Oriented Gamma Phase in Isotactic Polypropylene Homopolymer

Letter pubs.acs.org/macroletters

Oriented Gamma Phase in Isotactic Polypropylene Homopolymer Tim 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, 5600 MB, Eindhoven, The Netherlands ‡ DSM Material Science Center, Urmonderbaan 22, Geleen 6167 RD, The Netherlands ABSTRACT: The presence of γ-phase in isotactic polypropylene is well-known but, up until now, could only be induced by specific processing conditions or material modifications. Typically, for Ziegler−Natta (ZN) iPPs pressures of 2000 bar are required, otherwise, metallocene (M) iPPs and copolymerization using olefin-type counits should be used. Here we show that crystallization under the unique combination of moderate pressure (p ≥ 900 bar) and strong shear flow oriented specimens with high contents of γ-phase are created in ZN-iPPs. The oriented morphology is qualified as a shish-kebab structure that templates densely branched γ-lamellae on parent α-lamellae as well as directly to the shish backbone.

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another, giving rise to a tilting of the chain axis of 40° respect to the lamellar normal.4 Lamellae comprising of γ-phase are known to nucleate on α-lamellae by an epitaxial mechanism, resulting in an angle of 40° with respect to the α-lamellae.5,6 It is worth underlining that γ-lamellae do not show any homoepitaxy, in contrast to α-lamellae. Crystallization of the γ-phase can be achieved in several manners such as copolymerization with small amounts of 1olefin counits,7−9 introducing stereo- and regio-irregularities controlled by a metallocene catalyst10−13 in materials of very low molecular weight,6 or crystallizing under elevated pressure and high temperatures.14−16 Comprehensive studies of Alamo et al. and De Rosa et al. revealed the details behind the formation and relative amount of the α- and γ-phase in metallocene-derived iPPs by varying the amount and nature of chain defects and comonomers.7−12 A bell-shaped dependence of the γ-crystal fraction on the crystallization temperature is found and explained by the competition between kinetic and thermodynamic effects; an interplay between the individual growth rates and thermodynamic stability of both phases at higher temperatures. The effect of pressure on the formation of γ-crystals in a highly stereoregular ZN-iPP homopolymer is studied by Mezghani et al.14−16 They found a competition between the growth of α- and γ-phase in the high temperature range with the latter predominant for pressures reaching 2000 bar and crystallization temperatures exceeding 170 °C, that is, at very low undercoolings. Their explanation is that the formation of the γ-phase is thermodynamically driven at higher crystallization temperatures due to a lower heat of fusion

olidification in industrial processes, like injection molding, involves complex flow fields, steep thermal gradients, and high pressures. Investigating polymer solidification under comparative processing conditions is a necessary step in order to understand or even predict the final polymer properties. In general, the separate effects of shear flow or pressure on polymer crystallization are well-known, in contrast to the simultaneous effect of flow and pressure on the crystallization kinetics for which accurate control during processing is a prerequisite. Therefore, in this study, dilatometry is used to crystallize isotactic polypropylene (iPP) nonisothermally after applying short-term shear at two different 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 in semicrystalline polymers. Isotactic polypropylene (iPP) shows an extremely rich and interesting polymorphic behavior, exhibiting multiple crystalline forms and a mesophase.1,2 All known modifications share the 3-fold helical conformation of the polymer chains, but differ in the packing mode. Chain packing in the different crystal forms is determined both by molecular variables, such as isotacticity or the presence of comonomers, 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 monoclinic α-phase. A peculiar feature of this phase is lamellar branching that results in cross-hatched morphologies containing initially formed lamellae (parents) that have branches (daughters) at an angle of 80° to the (010) lateral parent-lamellae surface.3 The orthorhombic γ-phase is less frequently observed in iPP, and this polymorph is rather unique, containing layers of nonparallel chain segments. The structure of the γ-phase is made up by a succession of bilayers tilted by 80 or 100° from one to © 2012 American Chemical Society

Received: February 29, 2012 Accepted: April 9, 2012 Published: April 19, 2012 618

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

the superposition of the (040)α and (008)γ reflections, displays a strong maxima at the equator. All these WAXD observations suggest that the morphology is composed of a fiber like structure with epitaxially crystallized α-daughters and γlamellae, for which the α-axis and chain direction are parallel with flow direction, respectively.11,28 Two types of molecular orientation can be distinguished in γ-crystals: parallel and perpendicular (“cross-β”) chain axes orientation.28 For the “cross-β” configuration, a strong meridional spot of the (008)γ reflection should be present, which is absent in our diffraction patterns. Therefore, only parallel chain axis orientation of the γform is present. A remarkable and unique SAXS image is shown in Figure 1, which contains the typical features of a shish-kebab structure,29 however, with additional scattering in the diagonal direction of ∼40°. It is known that the γ-lamellae branch at an angle of 40.7° to α-phase parents.5,6 Here, the parent lamellae are well oriented in flow, and consequently, the diagonal SAXS scattering originates from the densely branched γ-lamellae templated on the oriented shish-kebab. Furthermore, SAXS integrated data show that the kebabs possess a long period of ∼33 nm, while long periods half of that are found in equatorial and diagonal direction, meaning that both the α-daughters and γ-lamellae are more densely spaced, confirming the α−γ branching scheme proposed by Lotz et al.5,6 Two conclusions can be drawn from the coexistence of such large amounts of γ-phase with a fair amount of oriented αphase, originating from shish-kebabs and daughter lamellae, First, a significant amount of crystallization takes place under “unperturbed”, that is, partially relaxed, melt creating α-phase parents (kebabs) and daughter lamellae as well as the γlamellae. Second, the α−γ branching mechanism still takes place under combined conditions of pressure and flow. Combining these conclusions with the X-ray observations leads to a schematic of the morphology visualized in Figure 2. The basis is the well-known shish-kebab structure when, under flow, the stretched chain segments crystallize into the extended chain shish structure, and subsequently, the less oriented chains crystallize into folded chain kebabs growing perpendicular to

leading to a lower Gibbs free energy above a transition temperature of 174 °C. Typical processing of polymers usually involves application of a flow field; however, only a few studies focus on the influence of shear stress on the formation of γ-phase in both ZN-iPP,17 as well as M-iPP.18,19 Spatial distributions of γcontent are found in injection molded samples in which the level of γ-crystals is determined by thermodynamics, regio defects, and (packing) pressure. Nevertheless, it is clear that the presence of shear flow is not a mechanism for the formation of the γ-phase, but only results in an oriented morphology in which 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 γ̇ = 180 s−1 for 1 s at ΔT = 30 °C (Tγ̇ = 201 °C), resulting in an onset crystallization temperature of 179.3 °C (according to specific volume data) is discussed. A characteristic feature of crystallization, measured by dilatometry, is the significant drop in the evolution of specific volume. The temperature at which the drop in specific volume initiates is frequently called the transition temperature or onset crystallization temperature.26,27 The corresponding X-ray scattering data is presented in Figure 1 in which clear evidence of densely branched γcrystals on an oriented α-phase shish-kebab is observed. The coexistence of both α- and γ-crystals causes overlapping of the main WAXD reflections, due to the structural similarity of these crystal phases, except for the characteristic (130)α and (117)γ reflections. Therefore, WAXD deconvolution is applied to extract the relative fraction of different crystalline phases using the ratio of the fitted peak area under these reflections,7 resulting in fγ = 84.5% for these particular processing conditions. Radial integration of both reflections show maxima at the equator and at ∼45° and, additionally, the overall intensity level is stronger for the (117)γ than the (130)α reflection. Meanwhile, the radial intensity at 2θ ≈ 9.3°, composed of the superposition of the (110)α and (111)γ reflections, also displays maxima at the equator and, in this case, at ∼80°. The radial intensity at 2θ ≈ 11.2°, composed of 619

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shish is 35.5 ± 0.5 nm. These observations are in line with proposed shish core structures and originate from amorphous (or high concentration of defects) and crystalline regions in the backbone.36,37 The meridional SAXS peak at 0.19 nm−1 might be the result of this periodicity within the shish backbone. Morover, the diameter of a (fibril) structure can be probed with SAXS,38 using D = (8π/w)0.5, with w, the peak full width at half max, yielding a diameter of 29 nm, and a long spacing of LSAXS p = 33.1 nm, both well in agreement with TEM observations. Perpendicular to the shish, numerous kebabs are present on which countless γ-lamellae are grown at an angle of ∼40°, displaying a “feather”-like structure,15 which give rise to the distinct diagonal scattering in SAXS. The γ-phase long period obtained from TEM is 11.0 ± 1.0 nm being close but lower than SAXS observations, which shows a long period of 14.4 nm. Note from the TEM pictures it is ambiguous if γ-lamellae are directly 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), multiple conditions are found for which a high amount of γ-phase gives rise to distinct small angle scattering at ∼40° respect to flow and these SAXS images, and corresponding processing conditions, are presented in Figure 4. Only for these four samples, a distinct maximum, at β = 40.7°, is observed upon radial integration of the SAXS intensity at q = 0.43 nm−1 (as shown in Figure 1 top right), while WAXD still reveal oriented γ-crystals (equatorial and diagonal arching of (117)γ reflection) for less harsh processing conditions, that is, lower pressure and shear rate. For instance, the PVT experiments, under isobaric conditions of 1200 bar and absence of flow, show that ZN-iPP crystallizes at Tc = 149.2 °C into an isotropic (spherulitic) αand γ-phase mixture with fγ = 66.3%. It appears that the creation of γ-phase lamellae produce diagonal maxima in SAXS only when they exhibit a preferred orientation and when their content exceeds ∼80%. Such high amounts of γ-phase in ZNiPP can only be created at elevated pressure and high crystallization temperatures, approaching or exceeding the transition temperature, Tα→γ = 174 °C, where for the γ-phase the growth rate and the thermodynamic stability are higher than the α-phase.16,40 The consequence of an additional strong shear pulse is not only to induce oriented structures, but it also plays a key role in creating favorable conditions for γ-phase formation, that is, increasing the crystallization temperature. It is well-known that the crystallization kinetics are enhanced with applied flow29,41,42 and, consequently, for similar nonisothermal conditions, the crystallization temperature increases.22 On the other hand, during the shear flow the chains will align parallel to flow direction, which is unfavorable to the nonparallel chain arrangement of the γ-crystal, and as a result in the initial stage of the crystallization process α-phase oriented structures are formed. Subsequently, after cessation of flow, the combination of elevated pressure and the increase of crystallization temperature favor the formation of γ-crystals. To conclude, we show that a sheared and pressurized ZN-iPP homopolymer melt crystallizes, at high temperatures, into oriented α-phase shish-kebab structures with high amounts of branched, oriented γ-crystals on the parent lamellae, as well as directly on the shish backbone.

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

the surface of the shish. The kebabs are aligned with their c-axes in flow direction and the daughter lamellae, considered to grow epitaxially on the formed parent lamellae with an angle of 80.7°,3 with their a-axes parallel to the flow direction.30−32 The high content of γ-crystals (84.5%) suggests that branching does not only take place on the parent and daughter lamellae but is also possible directly on the shish. This is supported by the fact that the (010) lateral face of the α-phase structures is the ideal surface for epitaxial crystallization of γ-lamellae.5,6 This particular face is abundantly present at the side surface of a shish because it is shown that the shish backbone is already partially crystalline, with a well developed α-crystal structure, during the initial stages of formation.20 Additionally, the formation of γ-crystals is possible on oriented substrates, such as fibers and carbon nanotubes, driven by epitaxy lattice matching.33−35 Note, in composite α−γ single crystals, the relationship between lamellar orientations of the α- and γ-phase is uniquely defined; however, in our samples, due to the fiber symmetry, all orientations of the α-phase at 80° and γ-phase at 40° are generated. To support the suggested structural relationship between shish-kebab and γ-phase, detailed TEM pictures are taken (on 70 nm thick coupes) to visualize the morphology and are shown in Figure 3. As expected, the TEM pictures show an oriented 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

Figure 3. TEM pictures of a ZN-iPP sample crystallized at p = 1200 bar, γ̇ = 180 s−1 for 1 s, Tγ̇ = 201 °C, and Ṫ = 1 °C·s−1. The samples were trimmed at low temperature (−100 °C) and subsequently stained for 24 h with a RuO4 solution, prepared according to Montezinos et al.39 Ultrathin sections (70 nm) were obtained at −50 °C using a Leica Ultracut S/FCS microtome. The sections were put on a 200 mesh copper grid with a carbon support layer and examined in a Tecnai 20 transmission electron microscope, operated at 200 kV.

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EXPERIMENTAL SECTION

In this Letter, we describe the processing conditions necessary to obtain almost fully γ-phase crystallized samples of a highly stereoregular ZN-iPP homopolymer (Borealis HD601CF, Mw = 365 620

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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. 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 Molten samples, ring-shaped with mass m ∼ 75 mg, were crystallized in a dilatometer (Pirouette PVT apparatus, IME Technogolgies22,23) under similar nonisothermal but varying isobaric conditions, while a brief pulse of shear is applied at two fixed undercoolings of ΔTγ̇ = 30 and 60 °C with the shear temperature Tγ̇ = Tm(p) − ΔTγ̇. Note that the equilibrium melting temperature is, in first approximation, linearly dependent on pressure.16,24 Morphological and structural details were extracted with X-ray scattering (WAXD and SAXS) at the beamline BM2625 of the European Synchroton Radiation Facility (ESRF, Grenoble, France) using a wavelength λ = 1.033 Å and 2D patterns were recorded with a low noise Pilatus 1 M detector with pixel size of 172 × 172 μm2.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +31 (0)40 247 4840. Fax: +31 (0)40 244 7355. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Prof. Bernard Lotz (CNRS) for helpful discussion on the crystallography, Anne Spoelstra (TUE) for providing TEM pictures, Guiseppe Portale (BM26, ESRF) for support during the beam time, and The Netherlands Organisation for Scientific Research (NWO) and DUBBLE are acknowledged granting the beam time. This research was supported by the Dutch Technology Foundation STW, applied science division of NWO, and the Technology Program of the Ministry of Economic Affairs (under Grant No. 07730).

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