Oriented Gamma Phase in Isotactic Polypropylene HomopolymerTim B. van Erp,† Luigi Balzano,‡ and Gerrit W.M. Peters*,†

†Department of Mechanical Engineering, Materials Technology Institute, Eindhoven University of Technology, P.O.Box 513, 5600MB, Eindhoven, The Netherlands‡DSM Material Science Center, Urmonderbaan 22, Geleen 6167 RD, The Netherlands

ABSTRACT: The presence of γ-phase in isotactic polypropyleneis well-known but, up until now, could only be induced by specificprocessing conditions or material modifications. Typically, forZiegler−Natta (ZN) iPPs pressures of 2000 bar are required,otherwise, metallocene (M) iPPs and copolymerization usingolefin-type counits should be used. Here we show thatcrystallization under the unique combination of moderatepressure (p ≥ 900 bar) and strong shear flow oriented specimenswith high contents of γ-phase are created in ZN-iPPs. Theoriented morphology is qualified as a shish-kebab structure thattemplates densely branched γ-lamellae on parent α-lamellae aswell as directly to the shish backbone.

Solidification in industrial processes, like injection molding,involves complex flow fields, steep thermal gradients, and

high pressures. Investigating polymer solidification undercomparative processing conditions is a necessary step inorder to understand or even predict the final polymerproperties. In general, the separate effects of shear flow orpressure on polymer crystallization are well-known, in contrastto the simultaneous effect of flow and pressure on thecrystallization kinetics for which accurate control duringprocessing is a prerequisite. Therefore, in this study,dilatometry is used to crystallize isotactic polypropylene(iPP) nonisothermally after applying short-term shear at twodifferent undercoolings and at elevated pressures.Polymorphism is the ability of a substance to exist in more

than one form or crystal structure and is common insemicrystalline polymers. Isotactic polypropylene (iPP) showsan extremely rich and interesting polymorphic behavior,exhibiting multiple crystalline forms and a mesophase.1,2 Allknown modifications share the 3-fold helical conformation ofthe polymer chains, but differ in the packing mode. Chainpacking in the different crystal forms is determined both bymolecular variables, such as isotacticity or the presence ofcomonomers, and crystallization conditions, for example,cooling rate or flow.The most common and stable crystal modification of iPP,

obtained at standard processing conditions, is the monoclinicα-phase. A peculiar feature of this phase is lamellar branchingthat results in cross-hatched morphologies containing initiallyformed lamellae (parents) that have branches (daughters) at anangle of 80° to the (010) lateral parent-lamellae surface.3 Theorthorhombic γ-phase is less frequently observed in iPP, andthis polymorph is rather unique, containing layers of non-parallel chain segments. The structure of the γ-phase is madeup by a succession of bilayers tilted by 80 or 100° from one to

another, giving rise to a tilting of the chain axis of 40° respect tothe lamellar normal.4 Lamellae comprising of γ-phase areknown to nucleate on α-lamellae by an epitaxial mechanism,resulting in an angle of 40° with respect to the α-lamellae.5,6 Itis worth underlining that γ-lamellae do not show anyhomoepitaxy, in contrast to α-lamellae.Crystallization of the γ-phase can be achieved in several

manners such as copolymerization with small amounts of 1-olefin counits,7−9 introducing stereo- and regio-irregularitiescontrolled by a metallocene catalyst10−13 in materials of verylow molecular weight,6 or crystallizing under elevated pressureand high temperatures.14−16 Comprehensive studies of Alamoet al. and De Rosa et al. revealed the details behind theformation and relative amount of the α- and γ-phase inmetallocene-derived iPPs by varying the amount and nature ofchain defects and comonomers.7−12 A bell-shaped dependenceof the γ-crystal fraction on the crystallization temperature isfound and explained by the competition between kinetic andthermodynamic effects; an interplay between the individualgrowth rates and thermodynamic stability of both phases athigher temperatures. The effect of pressure on the formation ofγ-crystals in a highly stereoregular ZN-iPP homopolymer isstudied by Mezghani et al.14−16 They found a competitionbetween the growth of α- and γ-phase in the high temperaturerange with the latter predominant for pressures reaching 2000bar and crystallization temperatures exceeding 170 °C, that is,at very low undercoolings. Their explanation is that theformation of the γ-phase is thermodynamically driven at highercrystallization temperatures due to a lower heat of fusion

Received: February 29, 2012Accepted: April 9, 2012

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leading to a lower Gibbs free energy above a transitiontemperature of 174 °C.Typical processing of polymers usually involves application

of a flow field; however, only a few studies focus on theinfluence of shear stress on the formation of γ-phase in bothZN-iPP,17 as well as M-iPP.18,19 Spatial distributions of γ-content are found in injection molded samples in which thelevel of γ-crystals is determined by thermodynamics, regiodefects, and (packing) pressure. Nevertheless, it is clear that thepresence of shear flow is not a mechanism for the formation ofthe γ-phase, but only results in an oriented morphology inwhich oriented γ-crystals are incorporated.In the following, the structure of a sample crystallized at p =1200 bar after application of a shear pulse of γ = 180 s−1 for 1 sat ΔT = 30 °C (Tγ = 201 °C), resulting in an onsetcrystallization temperature of 179.3 °C (according to specificvolume data) is discussed. A characteristic feature ofcrystallization, measured by dilatometry, is the significantdrop in the evolution of specific volume. The temperature atwhich the drop in specific volume initiates is frequently calledthe transition temperature or onset crystallization temper-ature.26,27 The corresponding X-ray scattering data is presentedin Figure 1 in which clear evidence of densely branched γ-crystals on an oriented α-phase shish-kebab is observed. Thecoexistence of both α- and γ-crystals causes overlapping of themain WAXD reflections, due to the structural similarity of thesecrystal phases, except for the characteristic (130)α and (117)γreflections. Therefore, WAXD deconvolution is applied toextract the relative fraction of different crystalline phases usingthe ratio of the fitted peak area under these reflections,7

resulting in fγ = 84.5% for these particular processingconditions. Radial integration of both reflections show maximaat the equator and at ∼45° and, additionally, the overallintensity level is stronger for the (117)γ than the (130)αreflection. Meanwhile, the radial intensity at 2θ ≈ 9.3°,composed of the superposition of the (110)α and (111)γreflections, also displays maxima at the equator and, in thiscase, at ∼80°. The radial intensity at 2θ ≈ 11.2°, composed of

the superposition of the (040)α and (008)γ reflections, displaysa strong maxima at the equator. All these WAXD observationssuggest that the morphology is composed of a fiber likestructure with epitaxially crystallized α-daughters and γ-lamellae, for which the α-axis and chain direction are parallelwith flow direction, respectively.11,28 Two types of molecularorientation can be distinguished in γ-crystals: parallel andperpendicular (“cross-β”) chain axes orientation.28 For the“cross-β” configuration, a strong meridional spot of the (008)γreflection should be present, which is absent in our diffractionpatterns. Therefore, only parallel chain axis orientation of the γ-form is present. A remarkable and unique SAXS image is shownin Figure 1, which contains the typical features of a shish-kebabstructure,29 however, with additional scattering in the diagonaldirection of ∼40°. It is known that the γ-lamellae branch at anangle of 40.7° to α-phase parents.5,6 Here, the parent lamellaeare well oriented in flow, and consequently, the diagonal SAXSscattering originates from the densely branched γ-lamellaetemplated on the oriented shish-kebab. Furthermore, SAXSintegrated data show that the kebabs possess a long period of∼33 nm, while long periods half of that are found in equatorialand diagonal direction, meaning that both the α-daughters andγ-lamellae are more densely spaced, confirming the α−γbranching scheme proposed by Lotz et al.5,6

Two conclusions can be drawn from the coexistence of suchlarge amounts of γ-phase with a fair amount of oriented α-phase, originating from shish-kebabs and daughter lamellae,First, a significant amount of crystallization takes place under“unperturbed”, that is, partially relaxed, melt creating α-phaseparents (kebabs) and daughter lamellae as well as the γ-lamellae. Second, the α−γ branching mechanism still takesplace under combined conditions of pressure and flow.Combining these conclusions with the X-ray observationsleads to a schematic of the morphology visualized in Figure 2.The basis is the well-known shish-kebab structure when, underflow, the stretched chain segments crystallize into the extendedchain shish structure, and subsequently, the less oriented chainscrystallize into folded chain kebabs growing perpendicular to

Figure 1. X-ray scattering data for a ZN-iPP sample crystallized at p = 1200 bar, γ = 180 s−1 for 1 s, Tγ = 201 °C, and T = 1 °C·s−1. Top row (left toright): 2D SAXS image, total integrated intensity and radial integration at two q values. Bottom row (left to right): 2D WAXD image, total integratedintensity, and radial intensity of the (110)α, (040)α, (130)α, and (117)γ reflections.

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the surface of the shish. The kebabs are aligned with their c-axesin flow direction and the daughter lamellae, considered to growepitaxially on the formed parent lamellae with an angle of80.7°,3 with their a-axes parallel to the flow direction.30−32 Thehigh content of γ-crystals (84.5%) suggests that branching doesnot only take place on the parent and daughter lamellae but isalso possible directly on the shish. This is supported by the factthat the (010) lateral face of the α-phase structures is the idealsurface for epitaxial crystallization of γ-lamellae.5,6 Thisparticular face is abundantly present at the side surface of ashish because it is shown that the shish backbone is alreadypartially crystalline, with a well developed α-crystal structure,during the initial stages of formation.20 Additionally, theformation of γ-crystals is possible on oriented substrates, suchas fibers and carbon nanotubes, driven by epitaxy latticematching.33−35 Note, in composite α−γ single crystals, therelationship between lamellar orientations of the α- and γ-phaseis uniquely defined; however, in our samples, due to the fibersymmetry, all orientations of the α-phase at 80° and γ-phase at40° are generated.To support the suggested structural relationship between

shish-kebab and γ-phase, detailed TEM pictures are taken (on70 nm thick coupes) to visualize the morphology and areshown in Figure 3. As expected, the TEM pictures show anoriented morphology composed of long crystalline structures(shish) parallel to flow with perpendicular nucleated crystals(kebab) and branched crystals at a given angle (γ-lamellae).The lateral dimension of the shish backbones is 30.5 ± 6.4 nm,while the spacing of alternating dark and bright areas in the

shish is 35.5 ± 0.5 nm. These observations are in line withproposed shish core structures and originate from amorphous(or high concentration of defects) and crystalline regions in thebackbone.36,37 The meridional SAXS peak at 0.19 nm−1 mightbe the result of this periodicity within the shish backbone.Morover, the diameter of a (fibril) structure can be probed withSAXS,38 using D = (8π/w)0.5, with w, the peak full width at halfmax, yielding a diameter of 29 nm, and a long spacing of Lp

SAXS

= 33.1 nm, both well in agreement with TEM observations.Perpendicular to the shish, numerous kebabs are present onwhich countless γ-lamellae are grown at an angle of ∼40°,displaying a “feather”-like structure,15 which give rise to thedistinct diagonal scattering in SAXS. The γ-phase long periodobtained from TEM is 11.0 ± 1.0 nm being close but lowerthan SAXS observations, which shows a long period of 14.4 nm.Note from the TEM pictures it is ambiguous if γ-lamellae aredirectly nucleated on the shish, however, it cannot be excluded.In the complete series of performed PVT experiments (the

detailed results will be reported in the near future), multipleconditions are found for which a high amount of γ-phase givesrise to distinct small angle scattering at ∼40° respect to flowand these SAXS images, and corresponding processingconditions, are presented in Figure 4. Only for these foursamples, a distinct maximum, at β = 40.7°, is observed uponradial integration of the SAXS intensity at q = 0.43 nm−1 (asshown in Figure 1 top right), while WAXD still reveal orientedγ-crystals (equatorial and diagonal arching of (117)γ reflection)for less harsh processing conditions, that is, lower pressure andshear rate. For instance, the PVT experiments, under isobaricconditions of 1200 bar and absence of flow, show that ZN-iPPcrystallizes at Tc = 149.2 °C into an isotropic (spherulitic) α-and γ-phase mixture with fγ = 66.3%. It appears that thecreation of γ-phase lamellae produce diagonal maxima in SAXSonly when they exhibit a preferred orientation and when theircontent exceeds ∼80%. Such high amounts of γ-phase in ZN-iPP can only be created at elevated pressure and highcrystallization temperatures, approaching or exceeding thetransition temperature, Tα→γ = 174 °C, where for the γ-phasethe growth rate and the thermodynamic stability are higherthan the α-phase.16,40 The consequence of an additional strongshear pulse is not only to induce oriented structures, but it alsoplays a key role in creating favorable conditions for γ-phaseformation, that is, increasing the crystallization temperature. Itis well-known that the crystallization kinetics are enhanced withapplied flow29,41,42 and, consequently, for similar nonisothermalconditions, the crystallization temperature increases.22 On theother hand, during the shear flow the chains will align parallelto flow direction, which is unfavorable to the nonparallel chainarrangement of the γ-crystal, and as a result in the initial stageof the crystallization process α-phase oriented structures areformed. Subsequently, after cessation of flow, the combinationof elevated pressure and the increase of crystallizationtemperature favor the formation of γ-crystals. To conclude,we show that a sheared and pressurized ZN-iPP homopolymermelt crystallizes, at high temperatures, into oriented α-phaseshish-kebab structures with high amounts of branched, orientedγ-crystals on the parent lamellae, as well as directly on the shishbackbone.

■ EXPERIMENTAL SECTIONIn this Letter, we describe the processing conditions necessary toobtain almost fully γ-phase crystallized samples of a highly stereo-regular ZN-iPP homopolymer (Borealis HD601CF, Mw = 365

Figure 2. Schematic of the structural relationship between α-phaseshish-kebab and γ-lamellae.

Figure 3. TEM pictures of a ZN-iPP sample crystallized at p = 1200bar, γ = 180 s−1 for 1 s, Tγ = 201 °C, and T = 1 °C·s−1. The sampleswere trimmed at low temperature (−100 °C) and subsequently stainedfor 24 h with a RuO4 solution, prepared according to Montezinos etal.39 Ultrathin sections (70 nm) were obtained at −50 °C using a LeicaUltracut S/FCS microtome. The sections were put on a 200 meshcopper grid with a carbon support layer and examined in a Tecnai 20transmission electron microscope, operated at 200 kV.

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kg·mol−1, Mn = 68 kg·mol−1, Mw/Mn = 5.4, and tacticity 97.5%[mmmm] pentads) used in several crystallization studies.20,21 Moltensamples, ring-shaped with mass m ∼ 75 mg, were crystallized in adilatometer (Pirouette PVT apparatus, IME Technogolgies22,23) undersimilar nonisothermal but varying isobaric conditions, while a briefpulse of shear is applied at two fixed undercoolings of ΔTγ = 30 and 60°C with the shear temperature Tγ = Tm(p) − ΔTγ. Note that theequilibrium melting temperature is, in first approximation, linearlydependent on pressure.16,24 Morphological and structural details wereextracted with X-ray scattering (WAXD and SAXS) at the beamlineBM2625 of the European Synchroton Radiation Facility (ESRF,Grenoble, France) using a wavelength λ = 1.033 Å and 2D patternswere recorded with a low noise Pilatus 1 M detector with pixel size of172 × 172 μm2.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: g.w.m.peters@tue.nl. Phone: +31 (0)40 247 4840.Fax: +31 (0)40 244 7355.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors thank Prof. Bernard Lotz (CNRS) for helpfuldiscussion on the crystallography, Anne Spoelstra (TUE) forproviding TEM pictures, Guiseppe Portale (BM26, ESRF) forsupport during the beam time, and The NetherlandsOrganisation for Scientific Research (NWO) and DUBBLEare acknowledged granting the beam time. This research wassupported by the Dutch Technology Foundation STW, appliedscience division of NWO, and the Technology Program of theMinistry of Economic Affairs (under Grant No. 07730).

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Figure 4. 2D SAXS images of ZN-iPP homopolymer showing distinct γ-phase lamellar scattering in the diagonal direction respect (∼40°) to flow.Accordingly, the applied processing conditions, under nonisothermal cooling of ∼1 °C·s−1, are given below the images.

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