Isothermal Crystallization Kinetics and MorphologyDevelopment of Isotactic Polypropylene Blendswith Atactic Polypropylene

Jean-Hong Chen, Yu-Lun Chang

Department of Polymer Materials, Kun Shan University, Tainan 710, Taiwan, Republic of China

Received 20 March 2006; accepted 17 August 2006DOI 10.1002/app.25354Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: The crystallization kinetics and morphologydevelopment of pure isotactic polypropylene (iPP) homo-polymer and iPP blended with atactic polypropylene (aPP)at different aPP contents and the isothermal crystallizationtemperatures were studied with differential scanning calo-rimetry, wide-angle X-ray diffraction, and polarized opti-cal microscopy. The spherulitic morphologies of pure iPPand larger amounts of aPP for iPP blends showed the neg-ative spherulite, whereas that of smaller amounts of aPPfor the iPP blends showed a combination of positive andnegative spherulites. This indicated that the morphologytransition of the spherulite may have been due to changesthe crystal forms of iPP in the iPP blends during crystalli-zation. Therefore, with smaller amounts of aPP, the spher-ulitic density and overall crystallinity of the iPP blends

increased with increasing aPP and presented a lower de-gree of perfection of the g form coexisting with the a formof iPP during crystallization. However, with larger amountsof aPP, the spherulitic density and overall crystallinity ofthe iPP blends decreased and reduced the g-form crystalswith increasing aPP. These results indicate that the aPPmolecules hindered the nucleation rate and promoted themolecular motion and growth rate of iPP with smalleramounts of aPP and hindered both the nucleation rateand growth rate of iPP with larger amounts of aPP duringisothermal crystallization. � 2006 Wiley Periodicals, Inc. J ApplPolym Sci 103: 1093–1104, 2007

Key words: crystal structures; morphology; poly(propyl-ene) (PP)

INTRODUCTION

It is well known that isotactic polypropylene (iPP)exhibits several crystalline forms, including a, b, andg forms, at different process conditions. In all these,the crystal forms of iPP are identical and correspondto the familiar threefold helix at different stackinggeometries of these helices.1–15 This is because thecrystal morphologies are affected not only by themolecular mass and molecular mass distribution ofiPP but also by different blending compounds andpreparation conditions, that is, isothermal tempera-ture and pressure.7,16–18

The g form of iPP was first reported during the1960s and was generated by crystallization at ele-vated pressures of the polymer. However, Meilleet al. reported19 that the structure of the g form didnot account for the diffraction peak at 2y ¼ 24.58.They also reported a g form generated at atmospheric

pressure from low-molecular-weight iPP, which ledto a reassignment of the structure as a modifiedtriclinic unit cell. Contrary to this unit cell, the modi-fied cell accounted for the diffraction peak at 2y¼ 24.58 because the a angle of the lattice constantswas higher in the modified triclinic cell.20 The modi-fied triclinic structure is unique and contains sheetsof parallel molecules, but the molecular orientationbetween adjacent sheets becomes nonparallel everytwo sheets. The angle between the nonparallel andparallel sheets is about 818; this angle is also ob-served at the contact planes between the radiallamellae and the branches of the a form.21–23

Recently, iPP blends have received much attentionbecause the morphology, crystallinity (Xc), micro-structure, and melting and crystallization behaviorsof iPP are strongly dependent on the process condi-tions and blend components.24–31 Time-resolved X-ray scattering techniques showing the relatively mod-est incorporation of atactic polypropylene (aPP) inthe interlamellar regions, depending on the crystalli-zation temperature (Tc) and blend composition werealso reported by Wang et al.25,26 However, they indi-cated that the partial miscibility of the aPP and iPPcomponents in the blends resulting the crystallizabil-ity and crystal morphology of iPP was not a strongfunction of aPP and the segregation of aPP on size

Correspondence to: J.-H. Chen (kelvench@mail.ksu.edu.tw).Contract grant sponsor: National Science Council of

the Republic of China; contract grant number: NSC 93-2216-E-168-004.

Journal of Applied Polymer Science, Vol. 103, 1093–1104 (2007)VVC 2006 Wiley Periodicals, Inc.

scales larger than the lamellar spacing. The segrega-tion of aPP to a size scale larger than lamellar spac-ing was consistent with literature observations show-ing both open spherulitic morphologies and thepooling of aPP within the spherulitic morphology,depending on the crystallization conditions.25,26 Morerecently, we31 reported that aPP was locally misciblewith iPP in the amorphous region of iPP/aPP blendsand the contents of the g form of iPP dependedremarkably on both the aPP content and isothermalTc. The effect of aPP addition on iPP was investigatedby Keith and Padden;32,33 they reported that withincreasing aPP in iPP/aPP blends, a more openspherulitic texture was observed due to the incorpo-ration of aPP diluent in the interfibrillar regions.Although the main structure of the iPP blends issimilar to that of pure iPP, the morphology develop-ment of iPP blends has not yet been studied indetail. Therefore, study of the relationship betweenthe aPP content and isothermal conditions and themorphology development of iPP blends is becomingmore important. From this point of view, a study ofthe effect of process conditions on the crystallizationkinetics of polymers is an important step in under-standing, predicting, and designing microstructureformation under differences processes conditions ofiPP blends. Generally, the crystallization kinetics ofpolymers have been well described by the Avramiequation, although they are limited when used todescribe the crystallization of the polymers.34,35

In this study, the effect of aPP contents and iso-thermal temperature on the spherulitic morphologyand isothermal crystallization kinetics of iPP blendswere studied. The Avrami equation was used to ana-lyze the isothermal crystallization kinetics of iPPblends. Dynamic differential scanning calorimetry(DSC) thermograms provided the thermal propertiesand necessary crystallization kinetics data in thisstudy. Polarized optical microscopy (POM) and wide-angle X-ray diffraction (WAXD) results were alsoused to understand the morphology development ofthe iPP in blends under various conditions.

EXPERIMENTAL

Material preparation

iPP with a weight-average molecular weight of1.9 � 105 g/mol and aPP with a weight-average mo-lecular weight of 1.96 � 104 g/mol were purchasedfrom Aldrich Chemical, USA. Melt-blended speci-mens of these homopolymers with various com-positions were prepared with a twin-screw appara-tus (MP2015, APV Chemical Machinery Co., USA) at2108C. The iPP/aPP (w/w %) mixing ratios were100/0, 90/10, 80/20, 70/30, 50/50, and 30/70; theblends were prepared and defined as iPP-100, iPP-90, iPP-80, iPP-70, iPP-50, and iPP-30, respectively.The composition and thermal properties of the iPPblends in this study are collected in Table I.

The compression-molded films were prepared bymelt-pressing of iPP blends for a molding of 120 �120 � 1 mm3. All samples were molten at 2108Cand were held at this temperature for 10 min to allowcompete melting, and then, these iPP blend moldswere taken out and immediately submerged in a tem-perature-controlled compression-molding machine atdifferent Tc’s, with temperature intervals of 58C from90 to 1358C under a pressure of 50 kg/cm2. They werethen placed between the two steel platens and held at120 min for necessary isothermal crystallization.

DSC measurement

Thermal behaviors of all of the isothermal crystal-lized iPP blends were measured with a differentialscanning calorimeter (Perkin-Elmer Pyris DiamondDSC, USA; with an intracooler for the lowermosttemperature of about �658C). A sample weight ofabout 5 mg was cut from the isothermal crystallizediPP blends at different isothermal Tc’s, put into asample pan, and then melted in the furnace in anitrogen atmosphere from 30 to 2108C at a rate of108C/min; then, the melting thermogram was meas-ured. The temperature and area of the endothermic

TABLE ICharacteristics and Thermal Properties for the iPP Blended Samples

Materials Tg (8C)a Tc (8C)

b DHc (J/g)b Tm

L (8C)c DHf (J/g)c Tm

o (8C)a Xc (%)d

iPP-100 �10.1 129.0 83.7 163.5 109.6 187.2 53.7iPP-90 �10.1 126.1 78.8 163.5 123.6 186.9 57.6iPP-80 �10.3 127.6 72.6 163.7 132.0 186.4 61.3iPP-70 �10.6 125.1 61.7 163.7 120.8 186.0 56.2iPP-50 �10.4 123.1 62.7 162.7 102.7 184.6 41.2iPP-30 �11.1 120.6 33.4 161.0 50.3 181.7 25.2

a Tg, Glass-transition temperature, and Tmo , equilibrium melting temperature, were reported in reference 31.

b Calculated from the crystallization exotherm of the DSC cooling trace after melting.c Calculated from the melting endotherm of the DSC melting trace of the isothermal crystallized iPP blend at 1308C.d Calculated from the X-ray diffraction intensity pattern of the isothermal crystallized iPP blend at 1308C.

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peak were taken as the melting temperature and theheat of fusion (DHf), respectively. The temperaturewas 2108C and was maintained for 10 min to elimi-nate any previous thermal history; then, the samplewas cooled at a rate of 108C/min, and the crystalli-zation thermogram was measured. The temperatureand area of the exothermic curve were taken as Tc

and the latent heat of crystallization (DHc), respec-tively. These measurement results are shown in Ta-ble I. The isothermal crystallization kinetics werealso measured with the Pyris Diamond DSC. Thesamples were melted in the furnace in a nitrogenatmosphere at 2108C for 10 min to eliminate any pre-vious thermal history, and then, they were rapidlycooled to Tc at a rate of 4008C/min and maintainedat this temperature for the time necessary for iso-thermal crystallization. These isothermal crystalliza-tion kinetics data are listed in Table II.

POM measurement

The spherulite morphologies of the iPP blends wereinvestigated with a polarized optical microscope(Zeiss Axioskop-40, UK, with a Linkam TH600 hotstage, Germany). The samples, inserted between twomicroscope cover glasses, were melted at 2108C andsqueezed to obtain a sample about 10 mm thick forthe iPP blend thin films. The thin-film samples wereinserted into the hot stage. Each sample was meltedat 2108C for 10 min to eliminate any previous ther-mal history; they were then cooled to isothermal Tc

at a rate of 908C/min and maintained at this temper-ature for the time necessary for crystallization.

WAXD measurement

WAXD intensity curves of isothermal crystallized iPPblends were measured with graphite-monochromat-ized Cu Ka radiation generated at 40 kV and 180 mAin a Rigaku D/Max 2500VL/pc diffractometer. WAXDintensities were recorded from 2y ¼ 5 to 358 with acontinuous scanning speed of 2y ¼ 18/min with datacollection at each 0.028 of 2y.

RESULTS AND DISCUSSION

Melting endotherm morphology

Figure 1(a–f) shows the DSC melting scans at a heat-ing rate of 108C/min for isothermal crystallized iPPblends at different isothermal Tc’s. However, a gen-eral feature of these curves was the appearance oftwo melting endotherms at temperatures below1208C; a single melting endotherm existed at temper-atures above 1208C. As shown in Figure 1, with in-creasing aPP content, the heat of fusion of the lowertemperature endotherm (DHf

L) decreased, whereasthe heat of fusion of the higher temperature endo-therm (DHf

H) increased. This result indicates that Xc

and the morphology for the iPP blends after isother-mal crystallization were affected by the aPP contentand isothermal Tc. The decrease in DHf

L with in-creasing aPP content was indicative of a decrease inrecrystallization or reorganization of the crystalsoriginally formed during crystallization. Therefore,DHf

L usually represented the melting of the crystalsformed during crystallization, whereas DHf

H wasprobably due to the melting of crystals of higherstability formed by the recrystallization of crystalsinitially obtained. Moreover, the area of DHf

L in-creased with increasing isothermal temperature be-cause a higher degree of perfection was achieved inthe crystals initially obtained, although the area ofDHf

H decreased with increasing isothermal tempera-ture. This was due to the melting of crystals formedduring recrystallization, and the results obtainedwere assumed to be because the same degree of per-

TABLE IIIsothermal Crystallization Kinetics for the iPP

Blended Samples

SampleTc

(8C)t1/2(min)

t1/2(min�1) n

k(8C/min) � 103

iPP-100 110 0.25 4.00 3.50 18.3115 0.32 3.13 2.68 12.0120 0.41 2.44 2.76 7.82125 0.53 1.89 2.78 6.47130 0.76 1.32 2.65 5.86135 2.73 0.37 2.70 1.36

iPP-90 110 0.19 5.26 3.98 92.3115 0.22 4.55 3.24 79.8120 0.27 3.70 2.87 19.1125 0.34 2.94 2.67 23.8130 1.12 0.89 2.64 1.32135 5.89 0.17 3.48 0.56

iPP-80 110 0.14 7.14 4.12 24.7115 0.17 5.88 3.76 18.3120 0.34 2.94 3.43 10.3125 0.47 2.13 3.09 4.51130 1.08 0.93 2.46 15.3135 7.04 0.14 3.12 1.12

iPP-70 110 0.18 5.56 4.03 32.6115 0.21 4.76 4.09 23.4120 0.29 3.45 3.57 14.5125 0.82 1.22 3.22 6.73130 2.05 0.49 2.83 5.26135 7.05 0.14 2.32 1.64

iPP-50 110 0.20 5.00 4.17 40.7115 0.29 3.45 4.01 23.9120 0.55 1.82 2.56 9.56125 0.90 1.11 2.76 1.23130 2.45 0.41 2.34 3.18135 7.11 0.14 2.12 2.03

iPP-30 110 0.23 4.35 3.45 36.5115 0.33 3.03 2.97 11.1120 0.88 1.14 3.06 3.03125 2.71 0.37 3.10 0.83130 4.46 0.22 2.94 0.41135 9.25 0.11 2.86 0.27

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fection was achieved in recrystallized crystals. DHfL

and the melting temperature of the lower tempera-ture endotherm (Tm

L) remained almost constant attemperatures below 1058C, and then, DHf

L and TmL

increased at higher isothermal temperatures. Thisindicated that a higher degree of perfection wasachieved in the crystals due to the higher thermody-namic mobility of iPP molecules necessary for therecrystallization to take place. DHf

L and TmL did not

changed at temperatures below 1058C, which impliedthat the previous thermal history or degree of per-fection achieved may have been the same. The melt-ing temperature of the higher temperature endo-therm remained unchanged, whereas DHf

H decreasedwith increasing isothermal Tc.

An increasing Tc led to the promotion of a higherdegree of perfection of crystals and fewer iPP chainsfor recrystallization to take place at higher tem-

peratures, so the obtained crystals were more perfectthan those formed at lower isothermal temperatures.However, the existence of double melting endo-therm peaks in the DSC profiles may have resultedbecause of the following: first, the presence of twodifferent crystal structures, the presence of two dif-ferent thicknesses of crystal lamellae with the sametype of crystal formed at the isothermal crystal-lization conditions,36 and second, the simultaneousmelting–reorganization/recrystallization–remelting ofthe lamellae originally formed during the crystalliza-tion process.37 The WAXD results, shown later inFigure 8, indicated that the g-form coexisting withthe a-form crystal structures for pure iPP and iPPblends with lower amounts of aPP during the crys-tallization process. However, the results of iPPblends with larger amounts of aPP show that onlythe a-form crystal structure was observed. Thus, the

Figure 1 Melting endotherms of iPP/aPP blends after isothermal crystallization for 120 min at different isothermal tem-peratures: (a) iPP-100, (b) iPP-90, (c) iPP-80, (d) iPP-70, (e) iPP-50, and (f) iPP-30.

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occurrence of the double melting peaks may havemainly been caused by coexistence with the differenttypes of crystal structures and different crystal thick-nesses with simultaneous melting–reorganization/recrystallization–remelting of the lamellae during theheating trace. The double endotherms were attrib-uted to the recrystallization of less perfected andmore perfected a- and/or g-form crystals and presenteda morphology transition temperature at Tc ¼ 1208C.However, below this transition temperature, imper-fect a- and g-form crystals were obtained and led totwo endotherms. However, above this transitiontemperature, more perfect a- and g-form crystalswere formed, in which only a single endotherm wasobserved. The development of Xc of the iPP blendexplained that the diluent aPP molecules suppressedthe entanglement between iPP molecules and pro-moted the mobility of iPP molecules during crystalli-zation. However, with larger amounts of aPP, thedecreasing Xc of the iPP blends may have been thelarger amount of diluent aPP action suppressedthe concentration of the nucleus more and inhibitedthe iPP molecular from molten region diffusing to thesurface of nucleus during crystallization.31

Isothermal crystallization kinetics

On the basis of the previous results, therefore, thisstudy was conducted to measure the effects of aPPcontent and isothermal temperature on the crystalli-zation kinetics and morphology development of theiPP blends. Generally, analysis of isothermal crystal-lization kinetics of polymer and polymer blends isperformed with a classical Avrami equation, as givenin eq. (1):34,35

1� XðtÞ ¼ expð�ktnÞ (1)

where X(t) is the development of crystallinity at timet and k and n are the crystallization rate constantand the Avrami exponent, respectively. Both k and ndepend on the nucleation and growth mechanismsof the spherulites. The fraction of X(t) was obtainedfrom the area of the exothermic peak in the DSCscans; the isothermal crystallization analysis at acrystallization time t was divided by the total areaunder the exothermic peak, as shown in eq. (2):

XðtÞ ¼ XcðtÞXcðt ¼ 1Þ ¼

R t0ðdH=dtÞdt

R10 ðdH=dtÞdt ¼ 1� expð�ktnÞ

(2)

where the numerator is the heat generated at time tand the denominator is the total heat generated untilcrystallization was complete. t is the time spent dur-ing the course of crystallization as measured from

the onset of crystallization. To deal convenientlywith the operation, eq. (1) is usually rewritten in adouble logarithmic form as follows:

logf�ln½1� XðtÞ�g ¼ log kþ n log t (3)

According to eq. (3), when log{�ln[l � X(t)]} isplotted against log t, the n and k values can be dir-ectly obtained as the slope and the intercept, respec-tively, of the linear plots of log{�ln[l � X(t)]} againstlog t. On the basis of eq. (1), if the time the polymerspends from the beginning of the crystallization pro-cess to the time at which a certain amount of relativeXc has developed is known, k can also be directlycalculated if eq. (1) is rearranged as follows:

k ¼ �ln½1� XðtÞ�tn

(4)

If X(t) ¼ 0.5, eq. (4) converts in to a more familiarequation, which is rewritten as follows:

k ¼ ln 2

tn1=2(5)

To analyze the effects of the aPP content and iso-thermal temperature on the crystallization kineticsfor the iPP blends, the exothermic profiles of iPP-100, iPP-80, and iPP-30 at various Tc’s are shown inFigure 2. As the results indicate, the time to reachthe maximum degree of crystalline order in the iPPblends increased with increasing Tc. Generally, thetime needed to reach the maximum degree of crys-talline order of iPP blends was slower than that ofiPP-100. However, interestingly, if the temperaturewas lower than 1208C, the time needed to reach themaximum degree of crystalline order of the iPP-80was shorter than that of pure iPP and iPP-30; how-ever, the amount of nuclei per unit area in iPP-80was higher than that in pure iPP under the sameconditions (as shown later in Fig. 6). These resultsindicate that the crystallization mechanism of iPP-80must have been different that of iPP-100 and iPP-30.Interestingly, at lower Tc’s, the viscosity of iPP mole-cules was high due to more entanglements betweeniPP chains; therefore, the small amount of moltenaPP molecules acting as the diluent role for iPPmolecules promoted the chain mobility of iPP andincreased the free volume between iPP chains, andso this led to an increase as the growth rate of iPPduring crystallization.31 The effect of aPP content andisothermal temperature on the relative Xc for the iPPblends with crystallization time is plotted in Figure 3.The results indicate that the overall crystallizationmechanism of the iPP blends showed a sigmoi-dal curve characterized by a primary crystallizationprocess during the initial stage and by a secondary

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crystallization process during the later stage. Theyalso show that the crystallization rate of iPP-80 wasfaster than that of pure iPP at lower Tc values, asshown by the half-time of crystallization (t1/2), listedin Table II. As shown in Table II, t1/2 of the iPPblends decreased and then increased with increasingaPP content with small and large amounts of aPP;particularly, iPP-80 showed the shortest t1/2 for allof the samples at lower Tc values. t1/2 is the time toreach the maximum rate of heat flow and corre-sponds to the change to a slower kinetic process dueto the impingement of adjacent spherulites.38 Theseresults indicate that the nucleation and growth ratesof iPP in the iPP blends depended strongly on theaPP content. Therefore, these facts imply that thedispersed aPP molecular played a diluent role in

the iPP blends. Further addition of aPP in the iPP ledto a drastic increase in t1/2 of the iPP blends. How-ever, in higher temperature regions, the thermody-namic or diffuse motion of the iPP chains dominatedthe crystallization mechanism; therefore, the nuclea-tion and growth rates of the iPP blends decreasedwith increasing aPP content at higher Tc values.

The effect of aPP content and isothermal tempera-ture on log{�ln[l � X(t)]} versus log t is shown inFigure 4. The overall crystallization rate of polymerswas a combination of the growth rate and nucleationrate during crystallization. However, the growth ratewas controlled by the diffusion of polymer chains tothe nuclear surface, and the nucleation rate was con-trolled by the specific interfacial energy difference ofheterogeneous nuclei and by the concentration ofnuclei. The results in Figure 4 show that the overall

Figure 2 Effect of aPP content and isothermal Tc on theexothermic profiles for the iPP blends, from left to right:110, 115, 120, 125, 130, and 1358C.

Figure 3 Effect of aPP content and isothermal Tc onrelative Xc for the iPP blends, from left to right: 110, 115,120, 125, 130, and 1358C.

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crystallization mechanism of the iPP blends gave anonlinear curve characterized by a primary crystalli-zation process during the initial stage and by asecondary crystallization process during the laterstage. The kinetic parameters of the Avrami equa-tion, including n and k, could be determined and fitthe initial-stage data.39 Therefore, the overall crystal-lization kinetics of the polymer and polymer blendcould be analyzed by the observation of rates of thenuclei and growth of crystalline polymers or the rateof formation and growth of a stable nucleus and therate at which the polymer chains were incorporatedinto the growing crystalline faces.

In this study, the n values of isothermal crystalli-zation were nonintegral in the range 2–4.40 Gener-ally, the n value of iPP is about 3–4,40,41 althoughsome authors have reported the n values in the

range of 240 to 442 for iPP blends. An increase in n isusually attributed in the literature to a change frominstantaneous to sporadic nucleation and to anincrease in the crystal dimentions.39 As shown inTable II, the n values were about 2.8 6 0.2 for iPP-100 and about 3.6 6 0.5 and 2.5 6 0.3 for iPP mix-tures with smaller and larger aPP contents, respec-tively. Normally, n values close to 3 of pure iPPindicate an athermal and sporadic nucleation processfollowed by three-dimensional crystal growth. Onthe other hand, n values close to 4 indicate a thermalnucleation process followed by three-dimensionalcrystal growth.40 The nonintegral of the n value maybe considered to be due to the crystal branching,two-stage crystal growth, or mixed growth and nu-cleation mechanisms.40,43 Moreover, it was clear thatthe slope of the plots of log{�ln[1 � X(t)]} versus logt for iPP blends remained unchanged until a highdegree of conversion was reached at higher Tc val-ues, which implied that secondary crystallization didnot play an important role for iPP and iPP blends athigher Tc values. The intercept value of the plotsof log{�ln[1 � X(t)]} versus log t for iPP blends (k)decreased with increasing Tc. This indicated a de-crease in the nucleation and the growth rate con-stants with increasing Tc; however, k increased withsmall amounts of aPP and decreased with increasingaPP contents in the iPP blends. These results implythat the overall isothermal crystallization rate of iPPwere markedly affected as aPP was added. Theisothermal crystallization kinetic parameters are allcollected in Table II.

The reciprocal of the half-time (t1/2) for the iPPblends versus Tc are shown in Figure 5. t1/2 is thetime to reach the maximum rate of heat flow and

Figure 4 Effect of aPP content and isothermal Tc on theAvrami plots for the iPP blends, from left to right: 110,115, 120, 125, 130, and 1358C.

Figure 5 Effect of aPP content and isothermal Tc on t1/2for the iPP blends.

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corresponds to the impingement of adjacent spheru-lites during crystallization. As shown in Table II, t1/2of the iPP blends increased and then decreased withincreasing aPP at the same Tc. However, t1/2 of theiPP blends decreased as Tc increased, which impliedthat the crystallization for the iPP blends was domi-nated by the nucleation-controlled mechanism.39

Therefore, these results indicate that the smalleramount of dispersed aPP molecules in the iPPblends acted as the diluent role for iPP moleculesand promoted the crystallization rate of iPP. How-ever, with larger amounts of dispersed aPP, t1/2 ofthe iPP blends decreased drastically as the largerloading of diluent aPP action suppressed the concen-tration of the nucleus more and inhibited iPP molec-ular diffusion to the surface of the nucleus duringcrystallization.

Morphology development

Figures 6 and 7 show the pictures of the spherulitemorphologies of the iPP blends under the polarizedoptical microscope with a quarter-wave plate (1/4l plate) at different aPP contents and isothermalTc’s, respectively. The sign of birefringence of thespherulites was determined by means of a quarter-wave plate located diagonally between crossedpolars. Depending on the birefringence, the spheru-lites could be optically positive or negative. Asshown in Figures 6 and 7, the morphologies of the

iPP blends depend on both the aPP content and Tc.As shown by POM analyzed for the iPP blends,there was no phase separation in the melt state. Thisresult proved that the aPP molecules were misciblewith the iPP chains in the melted state, as reportedrecently.31 When the iPP blends were crystallizedfrom the melt, the size and number of spherulitesalso depended on both the aPP content and Tc. It isinteresting that with smaller amounts of aPP, thedensity of the spherulites in the iPP blends increasedwith increasing aPP, whereas with larger amounts ofaPP, the density of the spherulites in the iPP blendsdecreased with increasing aPP content, as shown inFigure 6. The effect of aPP on the spherulite mor-phology of iPP was reported by Keith et al.32,33 Theyconcluded that with increasing aPP content in iPP/aPP blends, a more open spherulitic texture due tothe incorporation of aPP diluent in the interfibrillarregions was observed. However, this study showedincreases in the spherulite density or a more densespherulitic texture with increasing aPP with loweraPP contents, whereas with larger amounts of aPP,there was a decrease in the spherulite density or amore open spherulitic texture with increasing aPPwas observed. This fact demonstrates that the dilu-ent aPP molecules promoted the growth rate of iPPbecause the diluent aPP molecules reduced theentanglement between iPP molecules and increasedthe mobility of iPP with smaller amounts of aPP andinduced a more dense spherulite texture. However,

Figure 6 Effect of aPP content on spherulite morphology for the iPP blends at 1308C (magnification ¼ 400�): (a) iPP-100,(b) iPP-80, (c) iPP-50, and (d) iPP-30.

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with larger amounts of aPP, the decreasing Xc of theiPP blends may have been due to the larger amountof diluent aPP action, which suppressed the concen-tration of the nucleus more and inhibited iPP molec-ular diffusion to the surface of nucleus during crys-tallization; therefore, the spherulite morphologies ofiPP-50 and iPP-30 showed a more open spherulitictexture, which agreed with the results of Keith andPadden.32,33

On the other hand, the signs of birefringence ofthe spherulites for iPP-100, iPP-50, and iPP-30 pre-sented a negative birefringence, whereas that of thespherulites for iPP-80 presented a mix of positiveand negative birefringences (with similar results iniPP-90 and iPP-70), as shown in Figure 6. From sev-eral studies reported on the morphologies of melt-crystallized iPP, it is clear that the crystalline mor-phology of iPP is dominated by a highly characteris-

tic lamellar branching (crosshatching), with radialand tangential lamellae (chains perpendicular andparallel to spherulitic radius, respectively).7,14–15,44

According to Norton and Keller,7 the spherulites ofiPP-100, iPP-50, and iPP-30 may be dominated byradial lamellae (chains parallel to spherulitic radius),whereas the spherulites of the iPP-80 may be domi-nated by both crosshatched and radial lamellae(chains perpendicular to spherulitic radius). Thedifferent birefringences of spherulites have beenlinked to lamellae morphology through the balanceof crosshatched radial and tangential lamellae or thevarious feather contents in spherulites.

Figure 7 shows the effect of isothermal Tc on thespherulite morphologies for iPP-100 and iPP-80,respectively. The results indicate that the size of thespherulite increased and the amount of the spheru-lites decreased as Tc increased. This implies that the

Figure 7 Effect of isothermal Tc on spherulite morphology for the iPP blends (magnification ¼ 400�): (a) 110, (b) 120,and (c) 1308C for iPP-100 and (d) 110, (e) 120, and (f) 1308C for iPP-80.

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iPP-100 and iPP blends suppressed the concentrationof the nucleus and inhibited iPP molecule diffusionto the surface of the nucleus with increasing Tc

because crystallization proceeded for iPP blendsdominated by the nucleation-controlled mechanismduring crystallization, as discussed previously.

Microstructure

WAXD studies were carried out to obtain informa-tion about the crystal microstructure for the iPP andiPP blends. WAXD intensity curves of the iPP blendsafter isothermal crystallization at 1308C for 120 minare shown in Figure 8. The X-ray diffractograms ofsamples of the iPP blends showed nearly the a-formdiffractograms after they were isothermally crystal-lized at 1308C. For iPP isothermally crystallized at1308C, the characteristic peaks of the a form werefound at 2y angles of 14.088 (110), 16.958 (040), 18.58(130), 21.28 (111), and 21.858 (�131 and 041), asshown in Figure 8.27,45 Generally, in the typicalWAXD intensity pattern of the a form, the intensityof second peak (040) must be smaller than that offirst peak (110). In this study, the intensity of thesecond peak was stronger than the first peak. Thisfact indicates that other crystal forms have coexistedwith the a form of iPP in the iPP blends. The charac-teristic peaks of the g form of iPP were also found at2y angles of 13.848 (111), 15.058 (113), 16.728 (008),20.078 (117), 21.28 (202), and 21.888 (026).27,45 There-fore, these results indicate that the location of the

stronger second peak may have been due to the (008)of the g form coexisting with the (040) of a form ofiPP, which implies a lower degree of perfection ofthe g-form crystals/triclinic structure coexisting withthe a form of iPP at smaller amounts of aPP in theiPP blends during crystallization. However, aPP con-tent in even larger amounts led to a decrease in theg-form crystallization, which implied that the inten-sity of second peak was lower than that of the firstone for iPP-50 and iPP-30. The developing intensityof the second peak indicated that the aPP moleculesacted a diluent on the growth rate of the g-formcrystal because the diluent aPP molecules promotedXc and/or the crystallization rate of the g-form crys-tal of iPP in the iPP blends. This indicated that thesediluent aPP molecules were incorporated in theinterfibrillar and interlamellar regions to form anunstable g-form crystal. However, when aPP contentwas above 50 wt %, the diffraction pattern showedonly the a-form diffractogram.

From these results, we determined that the transi-tion from positive to negative spherulites occurredat smaller aPP contents and may have been due tothe appearance of coexisting a- and g-form crystals.Moreover, the highest intensity of the second peakand Xc were obtained for iPP-80; however, theg-form content almost disappeared for iPP-50. There-fore, the morphology of the spherulites of iPP-50 onlyshowed negative spherulites, whereas that for iPP-80combined positive with negative spherulites. Theseresult indicated that aPP molecules altered the crystalstructures of iPP, favoring the tangential lamellaeand, hence, changed the ratio between tangential andradial lamellae or the ratio of featherlike lamellarstructures radiating approximately from the centers ofthe spherulites during crystallization. The morphol-ogy transition from a negative to combination of posi-tive and negative spherulites may have been due toappearance of coexisting a- and g-form crystals of iPPin the iPP blends during crystallization.

In comparison, Figure 9 shows the effect of theisothermal temperature on the WAXD intensity pat-terns for iPP-100. The results indicate that at higherisothermal temperatures, lower supercooling wasmore favorable for the g-form crystal of iPP. Thefraction content of the g-form increased remarkably,whereas that of a-form decreased as the isothermaltemperature was increased. This fact indicates thatincreasing Tc promoted an a- to g-form transitionand induced a higher degree of perfection of the g-form crystals.31

Figure 10(a,b) shows the effect of aPP content onXc of the iPP blends and normalized Xc of iPP withinthe iPP blends. Figure 10(a) shows that with increas-ing aPP content, Xc of the iPP blend increased andthen decreased, as shown in Table1. This resultindicates that the crystallization ability of iPP was

Figure 8 Effect of aPP content on the WAXD intensitypatterns for the iPP blends after isothermal crystallizationat 1308C.

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strongly dependent on the aPP content. At smallamounts of aPP, Xc of the iPP blends were higherthan that of pure iPP. The developing Xc of the iPPblend showed that the diluent aPP molecules sup-pressed the entanglement between iPP moleculesand promoted the mobility of iPP molecules duringcrystallization. However, with larger amounts of aPP,the decreasing Xc of the iPP blends may have beendue to the larger amount of diluent aPP action thatsuppressed the concentration of the nucleus moreand inhibited the iPP molecules from molten regiondiffusing to the surface of nucleus during crystalliza-tion. In this study, however, aPP was a noncrystal-line polymer, and it could not exist in the crystallinestructure of iPP. Therefore, aPP must have residedin the noncrystalline regions, that is, the interlamel-lar and interfibrillar or interspherulitic regions.

Interestingly, the effect of aPP content on the totalXc and the spherulitic density of iPP blends acted asa diluent or plasticizer agent. Therefore, normalizedXc or the crystallizability of the iPP chain within theiPP blends are shown in Figure 10(b). Normalized Xc

was calculated with the result of the Xc of iPP blendsdivided by iPP content within iPP blends, as dis-cussed recently.31 This result showed that normal-ized Xc of the iPP molecules within the iPP blendsincreased with increasing aPP content. This resultshows that the aPP molecules promoted the growthrate of iPP because the diluent aPP moleculesincreased the mobility of iPP and reduced the entan-glement between iPP molecules and led to an in-crease in normalized Xc of iPP during crystallization.

CONCLUSIONS

The crystallization kinetics and morphology develop-ment for iPP and iPP blends with aPP were investi-gated with different aPP contents and isothermalconditions. The crystallization kinetics of iPP blendsfollowed Avrami behavior at early stages of primarycrystallization with exponents of 2.75 6 0.17 forpure iPP and 3.55 6 0.5 or 2.62 6 0.3 for iPP blendedwith smaller or larger amounts of aPP, respectively,over the Tc range studied. The differences in nvalues were ascribed to different nucleation andcrystal growth mechanisms. From these values of n,we established that spherulitic development arosefrom an athermal and instantaneous nucleation pro-cess followed by three-dimensional crystal growthfor pure iPP. However, spherulitic development ofthe iPP blend crystalline phase arose from a thermaland random nucleation process followed by three-dimensional crystal growth with increasing aPPcontent. From these facts, we concluded that withsmall amounts of aPP, the aPP molecules promotedthe mobility of iPP molecules and reduced the entan-glement between iPP molecules and led to an increas-ing crystallization ability of iPP, whereas with largeramounts of aPP, the decrease in Xc of the iPP blendsmay have been due to the larger amount of diluentaPP action that suppress the amount of nucleusmore and inhibited the iPP molecules from moltenregion diffusing to the surface of the nucleus duringcrystallization. This means that the aPP moleculesacted as a diluent agent in the iPP phase and had adrastic affect on Xc and structure morphology of theisothermal crystallized iPP.

Figure 9 Effect of isothermal Tc on the WAXD intensitypatterns for iPP-100.

Figure 10 Effect of aPP content on (a) Xc of the iPPblends and (b) normalized Xc of iPP for the iPP blends.

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Journal of Applied Polymer Science DOI 10.1002/app

Because aPP is a noncrystallizable component, itcannot exist in the crystalline structure of iPP; there-fore, aPP must reside in the noncrystalline regions ofiPP blends. However, with small amounts of aPP, Xc

and spherulitic density of the iPP blends were higherthan that of pure iPP and presented a lower degreeof perfection of the g-form crystals/triclinic structurecoexisting with the a-form of iPP during crystalliza-tion. The developing Xc and morphology of the iPPblends showed that the diluent aPP molecular sup-pressed the amount of entanglement between iPPchains and promoted the mobility of the iPP chainin the amorphous phase during crystallization. How-ever, with larger amounts of aPP, Xc and spheruliticdensity of the iPP blends decreased, and the reduc-tion in the g-form crystallization may have been dueto the larger amounts diluent aPP molecular actionthat suppressed the concentration of nucleus moreand inhibited the iPP chain from diffusing to thesurface of the nucleus during crystallization.

Interestingly, the birefringences of spherulites foriPP-100 and iPP-50 presented a negative profile,whereas that of iPP-80 showed a combination of thepositive with negative birefringences, which indicatedthat aPP molecules altered the spherulite structure ofiPP during crystallization and favored the formationof radial lamellae and hence modified the ratiobetween tangential and radial lamellae. The transi-tion of birefringence of the spherulites from negativeto mixed negative and positive spherulites may havebeen due to the appearance of a- and g-form crystalscoexisting within the iPP blend during crystalliza-tion. However, the total Xc of the iPP blends in-creased and then decreased, whereas the normalizedXc of the iPP molecules within the iPP blends in-creased with increasing aPP content.31 Therefore, theaPP content affected the crystallization process ofiPP in two opposite directions; that is, it hinderedthe nucleation rate and promoted the chain motion/growth rate with smaller aPP contents and hinderedboth the nucleation rate and growth rate at largeraPP contents. The action of these two effects changedthe crystal forms from coexisting a and g formsto the pure a form with increasing aPP content asdiscussed.

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