Deformation of Stereoirregular Isotactic Polypropylene across Length

Article pubs.acs.org/Macromolecules

Deformation of Stereoirregular Isotactic Polypropylene across Length Scales. Influence of Temperature Finizia Auriemma,*,† Claudio De Rosa,† Rocco Di Girolamo,† Anna Malafronte,† Miriam Scoti,† Geoffrey R. Mitchell,‡ and Simona Esposito† †

Dipartimento di Scienze Chimiche, Università di Napoli “Federico II”, Complesso Monte S.Angelo, via Cintia, 80126 Napoli, Italy CDRSP - Centre for Rapid and Sustainable Product Development, Polytechnic Institute of Leiria, Centro Empresarial da Marinha Grande, 2430-028 Marinha Grande, Portugal

‡

S Supporting Information *

ABSTRACT: The development of a highly oriented fiber morphology by stretching low stereoregular isotactic polypropylene (iPP) samples is studied at two different temperatures. The structural and morphological transformations occurring during deformation are followed in real time by collecting in situ wide (WAXS) and smallangle (SAXS) X-ray scattering data. WAXS analysis reveals that the disordered γ form initially present in the samples gradually transforms by stretching into the oriented mesomorphic form of iPP at room temperature and in well-oriented crystals of α form at 60 °C. SAXS analysis indicates that regardless of the stretching temperature, a cascade of events occurs at mesoscale during deformation, consisting in interlamellar separation and lamellar reorientation at low deformations, followed by formation of chevron-like textures, lamellar fragmentation, and cavitation. Moreover, different fibrillary morphologies develop at the different temperatures. In fact, the fibrillary morphology that develops at room temperature is characterized by rod-like fibrillary entities containing mesomorphic aggregates separated by amorphous regions, placed at uncorrelated longitudinal and lateral distances. Instead, the morphology that develops at 60 °C consists of a fibrillary network for the more stereoregular sample and of rod-like fibrillary entities for the less stereoregular sample, and both morphologies include well-oriented crystals of α form alternating with amorphous regions which are periodically stacked along the stretching direction and are placed at uncorrelated lateral distance. We argue that the difference in structural arrangement of amorphous and crystalline regions at mesoscale in the stretched samples are mainly due to the increase of the mobility of the chains in the amorphous matrix at higher temperature, which facilitates the relaxation of the amorphous phase and the changes in the relative arrangement of the crystalline domains.

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form.5−13 The hierarchy of these transformations entails that the transitions occurring at molecular level should be strongly correlated with those occurring at mesoscale, even though direct studies aimed at establishing these correlations are rare. The study of these correlations was the subject of a recent investigation,14 where the changes occurring at molecular level and lamellar length scale during uniaxial deformation were analyzed in the case of some isotactic polypropylene (iPP) samples of different stereoregularity. The study was performed at 25 °C, above the glass transition of the samples (Tg ≈ −10 °C), by collecting in real time, during stretching at constant deformation rate, wide-angle (WAXS) and small-angle (SAXS) X-ray scattering data. An analytical tool for the analysis of scattering data was devised that allowed us establishing precise correlations between the transformations occurring at mesoscale, such as formation of undulated structures for the lamellar stacks, lamellar breaking and cavitation, and microscopic phenomena such as chain orientation and phase transitions. The samples were synthesized using single site organometallic

INTRODUCTION Uniaxial deformation of semicrystalline polymers produces, in general, the transformation of an initially isotropic morphology consisting of crystalline lamellae alternating with amorphous layers, usually assembled to form spherulites, into a fibrillary morphology, characterized by polymeric chains aligned along the drawing direction.1,2 During this process, complex transformations occur both at lamellar and unit-cell length scales. In particular, the changes occurring at mesoscale may involve, at low deformations, slippage of crystal blocks inside the crystalline lamellae and interlamellar separation,2−4 whereas, with increasing deformation, formation of undulated structures and lamellar fragmentation occur.2−6 Above the glass transition temperature, the interwoven network formed by the crystalline and amorphous phase necessarily entails that the deformation modes of the crystals are made possible also by the nontrivial participation of the compliant interlamellar amorphous phase, the main deformation modes of which are shear deformation and interlamellar separation.1−8 At unit-cell length scale, changes generally occur after yielding and may involve mechanical melting followed by recrystallization into fibrils, which may be either of the same crystalline form originally present in the unstretched sample or of a new crystalline © XXXX American Chemical Society

Received: January 8, 2017 Revised: March 26, 2017

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DOI: 10.1021/acs.macromol.7b00045 Macromolecules XXXX, XXX, XXX−XXX

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effect of deformation at 60 °C, in order to establish a comparison with the stretching behavior of the samples at room temperatures. This approach will help understanding the role of the mobility of the amorphous phase and of the relative stability of the crystals in the emergence of the fibrillary morphology. The relevance of the present study stems not only from the industrial importance of iPP but also from the possibility to apply our tools to other systems because it helps understanding the mechanical behavior of semicrystalline polymers on the basis of molecular parameters.

catalysts activated with methylalumoxane (MAO) as described in refs 15 and 16 and had molecular mass in between 100 and 200 kg/mol, polydispersity index close to two, and a high level of crystallinity. It was shown that the samples, initially crystallized form the melt in the α and/or γ forms of iPP, transform by effect of stretching into the mesomorphic form of iPP at high deformations, achieving a high degree of a fiber-like orientational order.14 Two critical deformations were identified, that is, the deformation at which the polymorphic transitions start εstart and the deformations at which the transformations end εend. The dependence of these critical values of deformation on the stereoregularity of the samples was also established. The changes occurring at the molecular level were correlated with the changes at mesoscale, addressing the formation of undulated (chevron-like structures) at deformations lower than εstart and cavitation at deformation higher than εend. In ref 14, the role of the amorphous component in the stretching behavior of iPP was neglected. It is worth noting that in ref 17 the plastic deformation of γ form was studied at room temperature by effect of plane-strain compression in the case of a commercial grade of iPP, obtained by heterogeneous Ziegler−Natta catalysts. The sample was crystallized from the melt almost completely in the γ form, at high pressure (200 MPa), in the temperature range of 170−200 °C. It was evidenced that the deformation mechanism was leaded by the interlamellar shear of the amorphous layers, inducing formation of fine shear bands already at deformation close to yielding. With increasing deformation, the progressive fragmentation of lamellae occurs, associated with kinking and rotation, formation of a chevron-like texture, up to achieve orientation of γ form with the chain axes perpendicular to the constrained direction, and directed in between the flow and loading directions. No crystallographic slip events were detected during the deformation by plane-strain compression in Ziegler−Natta iPP crystallized in the γ form, pointing out at the important role of interlamellar amorphous phase. Moreover, in the sequence of transformation events by effect of deformation, only a small fraction of the initial γ form was transformed into mesomorphic form of iPP. It was argued that the low amount of crystals in the γ form experiencing destruction and successive transformation into mesophase, and the lack of crystallographic slip events, entails high stability and high plastic resistance of the crystals of the γ form. Therefore, the easy deformability and the complete transformation of the initial α and/or γ forms into the mesophase observed in the streodefective iPP samples of ref 14 by effect of uniaxial stretching, is only in part due to the different deformation mode adopted in ref 17. The difference in the deformation behavior of stereodefective iPP samples14 with respect to Ziegler−Natta iPP,17 indeed, is also due to the effect that different concentrations of stereodefects play on the intrinsic stability of crystals. In this paper, the experimental approach and the analytical tool devised in ref 14 are extended to the study of the stretching behavior of low stereoregular iPP samples at a temperature significantly higher than the glass transition, but lower than the melting temperature. In particular, we focus on iPP samples containing 5.92 and 11.01 mol % of rr stereodefects (iPP-5.9 and iPP-11.0, respectively).14 The stretching temperature of 60 °C is selected in order to prevent the transformation of the initial crystalline forms into the mesophase because at this temperature the mesomorphic form is unstable.18−21 The aim is of studying the structural changes occurring at mesoscale by

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EXPERIMENTAL SECTION

Samples of iPP characterized by different amounts of randomly distributed stereoirregularities (only isolated rr triads), absence of regio-defects, and similar molecular mass (100−200 kg/mol) were prepared with the complexes of Scheme 1, activated with

Scheme 1. Structures of Precatalysts

methylaluminoxane (MAO) as detailed in refs 15 and 16. In particular, we focused on the samples iPP-5.9 and iPP-11.0, containing 5.92 and 11.01 mol % of rr stereodefects (Table 1). The viscosimetric molecular mass of the samples is 211 and 123 kg/mol, respectively, and the polydispersity index is close to 2. They crystallize from the melt in disordered modifications of γ form16,22 and show melting temperatures of 114 and 84 °C, respectively (Table 1), whereas the glass transition temperature is around −10 °C.16,21 Disorder consists in the random arrangement of layers of chains along the bα- or cγ-axis direction with the axes either parallel (like in the α form) or nearly perpendicular (like in the γ form).22 Rectangular specimens of initial gauge length l0 = 3.0 mm, initial width w0 = 1.6 mm, and initial thickness t0 = 0.25 mm were cut from films prepared by compression molding in a hot press using a low pressure, by fluxing cold water in the refrigerating system of the press (average cooling rate ≈10 °C/min). The maximum temperature reached in the melt was 20−30 °C higher than the melting temperature determined in the DSC scans. Compression-molded samples were crystallized in disordered modifications of γ form.16,22 The relative amount of γ form was determined, according to TurnerJones,23 using the intensities of (117)γ reflection relative to γ form Iγ and (130)α reflection relative to α form Iα, as fγ = Iγ/(Iα + Iγ). Stretching experiments were performed using a dynamometer consisting of two symmetrically related cross-heads moving in the opposite directions, in order to ensure that the center of the gauge length remains in the center of the X-ray beam. The dynamometer contains a heating/cooling camera working in the temperature range between −10 and 80 °C (temperature control ±1 °C). The samples were uniaxially deformed at 25 and 60 °C while collecting in situ wideangle (WAXS) and small-angle (SAXS) X-ray scattering patterns using the high flux available at the Synchrotron Radiation Source in Daresbury (Cheshire, UK, beamline 16.1). The parameters used in the experiments were deformation rate of 2.36 mm/min, incident wavelength λ of 1.4 Å, and collection rate of 1 frame/5 s for WAXS data and 1 frame/20 s for SAXS data. The size of the primary X-ray beam at the sample position was 0.1 × 0.1 mm2. In our setup, the beam size is smaller than the fiber width even at high deformations, in order to prevent any contribution to small-angle X-ray scattering on the equator due to edge scattering from surface refraction.24 The B


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Table 1. Viscosimetric Molecular Mass (Mv), Melting Temperature (Tm), and Content of rr Triads and mmmm Pentads Stereosequences of iPP Samples Prepared with the Catalysts of Scheme 1a sample

catalyst/cocatalyst

Mvb (kg/mol)

Tmc (°C)

Tgc (°C)

[mm] (mol %)

[mr] (mol %)

[rr] (mol %)

[mmmm] (mol %)

xcd

fγ d

iPP-5.9 iPP-11.0

1/MAO 2/MAO

211 123

114 84

−10 −10

82.2 66.9

11.8 22.0

5.92 11.01

72.2 51.0

0.55 0.42

0.96 1.0

a

The concentration of regio-errors (2,1 insertions) was not detectable in the 13C NMR spectra of the samples. bViscosimetric molecular mass.13,14 The melting temperatures were obtained with a differential scanning calorimeter (PerkinElmer DSC-7) performing scans in a flowing N2 atmosphere and heating rate of 10 °C/min.16,21 dCrystallinity index xc and relative amount of γ form with respect to the α form fγ evaluated from Xray powder diffraction profiles of compression-molded samples, used in the stretching experiments.14 c

Figure 1. 2D-WAXS patterns of fibers of samples iPP-5.9 (A−E and A′−D′) and iPP-11.0 (A″, A‴, B″, B‴), obtained by stretching at 25 (A−E, A″, A‴) and 60 °C (A′−D′, B″, B‴) compression-molded films at the indicated values of the strain ε. The stretching direction is vertical. The relevant reflections of α and γ forms are indicated in (A). Arrows in (B, C, B′) indicate the polarization of the reflection (008)γ of γ form in the four quadrants due to the quasi-perpendicular chain-axis orientation (cross β) of the crystals in γ form (Scheme 2A) occurring at low deformations, before the beginning of the transformation into the mesophase at 25 °C and into the α form at 60 °C. The azimuthal angle χ is defined in (A). For the sample iPP-11.0 data are recorded while cyclically stretching and relaxing the tension. The residual deformation in (A‴ and B‴) are indicated. Parts A−E, A″, and A‴ are reproduced with permission from ref 14. Copyright 2016 Springer. strong reflections of the unoriented film specimens were used to determine the correct beam center coordinates and the sample-todetector distance. In this study, the nominal values of stress and strain are used, which were measured directly in the dynamometer machine during WAXS and SAXS measurements. All samples undergo, at both 25 and 60 °C, nearly uniform deformation, in the whole deformation range. We checked, on independent samples and at fixed values of strain, that the thickness t of the samples decreases monotonously with deformation as t = t0(l0/l)ν with ν comprised between 0.4 and 0.5, depending on the deformation (where t and l are the thickness and gauge length of the deformed specimen, respectively). Raw SAXS and WAXS data were reduced and analyzed using the homemade software XESA.25 In particular, the SAXS data were analyzed resorting to a series expansion in terms of spherical harmonics P2n(cos χ) for the interpolation of the data collected during stretching:26

the meridian axis corresponds to χ = 0° and 180° whereas the equator corresponds χ = 90° and 270° (see also Figure 1A). This method allows separating the effects of preferred orientation from the dependence of the SAXS intensity on the spatial correlations as a function of q. Because of the inversion center intrinsic to X-ray scattering patterns typical of uniaxially symmetric samples, only the even terms contribute to eq 1. The amplitude of spherical harmonics components I2n(q) taking part to eq 1 are given by26,27

I2n(q) = (4n + 1)

∑ 2n = 0,2,4,...

π /2

I(q , χ )P2n(cos χ ) sin χ dχ

(2)

where P0(cos χ) = 1, P2(cos χ) = 1/2(3 cos2 χ − 1), P4(cos χ) = 1/ 8(35 cos4 χ − 30 cos2 χ + 3), etc. According to eq 2, the amplitude of the zero-order term I0(q) corresponds to isotropic contribution to the intensity as a function of q, whereas the terms of higher order account for the orientational anisotropy. In particular, from the amplitude of the second-order spherical harmonics I2(q) it is possible to calculate the order parameter P2(cos χ)q as a function of q as

∞

I(q , χ ) =

∫0

I2n(q)P2n(cos χ )

P2(cos χ )q =

(1)

where q is the modulus of the scattering vector equal to q = 4π sin θ/λ, θ being the halved angle between the incident and scattered beam at detector position, and χ is the azimuthal angle, defined as the angle between the radial vector q and the direction of stretching. Therefore,

I2(q) 5I0(q)

(3)

P2(cos χ)q denotes the second-order Legendre polynomial (argument ⟨cos χ⟩), which is equal to 1 in the case of perfect alignment of the chain axes in the stretching direction, equal to 0 in the isotropic case and equal to −0.5 in the case of perpendicular orientation. C


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Figure 2. Equatorial section profiles extracted from the 2D X-ray diffraction patterns of Figure 1 of the sample iPP-5.9, collected at 25 (A, B) and 60 °C (A′, B′). The relevant reflections of α and γ form are indicated. In (A′, B′) the horizontal lines mark the critical deformations at which the stress induced phase transformations of the initial crystalline form into a new one start and end (εstart ≈ 200% (B) or 180% (B′) and εend ≈ 400%). Part A is reproduced with permission from ref 14. Copyright 2016 Springer.

Figure 3. Distribution of intensity along the azimuthal angle χ extracted from the bidimensional X-ray diffraction patterns for the sample iPP-5.9, in the q region close to 12 nm−1 (A, B, A′, B′). The data are obtained by stretching at room temperature compression-molded films at the indicated values of the strain ε, at the temperature of 25 (A, B) and 60 °C (A′, B′). Arrows point out at off-meridional and off-equatorial polarization of the intensity at q ≈ 12 nm−1. Part A is reproduced with permission from ref 14. Copyright 2016 Springer. D


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RESULTS AND DISCUSSION WAXS Analysis. The bidimensional (2D) X-ray fiber diffraction patterns of the sample iPP-5.9 with 5.92 mol % of rr defects collected during stretching at 60 °C are compared with those collected at 25 °C in Figure 1. The corresponding equatorial profiles are shown in Figure 2 as a function of q. Unoriented specimens of the samples iPP-5.9 obtained from the melt by compression molding are in a disordered modification of γ form22 (Figure 1A,A′ and curves a of Figure 2A,A′). Upon stretching at 25 °C, the gradual transformation of the initial γ form into the mesomorphic form of iPP is observed in Figures 1A−E and 2A,B.15 At 60 °C, instead, the mesomorphic form is unstable,19−21 and the initial γ form transforms into the α form (Figures 1A′−D′, and 2A′,B′). These transformations start occurring at the critical deformation εstart of ≈200% (Figure 1C) at 25 °C and of ≈180% at 60 °C, whereas complete transformation occurs at εend ≈ 400% in both cases (Figures 1 and 2). The sample achieve a high degree of orientation of the chain axes of the new crystalline forms along the stretching direction (Figures 1 and 2) as indicated by the polarization of the intensity at q ≈ 10 nm−1 on the equator, which corresponds at low deformations to the (111)γ reflection of γ form and at high deformations to the tail of the mesomorphic halo for stretching at 25 °C and to the reflection (110)α of α form for the stretching at 60 °C (Figures 1 and 2). The distribution of the intensity of the reflection at q ≈ 12 nm−1 (q range 0.115−0.125 nm−1), corresponding to the (040)α and/or (008)γ reflections of α and/or γ forms, respectively (arrows in Figures 1), along the azimuthal coordinate (defined in Figure 1A) is shown in Figure 3, in the case of the sample iPP-5.9. It is apparent that already for low deformations, the azimuthal intensity distribution at q ≈ 12 nm−1 shows maxima located at oblique angles with respect to the meridian, at χ ≈ ±30°−40° and 180° ± (30−40)° (curves b, c of Figure 3A and curves a−c of Figure 3A′) . At higher deformations, the diffraction maxima at oblique angles disappear or become very low (Figure 3B,B′), and only when the transformation into the mesophase is complete (≈εend), the intensity at q ≈ 12 nm−1 becomes peaked on the equator (at χ = 90° and 270°), according to the standard fiber morphology (curves d, e of Figures 3A and curves d−f of Figure 3A′). We recall that in the crystals of γ form the chain axes run along two nearly perpendicular directions, both at right angle to the cγ-axis of the unit cell (bα-axis of α form), that is, the axis of stacking of the layers of chains.12 Therefore, the nearly meridional polarization of the intensity of the (008)γ reflections at q ≈ 12 nm−1 in the patterns of Figure 1 and azimuthal profiles of Figure 3 indicates that portion of the crystals of γ form assumes an orientation with the cγ-axis of γ form (bα-axis of α form) nearly parallel to the stretching direction (Scheme 2A), that is with the chain axes perpendicular to the stretching direction.10,11,28 This nonstandard mode of orientation of iPP crystals, corresponding to lamellae oriented with chain axes nearly perpendicular to the stretching direction,11,28 and not parallel as instead expected in a fiber morphology,10,11,29 is similar to cross β orientation of proteins29 (Scheme 2A). The stretching behavior of the low stereoregular sample iPP11.0 is similar to that of the sample iPP-5.9. Representative diffraction data are shown in Figure 1A″,A‴,B″,B‴. In particular, also for the sample iPP-11.0 the initial (disordered

Scheme 2. Perpendicular Chain Axis Orientation of Crystals in γ Form (Cross β) (A) and Undulated Chevron-like Stacks of Lamellar Crystals (B)

modification of) γ form transforms by stretching into the mesophase at 25 °C (Figure 1A‴)14 and into the α form at 60 °C (Figure 1B‴). Moreover, also for the sample iPP-11.0, at deformation lower than the critical strain marking the end of transformation, the crystals of the γ form tend to become oriented in the perpendicular chain axis orientation (data not shown). The main difference consists in the fact that whereas for the sample iPP-5.9 the mesomorphic or α form obtained upon stretching at 25 or 60 °C, respectively, remains stable by releasing the tension and no elastic recovery occurs, for the sample iPP-11.0, the mesomorphic form obtained by stretching at 25 °C transforms into the α form by releasing the tension (Figure 1A″,A‴),14 and partial elastic recovery of the initial dimensions of the sample occurs. This transformation is reversible, and the α form obtained releasing the tension (Figure 1A‴) transforms back into the mesomorphic form at 25 °C (Figure 1A″).10,11,14,28 However, the oriented α form obtained by stretching at 60 °C the sample iPP-11.0 does not undergo any transformation by releasing the tension, and the sample is not elastic (Figure 1B″,B‴). SAXS Analysis. The bidimensional SAXS data collected during tensile stretching at 60 °C for the samples iPP-5.9 and iPP-11.0 and the corresponding azimuthal profiles integrated over the q range 0.2−0.8 nm−1 are reported in Figures 4 and 5. They are compared with the relevant data collected at 25 °C (see also Figure S1).14 Undeformed samples show uniform distribution of intensity (Figure 4A,A′ and curves a of Figure 5). At low deformations up to ε ≈ 200% for the sample iPP-5.9 and ε ≈ 250% for the sample iPP-11.0, the intensity distribution assumes an elliptical geometry with four maxima (Figure 4B,B′,C′). The maxima are centered off the equator and off the meridian (arrows in Figure 4B,B′,C′) generating a four-lobe diffuse pattern. This behavior is common to the samples stretched at both 25 °C14 and 60 °C. In particular, the four-lobe pattern is better visualized in the azimuthal SAXS profiles of Figure 5A,A′ (curves b−e) for the sample iPP-5.9 stretched at 25 and 60 °C and of Figure 5B,B′ (curves b, c) for the sample iPP-11.0 stretched at the two temperatures. The four broad maxima are centered at χ ≈ ±30° and ±150°. By stretching at 60 °C, at deformations higher than 200% for the sample iPP-5.9 and 250% for the sample iPP-11.0, the fourE


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Figure 4. 2D SAXS patterns of the samples iPP-5.9 (A−E) and iPP-11.0 (A′−K′) stretched at 60 °C (A−D, A′−G′) and 25 °C (E, H′, K′). The stretching direction is vertical. Arrows in (B, B′, and C′) indicate the polarization of intensity off the equator and off the meridian due to the chevron-like texture (Scheme 2B). The azimuthal angle χ is defined in (A). The images G′ and K′ were recorded after unhooking the samples from the maximum deformation achieved during the stretching of the unoriented specimens. The residual deformations in G′ and K′ are indicated. Parts E, H′, and K′ are reproduced with permission from ref 14. Copyright 2016 Springer.

Figure 5. Intensity distribution along the azimuthal angle χ for the samples iPP-5.9 (A, A′) and iPP-11.0 (B, B′). Data are extracted from the 2D SAXS patterns of Figure S1 and Figure 4 collected during stretching at 25 (A, B) and 60 °C (A′, B′). Curves g−k in (B) and curve g in (B′) are collected during the relaxation step performed at the same deformation rate (2.36 mm/min) adopted during the stretching step. Parts A and B are reproduced with permission from ref 14. Copyright 2016 Springer.

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Macromolecules lobe pattern is replaced by a two-lobe pattern on the meridian (Figures 4C,D and 4D′−G′) whereas by stretching at 25 °C, the SAXS patterns become dominated by diffuse scattering in the shape of a streaked diamond located on the equator (Figure 4E,H′,K′). In the azimuthal profiles of Figure 5, the two-lobe pattern is visualized by the presence of maxima on the meridian for the stretching at 60 °C (curves f−i of Figure 5A′and curves d−g of Figure 5B′), whereas for the stretching at 25 °C, the diamond shape pattern corresponds to maxima centered on the equator (curves f−k of Figure 5A and curves e−k of Figure 5B). Therefore, no meridional maxima are apparent in the SAXS patterns collected for the samples stretched at 25 °C (Figure S1), and only the stretching at 60 °C produces a two-lobe pattern centered on the meridian. It is worth noting that in the case of the sample iPP-11.0 stretched at 60 °C, at deformations higher than 400%, the SAXS patterns show, along with the appearance of the two broad maxima on the meridian, also a narrow streak on the equator, whose relative intensity increases with increasing deformation (Figure 4D′−G′, and curves d−g of Figure 5B′). In the case of the sample iPP-5.9 stretched at 60 °C, instead, no equatorial streak appears besides the two lobes on the meridian (Figure 4C,D and curves f−i of Figure 5A′). The elliptical polarization and the four-lobe patterns in the SAXS images of Figure 4 observed at low deformations indicate the formation of undulated structures eventually evolving toward chevron-like textures of the lamellar stacks as shown in the Scheme 2B.30−32 As discussed in ref 14, the mechanism leading to the formation of chevron-like structures requires that the lamellar stacks are subjected to tensile deformation in the direction normal to their layers (Scheme 2B). These layers experience also a compressive stress in the parallel direction to the layers. When the concentration of transversal stress sustained by the crystalline (hard) layers becomes higher than a critical value, buckling instability occurs, generating the formation of undulated (chevron-like) structures (Scheme 2B).33 This mechanism holds at both 25 and 60 °C, with no relevant differences as far as the range of deformations in which it is involved; that is, undulated structures start appearing already at low deformations and disappear completely at deformation εend, marking the end of transformation of the initial γ form into the mesophase at 25 °C14 and into the α form at 60 °C. The appearance of meridional two lobes in the SAXS patterns collected at 60 °C (Figures 4A−D and 4A′−G′ and Figure 5A′,B′) indicates, in agreement with the WAXS results of Figure 1, the formation of a well-oriented fiber morphology, where the crystals of α form are periodically stacked along the stretching direction, separated by amorphous regions (Scheme 3A). On the other hand, also the patterns with the equatorial streaked diamond obtained by stretching at 25 °C (Figure 4E,H′,K′) are still due to the formation of a fiber morphology, but in this case, the absence of the meridional two lobes may be due partly to the low contrast between the amorphous and mesomorphic domains and partly to the fact that the aggregates of the mesomorphic form alternating with amorphous regions are stacked along the stretching direction at not correlated distances along the longitudinal (fiber) direction (Scheme 3B). Therefore, the main difference between the stretching behavior of the iPP samples at 25 and 60 °C at SAXS length scale consists in the formation of periodic stacks of well-oriented

Scheme 3. Periodic (A) and Nonperiodic (B) Stacking of Crystalline Aggregates

crystals of α form at 60 °C and of well-oriented mesomorphic aggregates with no longitudinal correlation at 25 °C. Moreover, the presence of streaked diffuse scattering on the equator in Figure 4E,H′−K′ for the stretching at 25 °C of both samples and Figure 4D′−G′ for the stretching at 60 °C of the sample iPP-11.0 indicates that crystalline and amorphous regions are organized at length scale of 6−60 nm in rod-like fibrillary entities, placed at uncorrelated lateral distance. Fibrillary entities with no lateral order are formed also in the case of the sample iPP-5.9 stretched at 60 °C (Figures 4A−D and 5A′). In this case, no equatorial streaks are apparent in the SAXS patterns up to high deformations. The lack of significant diffuse scattering on the equator for this sample entails either that the lateral interfibrillary distances are higher than 60 nm so that fibrillae scatter X-rays at q values lower than our SAXS detection limit or that the fibrillary entities are not rod-like, but form an interwoven network (vide infra), while keeping longitudinal order in the short range (Scheme 3A). It is worth noting that the presence equatorial streaks may also be attributed to the presence of elongated nanovoids having size of 6−60 nm.34−37 In this case, upon stretching, voids are torn open in the amorphous layers and act as a third phase with null electron density. Therefore, the diffuse scattering located on the equator in the small-angle region occurring at large deformations (Figure 4E,D′−K′) may be accounted for by both factors, that is, uncorrelated disorder for the relative position of the rod-like fibrillae and formation of elongated nanovoids (vide infra). On the other hand, the absence of equatorial streak in the SAXS patterns recorded at 60 °C for the sample iPP-5.9 stretched at 60 °C indicates either that cavitation does not occur as in Ziegler−Natta iPP stretched at high temperatures36 or that cavities are so large that most of them scatter X-ray outside of SAXS detection limit.36,37 The average radius of the fibrillae Rf was evaluated by fitting the SAXS equatorial intensity profiles at low q values (Figure S2), extracted from the SAXS patterns collected at high deformations (i.e., for qRf ≪ 1), with the Guinier equation (eq S1). It is assumed that the fibrillary morphology can be modeled as consisting of incoherent assemblies of parallel cylinders with uniform cross-section and infinite length Lf (Supporting Information). The so-calculated values are Rf ≈ 5.3 nm in the case of the sample iPP-5.9 and 4.7 nm in the case of the sample iPP-11.0, for the stretching at 25 °C, and Rf ≈ 2 nm in the case of the sample iPP-11.0, for the stretching at 60 °C. For the stretching at 60 °C, the values of Rf can be also obtained from the WAXS patterns by measuring the width at midheight of the (110)α and/or (040)α equatorial reflections of α form at q ≈ 10 and 12 nm−1, respectively (Figure 2A′,B′), G


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Figure 6. Zero-order spherical harmonics component I0(q) (A, C, A′, C′) and order parameter P2(cos χ)q (B, D, B′, D′) for the sample iPP-5.9 (A, A′, B, B′) and iPP-11.0 (C, C′, D, D′). Data are extracted from the 2D SAXS patterns of Figure 4 and Figure S1 collected during stretching at 25 (A−D) and 60 °C (A′−D′). Parts A−D are reproduced with permission from ref 14. Copyright 2016 Springer.

and using the Scherrer formula. The so-obtained values of Rf are ≈2−3 nm (Supporting Information). The morphological transformations occurring by stretching at 25 and 60 °C are irreversible. This is shown in Figure 4 in the case of the sample iPP-11.0, which shows large elastic recovery at 25 °C (Figure 4H′,K′)16,28 and no recovery at 60 °C (Figure 4F′,G′). Indeed, the SAXS data collected for the samples stretched at the maximum deformation (Figure 4F′,H′) are identical to those collected after removing the tension (Figure 4G′,K’ and curves g−k of Figure 5B and curve g of Figure 5B′), at both 25 and 60 °C. We recall, as already discussed, that in the case of the sample iPP-11.0 the structural transformations occurring by stretching at 25 °C are instead reversible since the mesomorphic form obtained at high deformations transforms into the α form upon releasing the tension, allowing elastic recovery of the sample (Figure 1A″,A‴).11,14,16,28 Spherical-Harmonics Analysis of SAXS Intensity. The SAXS intensity distribution is analyzed in terms of sphericalharmonics series expansion. Information related to the dependence of the SAXS intensity on the spatial correlations

is contained in the zero-order spherical harmonics component I0(q), whereas information related to the preferred orientation are contained in the second, fourth, and successive order of the zero-order spherical harmonics terms, which, after normalization, correspond to the order parameters P2(cos χ)q, P4(cos χ)q, etc., describing the orientational distribution function of chain axes along the stretching direction. Within the present context only the zero component and the order parameter P2(cos χ)q (see Experimental Section) are analyzed, as the successive order parameters would be affect by too large noise. The isotropic intensity component I0(q) and the order parameter P2(cos χ)q of iPP samples stretched at different deformations at 25 and 60 °C are reported in Figure 6. Undeformed samples at 25 °C (ε = 0%) present a correlation peak due to lamellar stacking, at qmax ≈ 0.5 nm−1 at 25 °C and qmax ≈ 0.45 nm−1 at 60 °C (Figure 6A,A′,C,C′), corresponding to a long period L of ≈13 and ≈14 nm, respectively (Scheme 3A). In both cases the distribution of intensity is isotropic (P2(cos χ)q ≈ 0) (Figure 6B,B′,D,D′). The correlation peak for the undeformed sample iPP-11.0 collected at 60 °C (Figure H


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Macromolecules 6C′) is less pronounced then that at 25 °C (Figure 6C) due to partial melting of the less perfect crystals of γ form. Upon stretching at deformations lower than 150−200% at both temperatures, a shift of the correlation peak to lower q values occurs (Figure 6A,A′,C,C′), indicating an increase of the long period, due to interlamellar separation.1−3 The order parameter P2(cos χ)q at these low deformations tends to become slightly positive, reaching values close to 0.1, in agreement with the four-lobe diffuse patterns observed in the bidimensional SAXS images (Figure 4B,B′C′ and curves b−e of Figure 5A,A′, and curves b, c of Figure 5B,B′) due to the formation of the chevron-like undulated structures (Scheme 2B). Therefore, Figure 6 evidences that regardless of deformation temperature and for deformations lower than εstart (≈200%), undulation and interlamellar separation occur simultaneously probably because they involve the same population of lamellae oriented with their layers perpendicular to the stretching direction.1−3 At deformation higher than 200%, for the samples stretched at 25 °C, the I0(q) component does not show any correlation peak but shows a smooth decay with increasing q (Figure 6A,C). Correspondingly, the order parameter P2(cos χ)q assumes negative values at any q, in the range between ≈−0.1 and −0.25 (Figure 6B,D). Negative values of P2(cos χ)q are in agreement with the fact that the polarization of intensity concentrates on the equator (Figure 4E,H′,K′ and curves f−k of Figure 5A and curves d−k of Figure 5B). As discussed in the preceding paragraph and in ref 14, this is due to formation of cylindrical (rod-like) fibrillary entities placed at uncorrelated lateral distance even though formation of nanocavities may not be excluded. The formation of elongated, rod-like entities, at high deformations, is also indicated by the decay of I0(q) amplitude at high q, according to the power law dependence q−D with D ≈ 2.38 The stretching at 60 °C, instead, produces, for deformation higher than 200%, bell-shaped curves for the I0 component and positive values of the order parameter P2(cos χ)q, in agreement with the two-lobe patterns located on the meridian observed in the SAXS patterns of Figure 4C,D,C′−G′. It is worth noting that in the case of the sample iPP-11.0 stretched at 60 °C, and at deformations higher than 800−900%, the negative values of the order parameter at q lower than 0.3 nm−1 reflect the strong equatorial streaks observed in the corresponding SAXS patterns of Figure 4F′,G′. Anisotropic Lamellar Deformation and Amorphous Relaxation. The anisotropy of deformation at lamellar length scale may be mapped considering the changes of the SAXS intensity distribution along the equator and the meridian, separately, as visualized in Figure S3 for the stretching at 60 °C. The same kind of analysis cannot be extended to the samples stretched at 25 °C because uncorrelated longitudinal disorder in the relative arrangement of mesomorphic domains and amorphous regions sets in already at low deformation (Scheme 3B). In particular, as shown in Figure S3 for the stretching at 60 °C, whereas a well-defined correlation peak is present on the meridian over the whole deformation range (Figure S3B,B′), a correlation peak is present on the equator only up to deformations of ≈200−300%, which disappears nearly concomitant with the beginning of transformation of the initial crystalline forms into the oriented α form and also with the onset of the transformation into the fibrillary morphology. Moreover, the equatorial correlation peak at low deformations is well-defined only for the sample iPP-5.9 whereas for the low-

stereoregular sample iPP-11.0 the correlation peak is ill-defined even at low deformations. The values of lamellar periodicity L calculated from the position of the intensity maxima qmax on the equator and meridian at 60 °C of Figure 4 are shown in Figure 7. The

Figure 7. Values of the lamellar periodicity extracted from meridional (a) and equatorial slices (b) of the SAXS patterns of Figure 4 for the sample iPP-5.9 (A) and iPP-11.0 (B) stretched at 60 °C. Lamellar stacks undergoing longitudinal separation (red) and transversal compression (blue) are indicated.

periodicity L extracted from the meridional profiles first increases with increasing deformation and then suddenly drops to values lower than those of the initial unoriented samples L0 (curves a of Figure 7A,B) in correspondence with the deformations marking the onset of the fibrillary morphology (≈200%), up to gradually recover the initial value L0 almost completely at large deformations. Since the lamellar stacks contributing to the intensity on the meridian are those characterized by layers oriented perpendicular to the stretching direction (see Figure 7), the initial increase of periodicity indicates lamellar separation, facilitated by the high compliance of the intralamellar amorphous phase, the chains of which easily assume elongated conformations. The maximum increase of periodicity occurs at ≈100% deformation and amounts to ≈1.20L0 for the sample iPP-5.9 and ≈1.50L0 for the sample iPP-11.0. Moreover, even though for deformations lower than 100%, the amorphous phase of the low stereoregular sample iPP-11.0 is more prone to deform at local scale according with a linear response than the sample iPP-5.9; in neither case the local deformation of the lamellar stacks is affine with macroscopic deformation, indicating that dissipative phenomena of viscoelastic origin may not be neglected. Afterword, the sudden decrease of the lamellar periodicity along the meridian, occurring immediately after reaching the maximum, is due to relaxation of the strained amorphous phase located in the intralamellar regions, probably triggered by crystal fragmentation.2,8,39 I


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Figure 8. Normalized invariant R(ε) obtained from the ratio between the SAXS scattering invariant at deformation ε and at zero deformation (ε = 0) as a function of strain (A, B) and nominal stress−strain curves (A′, B′), recorded in situ, for the samples iPP-5.9 (A, A′) and iPP-11.0 (B, B′) stretched at 25 °C (curves a, a′ in parts A, B and curves a in parts A′, B′) and 60 °C (curves b, b′ in parts A, B and curves b in parts A′, B′). Curves a′ and b′ in parts A, B are obtained from curves a, b by multiplication for the factor accounting for the thickness contraction t0/t = (l/l0)ν, with t0 and l0 the thickness and gauge length of the undeformed specimen, respectively, t and l the thickness and gauge length of the deformed specimen, respectively, and ν = 0.5 the Poisson’s ratio of an ideal rubbery material.42

thickness contraction (Poisson’s ratio = 0), and the case of thickness contraction in the rubbery limit (Poisson’s ratio = 0.5), since the effective change should behave in between these two limiting chances. In all cases the normalized invariant (curves a, b of Figure 8) decreases with increasing deformation according to a nearly sigmoidal or oscillating shape and reaches a quasi-plateau value at deformations close to the end of transformation of the initial γ and/or α forms into the oriented mesophase for the stretching at 25 °C, into the oriented α form for the stretching at 60 °C. The plateau value amounts to ≈0.2 (curves a, b of Figure 8A and curve a of Figure 8B) in all cases, with the exception of the sample iPP-11.0 stretched at 60 °C, which reaches a nearly plateau of ≈0.4 (curves b of Figure 8B). After correction for the thickness contraction, oscillations are amplified, and the total decrease of the normalized invariant is reduced. A plateau value of ≈0.6 (curves a′, b′ of Figure 8A and curve a′ of Figure 8B) is reached in all cases, except for the sample iPP-11.0 stretched at 60 °C, which show an oscillating behavior of R(ε) centered around a value of 1.1 (curves b′ of Figure 8B). The oscillating behavior of the invariant reflects the oscillations of the nominal stress−strain curves recorded in situ at 60°, shown in Figure 8A′,B′ (curves b). Oscillating behavior for the nominal stress−strain curves was observed also for Ziegler−Natta iPP crystallized in the α form, by effect of stretching at high temperature (160 °C), after lamellar fragmentation.43 It was attributed to chain disentanglement processes which become dominant in the stretching experiments at high temperatures and result in the relaxation of the restrained chains of the interlamellar amorphous regions. We argue that also in our case these oscillations arise from relaxation of amorphous phase occurring during stretching at

Concomitant with lamellar separation and consequent increase of lamellar periodicity on the meridian, the lamellar periodicity extracted from the equatorial profiles (curves b of Figure 7A) decreases. Since the lamellar stacks contributing to the intensity on the equator are oriented with the layers parallel to the stretching direction, the decrease of periodicity indicates that these stacks experience transversal compression at low deformations, before undergoing breaking and reorientation at higher deformation (Figure 7A). Transversal compression entails local rearrangements of the chains belonging to the intralamellar amorphous regions toward more compact conformations and consequent increase of the stress level at local scale. The transversal compression process ceases at ≈300% deformation, which is near to the onset of lamellar reorientation and/or lamellar fragmentation.39 SAXS Invariant and Nanovoid Concentration. The area subtending the I0(q) curves of Figure 6A,A′,C,C′ is proportional to the scattering invariant Q.40,41 We evaluated the normalized quantity R(ε) = Q(ε)/Q(0) as a function of deformation ε by calculating the ratio between the SAXS scattering invariant Q(ε) at deformation ε and the value Q(0) in the undeformed state. The so-obtained values of normalized invariant R(ε) are reported in Figure 8, before (curves a, b) and after correction for the thickness contraction. Since the deformation is nearly uniform for all samples, and the thickness scales as t = t0(l0/l)ν with ν comprised between 0.4 and 0.5, depending on the deformation, for the sake of simplicity the effect of thickness contraction of the specimen was accounted for in the limit hypothesis of the rubbery model (Poisson’s ratio ν = 0.5),42 by multiplying the reduced invariant R(ε) by the factor t0/t = (l/l0)ν, with ν = 0.5.42 Therefore, the curves shown in Figure 8A,B illustrate well the change of invariant with deformation, in the two limiting cases, that is the case of no J


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Macromolecules 60 °C, associated with occurrence of (mechanical) melting/ recrystallization phenomena. During stretching, in fact, the transformation of the initial γ form into mesophase at 25 °C and into the α form at 60 °C necessarily involve “mechanical melting” of the crystals through pulling out of the chains from lamellae.44−46 Mechanical melting processes, in turn, may be easily followed by disentanglement, with consequent relaxation of the chains located in the amorphous regions acting as tie chains43 and successive recrystallization in the new form. At 25 °C, the nominal stress−strain curve does not show undulations consequent to mechanical melting because the recrystallization process into mesophase becomes competitive with disentanglement and relaxation of amorphous tie chains and the chain mobility is not sufficiently high. At 60 °C, instead, that is, close to the melting points of the stereodefective iPP samples, the chain mobility increases, the recrystallization process is slowed down, and relaxation phenomena take place during deformation. Additionally, due to the high compliance of the amorphous phase at 60 °C, the healing of nanovoids created by effect of stretching are facilitated. Therefore, reversible mechanical melting/recrystallization and reversible cavitation during stretching, are responsible for the oscillation in the invariant (curves b, b′ of Figure 8A,B) showing minima and maxima at deformations close to the minima and maxima of the stress−strain curves recorded at 60 °C (curves b of Figure 8A′,B′). In general, the invariant depends on the number and relative amounts of phases and on the contrast between the electron densities of the phases. For a multiphase system, the invariant Q can be written as n

phase model, with no change in thickness, the value of invariant by effect of complete transformation of the initial γ form into the mesophase, upon stretching at 25 °C, is expected to decrease because of decrease of contrast, whereas by effect of complete transformation into the α form, upon stretching at 60 °C, the invariant is expected to increase because of increase of contrast. This indicates that the stress-induced transformation of γ form into a new phase, with no change of the degree of crystallinity and no thickness contraction, would lead to a decrease of invariant at 25 °C and an increase of invariant at 60 °C. More in general, for a three-phase model at high deformation, indicating with φvoids the volume fraction of nanovoids, the volume fractions of the final crystalline phases φ1ε, that is mesophase at 25 °C and α form at 60 °C (phase 1), and of the amorphous component φ2ε (phase 2) are related to the volume fraction of the initial γ form and amorphous component (φγ and φa respectively) at zero deformation, by eqs 5: φ1ε ≈ φγ (1 − φvoids) φ2ε ≈ φα(1 − φvoids)

Substituting eqs 5 into eq 4, and indicating with ρ1, ργ, and ρa the electron density of the final crystalline forms (mesophase at 25 °C, α form at 60 °C), γ form, and amorphous component, respectively, the theoretical value of the normalized invariant R(εmax) achieved at deformation εmax of 800−1000% results: R(εmax ) =

n

Q (ε) = 2π 2Vε ∑ ∑ φiεφjε(ρiε − ρjε )2 i=1 j=1

(5)

×

(4)

Q (εmax ) = (1 − φvoids) Q (ε = 0)

ρ2 ] Vε[φγ φa(1 − φvoids)(ρ1 − ρa )2 + φγ φvoidsρ12 + φφ a voids a Vφγ φa(ργ − ρa )2 (6)

In eq 4 the explicit dependence on deformation ε is indicated. In particular, n indicates the number of phases, φiε and φjε denote the volume fractions of the phases i and j, ρiε and ρjε denote the corresponding electron densities, and Vε denotes the total sampled volume in the scattering experiment at deformation ε. This ratio may change by effect of thickness contraction of the specimen, phase transitions, and also because of cavitation. In particular, cavitation is expected to increase the invariant because nanovoids produce an additional phase characterized by null electron density and consequently an increase of contrast with the surrounding matrix. Therefore, eq 4 was used to evaluate the volume fraction of voids at high deformations by modeling the observed changes of the invariant at high deformations (Figure 8). In particular, we analyzed the changes of the invariant at large deformation assuming the presence of two phases in the undeformed specimens (amorphous and γ form, n = 2 in eq 4) and the possible presence of three phases (n = 3 in eq 4) at high deformations, that is, mesophase, amorphous component, and nanovoids for the stretching at 25 °C and α form, amorphous component, and nanovoids for the stretching at 60 °C. The density of α and γ forms, mesophase, and amorphous phase in isotactic polypropylene are 0.946, 0.939, 0.91, and 0.854 g cm−3 at room temperature,47 respectively, and the corresponding values of electron density are 326, 323, 313, and 294 electrons/nm3, respectively. Since the relative amount of amorphous and crystalline components does not change by effect of stretching in our samples,16,28 in a hypothetical two-

Using eq 6, the volume fraction of nanovoids formed at deformations of 800−1000% may be calculated as it follows. We set in eq 6 the initial volume fraction of γ form φγ equal to 0.5 for both samples in agreement with data of Table 1 (φγ ≈ xc fγ).11,16,28 We have also to consider that the experimental value of the scattering invariant for the stretching at 25 °C decreases by an amount in between 80% (curve a of Figure 8A,B) without accounting for the effect of thickness contraction (that is, V coincides with Vε) and 40% (curve a′ of Figure 8A,B) considering thickness contraction in the limit hypothesis of the rubbery model42 (that is, Vε = V(l0/l)0.5). Accordingly, the decay of the SAXS invariant with deformation by 80%, in the absence of volume contraction (curves a of Figure 8A,B), may not be reproduced with eq 6, and only considering thickness contraction the decay by 40% (curves a′ of Figure 8A,B) may be accounted for. In particular, in the limit of the rubbery model a volume fraction of nanovoids φvoids less than 1% is enough to reproduce the plateau value of 0.6 for the normalized invariant R(ε) at 25 °C (curves a′ of Figure 8A,B). This indicates that the volume fraction of nanovoid contributing to the SAXS intensity in the sampled q range of 0.1−1 nm−1 (size range 6−60 nm) is very low. The low value of the volume fraction for these nanovoids is in agreement with the low intensity of the diamond-like SAXS patterns of these samples (Figure 4E,H′,K′) compared with the high intensity in the corresponding WAXS patterns (Figure 1D,E and ref 14). Therefore, the diamond shape of SAXS patterns in the range 0.1−1 nm−1 is due to a low correlation in the lateral and K


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Figure 9. (A−C) Models of the fibrillary morphology obtained by stretching at 25 (A) and 60 °C (B, C) the samples iPP-5.9 and iPP-11.0 and corresponding SAXS images (A′, A″, B′, C′) collected at the indicated deformations. Crystalline and mesomorphic domains are indicated in black. (A) Rod-like fibrillary morphology including aggregates of mesomorphic crystals alternating with amorphous layers with no lateral and no longitudinal correlations in the relative positions of crystalline (mesomorphic) aggregates. (B) Fibrillary network morphology, characterized by crystals of α form alternating with amorphous layers with no lateral order, but periodically arranged along the stretching direction. (C) Rod-like fibrillary morphology, characterized by crystals of α form alternating with amorphous layers with no lateral order, but periodically arranged along the stretching direction.

to those obtained at 25°C (Figure 9A) for the low stereoregular sample iPP-11.0. The different stretching behavior of the two iPP samples at 25 and 60 °C may be related to the different degree of mobility of the amorphous phase and the different mechanical stability of the initial crystals of the disordered γ form at the two different temperatures. In fact, 25 °C is far from the glass transition temperature of nearly −10 °C11,16,21 and lower than the melting temperature of 114 °C for the sample iPP-5.9 and of 84 °C for the sample iPP-11.0 (Table 1). Therefore, for both samples, at 25 °C, for deformations higher than a critical value, after fragmentation of the crystals of the initial γ form and mechanical melting, the chains previously belonging to the fragments are not able to relax because of the restricted mobility of the amorphous phase but readily form coarse mesomorphic aggregates embedded in coarse fibrillar entities with diameter close to 10 nm, oriented with the chain axes parallel to the stretching direction, with no lateral and no longitudinal order. The selected temperature of 60 °C is instead sufficiently higher than the glass transition temperature and sufficiently lower than the melting temperature of the sample iPP-5.9, but close to the melting temperature of the sample iPP11.0. At this temperature and above a critical deformation, the initial crystals of γ form experience a less coarse fragmentation than that at 25 °C probably because the thermally activated process of nucleation and propagation of screw dislocations at lateral surfaces of the crystals becomes favored.48,49 Therefore, at 60 °C, the less coarse fragmentation of the initial crystals produces, upon mechanical melting and recrystallization, formation of well-oriented crystals of the α form with low lateral dimensions in which the fibrillary entities have average diameter (4−6 nm) lower than that at 25 °C. Moreover, the high mobility of the amorphous phase facilitates the relative reorganization of the oriented crystals of α-form obtained by effect of deformation, according to a well-defined periodic arrangement in the oriented fibers, characterized by long spacing close to the long spacing of the unoriented samples. The mechanism involved in this process corresponds

longitudinal distance of the mesomorphic aggregates of the fibrillary morphology, and to the presence of less than 1% elongated nanovoids with size in the range 6−60 nm as depicted in the model of Figure 9A. Similarly, for the stretching at 60 °C, the experimental value of the scattering invariant decreases by ≈80 and 60% for the samples iPP-5.9 and iPP-11.0, respectively (curve b of Figure 8A,B), without considering the effect of thickness contraction and, considering thickness contraction, by ≈40% for the sample iPP-5.9 (curve b′ of Figure 8A) and oscillates around values higher than 1 for the sample iPP-11.0. Also in this case, the values of the normalized invariant R(ε) at high deformation may be accounted for only considering volume contraction. In particular, in the limit of the rubbery model, the calculated volume fraction of nanovoid formed at 900% deformation is close to zero for the sample iPP-5.9 and ≈1% for the sample iPP-11.0. Therefore, also the stretching at 60 °C produces a fibrillary morphology with a low amount of nanovoids even though the stretching temperature produces different effects on the morphology. Hypothetical models for the different kinds of morphology obtained at 25 and 60 °C are sketched in Figure 9. The most remarkable difference between the fibrillary morphologies which develop at 25 and 60 °C consists of the fact that whereas the rod-like fibrillae obtained for the stretching at 25 °C are arranged with no lateral order and are disorderly stacked along the stretching direction (Figure 9A), the fibrillary entities obtained at 60 °C are still characterized by no lateral order but consist of well oriented crystals of α form alternating with amorphous regions, according to a well-defined periodic arrangement parallel to the stretching direction (Figure 9B,C). A second important difference consists in the fact that whereas the stretching at 25 °C produces a similar fibrillary morphology for the two samples, regardless of stereoregularity (Figure 9A), the stretching at 60 °C produces a fibrillary network for the more stereoregular sample iPP-5.9 (Figure 9B), rod-like fibrillary entities (Figure 9C), which are more similar L


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lated lateral and longitudinal distances, regardless of stereoregularity. The corresponding SAXS pattern show only diffuse (diamond-like) streaked scattering on the equator. The stretching at 60 °C up to high deformations produces a fibrillary network in the case of the less stereoirregular sample with [rr] = 5.9 mol % and a rod-like fibrillary morphology in the case of the more stereoirregular sample with [rr] = 11.0 mol %. The fibrillary entities consist in both cases of well-oriented crystals of α form alternating with amorphous regions, and in both cases the fibrillary entities are characterized by no lateral order, but by well-defined periodicity in the direction parallel to the stretching direction. The corresponding SAXS patterns show two lobes on the meridian and no equatorial streaks in the case of the fibrillary network and two lobes on the meridian and strong equatorial streaks in the case of the rod-like morphology. Hence, the difference between the rod-like fibrillary morphology obtained at 60 °C with respect to that obtained at 25 °C consist in the tendency of the amorphous and crystalline regions to organize in a well-defined periodic arrangement in the direction parallel to the stretching direction. The different fibrillary morphology obtained at 60 °C for the two samples, instead, is the result of the different degree of stereoregularity, which affects, under identical crystallization conditions, the entanglement density in the amorphous interlamellar regions to a different extent, even though the entanglement density in the melt are similar. In particular, since the entanglement density in the interlamellar amorphous regions for the sample iPP-5.9 is higher than that for the sample iPP-11.0, the chain mobility results damped, with the results that the fibrillary morphology which sets in by effect of the stretching at 60 °C is not rod-like, but it is in somehow reminiscent of the highly entangled network fixed during the crystallization step. A small fraction of nanovoids is also formed by stretching at 25 and 60 °C, the amount of which depends on the competition between the tendency of the crystalline aggregates to break and to reassemble, giving rise to aggregates of variable size and the tendency of the amorphous phase to heal the vacancies. Finally, we also observed by effect of the temperature a transition from a more heterogeneous crystal slip at 25 °C to a more homogeneous crystal slip at 60 °C, probably triggered by thermally activated process of nucleation and propagation of screw dislocations. In fact, since the preliminary step of plastic deformation is lamellar fragmentation, heterogeneous slip is active at 25 °C and produces coarse fragments, which give rise to fibrillary entities with diameter of ≈10 nm, whereas more homogeneous slip becomes active at 60 °C and produces a less coarse fragmentation with the result that the fibrillar entities formed at this temperature have a lower diameter of ≈4−6 nm.

to collective movements of the amorphous regions facilitated by the low lateral dimensions of the crystals. Finally, the different fibrillary morphology obtained for the two samples by stretching at 60 °C, that is, a fibrillary network for the sample iPP-5.9 (Figure 9B) and a rod-like fibrillary morphology in the case of the sample iPP-11.0 (Figure 9C), may be due to different factors. First of all, the mobility of the amorphous phase for the sample iPP-5.9 at 60 °C is more damped than that of the less stereoregular sample iPP-11.0 because of the higher melting temperature. Moreover, the higher melting temperature and the lower content of stereodefects suggest that the lamellar thickness of the sample iPP-5.9 is higher than that of the sample iPP-11.0. Since the lamellar periodicity of unoriented samples and that one achieved at high deformations for the two samples are similar (≈14−15 nm, Figure 7), the thickness of interlamellar amorphous layers for the sample iPP-5.9 is lower than that of the sample iPP-11.0. As a consequence, although the entanglement densities in the melt are similar,50 the rejection of entanglements outside the crystals entails that the density of entanglements in the interlamellar amorphous regions for the sample iPP-5.9 is higher than that for the sample iPP-11.0, resulting in a more damped mobility of the chains in the amorphous phase. Therefore, the fibrillary morphology which sets in for the sample iPP-5.9 by effect of the stretching at 60 °C is not rod-like, but it is in somehow reminiscent of the highly entangled network fixed during the step adopted for preparation of the unoriented specimens. We argue that for the sample iPP-5.9 the rate of the process involving mechanical melting/recrystallization into the oriented α-form is competitive with the rate of the typical processes involving the relative rearrangement of the amorphous and crystalline domains, chain unfolding, disentanglement, because of the damped mobility of the chains in the interlamellar amorphous layers. These movements are much easier for the more stereodefective sample iPP-11.0, since the stretching temperature is close to the melting, and the plastic resistance of the crystals, the density of entanglements, and the recrystallization rate are lower. At 25 °C, instead, the samples develop a similar rod-like fibrillary morphology because coarse slip process produces bigger fragments, and the stress level coming into play is high enough to efficiently promote disentangling and chain unfolding in both samples.

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CONCLUDING REMARKS The structural and morphological transformations of metallocene-made iPP samples of different stereoregularity, containing only isolated rr triads stereodefects, with a truly random distribution, occurring during deformation at 25 and 60 °C were followed in real time by collecting WAXS and SAXS patterns as a function of deformation. All samples undergo plastic deformation, and the disordered crystalline γ form initially obtained from the melt in an isotropic orientation transforms by stretching in well-oriented fibers of the mesomorphic form by stretching at 25 °C, and of the α form by stretching at 60 °C, regardless of stereoregularity. The degree of stereoregularity of the samples and the selected stretching temperatures allowed identifying the formation at high deformations of three different types of fiber morphologies. The stretching at 25 °C produces a fibrillary morphology consisting of rod-like entities, which contain well-oriented crystalline aggregates of the mesomorphic form alternating with amorphous layers, arranged at uncorre-

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00045. Additional SAXS data; evaluation of the radius and length of fibrillary entities; equatorial and meridional SAXS intensity distribution (PDF) M


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AUTHOR INFORMATION

Corresponding Author

*E-mail: fi[email protected] (F.A.). ORCID

Finizia Auriemma: 0000-0003-4604-2057 Claudio De Rosa: 0000-0002-5375-7475 Rocco Di Girolamo: 0000-0001-8815-2997 Present Address

S.E.: Lamberti S.p.A, Via Piave 18, 21041 Albizzate (VA), Italy. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The synchrotron measurements were made at the Science and Technology Facilities Council (UK) at Daresbury, and we thank the beamline staff for their help with the experiments. The support by Fondazione Cariplo (Cariplo project 2013 “Crystalline Elastomers”) (F.A.) and the Portuguese Foundation for Science and Technology (FCT) through the Project reference “UID/Multi/04044/2013” (G.R.M.) is acknowledged.

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Deformation of Stereoirregular Isotactic Polypropylene across Length

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