Controlling Size and Orientation of Lamellar Microdomains in

Feb 14, 2017 - *E-mail: [email protected]. This article is part of the Block ... of the amorphous EP block. Epitaxy has been coupled with the te...
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Controlling Size and Orientation of Lamellar Microdomains in Crystalline Block Copolymers Claudio De Rosa,* Rocco Di Girolamo, Finizia Auriemma, Giovanni Talarico, Anna Malafronte, Carmela Scarica, and Miriam Scoti Dipartimento di Scienze Chimiche, Università di Napoli Federico II, Complesso Monte S. Angelo, Via Cintia, I-80126 Napoli, Italy S Supporting Information *

ABSTRACT: Highly ordered lamellar nanostructures with high orientation of lamellar microdomains have been obtained by epitaxial crystallization of crystalline diblock copolymers constituted by crystalline polyethylene (PE) linked to an amorphous block of a propene-ethene random copolymer (EP). The epitaxial crystallization onto crystals of pchlorobenzoic acid induces formation of crystalline PE lamellae highly aligned along one direction, resulting in ordered lamellar nanostructures with perfectly aligned layers of crystalline PE alternating to amorphous layers of EP block. The periodicity of the lamellar structure can be modulated by modifying the molecular mass of the amorphous EP block. Epitaxy has been coupled with the technique of gold decoration so that the ordered nanostructures produced by epitaxy act as template for the formation of long, straight, and parallel rows of gold nanoparticles. KEYWORDS: block copolymers, crystallization, polyethylene, epitaxy, ordered lamellar nanostructure

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INTRODUCTION Phase separation of immiscible linked polymer blocks in block copolymers (BCP) results in formation of ordered nanostructures with self-assembly in lamellar, spherical, cylindrical, gyroid nanodomains, whose size and shape may be tuned by changing BCP molecular mass and composition.1,2 These self-assembled nanostructures having periodicity in the nanoscale, have already shown their potential for fabrication of nanomaterials.3−9 However, regular structures may be obtained only by controlling the microdomains orientation during phase separation. The self-assembly of BCPs in bulk, indeed, generally produces grains with high local order, but short-range correlation order in the relative orientation of adjacent grains. High degrees of order in the microdomain orientation and ordered nanopatterns may be obtained by introducing external bias fields during phase separation or processing. Various different methods have been proposed based on the application of mechanical, electrical, magnetic biases and on surface interactions with substrates.10,11 In semicrystalline block copolymers containing crystalline blocks, microphase separation arises from incompatibility of the blocks as in amorphous BCPs, or by crystallization of one or more blocks. The final morphology is path dependent and is the result of the competition and interplay between at least two thermodynamic transitions, microphase separation and crystallization.11−14 A wide range of morphologies are possible depending on the composition of the BCP, the crystallization temperature, the glass transition temperature and the order− disorder transition temperature. Different structures can be © XXXX American Chemical Society

obtained if the crystallization takes place from a single-phase melt or from an already microphase separated heterogeneous melt.15−46 In the latter case, microphase separation precedes crystallization and provides a microstructure within which crystallization must take place, resulting in a crystallization confined within preformed microdomains, or breaking out of the microphase separated structure formed in the melt.15−46 Crystallizable block copolymers have been mainly studied in the past for their possible application as thermoplastic elastomers because of their improved mechanical properties as well as better thermal stability.15,20,23,24,30,34,37 However, the presence of a crystallizable component can be exploited for controlling the final morphology through the control of crystallization and orientation of the crystals.40 In particular, a method for controlling the crystallization and crystal orientation of semicrystalline polymers in thin films is the epitaxial crystallization on suitable crystalline substrates.47−51 This method allows inducing preferred orientation of crystals on the substrate and/or crystallization of unstable crystal modifications. Epitaxial crystallization of polyethylene (PE) homopolymer onto crystals of various organic substances has been well-described and has been used for obtaining crystals with single-crystal or fiberlike orientations.47−51 In particular, in Special Issue: Block Copolymers for Nanotechnology Applications Received: December 13, 2016 Accepted: February 1, 2017

A

DOI: 10.1021/acsami.6b15913 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces the case of epitaxial crystallization of PE onto crystals of substituted aromatic acids or salts (p-chloro and p-bromobenzoic acid salts) a unique chain axis orientation has been obtained.50 The (100) plane of orthorhombic form of PE is in contact with the (100) face of the p-chlorobenzoic acid (CBA) crystal, with the c axis of PE oriented along the [001] direction of the substrate crystal.50 Epitaxy induces crystallization of the monoclinic phase of PE, but phase transition during crystal growth from the epitaxially induced monoclinic to the more stable orthorhombic form takes place within a few nanometers of the substrate surface.50,52 The single orientation of PE crystals onto the substrate surface is induced by lattice matching between the polymer interchain distance of 4.94 Å (b axis of the orthorhombic form of PE), or 5.234 Å of the (2̅10) plane of the monoclinic form, which is in contact with the (100) plane of the CBA crystal, and the substrate periodicity of 5.60 Å.50 The epitaxial crystallization of PE has also been used for the processing of crystalline/amorphous block copolymers.40−43 These studies have been performed on crystalline block copolymers containing PE crystallizable blocks obtained by hydrogenation of BCPs containing 1,4-polybutadiene blocks prepared by classic anionic living polymerization.40−43 This results in highly defective PE blocks with low melting temperature (about 90 °C) due to the presence of high amount of constitutional defects of 1-butene units arising from hydrogenation of 1,2-butadiene units present as defect in the precursor 1,4-polybutadiene blocks. In recent years, BCPs containing crystallizable polyolefins have been synthesized thanks to the development of metalbased coordination polymerization methods able to ensure a high stereochemical control, as in Ziegler−Natta catalysis, in living olefin polymerization.53−57 Depending on the ligand framework of the catalyst system and the nature of the coordination metal center, linear or branched polyethylene, atactic, isotactic, and syndiotactic poly(α-olefins), poly(cycloolefins), random copolymers of ethylene with higher olefins can be synthesized in living manner, allowing the synthesis of semicrystalline block copolymers.53−57 In this paper, we report a study of the morphology of BCPs constituted by a crystallizable block of PE and an amorphous block of propene-ethene random copolymer of different block lengths, synthesized using the bis(phenoxyimine) titanium complex of Scheme 1 as catalyst in coordination polymerization. The crystallizable PE block has been epitaxially crystallized on p-chlorobenzoic acid crystals resulting in highly ordered morphology with highly aligned crystalline lamellae of PE alternated with amorphous layers of the ethylene-propylene

random copolymer block. The use of the technique of gold deposition has allowed producing highly ordered rows of gold nanoparticles included only onto the amorphous layers of the ordered nanostructure.

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EXPERIMENTAL SECTION

Samples of polyethylene-block-(ethylene-co-propylene) (PE-block-EP) have been prepared with a living organometallic catalyst, bis[N-(3-tertbutylsalicylidene)-2,3,4,5,6-pentafluoroanilinato]-titanium(IV) dichloride complex (Scheme 1), activated with methylalumoxane (MAO).53 The polymerizations were carried out following the procedure described in refs 45 and 46 at a polymerization temperature of 10 °C and 15−18 min of homopolymerization under a constant ethylene feed (with formation of the PE block), and 4−7 h of ethylenepropylene copolymerization, depending on the desired block length, by discontinuously feeding ethylene to ensure a constant ethylene/ propene comonomer ratio (≈ 5−8 mol % of ethylene) in the liquid phase (with formation of the EP block). A sample of PE homopolymer has been prepared in the same polymerization conditions. The synthesized samples and their molecular characteristics are reported in Table 1. The molecular masses of the samples were determined by gel permeation chromatography (GPC 220 by Polymer Laboratories), on polymer solutions in 1,2,4-trichlorobenzene at 135 °C. 13C NMR spectra were obtained with a Bruker Advance spectrometer (400 MHz) equipped with a 5 mm high temperature cryoprobe operating on 50 mg/mL solutions in 1,1,2,2-tetrachloroethane-d2 at 120 °C. The ethylene concentration was evaluated from the 13C NMR spectra according to ref 58 (see the Supporting Information). DSC curves were obtained with a differential scanning calorimeter (DSC-822 by Mettler Toledo) in a flowing N2 atmosphere at a scanning rate of 10 °C/min. X-ray powder diffraction profiles were obtained with Ni-filtered CuKα radiation with a X-Pert diffractometer by Panalytical. Films of PE and PE-block-EP samples of thickness of ≈200 μm were prepared by compression molding under a press at very low pressure. Powder samples were melted at 180 °C under the press, kept at 180 °C for 2 min and crystallized by slow cooling to room temperature at cooling rate of about 15 °C/min. In the case of the BCP samples, parts of the so obtained compression molded films were annealed at high temperatures. The films were sandwiched between two kapton foils and heated again to ≈180 °C and kept at 180 °C for 2 min. The samples were then cooled at cooling rate of 20 °C/h to the annealing temperature (114 °C for the sample PE-b-EP-1 and 120 °C for the sample PE-b-EP-2, that is, 5 °C higher than the crystallization temperature peak evaluated by DSC reported in Table 1) and the samples were annealed for 24 h. The bulk morphology which develops at 25 °C in the so-obtained films was probed by performing small-angle X-ray scattering (SAXS) measurements, using a Kratky compact camera SAXSess (Anton Paar, Graz, Austria) with CuKα radiation in the range of scattering vector 0.1 nm−1 ≤ q ≤ 2 nm−1, where q = (4πsin θ/λ) and 2θ is the scattering angle. Data were collected on a MS (MultiSensitive) storage phosphor screens (PerkinElmer) and processed with a digital imaging reader Cyclone Plus (PerkinElmer). The lamellar peridicity L was determined from the SAXS intensity distribution corrected for the Lorentz factor (proportional to q2) from the position of the correlation peaks q* using the Bragg law, L = 2π/q*. Epitaxial crystallizations were performed using as substrate crystals of p-chlorobenzoic acid (CBA). The PE homopolymer and the two samples of PE-block-EP were epitaxially crystallized onto the surface of CBA crystals following the procedure used for the PE homopolymer,49,50 and also described in refs 45, 46 for block copolymers. The so obtained thin films crystallized onto CBA were analyzed by transmission electron microscopy (TEM) (Philips EM 208S operating at 120 kV) and atomic force microscopy (AFM) (Veeco Caliber microscope in tapping mode). The TEM specimens of the PE and PEblock-EP were exposed to RuO4 for 2 h to stain the amorphous phase in PE and the amorphous EP microdomains.

Scheme 1. Structure of the Titanium Complex Used As Catalyst for the Preparation of the PE-block-EP Block Copolymer

B

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Table 1. Total Molecular Mass (Mn), Molecular Masses of PE (Mn(PE)) and EP (Mn(EP)) Blocks, Polydispersity (Mw/Mn), Volume Fraction of the PE Block (f PE), Total Concentration of Ethylene ([E]), Melting Temperature of As-Prepared Samples (Tm), Crystallization Temperature (Tc) and Degree of Crystallinity (xc) of the samples of PE-block-EP and of PE Homopolymer samples

Mna (kDa)

Mn(PE)a (kDa)

Mn(EP)a (kDa)

Mw/Mna

f PEb (v/v%)

[E]totc (mol %)

[E]EPd (mol %)

T me (°C)

Tcf (°C)

Tge (°C)

xcg (%)

PE PE-b-EP-1 PE-b-EP-2

122 321.4 254.5

122 120 130

201.4 124.5

1.3 1.4 1.4

100 34 48

100 69 73

45 36

136 138 139

120 109 115

−41 −45

60 30 39

a

From GPC analysis. bCalculated from the molecular masses Mn(PE) and Mn(PE), the densities of PE (0.997 g/cm3) and EP (0.855 g/cm3) such that f PE = (Mn(PE)/0.997)/(Mn(PE)/0.997 + Mn(EP)/0.855). cDetermined from 13C NMR spectra according to ref 57 (see the Supporting Information). d Determined from [E]tot and the molecular mass of the blocks. eDetermined from the DSC heating scans of as-prepared samples at heating rate of 10 °C/min. fDetermined from the DSC cooling scans at cooling rate of 10 °C/min. gDetermined from the X-ray powder diffraction profiles. To improve contrast in the TEM images, we also decorated the thin films crystallized onto CBA with gold nanoparticles by using the method of vacuum evaporation and condensation.59,60 As described in ref 45, the produced gold nanoparticles deposit onto the target amorphous EP domains, allowing evidence of the BCP morphology.

PE-block-(ethylene-co-propylene) crystallization occurs from a single phase melt and drives phase separation. The DSC curves recorded during heating of the as-prepared samples and successive cooling from the melt, all recorded at scanning rate of 10 °C/min, are reported in Figure 2. The

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RESULTS AND DISCUSSION Two samples of PE-block-EP of different molecular mass have been synthesized (Table 1). The X-ray powder diffraction profiles of as-prepared samples of the PE homopolymer and of PE-block-EP are reported in Figure 1. All samples are

Figure 2. DSC curves recorded during (A) heating and (B) successive cooling from the melt of as-prepared samples of PE homopolymer and PE-block-EP.

samples of PE-block-EP show melting of the PE blocks at similar values of temperature, and similar to that of the PE homopolymer. The crystallization temperatures of the PEblock-EP are instead lower than that of the PE sample. Moreover, the sample PE-b-EP-2, with lower molecular mass, crystallizes from the melt at temperature (115 °C) slightly higher than that of the sample PE-b-EP-1 (Table 1). The DSC heating curves of the samples crystallized from the melt are very similar to those of the as-prepared samples. From the DSC heating curves values of the glass transition temperature of about −40 to −45 °C have been determined for the two samples of PE-block-EP. The morphology that develops in bulk samples crystallized from the melt has been analyzed by SAXS measurements on compression-molded films. The SAXS profiles of compressionmolded samples crystallized from the melt by cooling at 15 °C/ min of PE homopolymer and PE-block-EP copolymers are reported in Figure 3A (curves a−c). Because of the expected lamellar morphology for the PE and BCPs samples, the data were corrected for the Lorentz factor. The SAXS profile of the homopolymer PE (curve a) shows a strong correlation peak at q* ≈ 0.21 nm−1 that partially overlaps with a broad second order peak at q* ≈ 0.42 nm−1. This indicates a lamellar morphology with average periodicity LSAXS of nearly 30 nm due to crystalline lamellar stacks.

Figure 1. X-ray powder diffraction profiles of as-prepared samples of PE homopolymer and PE-block-EP. The 110 and 200 reflections at 2θ = 21.4 and 23.9°, respectively, of the orthorhombic form of PE are indicated.

crystallized in the orthorhombic form of PE as demonstrated by the presence in the diffraction profiles of Figure 1 of the 110 and 200 reflections at 2θ = 21.4 and 23.9° of PE. The BCP samples show in addition broad haloes centered at 2θ = 18° due to the scattering of the amorphous block of EP, and correspondingly present degrees of crystallinity lower than that of the homopolymer (Table 1). A slightly higher value of the degree of crystallinity is observed for the sample PE-b-EP-2, in accordance with the higher length of the PE block (Table 1 and the Supporting Information). In similar BCP samples, such as PE-block-(ethylene-alt-propylene), the low value of the Flory interaction parameter χ of 0.007 at 120 °C has been determined,20,61 indicating that in a rather wide range of molecular weights and compositions these BCPs form singlephase melt and phase separation is driven by crystallization of the PE block from the homogeneous melt. This results in the formation of alternating lamellar microdomains regardless of the copolymer composition.20,23 It is expected that also in our C

DOI: 10.1021/acsami.6b15913 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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for the sample PE-b-EP-1, and ≈57 nm for the sample PE-bEP-2, most likely due to the increase of the thickness of PE lamellar crystals. Thin films of thickness in the range 30−50 nm of samples of PE-block-EP and of PE homopolymer have been epitaxially crystallized onto the surface of preformed crystals of pchlorobenzoic acid (CBA), following the method used for PE49,50 homopolymer. As described above, the epitaxial crystallization of PE homopolymer onto crystals of substituted aromatic acids has been shown in the literature.50 It has been found that PE crystallizes onto CBA crystals with a unique chain axis orientation, with the c axis of PE oriented along the [001] direction of the substrate crystal, according to the epitaxial relationships between PE and CBA cystals.50 The TEM bright-field images of PE homopolymer and of samples PE-b-EP-1 and PE-b-EP-2 epitaxially crystallized onto CBA and stained with RuO4 are shown in Figure 4. The dark

Figure 3. (A) SAXS profiles recorded at 25 °C after correction for the Lorentz factor of samples of (a) PE homopolymer and of samples (b, b′) PE-b-EP-1 and (c, c′) PE-b-EP-2. The samples were crystallized from the melt (at 180 °C) by compression molding and cooling at 15 °C/min cooling rate down to room temperature (a−c) or slowly crystallized by cooling from the melt (at 180 °C) at cooling rate of 20 °C/h down to 114 °C for the sample PE-b-EP-1 (b′) and 120 °C for the sample PE-b-EP-2 (c′) and annealed at these temperatures for 24 h. (B) Scheme of the lamellar morphology of PE-block-EP copolymers.

A strong correlation peak is also present in the SAXS profiles of the samples PE-b-EP-1 and PE-b-EP-2 (curves b and c of Figure 3A, respectively) at value of the scattering vector q* ≈ 0.11 nm−1, lower than that observed for the PE homopolymer, corresponding to lamellar periodicity LSAXS of ∼57 and 53 nm for the samples PE-b-EP-1 and PE-b-EP-2, respectively, higher than the periodicity of the PE homopolymer. This indicates a lamellar morphology of the BCP samples, characterized by alternating crystalline PE layers and amorphous EP layers, as shown in Figure 3B, with crystalline PE lamellae separated by amorphous regions that include the amorphous portions of chains belonging to the PE blocks and the EP domains (Figure 3B),20 resulting in amorphous layers thicker than in the PE homopolymer. The absence of higher order peaks in the SAXS profiles indicates that the lamellar morphology formed by the BCPs under similar crystallization conditions is more disordered than in PE homopolymer, probably because the PE crystalline lamellae are separated by thicker amorphous regions (Figure 3B). SAXS analysis has been extended to BCPs samples slowly crystallized from the melt by cooling at low cooling rate of 20 °C/h down to 114 °C for the sample PE-b-EP-1 and 120 °C for the sample PE-b-EP-2 (that is, 5 °C higher than the crystallization temperatures evaluate by the DSC cooling curves of Figure 2B) and, then, annealed at these temperatures for 24 h. The annealing induces a small shift of the SAXS correlation peak toward lower values of q (curves b′ and c′ of Figure 3A) and a higher order peak appears in the case of the low molecular mass sample PE-b-EP-2. This corresponds to a slight increase of the lamellar periodicity LSAXS to values of ≈59 nm

Figure 4. Transmission electron microscopy images of thin films of (A) PE and BCP samples (B) PE-b-EP-1 and (C) PE-b-EP-2 epitaxially crystallized onto CBA crystals and stained with RuO4 for 2 h. The dark layers correspond to the stained amorphous phase of PE or amorphous EP microdomains, and the lighter layers to the crystalline PE lamellae. D

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samples that develops by crystallization from the melt, determined by SAXS measurements. In the case of PE homopolymer, the bulk periodicity of ∼30 nm, is significantly lower than the lamellar periodicity achieved in the epitaxially crystallized thin films. We argue that single crystals of CBA not only act as a template able to induce the parallel orientation of the lamellar morphology, through epitaxial crystallization, but also influence the thickness of lamellar crystals especially in the case of the homopolymer PE. Improvement of the contrast in the TEM observation and details of the morphology obtained after epitaxial crystallization of PE homopolymer and PE-block-EP have been obtained by decoration with gold of the epitaxially crystallized films. The bright field TEM image of films of PE homopolymer epitaxially crystallized onto CBA and decorated with gold is shown in Figure 6. The technique of gold decoration59,60 is used to

regions correspond to the stained amorphous phase of PE for the homopolymer (Figure 4A) and to the amorphous EP microdomains for the two BCP samples (Figure 4B,C), whereas the lighter regions correspond to the crystalline PE lamellae. Although the poor contrast, the TEM image of the homopolymer of Figure 4A shows lamellae highly oriented along one direction that correspond, according to the epitaxial relationships between PE and CBA crystals, to crystalline lamellae of PE oriented edge-on and aligned with the b axis along the b axis of the substrate.50 In the case of the two BCP samples PE-b-EP-1 and PE-b-EP-2, the images of Figure 4B,C also show the formation of crystalline PE lamellae oriented edge-on on the substrate and highly aligned along one direction. The crystalline layers alternate to amorphous layers that correspond to nanodomains of the EP blocks, resulting in ordered lamellar nanostructures of alternating parallel crystalline and amorphous layers aligned along one direction. The AFM images of the epitaxially crystallized films of PE homopolymer and of samples PE-b-EP-1 and PE-b-EP-2 are shown in Figure 5. It is apparent that also the AFM images

Figure 5. (A−C) AFM phase images of thin films of samples of (A) PE, (B) PE-b-EP-2, and (C) PE-b-EP-1 epitaxially crystallized onto crystals of CBA and (D−F) corresponding schemes of the obtained morphology with alternating layers of crystalline PE lamellae and amorphous EP domains aligned along one direction. The values of periodicity L of the lamellar nanostructures are indicated.

Figure 6. (A) Transmission electron microscopy images of golddecorated thin films of PE epitaxially crystallized onto CBA crystals and (B) scheme of the obtained morphology with alternating layers of crystalline lamellae and amorphous phase aligned along one direction and row of gold particles (dark circles) positioned at the interphase between crystalline and amorphous layers.

clearly show the formation of the ordered lamellar nanostructure with highly aligned alternating parallel crystalline and amorphous layers. From the AFM images, values of periodicity L of the lamellar nanostructures, and of thicknesses of crystalline lc and amorphous la layers have been determined (see the Supporting Information). In particular, for the PE homopolymer la = 19 nm, lc = 24 nm, and L = 43 nm, for the sample PE-b-EP-2, la = 24 nm, lc = 25 nm and L = 49 nm, and for the sample PE-b-EP-1, la = 34 nm, lc = 24 nm and L = 58 nm (Figure 5D−F). The thicknesses of crystalline layers lc are similar, whereas the thickness of the amorphous layer la and the periodicity L increase from the homopolymer PE, to the sample PE-b-EP-2, to the sample PE-b-EP-1. These data indicate that in the PE-block-EP samples the thickness of the amorphous layers and the periodicity of the ordered structure induced by epitaxy may be modulated by modifying the molecular mass of the EP block. It is worth noting that in the case of PE-block-EP copolymers the periodicity of the lamellar morphology that develops in thin films by epitaxial crystallization is similar to that of the bulk

visualize edge-on crystalline lamellae of polymers in TEM bright-field images and obtain reliable value of the lamellar periodicity.59,60 The gold particles after evaporation and condensation under vacuum deposit on the amorphous regions at the interface with the crystalline lamellae.45,59,60,62 In the case of linear low density PE or linear PE, the formation of a single row or a double row of gold particles, respectively, has been described.62 The rows of gold particles are separated by brighter zones that correspond to the crystalline lamellae.59,62 In the image of Figure 6A, the dark spots correspond to the gold particles that presumably are located into the amorphous intralamellar phases, that is, in between the crystalline domains of PE that appear white. With respect to the TEM image of Figure 4A, the gold decoration in Figure 6A allows for a better visualization of the crystalline lamellae of PE highly aligned E

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Figure 7. Transmission electron microscopy images of gold decorated samples (A) PE-b-EP-1 and (C) PE-b-EP-2 epitaxially crystallized onto CBA crystals and (B, D) corresponding schemes of the obtained morphology with alternating layers of crystalline PE lamellae and amorphous EP domains aligned along one direction. (B) A double row and a (D) single row of gold particles (dark circles) for the samples PE-b-EP-1 and PE-b-EP-2, respectively, are included in the amorphous layers and positioned at the interphase between crystalline and amorphous layers. The periodicity L of the lamellar nanostructure is indicated.

along one direction, oriented edge-on and aligned with the b axis along the b axis of the substrate, according to epitaxy.50 A single row of gold particles is positioned between amorphous and crystalline layers with preference to be included in the amorphous layer, as in the scheme of Figure 6B. From the distance between the parallel dark rows of gold particles a rough value of periodicity (equal to the sum of the similar thicknesses of crystalline lc and amorphous la layers) of nearly 30 nm, slightly lower than that obtained by AFM, has been evaluated. The TEM images of the two samples of PE-block-EP after epitaxial crystallization onto CBA and gold decoration are reported in Figure 7. Also for block copolymer samples long, straight and parallel rows of dark spots corresponding to gold particles included in the amorphous layers are observed. This allows the clear visualization of crystalline PE lamellae oriented edge-on on the substrate and highly aligned along one direction (white layers of Figure 7), formed by epitaxy. The crystalline layers alternate to amorphous layers that, in this case, correspond mainly to nanodomains of the EP blocks. Therefore, in the block copolymer samples the gold particles are included in the ticker amorphous EP layers. In the case of the sample PE-b-EP-2, the TEM image of Figure 7C still shows thin rows of gold particles of thickness corresponding to one particle (Figure 7D), as in the case of the homopolymer (Figure 6B). A value of the periodicity L of the lamellar nanostructure, corresponding to the sum of the thicknesses of crystalline lc and amorphous la layers, of nearly 35 nm has been evaluated from the distance between two adjacent rows of gold particles. For the sample PE-b-EP-1, the image of Figure 7A shows layers of gold particles inside the amorphous phase containing a double row of gold particles, as in the scheme of Figure 7B. These layers of amorphous domains are thicker than those in

the image of Figure 6A of the PE homopolymer and in the image of Figure 7C of the sample PE-b-EP-2 and are able to host a double row of gold particles.62 A value of periodicity L of 55 nm of the lamellar nanostructure of the sample PE-b-EP-1 has been evaluated from the distance between two adjacent double rows of gold particles (Figure 7B). The periodicity of the sample PE-b-EP-1 (55 nm) is higher than that of the sample PE-b-EP-2 (35 nm) mainly due to the larger thickness of the amorphous layers la (Figure 7). The values of periodicity L evaluated by TEM images of Figures 6 and 7 are only slightly lower than those evaluated from AFM images of Figure 5 but confirm that the periodicity of the sample PE-b-EP-1 is higher than that of the sample PE-bEP-2, due to the higher thickness of the amorphous layer. The experimental results of Figures 4−7 confirm that the periodicity of the alternating nanostructure can be modulated by changing the length of the amorphous block. As shown by SAXS data of Figure 3 and assumed in theoretical treatments,63−65 alternating lamellar microdomains are obtained also in bulk samples crystallized without epitaxy.20,23 The epitaxial crystallization allows for a better visualization of the lamellar domains thanks to the induced high alignment. The data of Figure 7 also indicate that the ordered BCP nanostructures act as template for the fabrication of ordered patterns of metal particles, which are of potential interest for many applications.

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CONCLUSION Crystalline PE-block-EP block copolymers constituted by a crystalline block of polyethylene and an amorphous block of a propene-ethene random copolymer with different molecular masses have been synthesized with a living metallorganic coordination catalyst. The samples crystallize by cooling from F

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the melt in the orthorhombic form of PE at similar values of temperature and show similar values of melting temperatures. The BCP samples and the corresponding PE homopolymer samples have been epitaxially crystallized onto the surface of crystals of p-chloro benzoic acid resulting in formation of highly ordered lamellar nanostructures. Epitaxy induces formation of crystalline PE lamellae oriented edge-on on the substrate and highly aligned along one direction. The crystalline layers alternate to amorphous layers of EP block resulting in perfectly aligned and parallel layers of the two nanodomains. The thickness of the layers can be modulated by changing the molecular mass of the blocks. Epitaxy has been coupled with the technique of gold decoration such that the ordered BCP nanostructures act as template for the fabrication of long, straight, and parallel rows of gold nanoclusters that could be used in many applications.

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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/acsami.6b15913. 13 C NMR spectra, details of NMR analysis, AFM image analysis (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

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

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acsami.6b15913 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.6b15913 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX