS at 23 �C (kJ/m2) 22.6 � 1.1 23.2 � 1.3 23.5 � 1.2 30.8 � 1.5 40.2 � 2.4 42.7 � 2.3IS at 0 �C (kJ/m2) 3.3 � 0.4 3.1 � 0.3 4.1 � 0.4 3.5 � 0.3 4.3 � 0.4 6.5 � 0.5IS at �15 �C (kJ/m2) 2.3 � 0.3 2.0 � 0.3 2.2 � 0.4 2.3 � 0.4 2.4 � 0.3 3.3 � 0.4

Fig. 10. The stressestrain curves for varous samples, the inset pictures are the enlarged ones at the yield zone. (a) PPR/iPP/WBG system, (b) PPR/IPC/WBG system.

F. Luo et al. / Polymer 53 (2012) 4861e4870 4869

There are only a few b-nuclei formed in the early crystallizationstage. And then, a lot of a-nuclei appear during cooling. Because ofthe competitive growth between a-crystal and b-crystal, dominanta-crystals are formed in the sample. In contrast, a lot of b-nucleiconsisted of the importing iPP chain can be formed in the earlycrystallization stage in blends (Fig. 8(b)). In sequent, the b-nucleiinduce thegrowthofb-crystal and restrict thegrowthofa-crystal. Atlast, the b-crystal is dominant in the samples.

3.4. Assistance of b-crystallization capability of PPR by usingimpact polypropylene copolymer

Because PP homopolymerwith high chain stereoregularity actingas the matrix component of impact polypropylene copolymer (IPC)exhibited strong b-crystallization ability with WBG and a superiortoughness at low temperature of IPC is achieved [30], an expectationthat IPCmayenhance the b-crystallization ability of PPRwithWBG isinspired. Thus, a superior toughness at low temperature of PPRblends is alsoexpected to enlarge its applicationsat lowtemperature.As expected, shown in Fig. 9(a) similar result to that for PPR/10IPC/0.05WBG is observed, but the increased efficiency is a little lowerthan that of iPP. It should be noticed that, in Fig. 9(b), the very tinyholes belong to the etched EPR phase particles of PPR, while therelativebiggerholespresentedonly inPPR/10IPC/0.05WBGbelong tothe EPR phase particles in IPC [26,29].Since the whole stereoregu-larity of IPC is lower than that of iPP, such amount of enhancement ofb-form content is exciting. Therefore, the mechanical properties ofthese samples are investigated and discussed below.

3.5. Mechanical properties

Fromaviewpointofpracticeapplication, it isnecessary toestimatethemechanical properties of PPR-based blendswith different b-form

contents. The Izod impact strength (IS) of PPR-based blends withvarious b-formcontents tested at different temperatures is illustratedin Table 4. For PPR/10iPP and PPR/10IPC samples, since the b-formcontent is nearly zero, their impact strengths change a little from thatof neat PPR at all three test temperatures.WhenKb is increased to 36%forPPR/0.05WBG, amoderate increment in impact toughness,30.8kJ/m2, is attained at 23 �C. But, the toughness at low temperatures (both0 �C and�15 �C) can not be increased. For PPR/10 iPP/0.05WBGwithhigh b-form content, the impact strength increases remarkably to40.2 kJ/m2 at 23 �C, which is about twice as high as that of neat PPR(22.6 kJ/m2). Since IPC possesses excellent toughness, the impactstrength of PPR/10IPC/0.05WBG is higher than that of PPR/10iPP/0.05WBG though the Kb value in the former (69%) is lower than thelatter (82%). More importantly, a superior toughness at low temper-ature of PPR/10IPC/0.05WBG is obtained. The toughnesses at lowtemperatures of PPR/10IPC/0.05WBG, which are 6.5 kJ/m2 and 3.3 kJ/m2 tested at 0 �C and�15 �C respectively, aremuch higher than thoseof neat PPR (3.3 kJ/m2 and 2.3 kJ/m2 tested at 0 �C and �15 �Crespectively). On the other hand, the tensile stressestrain curves areplotted in Fig. 10 and the inset pictures illustrate the yield zone. It isfound that the introduction of iPP into PPR/WBG system can improverigidity. Both the yield strength and tensilemodulus are increased. Asto PPR/IPC/WBG system, the yield strength can be maintained at thesame level compared to the neat PPR. Therefore, the introduction ofstereoregular PP chains is an effective pathway to enhance theb-crystallization capability significantly and bring superior impacttoughness, without to sacrifice the initial rigidity of PPR.

4. Conclusions

A surprising enhancement in the b-crystallization capability ofPPR with WBG is achieved by adding moderate amounts of iPP. TheKb value can reach the highest level of 92% and the b-crystallization

F. Luo et al. / Polymer 53 (2012) 4861e48704870

capability of PPR is improved by 56%. The fractional crystallizationexperiment and observation of crystallization behavior illustratethat stereoregular PP chains in iPP assist the formation of primaryb-nuclei at the very early stage of crystallization. This specialcrystallization event dominates the final polymorphic compositionin PPR-based blends. Moreover, this idea can be further extended toPPR/IPC/WBG system, which shows a notable improvement oftoughness at low temperature of PPR without loss of rigidity. Ourstudy possibly provides a new pathway to tune the crystallizationbehavior of PPR and bring excellent comprehensive mechanics inPPR materials.

Acknowledgment

This work was supported by the Special Funds for Major StateBasic Research Projects of China (2011CB606006), and NationalNatural Science Foundation of China (51121001, 50903048).

References

[1] Varga J. J Macro Sci Part B 2002;41(4e6):1121e71.[2] Luo F, Wang J, Bai H, Wang K, Deng H, Zhang Q, et al. Mater Sci Eng A 2011;

528:7052e9.[3] Luo F, Wang K, Ning N, Geng C, Deng H, Chen F, et al. Polym Adv Tech 2011;22:

2044e54.[4] Fujiyama M. Int Polym Process 1995;10:172e8.[5] Su J, YangG,GengC, DengH,WangK, FuQ. Chin J PolymSci 2011;29(6):732e40.[6] Varga J, Menyhárd A. Macromolecules 2007;40(7):2422e31.[7] Bai H, Luo F, Zhou T, Deng H, Wang K, Fu Q. Polymer 2011;52(10):2351e60.[8] Yamaguchi M, Fukui T, Okamoto K, Sasaki S, Uchiyama Y, Ueoka C. Polymer

2009;50(6):1497e504.[9] Phillips A, Zhu PW, Edward G. Polymer 2010;51(7):1599e607.

[10] Nomura T, Nishio T, Iwanami K, Yokomizo K, Kitano K, Toki S. J Appl Polym Sci1995;55(9):1307e15.

[11] McNally T, McShane P, Nally G, Murphy W, Cook M, Miller A. Polymer 2002;43(13):3785e93.

[12] Silvestre C, Cimmino S, Triolo R. J Polym Sci Part B: Polym Phys 2003;41(5):493e500.

[13] Guidetti G, Busi P, Giulianelli I, Zannetti R. Eur Polym J 1983;19(9):757e9.[14] Galli P, Vecellio G. Prog Polym Sci 2001;26(8):1287e336.[15] Kontopoulou M, Wang W, Gopakumar TG, Cheung C. Polymer 2003;44(24):

7495e504.[16] Starke J, Michler G, Grellmann W, Seidler S, Gahleitner M, Fiebig J, et al.

Polymer 1998;39(1):75e82.[17] Fan ZQ, Zhang YQ, Xu JT, Wang HT, Feng LX. Polymer 2001;42(13):5559e66.[18] Cai HJ, Luo XL, Chen XX, Ma DZ, Wang JM, Tan HS. J Appl Polym Sci 1999;

71(1):103e13.[19] Zhou T, YangH, Ning N, Xiang Y, Du R, Fu Q. Chin J Polym Sci 2010;28(1):77e83.[20] Mileva D, Androsch R, Zhuravlev E, Schick C, Wunderlich B. Polymer 2012;

53(2):277e82.[21] Zhao S, Cai Z, Xin Z. Polymer 2008;49(11):2745e54.[22] Naffakh M, Marco C, Ellis G. J Phys Chem B 2011;115(37):10836e43.[23] Fujiyama M. Int Polym Process 1998;13:291e8.[24] Chen H, Karger-Kocsis J, Wu J, Varga J. Polymer 2002;43(24):6505e14.[25] Mahdavi H, Nook ME. Polym Int 2010;59(12):1701e8.[26] Zhang C, Shangguan Y, Chen R, Wu Y, Chen F, Zheng Q, et al. Polymer 2010;

51(21):4969e77.[27] Song S, Feng J, Wu P, Yang Y. Macromolecules 2009;42(18):7067e78.[28] Zhu H, Monrabal B, Han CC, Wang D. Macromolecules 2008;41(3):826e33.[29] Zhang C, Chen R, Chen F, Shangguan Y, Zheng Q, Hu G. Chin J Polym Sci 2011;

29(4):497e505.[30] Luo F, Xu C, Wang K, Deng H, Chen F, Fu Q. Polymer 2012;53:1783e90.[31] Luo F, Geng CZ, Wang K, Deng H, Chen F, Fu Q, et al. Macromolecules 2009;

42(23):9325e31.[32] Xiao W, Wu P, Feng J. J Appl Polym Sci 2008;108(5):3370e9.[33] Huo H, Jiang S, An L, Feng J. Macromolecules 2004;37(7):2478e83.[34] Turner-Jones A, Aizlewood J, Beckett D. Makromol Chem 1964;75:134e58.[35] Olley R, Bassett D. Polymer 1982;23(12):1707e10.[36] Varga J, Mudra I, Ehrenstein G. J Therm Anal Calor 1999;56(3):1047e57.[37] Padden FJ, Keith HD. J Appl Phys 1959;30(10):1479e84.[38] Varga J. J Mater Sci 1992;27(10):2557e79.[39] Shi G, Zhang X, Qiu Z. Die Makromol Chem 1992;193(3):583e91.[40] Duplay C, Monasse B, Haudin JM, Costa JL. J Mater Sci 2000;35(24):6093e103.[41] Wool RP. Macromolecules 1993;26(7):1564e9.

本文献由“学霸图书馆-文献云下载”收集自网络，仅供学习交流使用。

学霸图书馆（www.xuebalib.com）是一个“整合众多图书馆数据库资源，

提供一站式文献检索和下载服务”的24 小时在线不限IP

图书馆。

图书馆致力于便利、促进学习与科研，提供最强文献下载服务。

图书馆导航：

图书馆首页 文献云下载 图书馆入口 外文数据库大全 疑难文献辅助工具