Architectural Effects of Organic Nanoparticles on Block Copolymer

Jun 20, 2017 - Department of Chemical and Biological Engineering, Korea University, Seoul ... molecular weight, architecture, and interaction paramete...
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Architectural Effects of Organic Nanoparticles on Block Copolymer Orientation Hyun Suk Wang,† Anzar Khan,† Youngson Choe,‡ June Huh,† and Joona Bang*,† †

Department of Chemical and Biological Engineering, Korea University, Seoul 136-713, Republic of Korea Department of Chemical Engineering, Pusan National University, Kumjeong-ku, Busan 609-735, Republic of Korea

‡

ABSTRACT: Organic nanoparticles (ONPs) in the form of star polymers and single chain nanoparticles (SCNPs) are used as fillers in block copolymer (BCP)/ONP nanocomposite thin films to induce perpendicular microdomains without any substrate treatment. The nonselective ONPs for both blocks of BCP neutralize the substrate and the free surface via an entropy-driven boundary surface segregation process, which differs markedly from the conventional neutralization process relying on surface chemistry. To examine the architectural effect of ONPs for surface segregation, neutral star polymers with ∼30, 21, and 6 arms and singlechain nanoparticles (SCNPs) are used as fillers in PS-b-PMMA thin films in an attempt to produce perpendicular microdomains. Consequently, it was observed that ∼30- and 21-arm star polymers and SCNPs, which may behave like hard particles having excluded volume interactions with host BCPs, effectively induced perpendicular microdomains, while soft particle-like 6-arm stars led to morphological compatibilization with BCPs.

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perpendicular orientation of BCP thin films. However, the multistep process of such methods often leaves much to be desired. Other emerging approaches utilize additives15 in the form of polarity-switching top coats,16 nanoparticles (NPs),17 salts,18 or low surface energy moieties19 to overcome any preferential interaction between one block with the substrate. Previously, our group reported the perpendicular orientation of lamellae-forming PS-b-PMMA thin films via the segregation of thermally stable Au NPs tuned with neutrally selective ligands to the boundary surfaces.17,20 Furthermore, we recently reproduced the results using organic counterparts in the form of core-cross-linked star polymers to produce perpendicular, etch-friendly thin films.21 The boundary surface segregation of NPs in the polymer−NP mixture has been reported in various studies.22−24 Such boundary surface enrichment of NPs prevents polymers from direct contacting with the boundary surface and may provide a “neutral” surface at the appropriate conditions. Both experimental25 and theoretical26 studies have been conducted to observe the factors that influence the boundary surface segregation of NPs in homopolymers thin films. The boundary surface segregation of organic and inorganic NPs in BCP thin films have shown to affect the orientation of the microdomains. Park et al. demonstrated the effect of the number of arms in a poly(ethylene oxide) star polymer on the orientation of cylinder-forming PS-b-PMMA thin films under high-humidity solvent annealing conditions.27 In the study, star

INTRODUCTION Block copolymers (BCPs) are widely considered as a route to aid conventional top-down photolithography due to their ability to form spatially periodic nanostructures in the sub-20 nm regime via self-assembly. In addition, BCP thin films can act as scaffolds for various applications including nanolithography, microelectronics, nanoreactors, and separation membranes.1−7 Various periodic nanostructures such as hexagonally arrayed cylinders, lamellae, BCC spheres, and bicontinuous gyroids can be formed by controlling parameters such as the BCP molecular weight, architecture, and interaction parameter as well as the volume fraction of each block. However, one of the biggest challenges in BCP nanolithography lies in controlling the orientation of microdomains in BCP thin films with respect to the substrate. Because of the typical preferential wetting of one block to the substrate and/or free surface, the BCP microdomains orient parallel to the film boundary surfaces (top free surface and bottom substrate) to achieve the lowest free energy state. From a practical viewpoint, cylinders and lamellae oriented perpendicular to the substrate are favorable because they can be applied to the fabrication of nanoporous membranes and pattern transfer in nanolithography. In the case of polystyrene-block-poly(methyl methacrylate) (PS-bPMMA), a ubiquitous BCP with high potential for pattern transfer applications, PMMA preferentially wets the SiOx substrate while both blocks have similar surface energies, and thus only substrate neutralization is required to obtain perpendicular microdomains. Substrate neutralization with anchored random copolymers,8−11 solvent annealing,12 application of external fields,13 and surface roughening14 are some of the most conventionally used approaches to inducing © XXXX American Chemical Society

Received: April 26, 2017 Revised: June 20, 2017

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DOI: 10.1021/acs.macromol.7b00865 Macromolecules XXXX, XXX, XXX−XXX

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in an ice−water bath. After addition, the reactor was left at room temperature for 5 days and then under reduced pressure for 1 day. The brownish solution was diluted with dichloromethane and washed successively with 1 N HCl, saturated aqueous NaHCO3 solution, 1 N aqueous NaCl, and finally water. The organic phase was dried over NaHCO3 and concentrated using rotary evaporation after filtration. The crude product was purified using gel column chromatography using THF:hexane (5:5 volume fraction) as the mobile phase. After drying under reduced pressure for 2 days, star ONPs were synthesized via ATRP of styrene and MMA at 70 °C. Synthesis of 6-Arm Dipentaerythritol-Based Star ONPs (6AONPs). Dipentaerythritol was dissolved in water and filtered and subsequently recrystallized twice before drying at 120 °C for 2 days prior to use. The synthetic procedure is similar to that of 21A-ONP, only differing in the core (dipentaerythritol instead of β-CD) and solvent (THF instead of NMP) used. Synthesis of Intrachain Cross-Linked SCNPs. Styrene, MMA, and 4-vinylbenzyl chloride were copolymerized via reversible addition−fragmentation chain transfer (RAFT) polymerization using methyl 2-phenyl-2-(phenylcarbonothioylthio)acetate36 as the RAFT agent and precipitated into cold methanol to produce the linear precursor. After drying under reduced pressure overnight, the terminal RAFT agent group was removed by reaction with excess AIBN in toluene at 60 °C for 12 h. After precipitation and drying under reduced pressure, the copolymer was dissolved in DMF (ca. 100 mg/ mL) and reacted with excess NaN3 at room temperature for 4 days and filtered. The filtrate was precipitated into cold methanol and concentrated and dried under reduced pressure overnight. The dried product was dissolved in DMF and added dropwise into dibenzyl ether at 250 °C. The reaction was left for 1 h, and the resulting yellow solution was poured into cold methanol for precipitation. The precipitate was filtered and dried under vacuum for 3 days. Characterization of P(S-r-MMA) Arms and ONPs. Gel permeation chromatography (GPC) was performed using a Waters GPC with tetrahydrofuran (THF) as the mobile phase at 40 °C. A refractive index detector was used, and the molecular weight of the eluted polymer was calculated relative to linear PS standards. The hydrodynamic diameters of the ONPs were measured by dynamic light scattering (DLS) using BI-200SM (Brookhaven Instruments Corporation) with a 633 nm laser. The scattered light was collected at 90°. Samples were analyzed in toluene and diluted until the apparent size was independent of concentration. All measurements were performed at room temperature. Preparation of BCP/ONP Thin Films and Characterization. BCP and ONP solutions were separately prepared in toluene and mixed at the appropriate proportions to yield BCP/ONP solutions. Silicon substrates were sequentially washed with acetone, methanol, acetone, and toluene to clean the surface and to remove any organic contaminants. The BCP/ONP thin films were prepared by spin coating from the toluene solution onto silicon substrates without a neutral layer. The ONP concentration with respect to the BCP ranged from 0 to 60 wt %, and the thickness of the film was varied from 1L0 to 3L0, where L0 denotes the domain spacing of PS-b-PMMA microdomains, by controlling the solution concentration and the spin rate. Thin films were spin-casted onto silicon wafer and thermally annealed at 180 or 200 °C for at least 2 days. For film backside analysis, Si substrate with 300 nm SiO2 layer was used. Film were floated onto the surface of buffered oxide etch solution (7.3 wt % HF, 33.5 wt % NH4F) and transferred to deionized water before being recovered upside-down with a fresh Si substrate. BCP films were observed by a field emission-scanning electron microscope (FE-SEM, Hitach S-4800) operated at 15 kV and optical microscope (Nikon Eclipse E400POL). To remove the PMMA block, the reactive ion etching (RIE) was conducted with Ar (3 sccm)/O2 (15 sccm) by a RF power of 20 W at 0.1 Torr (RIE 5000, SNTEK). The etch rates for PMMA and PS are ∼7.0 and ∼1.2 nm/s, respectively. The etched samples for FE-SEM were coated with a thin Pt film to avoid charging effects. Scanning force microscopy (SFM) images were obtained for unetched BCP films on a Digital Instruments Multimode scanning force microscope in tapping mode. Grazing incidence small-angle X-

polymers with more than 8 arms led to a perpendicular orientation of the cylindrical microdomains on a native silicon oxide substrate under high humidity conditions. In another study, Horechyy et al. used supramagnetic Fe3O4 NPs with trin-octylphosphine oxide and oleylamine ligands to vertically orient cylindrical microdomains of poly(n-pentyl methacrylate)-block-poly(methyl methacrylate) (PPMA-b-PMMA) using solvent annealing.28 Although these works demonstrate that the addition of NPs may induce perpendicular orientations of microdomains under some solvent fields, it is still unclear that the sole effect of NPs, apart from solvent effects, are significant enough to cause the vertical domain orientation. Also, the difficulty in controlling interfacial tensions of NPs, which are interfaced with other mixing components and surroundings, makes it even more complicated for interpretation, to isolate the entropy-driven effects of NPs from their overall behaviors. In this respect, thermally annealed AB-type BCP nanocomposites with AB-type particulate additives, where all host and particulate species are made up only of A- and B-type monomers, provide an ideal platform to investigate such effects, offering an opportunity to establish a novel strategy for controlling the pattern orientation of BCP structure. Herein, we report the thin film morphology of cylinder-forming PS-bPMMA BCP mixed with various type of organic nanoparticles (ONPs) made from P(S-r-MMA) random copolymer. Boundary surface segregation of ONPs and the orientation of cylinders in the film are investigated as a function of various molecular variables such as the composition of random copolymer and the number of arms of star-shaped ONPs. Finally, a family of ONPs that has recently received much research momentum, single-chain nanoparticles (SCNPs),29−34 will also be used to further shed light on the architectural effect of ONPs.

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EXPERIMENTAL SECTION

Materials. Both cylinder- and lamellae-forming PS-b-PMMA were purchased from Polymer Source Inc. and used as received. The molecular weights of the blocks were Mn(PS) = 57 000 g/mol and Mn(PMMA) = 25 000 g/mol for the cylinder-forming BCP, and Mn(PS) = 45 000 g/mol and Mn(PMMA) = 44 000 g/mol for the lamellae-forming BCP. Toluene (99.9%), anhydrous anisole (99.7%), anhydrous dimethylformamide (DMF) (99.8%), dipentaerythritol (technical grade), styrene (≥99%), MMA (99%), divinylbenzene (DVB) (80%), 4-vinylbenzyl chloride (≥90%), anhydrous 1-methyl-2pyrrolidinone (NMP) (99.5%), 4-(dimethylamino)pyridine (DMAP) (≥99%), triethylamine (TEA) (≥99%), and α-bromoisobutryl bromide (BBIB) (98%) were purchased from Sigma-Aldrich. All commercially obtained solvents and reagents were used without further purification unless specified otherwise. Synthesis of Core-Cross-Linked Star ONPs (30A-ONPs). Corecross-linked star ONPs were synthesized by the “arm-first” method developed by Matyjaszewski’s group.35 P(S-r-MMA)-Br arms were synthesized via ATRP and precipitated into cold methanol. After drying in a vacuum oven at room temperature overnight, the arms were used as macroinitiators to DVB monomers to create a star ONP with P(S-r-MMA) arms and DVB core. Residual arms were removed by fractional precipitation using dichloromethane and methanol as the good and poor solvent, respectively. The number of arms and the core size can be readily controlled by the ratio of cross-linker to the arms, conversion, and choice of ligand. Synthesis of 21-Arm β-Cyclodextrin-Based Star ONPs (21AONPs). β-Cyclodextrin (β-CD) was purified by recrystallization in water and dried under reduced pressure at 120 °C for 2 days. A solution of EBIB (29.43 g, 128 mmol) in dry NMP (20 mL) was added dropwise to a solution of β-CD (2.27 g, 2 mmol), DMAP (catalytic amounts), and TEA (77.71 g, 768 mmol) in 40 mL of NMP B

DOI: 10.1021/acs.macromol.7b00865 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Characteristics of Synthesized ONPs feed mole ratio in P(S-r-MMA) no. of arms ∼30

21 6 0

S2-8 S3-7 S6-4 S7-3 S8-2 21A-ONP 6A-ONP SCNP

ONP composition by NMR

ONP characterization

styrene

MMA

styrene

MMA

Mnapp a (g/mol)

Đ

Dhb (nm)

20 30 60 70 80 60 60 60

80 70 40 30 20 40 40 40

15 27 57 66 70 58 59 57

85 73 43 34 30 42 41 43

35 000 33 000 28 000 28 000 34 000 30 000 23 000 12 000

1.34 1.28 1.31 1.22 1.25 1.19 1.43 1.4

10.8 10.4 10.0 10.0 10.5 10.2 8.4 n.a.c

a Apparent number-average molecular weight by GPC. bHydrodynamic diameter in toluene characterized by DLS. cSize analysis of SCNP falls well below 10 nm and could not be analyzed due to instrumental limitations.

ray scattering (GISAXS) measurements were carried out at the Pohang Accelerator Laboratory 9A beamline (Korea). A CCD detector was used to record 2-D GISAXS patterns, which was positioned at the end of a vacuum guide tube where the X-ray passed through the sample.

BCP microdomains from parallel to perpendicular and then back to parallel orientation is observed as the S:MMA molar feed ratio for the arms changes from 8:2 to 2:8, with the corresponding star ONPs denoted as S8-2 and S2-8, respectively. S8-2 and S2-8 thus represent ONPs with compositions belonging to the PS- and PMMA-selective regime in which the enthalpic selectivity of the star ONPs outweighs nonselectivity driven by the configurational entropy. Although the organic nature of the star ONPs as well as their compositional similarity to the matrix forbids their exact positional analysis via SEM, domain selectivity of the star ONPs is supported by the noticeable swelling of the PS domain containing PS-selective S8-2 as well as our previous study17 in which inorganic Au NPs were used. In contrast to the parallel cylinders induced by selective star ONPs, S6-4 induced a perpendicular orientation of cylinders. This phenomenon can be explained by the neutralization of the substrate by the boundary surface segregation of S6-4, which is presumably attributed to two entropic effects associated with chain conformations of BCPs and ONPs. It is generally understood by an entropic argument that molecules having less conformational ensembles tend to segregate to the boundary surface when they are mixed with conformationally richer molecules. Such conformational contrast involves the difference in chain architecture by which chains with larger architectural constraints (e.g., star, ring, branch, etc.) have a preferential attraction to the boundary surface.37−39 This entropic consequence is shown experimentally in a study on the free surface segregation of branched PS in a linear deuterated PS matrix, where the free surface excess, characterized by neutron reflectivity, of branched polymers increases predominantly with the number of chain ends and secondarily with the number of branch points.38 In addition, an effective repulsive interaction between BCPs and ONPs, which arises from extra stretching energy of microphase-separated BCP chains detouring around the excluded volume taken by particles, leads to a segregation of ONPs that are presumably expelled to the substrate or the free surface where minimal number of BCP chains incur the extra stretching energy. The resulting boundary surface enrichment of neutral ONPs leads to nonpreferential wetting of either block and thus the perpendicular orientation of microdomains. Compositional analysis of S6-4 using 1H NMR (not shown) shows that S:MMA = 59:41, close to the well-established neutral composition reported previously. The considerable MMA content prevents any significant repulsive interaction of the ONP with the substrate that may interfere with the boundary segregation.

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RESULTS AND DISCUSSION In our previous work, perpendicular orientation of lamellaeforming PS-b-PMMA on native silicon substrate was achieved using P(S-r-MMA)-based star ONPs with polyDVB cores.21 The overall molar composition and content of the optimal star ONP were S + DVB:MMA = 59:41 and 20 wt % relative to the BCP, respectively, similar to the previously reported “neutral” composition of S:MMA = 58:42 styrene in substrate-anchored P(S-r-MMA).8 To investigate the versatility of this approach, star ONPs were incorporated into thin films of cylinder-forming PS-bPMMA. The average number of arms for star ONPs synthesized using the arm-first method was calculated to be around 30,35 and the Mn of each arm was fixed at 3 kg/mol. DLS analysis in toluene showed that the final star ONPs had a hydrodynamic diameter of ∼10 nm. Figure 1 shows the SEM images of the cylinder-forming PSb-PMMA thin films incorporated with 20 wt % star ONPs of various PS/PMMA compositions. Prior to SEM analysis, the PMMA domains were etched away via reactive ion etching (RIE) with oxygen and argon gas. The orientation change in

Figure 1. SEM image of cylinder-forming PS-b-PMMA thin films with 20 wt % (a) S8-2, (b) S7-3, (c) S6-4, (d) S3-7, and (e) S2-8 after thermal annealing at 200 °C for 2 days and subsequent removal of PMMA block with RIE. Film thicknesses are ∼80 nm. Scale bars are 200 nm. C

DOI: 10.1021/acs.macromol.7b00865 Macromolecules XXXX, XXX, XXX−XXX

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suggesting that negligible amounts of star ONPs reside within the matrix. The lack of change in lateral ordering contrasts to that of our previous lamellar system, in which ordering defects were clearly visible at 40 wt % of its respective neutral star ONP. We attribute this difference to the more unfavorable accommodation of S6-4 in the discontinuous, highly curved cylindrical interface, which may incur an additional driving force for the ONPs to be expelled from the matrix to the boundary surfaces. SFM phase imaging of thin films with various contents of S64 finally confirmed the segregation phenomenon, as shown in Figure 3. At 5 wt %, patches of cylinders were covered by what

It is worth noting that topographic roughening effects caused by ONP segregation to the boundary surfaces can be considered negligible because these effects occur in periodic or random topographies having the size regime from BCP period to micrometer scale.40,41 Furthermore, the dimensions of the ONPs used in this study would form a surface that is considered “smooth”, especially when we account for the fact that these are polymeric nanoparticles that can deform to accommodate any spatial voids.42 To confirm the perpendicular orientation throughout the film and film thickness, grazing incidence small-angle X-ray scattering (GISAXS) was performed at 0.18°, above the critical angle. A sharp Bragg rod and multiple higher-order peaks corresponding to ordered hexagonally packed perpendicular cylinders were observed (Figure 2), indicating that the

Figure 3. SFM phase images of unetched cylinder-forming PS-bPMMA thin films with (a) 5, (c) 20, and (d) 60 wt % S6-4 at the film/ air interface. (b) SFM phase image of the film with 5 wt % S6-4 at the film/substrate interface. (e, f, g, h) SFM phase images of films with 20 wt % S8-2, S7-3, S3-7, and S2-8 at the film/air interface, respectively. Samples were thermally annealed at 200 °C for 2 days. Film thicknesses are ∼80 nm. Scale bars are 100 nm.

seems to be the agglomerations of free surface-segregated S6-4 (Figure 3a). To observe the segregation of S6-4 to the substrate, films were released from the substrate and recovered upside-down onto a fresh Si substrate for SFM analysis. At 5 wt %, no phase contrast could be discerned at the film bottom, indicating high surface coverage of S6-4 at the substrate interface (Figure 3b). This corresponds well to the perpendicular cylinders observed at the top of the film and shows that full coverage of the free surface is unnecessary for the perpendicular orientation of PS-b-PMMA (as expected, due to the negligibly different surface energies of the two blocks). Above 20 wt %, full surface coverage at the free surface was observed, and no cylinders were visible (Figures 3c and 3d). In contrast, 20 wt % selective star ONPs (Figures 3e−h) show inefficient segregation to the free surface due to the favorable enthalpic interactions with one of the blocks of the BCP matrix. In the case of star ONPs that are only “slightly” off-neutral, S73 and S3-7, there seems to be some segregation to the free surface although this contrasts directly to the full surface coverage seen for S6-4 (Figures 3f and 3g). Here, it is worth noting a conceptually similar work, although an enthalpy-driven one, reported by Zhang et al.19 In that case, it was shown that the low surface energy of the semifluorinated fillers resulted in their segregation to the free surface where they could act as neutral layers. Since this process is driven by an enthalpydictated segregation of the semifluorinated fillers, the basic principle is different from ours, an entropy-dictated segregation caused by architectural asymmetry between the ONP and BCP. Furthermore, because our system is entropy-dictated process, ONPs also segregate to the substrate and therefore substrate neutralization is unnecessary.

Figure 2. GISAXS 2-D images (a, b) and corresponding 1-D profiles (c) of cylinder-forming PS-b-PMMA incorporated with S6-4 after RIE etching. 15 and 60 wt % indicate the weight percent of S6-4 ONP relative to the BCP.

perpendicular orientation persisted throughout the film thickness. Remarkably, this ordering remained unchanged and even improved at superhigh loadings (60 wt %), strongly suggesting that S6-4 imposes a high entropic penalty on BCPs inside the matrix and thus are expelled efficiently to the film boundary surfaces (substrate or free surface), leaving behind little or no S6-4. It should be noted that RIE of the film was performed prior to GISAXS analysis, so any layer of star ONPs on the free surface or substrate was completely removed and thus does not contribute to the scattering image. Further evidence on the boundary surface segregation of S6-4 is given by the 1-D GISAXS profiles, which shows no shift in the 40 nm domain spacing when the S6-4 content is increased from 15 to 60 wt % (domain spacing of 82 kg/mol PS-b-PMMA = 40 nm), strongly D

DOI: 10.1021/acs.macromol.7b00865 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthetic Scheme of (a) 21-Arm Star ONP (21A-ONP) and (b) 6-Arm Star ONP (6A-ONP)a

a Multifunctional initiators where synthesized by esterification of the hydroxyl-terminated precursors and subsequently used in the ATRP of styrene and MMA to yield multiarm ONPs.

Now, due to the fact that the multiple arms of the star ONPs are what create sharp NP-like segregation, it is important to define the boundary between ONPs and non-ONPs in BCP/ ONP nanocomposites, where we define the former as an organic object having a hard particle-like excluded volume. Conversely, non-ONPs such as linear polymers entangle with the BCP matrix and do not have excluded volume in molten state. For star ONPs, the scaling theory by Daoud and Cotton predicts that the radius of excluded volume R, within which the segmental density of an isolated star is constant, scales with the number of arms n as R ∼ n1/2,43 which suggests that the free energy cost Δf for a chain detouring around a star polymer scales as Δf ∼ n.44 To see how the excluded volume size of star ONP affects the BCP morphology, we synthesized neutral 6and 21-arm star ONPs, denoted as 6A-ONPs and 21A-ONPs, respectively (Scheme 1). Figure 4 shows the SEM images of BCP films with 6A-ONPs and with 21A-ONP. In contrast to 30A-ONP, 6A-ONP did not produce any reorientation but in fact compatibilized the matrix. Even at 5 wt %, most of the cylinder morphology appears distorted (Figure 4a), and at 20 wt %, the BCP morphology disappeared completely (Figure 4c). A starkly different behavior was seen for films containing 21A-ONP, in which perpendicular cylinders were produced at 10 wt % (Figure 4d). Such behaviors suggest that 21A-ONP introduce significant excluded volume to the matrix, leading to a large entropic penalty to the BCP chains, whereas 6A-ONP entangle with the BCP chains, presumably due to negligibly small excluded volume effect, thus relieving the any entropic penalty incurred. The importance of arm number of star ONPs is further evidenced in an in situ cleavage of arms from the core in 21A-ONP/BCP nanocomposite thin films during thermal annealing. Thermogravimetric analysis (TGA) of 21A-ONP reveals a small weight loss at around 180 °C which corresponds to a cleavage of arms from the core and subsequent core degradation (Figure 5b).

Figure 4. SEM images of cylinder-forming PS-b-PMMA thin films with (a) 5, (b) 10, and (c) 20 wt % 6A-ONP after thermal annealing at 180 °C for 24 h. (d, e) SEM images of cylinder-forming PS-b-PMMA with 10 wt % 21A-ONP after 24 and 120 h of thermal annealing, respectively. The PMMA block was removed for all samples using RIE. Scale bars are 200 nm for (a, b, d, e), and 800 nm for (c).

TGA of a similarly synthesized ONP in another study supports this result, strongly suggesting that core scission is the reason behind the weight loss.45 Thus, it can be inferred that cleavage of arms from the core occurs during thermal annealing of the nanocomposite thin film. The degree of cleavage was monitored qualitatively by heating 21A-ONP at 200 °C in vacuo and obtaining samples at 24 and 120 h for GPC analysis. As shown in Figure 5a, after 24 h of heating, a significant peak at a higher GPC retention time corresponding to the cleaved P(S-rMMA) arm appeared. Thin films containing 21A-ONPs annealed for the same 24 h (Figure 4d) maintained perpendicular orientation either because sufficient amounts of intact 21A-ONPs were present or because of insufficient time E

DOI: 10.1021/acs.macromol.7b00865 Macromolecules XXXX, XXX, XXX−XXX

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Figure 5. (a) GPC traces of the synthesized 21A-ONP and degradation product after heating in vacuo at 200 °C for 24 and 120 h. (b) TGA of 21AONP showing a ∼4% weight loss around 200 °C corresponding to arm cleavage from the core and core degradation.

Figure 6. (a) Synthetic scheme and (b) GPC trace of linear P(S-r-S-N3-r-MMA) and the collapsed SCNP. A shift to a higher retention time confirms the successful intrachain cross-linking of the linear precursor.

for the back-diffusion of cleaved 21A-ONPs into the BCP matrix. However, after 120 h of annealing, fully parallel cylinders were observed (Figure 4e), and the corresponding GPC trace of 21A-ONP showed a much more significant arm peak. At this point, there is insufficient 21A-ONP at the boundary surface to neutralize the preferential interaction of the PMMA block to the substrate, reverting the orientation back to the more favorable parallel orientation. This transition from perpendicular to parallel cylinders highlights the significance of having a high number of arms that renders a large excluded volume. The importance of the number of arms on star ONPs has thus far been highlighted. At this point, one may wonder if there is an alternative way to introduce significant excluded volume, i.e., if a “hard” ONP without arms can be used. If possible, it may pave a new path to BCP/ONP nanocomposites. To the best of our knowledge, the vast majority of reports on BCP/NP nanocomposites have been based on ligand-stabilized inorganic/organic NPs mainly because of the instability of bare metal NPs especially at high temperatures during thermal treatment. Stable, ligand-less NP behavior in a host matrix has not yet been studied. Here, intrachain crosslinking (single chain collapse) of 30 kg/mol poly(styrene-r-4vinylbenzyl azide-r-methyl methacrylate) P(S-r-S-N3-r-MMA) containing 10 mol % azide functionality was used to create a densely cross-linked SCNP which acts as a hard ONP without the need of arms.

Figure 6b shows the GPC trace of the P(S-r-S-N3-r-MMA) precursor (linear) and the SCNP after collapse under dilute conditions at 250 °C. A shift to a lower apparent molecular weight corresponds to the shrinkage in hydrodynamic volume upon collapse. Figure 7b shows a thin film image of the BCP nanocomposite comprising 10 wt % SCNP. Remarkably, perpendicular orientation was yet again achieved with the SCNP whereas addition of linear 30 kg/mol P(S-r-MMA) did not produce any reorientation (Figure 7a). We postulate that the densely cross-linked polymer network of the SCNP endows rigid conformation and thus prevents significant interpenetration of the BCP chains, forcing them to detour and stretch in order to accommodate the SCNP. This incurs a driving force for the segregation to the boundary surface where the SCNPs act as a neutralizing layer. Furthermore, the rigid nature of the SCNP having severe conformational restrictions may reduce the loss in conformational entropy that the SCNP experiences at the substrate, contributing to the surface segregation. To test the versatility of the system, SCNP was incorporated into lamellae-forming PSb-PMMA. Similarly, linear P(S-r-MMA) did not affect the orientation (Figure 7c) whereas well-ordered perpendicular lamellae were produced at 10 wt % (Figure 7d) SCNP, confirming the ability of the SCNP to segregate to the boundary surface. F

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Future Planning. J.B. also acknowledges the support by a Korea University Grant.

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(1) Bang, J.; Jeong, U.; Ryu, D. Y.; Russell, T. P.; Hawker, C. J. Block copolymer nanolithography: translation of molecular level control to nanoscale patterns. Adv. Mater. 2009, 21, 4769−4792. (2) Phillip, W. A.; O’Neill, B.; Rodwogin, M.; Hillmyer, M. A.; Cussler, E. Self-assembled block copolymer thin films as water filtration membranes. ACS Appl. Mater. Interfaces 2010, 2, 847−853. (3) Kim, M.; Safron, N. S.; Han, E.; Arnold, M. S.; Gopalan, P. Fabrication and characterization of large-area, semiconducting nanoperforated graphene materials. Nano Lett. 2010, 10, 1125−1131. (4) Fan, H. J.; Werner, P.; Zacharias, M. Semiconductor nanowires: from self-organization to patterned growth. Small 2006, 2, 700−717. (5) Hong, A. J.; Liu, C.-C.; Wang, Y.; Kim, J.; Xiu, F.; Ji, S.; Zou, J.; Nealey, P. F.; Wang, K. L. Metal nanodot memory by self-assembled block copolymer lift-off. Nano Lett. 2010, 10, 224−229. (6) Luo, M.; Epps, T. H., III Directed block copolymer thin film selfassembly: emerging trends in nanopattern fabrication. Macromolecules 2013, 46, 7567−7579. (7) Mastroianni, S. E.; Epps, T. H., III Interfacial manipulations: controlling nanoscale assembly in bulk, thin film, and solution block copolymer systems. Langmuir 2013, 29, 3864−3878. (8) Mansky, P.; Liu, Y.; Huang, E.; Russell, T.; Hawker, C. Controlling polymer-surface interactions with random copolymer brushes. Science 1997, 275, 1458−1460. (9) Ryu, D. Y.; Wang, J.-Y.; Lavery, K. A.; Drockenmuller, E.; Satija, S. K.; Hawker, C. J.; Russell, T. P. Surface modification with crosslinked random copolymers: minimum effective thickness. Macromolecules 2007, 40, 4296−4300. (10) Ryu, D. Y.; Shin, K.; Drockenmuller, E.; Hawker, C. J.; Russell, T. P. A generalized approach to the modification of solid surfaces. Science 2005, 308, 236−239. (11) Han, E.; In, I.; Park, S. M.; La, Y. H.; Wang, Y.; Nealey, P. F.; Gopalan, P. Photopatternable imaging layers for controlling block copolymer microdomain orientation. Adv. Mater. 2007, 19, 4448− 4452. (12) Bang, J.; Kim, S. H.; Drockenmuller, E.; Misner, M. J.; Russell, T. P.; Hawker, C. J. Defect-free nanoporous thin films from ABC triblock copolymers. J. Am. Chem. Soc. 2006, 128, 7622−7629. (13) Thurn-Albrecht, T.; DeRouchey, J.; Russell, T.; Jaeger, H. Overcoming interfacial interactions with electric fields. Macromolecules 2000, 33, 3250−3253. (14) Sivaniah, E.; Hayashi, Y.; Iino, M.; Hashimoto, T.; Fukunaga, K. Observation of perpendicular orientation in symmetric diblock copolymer thin films on rough substrates. Macromolecules 2003, 36, 5894−5896. (15) Sarkar, B.; Alexandridis, P. Block copolymer−nanoparticle composites: Structure, functional properties, and processing. Prog. Polym. Sci. 2015, 40, 33−62. (16) Bates, C. M.; Seshimo, T.; Maher, M. J.; Durand, W. J.; Cushen, J. D.; Dean, L. M.; Blachut, G.; Ellison, C. J.; Willson, C. G. Polarityswitching top coats enable orientation of sub−10-nm block copolymer domains. Science 2012, 338, 775−779. (17) Yoo, M.; Kim, S.; Jang, S. G.; Choi, S.-H.; Yang, H.; Kramer, E. J.; Lee, W. B.; Kim, B. J.; Bang, J. Controlling the orientation of block copolymer thin films using thermally-stable gold nanoparticles with tuned surface chemistry. Macromolecules 2011, 44, 9356−9365. (18) Kim, S. H.; Misner, M. J.; Yang, L.; Gang, O.; Ocko, B. M.; Russell, T. P. Salt complexation in block copolymer thin films. Macromolecules 2006, 39, 8473−8479. (19) Zhang, J.; Clark, M. B.; Wu, C.; Li, M.; Trefonas, P., III; Hustad, P. D. Orientation control in thin films of a high-χ block copolymer with a surface active embedded neutral layer. Nano Lett. 2016, 16, 728−735. (20) Kim, S.; Yoo, M.; Kang, N.; Moon, B.; Kim, B. J.; Choi, S.-H.; Kim, J. U.; Bang, J. Nanoporous bicontinuous structures via addition of

Figure 7. SEM images of cylinder- (a, b) and lamellae-forming (c, d) PS-b-PMMA thin films with 5 wt % linear 30 kg/mol P(S-r-MMA) (a, c) and 5 wt % SCNP (b, d). Film thicknesses were matched to 2L0 and subsequently etched with RIE to remove the PMMA block. Scale bars are 200 nm for (a, b) and 500 nm for (c, d).

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CONCLUSION Perpendicular orientation of cylinder-forming PS-b-PMMA was achieved using 21- and ∼30-arm star ONPs as well as the armless SCNP. Each of the ONPs, including the SCNP, induced enough entropic penalty inside the BCP matrix to be expelled efficiently to the boundary surface and neutralize the interfacial energies of the BCP blocks. In contrast to these ONPs, 6-arm star ONPs compatibilized the BCP matrix at high loadings possibly due to the absence of an excluded volume effect. In conclusion, the ONP characteristics appropriate for boundary surface segregation in BCP/ONP nanocomposite thin films, and thus the requirements for perpendicular orientation have been discussed. We believe that this systematic study further elucidates the mechanism behind the reorientation of BCP microdomains and, furthermore, that this approach can be generally applied to other BCPs having different surface energies.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (J.B.). ORCID

Youngson Choe: 0000-0001-9536-113X Joona Bang: 0000-0002-2301-6190 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was by the National Research Foundation of Korea grant funded by the Korean government (MSIP; Nos. 2015R1A2A2A01006008 and 2016M3A7B4910619) and also by the Global Frontier R&D Program (No. 2013M3A6B1078869) on Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT & G

DOI: 10.1021/acs.macromol.7b00865 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules thermally-stable amphiphilic nanoparticles within block copolymer templates. ACS Appl. Mater. Interfaces 2013, 5, 5659−5666. (21) Kim, S.; Yoo, M.; Baettig, J.; Kang, E.-H.; Koo, J.; Choe, Y.; Choi, T.-L.; Khan, A.; Son, J. G.; Bang, J. Perpendicularly oriented block copolymer thin films induced by neutral star copolymer nanoparticles. ACS Macro Lett. 2015, 4, 133−137. (22) Meli, L.; Arceo, A.; Green, P. F. Control of the entropic interactions and phase behavior of athermal nanoparticle/homopolymer thin film mixtures. Soft Matter 2009, 5, 533−537. (23) Xu, C.; Ohno, K.; Ladmiral, V.; Milkie, D. E.; Kikkawa, J. M.; Composto, R. J. Simultaneous block copolymer and magnetic nanoparticle assembly in nanocomposite films. Macromolecules 2009, 42, 1219−1228. (24) Chang, C.-C.; Lo, C.-T. Effect of particles on the structure of solvent-annealed block copolymer/nanoparticle composite thin film. J. Phys. Chem. B 2011, 115, 2485−2493. (25) Foster, M. D.; Greenberg, C. C.; Teale, D. M.; Turner, C. M.; Corona-Galvan, S.; Cloutet, E.; Butler, P. D.; Hammouda, B.; Quirk, R. P. Effective chi and surface segregation in blends of star and linear polystyrene. Macromol. Symp. 2000, 149, 263−268. (26) Wu, D. T.; Fredrickson, G. H. Effect of architecture in the surface segregation of polymer blends. Macromolecules 1996, 29, 7919−7930. (27) Park, S. C.; Jung, H.; Fukukawa, K. i.; Campos, L. M.; Lee, K.; Shin, K.; Hawker, C. J.; Ha, J. S.; Bang, J. Highly ordered nanoporous thin films by blending of PSt-b-PMMA block copolymers and PEO additives as structure directing agents. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 8041−8048. (28) Horechyy, A.; Nandan, B.; Zafeiropoulos, N. E.; Jehnichen, D.; Göbel, M.; Stamm, M.; Pospiech, D. Nanoparticle directed domain orientation in thin films of asymmetric block copolymers. Colloid Polym. Sci. 2014, 292, 2249−2260. (29) Moreno, A. J.; Lo Verso, F.; Arbe, A.; Pomposo, J. A.; Colmenero, J. Concentrated solutions of single-chain nanoparticles: a simple model for intrinsically disordered proteins under crowding conditions. J. Phys. Chem. Lett. 2016, 7, 838−844. (30) Wedler-Jasinski, N.; Lueckerath, T.; Mutlu, H.; Goldmann, A. S.; Walther, A.; Stenzel, M. H.; Barner-Kowollik, C. Dynamic covalent single chain nanoparticles based on hetero Diels−Alder chemistry. Chem. Commun. 2017, 53, 157−160. (31) Sanchez-Sanchez, A.; Arbe, A.; Colmenero, J.; Pomposo, J. A. Metallo-folded single-chain nanoparticles with catalytic selectivity. ACS Macro Lett. 2014, 3, 439−443. (32) Willenbacher, J.; Wuest, K. N.; Mueller, J. O.; Kaupp, M.; Wagenknecht, H.-A.; Barner-Kowollik, C. Photochemical design of functional fluorescent single-chain nanoparticles. ACS Macro Lett. 2014, 3, 574−579. (33) Hanlon, A. M.; Chen, R.; Rodriguez, K. J.; Willis, C.; Dickinson, J. G.; Cashman, M.; Berda, E. B. Scalable synthesis of single-chain nanoparticles under mild conditions. Macromolecules 2017, 50, 2996− 3003. (34) Fischer, T. S.; Schulze-Sünninghausen, D.; Luy, B.; Altintas, O.; Barner-Kowollik, C. Stepwise Unfolding of Single-Chain Nanoparticles by Chemically Triggered Gates. Angew. Chem., Int. Ed. 2016, 55, 11276−11280. (35) Gao, H.; Matyjaszewski, K. Arm-first method as a simple and general method for synthesis of miktoarm star copolymers. J. Am. Chem. Soc. 2007, 129, 11828−11834. (36) Perrier, S.; Takolpuckdee, P.; Westwood, J.; Lewis, D. M. Versatile chain transfer agents for reversible addition fragmentation chain transfer (RAFT) polymerization to synthesize functional polymeric architectures. Macromolecules 2004, 37, 2709−2717. (37) Wei, B.; Gurr, P. A.; Gozen, A. O.; Blencowe, A.; Solomon, D. H.; Qiao, G. G.; Spontak, R. J.; Genzer, J. Autophobicity-driven surface segregation and patterning of core-shell microgel nanoparticles. Nano Lett. 2008, 8, 3010−3016. (38) Lee, J. S.; Lee, N.-H.; Peri, S.; Foster, M. D.; Majkrzak, C. F.; Hu, R.; Wu, D. T. Surface segregation driven by molecular architecture asymmetry in polymer blends. Phys. Rev. Lett. 2014, 113, 225702.

(39) Teng, C.-Y.; Sheng, Y.-J.; Tsao, H.-K. Boundary-induced segregation in nanoscale thin films of athermal polymer blends. Soft Matter 2016, 12, 4603−4610. (40) Sivaniah, E.; Hayashi, Y.; Matsubara, S.; Kiyono, S.; Hashimoto, T.; Fukunaga, K.; Kramer, E.; Mates, T. Symmetric diblock copolymer thin films on rough substrates. Kinetics and structure formation in pure block copolymer thin films. Macromolecules 2005, 38, 1837− 1849. (41) Man, X.; Zhou, P.; Tang, J.; Yan, D.; Andelman, D. Defect-Free Perpendicular Diblock Copolymer Films: The Synergy Effect of Surface Topography and Chemistry. Macromolecules 2016, 49, 8241− 8248. (42) Kim, T.; Wooh, S.; Son, J. G.; Char, K. Orientation control of block copolymer thin films placed on ordered nanoparticle monolayers. Macromolecules 2013, 46, 8144−8151. (43) Daoud, M.; Cotton, J. Star shaped polymers: a model for the conformation and its concentration dependence. J. Phys. (Paris) 1982, 43, 531−538. (44) Huh, J.; Ginzburg, V. V.; Balazs, A. C. Thermodynamic behavior of particle/diblock copolymer mixtures: Simulation and theory. Macromolecules 2000, 33, 8085−8096. (45) Jankova, K.; Bednarek, M.; Hvilsted, S. Star polymers by ATRP of styrene and acrylates employing multifunctional initiators. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 3748−3759.

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DOI: 10.1021/acs.macromol.7b00865 Macromolecules XXXX, XXX, XXX−XXX