Oriented Microstructures of CrystallineâCrystalline Block Copolymers

Article pubs.acs.org/Macromolecules

Oriented Microstructures of Crystalline−Crystalline Block Copolymers Induced by Epitaxy and Competitive and Confined Crystallization Claudio De Rosa,*,† Rocco Di Girolamo,† Finizia Auriemma,† Maria D’Avino,† Giovanni Talarico,† Claudia Cioce,† Miriam Scoti,† Geoffrey W. Coates,‡ and Bernard Lotz§ †

Dipartimento di Scienze Chimiche, Università di Napoli Federico II, Complesso Monte S. Angelo, Via Cintia, I-80126 Napoli, Italy Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, United States § Institut Charles Sadron, CNRS-Université Louis Pasteur, 23 Rue du Lœss, 67034 Strasbourg, France ‡

S Supporting Information *

ABSTRACT: Polyethylene-block-syndiotactic polypropylene (PE-block-sPP) crystalline−crystalline block copolymers with different block lengths have been synthesized with a stereospecific living organometallic catalyst. Samples of PE-block-sPP have been epitaxially crystallized onto crystals of p-terphenyl (3Ph) to achieve a control over the crystallization of both blocks and study the dependence of the thin film morphology on the sequential crystallization of the two blocks by cooling from the melt. The epitaxial crystallization generates oriented overgrowth of both crystals of sPP and PE, with a highly ordered single orientation of sPP lamellae and a double orientation of PE lamellae onto the (001) face of 3Ph. The final morphology depends on which polymer block crystallizes first, a sequence that depends on the block copolymer composition and block lengths. Ordered nanostructures with alternating lamellar domains are obtained and oriented by the lamellae that crystallize first.

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genated polynorbornene (hPN)12 has been investigated in some detail. By contrast, BCPs containing blocks based on crystallizable stereoregular polyolefin have received less attention13 because this new class of BCPs has been synthesized only recently thanks to the development of metal-based insertion polymerization methods able to ensure a high stereochemical control in living olefin polymerization.14−22 Many efficient and selective catalysts have been developed for living olefin polymerization that, depending on the ligand framework and the nature of the coordination metal center, may produce linear or branched polyethylene, stereoregular poly(α-olefins), poly(cycloolefins), and random copolymers of ethylene with higher olefins. This has allowed creating a range of new polymer architectures (including semicrystalline block copolymers) by sequential monomer addition and/or end-functionalized macromolecules.14−22

INTRODUCTION Block copolymers (BCPs) containing more than one crystallizable block have attracted much interest in recent years. 1 Amorphous and incompatible BCPs may form nanostructures by microphase separation and self-assembly in lamellar, spherical, and cylindrical microdomains depending on the composition of the BCP.2 When BCPs contain one or more crystallizable blocks, microphase separation in the melt and crystallization may compete and generate a wide range of morphologies.3−6 Different structures can be obtained depending on which process occurs first. In addition, crystallization may remain confined within preformed microdomains or may disrupt or break the first formed microphase-separated structure.3−6 In crystalline−crystalline (CC) block copolymers (CC-BCP) the crystallization of the first block may define the final morphology or be modified by the subsequent crystallization of the other block. The crystallization behavior of CC-BCPs containing poly(ethylene oxide) (PEO),7−10 poly(ε-caprolactone) (PCL),7,9 polyethylene (PE),10−12 poly(L-lactide) (PLLA),8,11 and hydro© XXXX American Chemical Society

Received: April 6, 2016 Revised: June 20, 2016

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DOI: 10.1021/acs.macromol.6b00705 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Structure of the Titanium Complex Used as Catalyst for the Preparation of the PE-block-sPP Block Copolymer and Sequential Polymerization Procedure

Table 1. Total Molecular Mass (Mn), Polydispersity (Mw/Mn), Total Number of the Four Carbon Atoms Segments (N), Weight Fraction (wsPP) and Volume Fraction (fsPP) of the sPP Block, Molecular Masses of PE (Mn(PE)) and sPP (Mn(sPP)) Blocks, Melting Temperature (Tm) and Crystallization Temperature (Tc) of the Samples of PE-block-sPP sample

Mna (Da)

Mw/Mna

Nb

wsPPc (wt %)

fsPPd (v/v %)

Mn(sPP)e (Da)

Mn(PE)e (Da)

Tmf (°C)

Tcg (°C)

PE-b-sPP-1 PE-b-sPP-2

20000 64000

1.2 1.2

357 1142

51 73

53 75

10200 46700

9800 17300

134 126

113 110

a

From GPC analysis. bConsidering the BCP composed of N monomeric units (ethylene and propylene) assumed on average as a four-carbon atom segment, N is calculated, according with ref 13, from the total molecular mass Mn as N = Mn/56 where 56 is the molar mass of the four-carbon atom segment. cDetermined from 13C NMR spectrum. dCalculated from the molecular masses Mn(PE) and Mn(sPP), the densities of PE (0.997 g/cm3) and sPP (0.9 g/cm3) such that fsPP = (Mn(sPP)/0.9)/(Mn(sPP)/0.9 + Mn(PE)/0.997). eEstimated from total Mn and wt % of sPP or PE such that Mn(sPP) = MnwsPP. fMain peak temperatures determined from the DSC scans of melt crystallized samples (II heating) at heating rate of 10 °C/min. g Determined from the DSC cooling scans at cooling rate of 10 °C/min. polymerization grade) at a partial pressure of 0.25 bar. Polymerization is initiated by adding 5 mL of MAO (Crompton, 10% w/w solution in toluene) followed by 40 mg of complex 1 previously dissolved in toluene (5.0 mL). After 10−15 min polymerization under a constant ethylene feed, the reactor is evacuated and flushed three times with nitrogen in order to remove ethylene still present. At this point the temperature is decreased to 0 °C, and the polymerization proceeds under a constant feed of propylene (2.8 bar) for 4−6 h, depending on the desired block length. The polymerization is terminated by injection of methanol/HCl (95/5 v/v). The polymer is precipitated in methanol, stirred overnight, filtered and dried in vacuo to constant weight. All polymer samples were characterized by gel permeation chromatography (GPC), using a Polymer Laboratories GPC220 apparatus equipped with a Viscotek 220R viscometer, on polymer solutions in 1,2,4-trichlorobenzene at 135 °C. 13C NMR was carried out with a Varian VXR 200 spectometer (see Supporting Information). The molecular characteristics of PE-block-sPP samples are reported in Table 1. The sample PE-b-sPP-1 has PE and sPP blocks with similar molecular mass (Mn(sPP or PE) ≈ 10 000), whereas the sample PE-b-sPP2 has a higher total molecular mass (Mn = 64 000) and a sPP block longer than the PE block (Mn(sPP) = 46 700) with 73 wt % of sPP. Data of other two samples (PE-b-sPP-3 and PE-b-sPP-4) with much higher total molecular masses (Mn = 135 600 and 151 100, respectively) and similar lengths of the sPP blocks are reported in the Supporting Information. The sample PE-b-sPP-3 has the sPP block longer than the PE block (Mn(sPP) = 83 000) with 61 wt % of sPP, whereas the sample PE-b-sPP-4 has the PE block slightly longer than the sPP block (Mn(PE) = 80 600). Calorimetric measurements (DSC-822 by Mettler Toledo) were performed under flowing N2 at heating and cooling rates of 10 °C/ min. X-ray powder diffraction profiles were obtained with Ni-filtered Cu Kα radiation with automated diffractometer (X-Pert by Panalytical). The average sizes of crystals of PE and sPP were determined from the broadness of reflection peaks in the X-ray powder diffraction profiles of melt-crystallized samples. The half-height widths of the (110)PE reflection at 2θ = 21° of PE and of the (200)sPP reflection at 2θ = 12° of sPP were evaluated, and the sizes of crystals of PE along directions normal to the (110)PE planes (L(110)PE) and of crystals of sPP along

Interesting catalyst precursors for living polymerization of 1alkene with high steric control are the bis(phenoxyimine) titanium complexes (as that of Scheme 1) that were developed by Fujita et al.14 in 1999. These complexes activated with methylaluminoxane (MAO) produce living syndiotactic polypropylene (sPP) or PE14,15,17,18 and have been used to synthesize BCPs composed of crystallizable blocks of sPP or PE linked to noncrystallizable blocks of ethylene−propylene random copolymers.14,15,17,18 In this paper we report on a structural characterization of CC-BCPs composed of blocks of crystallizable sPP and PE (PE-block-sPP) of different block lengths. The two crystallizable PE and sPP components have been epitaxially crystallized on pterphenyl crystals. Epitaxial crystallization of PE and sPP homopolymers onto crystals of various organic substances has been well-described and used as a tool for growing in thin films crystals of various polymorphic forms with single-crystal or fiber-like orientations.23−29 Polymer−polymer epitaxy, involving heteroepitaxy of sPP with PE and homoepitaxy, is well documented.30 Epitaxial crystallizations of the sole sPP or PE blocks when they are parts of crystalline/amorphous block copolymers have also been studied.6,31−34 In this work samples of PE-block-sPP with blocks that melt and crystallize in similar temperature ranges are investigated. This makes it possible, by appropriate processing, to have one or the other block crystallizes first. Thus, use of epitaxial crystallization offers a means to investigate in detail the interplay between crystallization sequence and final morphology of the BCP.

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EXPERIMENTAL PART

Samples of PE-block-sPP have been prepared with a living organometallic coordination catalyst, bis[N-(3-tert-butylsalicylidene)2,3,4,5,6-pentafluoroanilinato]titanium(IV) dichloride complex 1 of Scheme 1, activated with methylalumoxane (MAO).14a,15b The synthesis of BCPs was carried in a 250 mL glass reactor. This was charged under nitrogen with 100 mL of dry toluene, thermostated at 5 °C under stirring, and saturated with ethylene (SON, B


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Macromolecules directions normal to the (200)sPP planes (L(200)sPP) were calculated by the Scherrer equation. Optical microphotographs in polarized light were taken using a Zeiss Axioskop 40 microscope provided with a Mettler FP82 hot stage. Thin films (90−120 μm thick) sandwiched between glass coverslips are melted at ≈200 °C and cooled to room temperature at 10 °C/min. Epitaxial crystallization of the block copolymer on the crystals of pterphenyl (3Ph) followed the procedure used for the PE25 and sPP28 homopolymers. Thin films (thickness lower than 50 nm) are cast at room temperature on microscope glass slides from a p-xylene solution (0.2−0.5 wt %). Single crystals of 3Ph are produced independently by slow cooling of a boiling acetone solution; a drop of the suspension is deposited onto the polymer film at room temperature. After evaporation of the solvent, large (≈10−100 μm), flat crystals of 3Ph delimited by large top and bottom (001) surfaces remain on the copolymer film. This composite material is heated to ≈180 °C to melt the sPP and PE for a short time in order to limit sublimation of the 3Ph substrate and then recrystallized by cooling at a controlled rate (10−15 °C/ min). In the process sPP and PE crystallize epitaxially at the interface with the 3Ph crystals. The 3Ph crystals are subsequently dissolved with hot acetone. The thin films are gold decorated under vacuum and carbon-coated in an Edwards E306A evaporator. The films are then floated off on water with the help of a poly(acrylic acid) backing and mounted on copper grids. Transmission electron microscope (TEM) images in bright field and in selected area diffraction (50 μm diameter aperture) modes are taken in a Philips CM12 transmission electron microscope operating at 120 kV.

Based on ref 13, the Flory−Huggins interaction parameter between sPP and PE (χsPP−PE) is given by χsPP − PE = 6.2/T − 0.0053

For copolymers with volume fractions fsPP = 0.53 and 0.75, the calculated mean-field value for χN at the order−disorder transition (ODT) is expected to be higher than 10.5. Therefore, the temperature at ODT (TODT) is expected to be slightly lower than 0 and 150 °C for the samples PE-b-sPP-1 and PE-bsPP-2, respectively. This indicates that crystallization most likely takes place from a homogeneous melt in the case of the sample PE-b-sPP-1 and probably from a phase-separated melt in the case of the sample PE-b-sPP-2. The DSC curves recorded during first heating, successive cooling from the melt, and second heating of the meltcrystallized samples, all recorded at 10 °C/min, are reported in Figure 2. The DSC thermograms show only one broad melting

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RESULTS AND DISCUSSION Crystallization Behavior. The X-ray powder diffraction profiles of as-prepared (as-precipitated from the polymerization solution) samples of PE-block-sPP of Table 1 are reported in Figure 1. The presence of the 200 and 020 reflections of form I

Figure 2. DSC curves recorded during first heating of as-prepared samples (a, b), successive cooling from the melt (c, d), and second heating of the melt-crystallized specimens (e, f) of samples of PE-bsPP-1 (a, c, e) and PE-b-sPP-2 (b, d, f).

and crystallization peak due to the overlapping of PE and sPP melting and crystallization. The asymmetric sample PE-b-sPP-2 shows endothermic peaks with shoulders in both first and second heating curves (curves b and f of Figure 2), indicating that melting of sPP and PE takes place at slightly different temperatures. The symmetric sample PE-b-sPP-1 with lower molecular mass and similar lengths of PE and sPP blocks has melting temperature (134−136 °C, curves a and e of Figure 2) and crystallization temperature higher than those of the asymmetric sample PE-b-sPP-2 with high molecular mass and longer sPP block (main peak at 126−128 °C, curves b and f of Figure 2). This is in agreement with DSC data on similar crystalline BCP samples sPP-block-poly(ethylene-co-propylene) (sPP-block-EP) and PE-block-sPP reported by Kramer and Coates,13 who have shown that higher molecular mass results in lower crystallization and melting temperatures due to the highly entangled nature of the block copolymer melt. Moreover, they suggested that in PE-block-sPP PE crystallizes first, but the

Figure 1. X-ray powder diffraction profiles of as-prepared samples PEb-sPP-1 (a) and PE-b-sPP-2 (b). The (200)sPP and (020)sPP reflections of form I of sPP at 2θ = 12.2° and 16.1° and the (110)PE and (200)PE reflections at 2θ = 21.4° and 23.9° of the orthorhombic form of PE are indicated.

of sPP at 2θ = 12.2° and 16.1° and of the 110 and 200 reflections at 2θ = 21.4° and 23.9° of the orthorhombic form of PE indicates that in both samples PE and sPP crystallize in their most stable polymorphic forms. The intensities of PE and sPP peaks reflect, qualitatively, their different volume fractions. The diffraction profiles also indicate that the degree of crystallinity of the lower molecular mass sample PE-b-sPP-1 (49%) is slightly higher than that of the sample PE-b-sPP-2 (40%). C


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Figure 3. X-ray powder diffraction profiles of samples of PE-b-sPP-1 (A) and PE-b-sPP-2 (B) recorded at the different indicated temperatures during cooling from the melt at 180 °C down to 25 °C.

depend on the composition and are lower than those of the respective homopolymers and also lower than in equivalents blends.36−44 Even when the two blocks are miscible in the melt, the crystallization of one block is confined within the lamellar crystals previously formed giving lower crystallization and melting temperatures.1a,40 For instance, in the case of PEO-bPCL the confined crystallization of the PEO block within the PCL lamellae previously formed, giving fractionated crystallization of PEO in spite of the miscibility among the blocks, has been described.40 This is particularly evident in the case of asymmetric BCPs when one of the two blocks crystallizes first,1a but even in symmetric PEO-b-PCL, crystalline PEO and PCL lamellae occupy the same crystallite, resulting in large reduction of lamellar thickness and melting temperature depression.3,9 The impact of confined crystallization has also been reviewed by Loo and Register.4 Typically, it has been reported a decrease of the crystallization temperature of PE or PEO blocks within spheres, cylinders, and lamellae in various BCPs such as PEO-b-polybutadiene,45 PEO-b-poly(butylene oxide),46 PE-b-poly(vinyl cyclohexane),47 PE-b-poly(styrene-rethylene-r-butene),48 and PE-b-poly(3-methyl-1-butene).48,49 The X-ray diffraction profiles of samples PE-b-sPP-1 and PEb-sPP-2 recorded at different temperatures during cooling from the melt from 180 °C down to room temperature are reported in Figure 3. Both sets of data indicate that the PE blocks crystallize first irrespective of their weight fraction in the copolymer. For the sample PE-b-sPP-1, PE is fully crystallized at 125 °C and crystalline sPP becomes detectable only at 115 °C (Figure 3A). For PE-b-sPP-2, at the same 125 °C sPP starts crystallizing whereas PE is already almost fully crystallized. Similar sequences had already been observed in ref 13. Figure 3 confirms the data of Figures 1 and 2 that a higher degree of crystallinity is achieved for the sample PE-b-sPP-1 also in the melt-crystallized samples (56% and 45% for PE-b-sPP-1 and PE-b-sPP-2, respectively).

corresponding crystallization heat released obscures the signal from any sPP crystallization during cooling in DSC.13 However, the DSC heating curve of the higher molecular mass sample PE-b-sPP-2 crystallized from the melt (curve f of Figure 2) shows a main melting endotherm at 126 °C with a shoulder or a small peak at 137 °C, similar to or slightly higher than the melting temperature of the sample PE-b-sPP-1 with lower molecular mass, due probably to recrystallization and or reorganization during heating. The DSC data of Figure 2 also confirm the higher crystallinity of the lower molecular mass sample PE-b-sPP-1 for both as-prepared and melt-crystallized samples. The melting temperature of the sPP homopolymer synthesized with the same catalyst and in the same reaction conditions is 144 °C (data not shown), consistent with a concentration of the syndiotactic pentad rrrr of 91% (see Supporting Information). A chain-end mechanism of control of stereoregularity is operative, producing mainly m dyads stereodefects.17,35 Since a similar stereoregularity is expected for both PE-block-sPP copolymers, the lower melting temperatures (134 and 126 °C) are not correlated with a decrease of stereoregularity but are probably due to confinement phenomena. This is in agreement with literature where many examples of the effect of confinement on the crystallization and melting temperatures of crystalline BCPs have been described.1a,3,4 Frequently, a substantial depression of the crystallization and melting temperatures from the values for the homopolymer is taken as evidence of confined crystallization.1,3,4 Most of the earlier studies are summarized in the reviews by Hamley,3 whereas in the more recent review by Muller et al.,1a the behavior of double crystalline BCPs based on PCL−PEO, PLLA−PEO, PLLA−PCL and PE-based BCPs with PE linked to PCL or PLLA or PEO has been reviewed. In many of these examples the melting temperatures of the crystalline blocks D


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Macromolecules From the peak widths of the (200)sPP and (110)PE reflections of sPP and PE at 2θ = 12° and 21°, respectively, in the X-ray diffraction profiles of melt-crystallized samples of Figure 3, values of coherence lengths of crystals of sPP and PE of L(200)sPP ≈ 19−20 nm and L(110)PE ≈ 26 nm, respectively, have been evaluated for both samples. These coherence lengths roughly indicate the average sizes of sPP and PE crystals along the directions normal to the (200)sPP planes of form I of sPP and normal to the (110)PE planes of PE, respectively. The size of sPP crystals results slightly lower than that of PE crystals and lower than the value evaluated for the melt-crystallized sample of the sPP homopolymer (L(200)sPP = 23 nm), whereas the size of PE crystals is similar to that evaluated for the PE homopolymer (L(110)PE = 26 nm). This may be in agreement with the hypothesis that PE crystallizes first and of confined crystallization of the sPP blocks within the PE lamellae previously formed. Morphology of Bulk Crystallized Samples. Bulk samples were analyzed by SAXS (see Supporting Information) and polarized optical microscopy (POM) in order to evaluate the impact of the melt phase separation on the final crystal morphology. SAXS of these block copolymers in the melt is not informative due to the poor electron density contrast between the two blocks.13 Nevertheless, Kramer and Coates13 report that similar PE-block-sPP samples but with higher molecular weight do phase separate in the melt. They investigated samples quenched from the melt, assuming that the melt morphology was preserved in the process. A lamellar morphology with alternating sPP and PE layers was observed for a symmetric PEblock-sPP sample with nearly 41 wt % of sPP.13 In the PE layers, approximately 20 nm thick, crystalline PE lamellae alternate with amorphous PE layers. For lower sPP fractions, the lamellar morphology changes to cylindrical, where the sPP cylinders are embedded in the semicrystalline PE matrix.13 Polarized optical microscope (POM) images of samples PEb-sPP-1 and PE-b-sPP-2 crystallized from the melt at cooling rate of 10 °C/min are shown in Figure 4. In both cases, banded spherulites with average diameter of 10 μm and smaller bundlelike entities are present. Banded spherulites with their concentric dark and bright rings overlaid on the usual Maltese cross are typical of PE50,51 and indicate radiating twisted crystalline lamellae.51−53 Crystallization of PE is therefore clearly not influenced by the phase-separated melt structure and determines the final morphology, as for weakly segregated BCPs or even melt miscible ones. Morphology and Structural Analysis of Epitaxially Crystallized Thin Films of BCPs on 3Ph. Thin films (thickness lower than 50 nm) of both samples of PE-block-sPP have been epitaxially crystallized onto the (001) surface of preformed crystals of p-terphenyl (3Ph).25,28 The complex morphologies generated in this process result from interactions between all three components involved, sPP, PE, and 3Ph. First, the sPP/3Ph and PE/3Ph structures are described, and then, possible impact of sPP/PE epitaxy will also be considered. 1. sPP/3Ph and PE/3Ph Epitaxy. The selected area electron diffraction (ED) patterns of the epitaxially crystallized films are shown in Figure 5. Both patterns feature reflections of sPP in its form I and PE in its orthorhombic form. The sPP part is similar to that observed for the sPP homopolymer,28 and the sPPblock-poly(ethylene-co-propylene) block copolymer31 epitaxially crystallized onto 3Ph. Only 0kl reflections of sPP (020, 021, and 002) are observed, which indicate that the (100) plane of sPP is in contact with the (001) plane of 3Ph and the chain

Figure 4. Optical microphotographs recorded at room temperature in polarized light (crossed polars) of the PE-b-sPP-1 (A) and PE-b-sPP-2 (B) crystallized from the melt by slow cooling (10 °C/min) to room temperature. The insets show details at higher magnification of PE banded spherulites. A higher density of banded spherulites is observed for PE-b-sPP-1 (A) with higher PE content, whereas a high concentration of bundle-like crystals, presumably of sPP, is evident for PE-b-sPP-2 (B).

axis of sPP crystals lies flat on the substrate surface. The sPP crystalline lamellae stand edge-on on the substrate surface, oriented with their b- and c-axes parallel to the b- and a-axes of 3Ph, respectively, as schematized in Figure 5C. This epitaxy corresponds to a two-dimensional matching of the a = 8.05 Å and b = 5.55 Å axes of 3Ph with the c = 7.4 Å and b = 5.6 Å axes, respectively, of form I of sPP28 (unit cell dimensions are for 3Ph: a = 8.05 Å, b = 5.55 Å, c = 13.59 Å, β = 91.9°; for form I of sPP: a = 14.5 Å, b = 5.6 or 11.2 Å, c = 7.4 Å).54,55 Two sets of PE reflections are also present in Figure 5. Two pairs of strong 002 reflections indicate two populations of PE crystals with their chain axes oriented along two directions 72° apart and symmetrically tilted by about 36°−37° relative to the chain axis of sPP. The presence of equatorial 210 and 310 reflections of PE indicates that the (1̅10), or equivalently, the (110) plane of PE, is the contact plane, that is, is in contact with the (001) plane of 3Ph, as observed for the epitaxy of the PE homopolymer on 3Ph.25 The PE lamellae stand edge-on with the chain axes oriented parallel to the [110] and [11̅0] directions of the 3Ph crystal about 72° apart, as shown in the scheme of Figure 5D. This epitaxy and the selection of the (110) plane as contact plane with the (001) plane of 3Ph are E


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Figure 5. (A, B) Selected area electron diffraction patterns, and corresponding schemes (A′, B′), of thin films of samples PE-b-sPP-1 (A) and PE-bsPP-2 (B) epitaxially crystallized onto 3Ph. In A′B′ black and gray spots correspond to sPP and PE reflections, respectively, and only one of the two sets of PE reflections is reported. The ring reflections (I, II, III, IV in A) correspond to the 111, 200, 220, and 311 reflections of gold. (C, D) Schemes of the orientations of lamellae of sPP (C) and PE (D) onto the (001) face of 3Ph.

between the pseudo n-pentane segments and the interchain distance in the (110) plane of PE.30 Therefore, the final crystallographic relationships between PE and sPP for PE-blocksPP crystallized onto 3Ph (Figure 5) are the same as for the PE−sPP heteroepitaxy.30 This similarity suggests that for PE-block-sPP/3Ph crystallization PE and sPP can crystallize independently on the 3Ph substrate, and/or if PE (sPP) crystallizes first during cooling from the melt, part of sPP (PE) may crystallize epitaxially onto both crystals of 3Ph and PE (sPP). The final orientation of crystals, with a single orientation of sPP lamellae and a double orientation of PE lamellae, is identical. Whatever the sequence of crystallization, the final crystallographic relationships are summarized by

due to the matching between the 4.45 Å interchain distance in the (110) plane of PE and the 4.60 Å interplanar distance of the {110} planes of 3Ph.25 Therefore, the epitaxial crystallization of PE-block-sPP gives oriented overgrowth of both crystals of sPP and PE, with a single orientation of sPP lamellae and a double orientation of PE lamellae onto the (001) surface of the substrate. 2. Role of sPP/PE Epitaxy. In addition to the sPP/3Ph and PE/3Ph epitaxies considered so far, a possible polymer− polymer heteroepitaxy between sPP and PE needs to be considered, since it has already been reported in the literature.30,56 Petermann et al.56 have crystallized sPP on a uniaxially oriented film of PE and report on sPP lamellae oriented at about 53° to the PE stretching direction (forming an angle of 37° between chain axes of sPP and PE), with the (100) plane of sPP as the probable sPP contact face.56 The epitaxial relationship could not be determined as the fiber-like structure of PE gives no clue about the PE contact plane. This epitaxy has been instead analyzed with PE evaporated and condensed on an exposed (100) face of sPP generated first by epitaxial crystallization on 3Ph.30 The resulting electron diffraction patter is similar to those of Figure 5. The PE/sPP heteroepitaxy is therefore30

(100)sPP //(001)3Ph //(110)PE bsPP // b3Ph ;

csPP // a3Ph __

c PE //[110]3Ph and //[1 1 0]3Ph ;

c PE //[021]sPP

3. TEM Bright Field Images. The sequence of the crystallization events can be read from the morphology of the material formed. The final morphology obtained by epitaxial crystallization is best observed by TEM bright field on the films decorated with gold. The gold decoration technique is a powerful means to visualize edge-on crystalline lamellae.57,58 The vaporized gold particles gather in the interlamellar amorphous ditches that are generated upon crystallization. The rows of gold particles are separated by brighter zones that correspond to the crystalline lamellae.57,58 The TEM images of PE-b-sPP-1 and PE-b-sPP-2 epitaxially crystallized onto 3Ph and decorated with gold, corresponding to the diffraction pattern of Figure 5, are reported in Figure 6. The dark spots correspond to the gold particles whereas the crystalline lamellae appear white.

(100)sPP //(110)PE c PE //[021]sPP

Even though the conformations of PE and sPP are very different, the (100) face of sPP is made up of parallel stretches of pseudo n-pentane segments (with exposed CH3, CH2, and CH3 groups) 4.45 Å apart and oriented at 45° to the chain axis direction, i.e., nearly along the [021] direction.30 The best interaction between PE and these pseudo PE segments is created when the (100)sPP and (110)PE planes are in contact.30 The dimensional matching is perfect: 4.45 Å for the distance F


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oriented along two directions 74° apart and symmetrically tilted by about 37° relative to the chain axis of sPP (Figure 7A). In fact, the lower-left part of Figure 6A (sample PE-b-sPP-1) and the whole Figure 6B (asymmetric PE-b-sPP-2) display a single set of parallel lamellae only ≈5 nm thick (white stripes). These lamellae are of the sPP blocks that based on the diffraction data of Figure 5 (and model of Figure 7A) have a single orientation. Since this sPP part defines the overall morphology of the whole epitaxially crystallized film, sPP must have crystallized first. None of the expected PE lamellae with two different orientations 74° apart is visible. PE crystallizes after sPP in the confined interlamellar regions prescribed by the oriented sPP lamellae. These trapped and rather short and thin PE lamellae are hardly visualized by the gold decoration. However, the diffraction data and the observed single orientation of lamellae of Figures 6B and 6A may not be antinomic considering the confined growth of PE lamellae. Nucleation is always a rare event compared to growth. The growth of PE is confined between sPP lamellae; therefore, it is most probable that PE lamellae are made of chains tilted with respect to their basal fold surface and are parallel to the sPP lamellae, as shown in the model of Figure 7B. Chain tilting with tilt angle of 45° to the lamellar normal is known in PE.59−61 It corresponds to a (301) fold plane.59−61 In the present case, the tilt angle is only 37°, but as seen in the (110) plane, i.e., the contact plane dictated by the PE epitaxy with 3Ph. This angle is very close, indeed, to 40.4°, the tilt calculated for a (301) fold surface intersecting the (110) plane. Thus, in these systems, the stem orientation is dictated by the epitaxy with 3Ph, but the fold surface orientation is dictated by the orientation of the lamellae of the first crystallized polymer. It turns out in addition that the orientation of the preformed sPP lamellae (Figure 7B) corresponds to a “natural” fold surface of PE. Therefore, in both samples of PE-block-sPP of Figure 6B and lower-left part of Figure 6A, sPP crystallizes first, defining the overall morphology, and PE is confined between sPP lamellae. In the confined sPP interlamellar regions, the trapped PE lamellae may be oriented along two directions 74° apart and tilted by 37° to the chain axis of sPP (as in Figure 7A) or may be parallel to the sPP lamellae and oriented along the direction dictated by the sPP crystallization, with the PE chains tilted at 37° to the lamellar normal (as in the model of Figure 7B) (see Supporting Information).

Figure 6. TEM bright field images of gold decorated thin films of samples PE-b-sPP-1 (A) and PE-b-sPP-2 (B) epitaxially crystallized onto crystals of p-terphenyl. The red arrows indicate the orientation of the crystalline lamellae of PE and sPP induced by epitaxy.

The images of Figure 6 show morphologies slightly different to those expected from the diffraction patterns of Figure 5. These patterns indicate a single orientation of sPP lamellae and a double orientation of PE lamellae with their chain axes

Figure 7. (A) Expected orientations of PE and sPP lamellae with respect to the 3Ph substrate. (B) sPP crystallizes first and PE is confined between sPP lamellae resulting in parallel PE and sPP lamellae with PE chains tilted by 37° to the global lamellar surface normal. (C) PE crystallizes first forming two sets of lamellae oriented along the two directions 74° apart, and sPP is confined between PE lamellae, resulting in parallel sPP and PE lamellae with sPP chains tilted by 37° to the lamellar normal. G


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potentially active sPP surface faces the glass cover slide, which leaves no possibility for the sPP/PE epitaxy to develop. As discussed above, the crystalline PE-block-sPP block copolymer is phase separated in the melt,13 and lamellar and cylindrical morphologies have been observed depending on the composition.13 Completely different morphologies have been observed in the samples epitaxially crystallized onto the 3Ph substrate (Figure 6): no cylindrical microstructures have been observed for the asymmetric sample PE-b-sPP-2. This indicates that the epitaxial crystallization destroys the phase-separated structure that probably is formed in the melt. Moreover, it appears that the crystallization of PE-block-sPP can be significantly altered by a proper control of the nucleation process. Bulk crystallization is dominated by the PE sequences that nucleate and grow first, as indicated by the formation of ringed spherulites and twisted lamellae. The high nucleation rate of PE at relatively low undercooling is known and is observed in blends of PE with isotactic polypropylene (iPP)62 and sPP.63 The melting temperatures of both iPP and sPP are higher than that of PE, but when blends of PE with iPP crystallize from the melt, PE crystallizes first and nucleates the crystallization of iPP by an epitaxial process.63 The spontaneous sequence of crystallization can be reversed by manipulation of the nucleation process, via the use in the present case of 3Ph substrate or, more generally, by nucleating agents. Nucleating agents are more efficient with materials such as sPP or iPP for which spontaneous nucleation takes place at large undercooling. The temperature rise depends on the quality of the polymer/nucleating interactions but can reverse the sequence of crystallization of the two components. In the present investigation this sequence reversal is vividly illustrated by the characteristic sPP lamellar orientations generated on the 3Ph substrate.

In the upper-right part of Figure 6A thinner regions reveal a different pattern with thicker lamellae (15 nm thick) oriented along two directions about 70° apart (white stripes) that are the two sets of PE lamellae (Figure 7A), as expected from the diffraction patterns of Figure 5. In this part of the image the crystalline sPP is not visible contrary to the diffraction evidence (Figure 5A and model of Figure 7A). In these regions PE crystallizes first, and sPP crystallizes in the PE lamellar interstices and does not set the morphological pattern. This crystallization scheme is quite rare in the present analysis and may well involve a PE-richer part of the material. Since the growth of sPP is confined between PE lamellae as infilling material, it is probable that PE and sPP lamellae are parallel and sPP lamellae are made of chains tilted at 37° to the lamellar normal, as shown in the model of Figure 7C, even though this sequence is not well supported by the experimental data. TEM images similar to those of Figure 6 have been obtained also for the samples PE-b-sPP-3 and PE-b-sPP-4 (see Supporting Information), which are characterized by total molecular masses higher than those of the samples PE-b-sPP-1 and PE-b-sPP-2 and similar lengths of the sPP blocks with sPP block longer than the PE block (Mn(sPP) = 83 000) in the sample PE-b-sPP-3, and the PE block slightly longer than the sPP block (Mn(PE) = 80 600) in sample PE-b-sPP-4. In particular, in the case of the sample PE-b-sPP-4 TEM images similar to that in the upper-right part of Figure 6A have been obtained, with the presence of two sets of PE lamellae oriented along the two directions 70° apart and, contrary to Figure 6A, the presence of crystalline sPP lamellae oriented with chain axis tilted by 37° to the chain axes of PE (as in the ideal model of Figure 7A), according to the diffraction patter of Figure 5A (see Figure S4B). It is worth noting that the thicknesses of the crystalline lamellae observed in the TEM images of Figure 6 and Figure S4 (5−15 nm) are in any case and for both samples smaller than the values of coherence lengths of crystals of PE and sPP evaluated from the broadness of the X-ray diffraction profiles of bulk melt-crystallized samples (26 and 20 nm, respectively). However, these data evaluated for bulk samples and epitaxially crystallized films are not directly comparable because the sPP and PE lamellae in the films of Figure 6 are oriented edge-on on the substrate with (100)sPP and (110)PE planes as contact planes (Figure 5). Therefore, the values of thickness evaluated from the TEM images correspond to the size of lamellae along directions parallel to the chain axes (Figure 5). The coherence lengths L(200)sPP and L(110)PE calculated from the peak widths in the X-ray diffraction profiles indicate, instead, the average sizes of sPP and PE crystals along directions normal to the (200)sPP plane of sPP (a-axis) and normal to the (110)PE plane of PE, respectively, that is, along directions normal to the substrate in Figure 5. The observed morphology of these systems has allowed revealing the crystallization sequences, which are characterized by the confined orientation of the lamellae that crystallize last and the maintenance of the chain orientation dictated by the epitaxy that remains active during the (confined) growth. Moreover, the use of thin films in our experimental procedure emphasizes the role of sPP/3Ph and PE/3Ph interactions, whereas the possible impact of sPP/PE epitaxy is limited. The crystallographic faces involved in this epitaxy are indeed parallel to the 3Ph substrate and glass cover slides. Growth of sPP, for instance, takes place through the film thickness, and the

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CONCLUSION The structure and morphology of crystalline−crystalline PE− sPP block copolymer have been investigated in the bulk and in thin films epitaxially crystallized on crystals of 3Ph substrate. Electron diffraction indicates that the epitaxial crystallization produces oriented growth of both crystals of sPP and PE, with a single orientation of sPP lamellae and a double orientation of PE lamellae onto the (001) surface of the 3Ph substrate, according to the epitaxy of sPP and PE with 3Ph. The process also produces development of ordered nanostructures composed of alternating lamellar domains of PE and sPP, guided by the orientation of the sPP crystalline lamellae. TEM bright-field images provide details of the resulting morphology and reveal the sequence of the crystallization events. In some regions the morphology is characterized by a double orientation of PE lamellae, when PE crystallizes first and sPP crystallizes in the PE interlamellar domains. In most regions crystalline lamellae oriented along only one direction develop when sPP crystallizes first and PE crystallizes in the confined interlamellar sPP domains. In the latter case, growth of PE is confined between sPP lamellae, so that PE and sPP lamellae are parallel but PE chains are tilted by 37° to the global lamellar surface normal. In such crystallization sequences, therefore, the stem orientation of the polymer crystallizing last is as usual, dictated by the epitaxy with 3Ph, but spatial constraints are at play: the fold surface orientation is not the spontaneous one since it is dictated by the polymer that crystallizes first. H


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(13) Ruokolainen, J.; Mezzenga, R.; Fredrickson, G. H.; Kramer, E. J.; Hustad, P. D.; Coates, G. W. Morphology and Thermodynamic Behavior of Syndiotactic Polypropylene-Poly(ethylene-co-propylene) Block Polymers Prepared by Living Olefin Polymerization. Macromolecules 2005, 38, 851. (14) (a) Matsui, S.; Tohi, Y.; Mitani, M.; Saito, J.; Makio, H.; Tanaka, H.; Nitabaru, M.; Nakano, T.; Fujita, T. New Bis(salicylaldiminato) Titanium Complexes for Ethylene Polymerization. Chem. Lett. 1999, 1065. (b) Matsui, S.; Mitani, M.; Saito, J.; Tohi, Y.; Makio, H.; Tanaka, H.; Fujita, T. Post-Metallocenes: A New Bis(salicylaldiminato) Zirconium Complex for Ethylene Polymerization. Chem. Lett. 1999, 1263. (c) Matsui, S.; Mitani, M.; Saito, J.; Matsukawa, N.; Tanaka, H.; Nakano, T.; Fujita, T. Post-Metallocenes: Catalytic Perfomance of New Bis(salicylaldiminato) Zirconium Complexes for Ethylene Polymerization. Chem. Lett. 2000, 554. (d) Saito, J.; Mitani, M.; Mohri, J.; Ishii, S.; Yoshida, Y.; Matsugi, T.; Kojoh, S.; Kashiwa, N.; Fujita, T. Highly Syndiospecific Living Polymerization of Propylene Using a Titanium Complex Having Two Phenoxy-Imine Chelate Ligands. Chem. Lett. 2001, 30, 576. (15) (a) Matsui, S.; Mitani, M.; Saito, J.; Tohi, Y.; Makio, H.; Matsukawa, N.; Takagi, Y.; Tsuru, K.; Nitabaru, M.; Nakano, T.; Tanaka, H.; Kashiwa, N.; Fujita, T. A Family of Zirconium Complexes Having Two Phenoxy−Imine Chelate Ligands for Olefin Polymerization. J. Am. Chem. Soc. 2001, 123, 6847. (b) Mitani, M.; Mohri, J.; Yoshida, Y.; Saito, J.; Ishii, S.; Tsuru, K.; Matsui, S.; Furuyama, R.; Nakano, T.; Tanaka, H.; Kojoh, S.; Matsugi, T.; Kashiwa, N.; Fujita, T. Living Polymerization of Ethylene Catalyzed by Titanium Complexes Having Fluorine-Containing Phenoxy−Imine Chelate Ligands. J. Am. Chem. Soc. 2002, 124, 3327. (c) Saito, J.; Mitani, M.; Mohri, J.; Yoshida, Y.; Matsui, S.; Ishii, S.; Kojoh, S.; Kashiwa, N.; Fujita, T. Living Polymerization of Ethylene with a Titanium Complex Containing Two Phenoxy-Imine Chelate Ligands. Angew. Chem., Int. Ed. 2001, 40, 2918. (d) Mitani, M.; Furuyama, R.; Mohri, J.; Saito, J.; Ishii, S.; Terao, H.; Nakano, T.; Tanaka, H.; Fujita, T. Syndiospecific Living Propylene Polymerization Catalyzed by Titanium Complexes Having Fluorine-Containing Phenoxy-Imine Chelate Ligands. J. Am. Chem. Soc. 2003, 125, 4293. (16) (a) Matsui, S.; Fujita, T. FI Catalysts: super active new ethylene polymerization catalysts. Catal. Today 2001, 66, 63. (b) Matsukawa, N.; Matsui, S.; Mitani, M.; Saito, J.; Tsuru, K.; Kashiwa, N.; Fujita, T. Ethylene polymerization activity under practical conditions displayed by zirconium complexes having two phenoxy-imine chelate ligands. J. Mol. Catal. A: Chem. 2001, 169, 99. (17) Tian, J.; Hustad, P. D.; Coates, G. W. A New Catalyst for Highly Syndiospecific Living Olefin Polymerization: Homopolymers and Block Copolymers from Ethylene and Propylene. J. Am. Chem. Soc. 2001, 123, 5134. (18) Coates, G. W.; Hustad, P. D.; Reinartz, S. Catalysts for the Living Insertion Polymerization of Alkenes: Access to New Polyolefin Architectures Using Ziegler - Natta Chemistry. Angew. Chem., Int. Ed. 2002, 41, 2236. (19) Domski, G. J.; Rose, J. M.; Coates, G. W.; Bolig, A. D.; Brookhart, M. Living alkene polymerization: New methods for the precision synthesis of polyolefins. Prog. Polym. Sci. 2007, 32, 30. (20) Makio, H.; Terao, H.; Iwashita, A.; Fujita, T. FI Catalysts for Olefin Polymerization, A Comprehensive Treatment. Chem. Rev. 2011, 111, 2363. (21) Yoon, J.; Mathers, R. T.; Coates, G. W.; Thomas, E. L. Optically Transparent and High Molecular Weight Polyolefin Block Copolymers toward Self-Assembled Photonic Band Gap Materials. Macromolecules 2006, 39, 1913. (22) Domski, G. J.; Edson, G. B.; Keresztes, I.; Lobkovsky, E. B.; Coates, G. W. Synthesis of a new olefin polymerization catalyst supported by an sp3-C donor via insertion of a ligand-appended alkene into the Hf−C bond of a neutral pyridylamidohafnium trimethyl complex. Chem. Commun. 2008, 6137. (23) Thierry, A.; Lotz, B. Epitaxial Crystallization of Polymers: Means and Issues. In Handbook of Polymer Crystallization; Piorkowska, E., Rutledge, C. G., Eds.; John Wiley & Sons, Ltd.: 2013; p 237.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00705. Figures S1−S5 and Tables S1−S3. NMR data, SAXS profiles and TEM images of additional samples. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel ++39 081 674346; Fax ++39 081 674090; e-mail claudio. [email protected] (C.D.R.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from “Ministero dell’Istruzione, dell’Università e della Ricerca, Italy” (project PRIN 2010-2011) is gratefully acknowledged.

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REFERENCES

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