Morphological Behavior of A2B Block Copolymers in Thin Films

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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Morphological Behavior of A2B Block Copolymers in Thin Films Hyeyoung Kim,† Beom-Goo Kang,‡ Jaewon Choi,† Zhiwei Sun,† Duk Man Yu,† Jimmy Mays,‡ and Thomas P. Russell*,†,§,∥ †

Department of Polymer Science and Engineering, University of Massachusetts, Amherst, 120 Governors Drive, Amherst, Massachusetts 01003, United States ‡ Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States § Materials Science Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States ∥ Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Bejing 100029, China S Supporting Information *

ABSTRACT: Copolymer composed of two poly(2-vinylpyridine) blocks and one polystyrene block ((P2VP)2−PS) with a PS volume fraction (ΦPS) of 0.44 was synthesized, and their morphologies in bulk and thin films were compared to linear block copolymers (BCPs) having a similar ΦPS (PS-b-P2VP). For the symmetric diblock copolymer, a lamellar morphology was observed after thermal annealing, as was expected. However, (P2VP)2−PS in the bulk showed a hexagonally packed cylindrical microdomain morphology with PS forming the cylindrical microdomains in a P2VP matrix. In thin films, after solvent annealing in the vapors of a chloroform/ethanol mixture, the cylindrical microdomain morphology was retained and the microdomains were oriented parallel to the substrate surface. With the addition of ethanol into the annealing solvent, the morphology of (P2VP)2−PS was transformed from a bicontinuous morphology to one with cylindrical microdomains of PS oriented parallel to the substrate. The lamellar microdomain of the PS-b-P2VP copolymer changed to a morphology composed of PS cylindrical microdomains oriented normal to the substrate. The different orientations of the morphologies arise from the preferential interaction of P2VP with the substrate and the mediation of these interactions by the solvent. The morphological evolution of (P2VP)2-PS thin films was also investigated as a function of SVA time to optimize welldefined microstructures where an orientation of the microdomains parallel to the substrate occurred within 180 s. By use of a faceted sapphire substrate having a sawtoothed topography, highly aligned line patterns were achieved.

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INTRODUCTION Block copolymers (BCPs) having lamellar, spherical, cylindrical, or gyroid morphologies can be used as templates and scaffolds for the fabrication of lithographic masks, nanoporous membranes, and nanophotonics.1−5 In the case of linear BCPs, their morphologies are typically dictated by the Flory− Huggins interaction parameter (χ), the degree of polymerization (N), the volume fraction of the blocks (Φ), and the rigidity of each block.6−10 However, for branched BCPs, such as A2B block copolymers or miktoarm star copolymers, branched copolymers containing more than two types of arms, the configurational constraints arising from the linking of the blocks at one junction point, must also be considered to understand their complex phase behavior. This gives rise to morphologies not seen with linear BCPs.11−14 For branched or miktoarm star copolymers, the effect of the chain configuration on the phase behavior of BCPs in bulk has been studied theoretically15−20 and experimentally.21−27 Milner showed that the phase diagram was shifted to higher volume fraction of the B block for A2B-type copolymers because of its asymmetric conformation.15 Matsen further showed that distinctive structures, such as Fddd (O70) or perforated lamellar, which are not stable for a linear AB diblock copolymer system, © XXXX American Chemical Society

can be obtained by alleviating packing frustration of the chains.20 Hadjichristidis and co-workers experimentally studied the microphase separation of the A2B copolymers composed of two polyisoprene (PI) blocks and one polystyrene (PS) block.26 They found that the (PI)2−PS copolymer with 40 vol % PS showed hexagonally packed PS cylindrical microdomains, whereas the linear BCP with the same volume fractions would be expected to produce lamellar morphologies. Goseki and co-workers investigated the bulk morphology of A2B-type copolymers, where A is PS and B is polyhedral oligomeric silsesquioxane (POSS) containing poly(methacrylate) (PMAPOSS), by changing the volume fractions of the PS block.27 After thermal annealing, it was found that (PS)2−PMAPOSS self-assembled into PMAPOSS cylinder-like microdomains with 40 vol % PMAPOSS, which is generally not observed in the linear PS-b-PMAPOSS system with a similar volume fraction due to the bulky POSS molecule. The differences in the morphologies were argued to arise from the perturbations to the chain configuration due to the architecture Received: December 7, 2017 Revised: January 14, 2018

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DOI: 10.1021/acs.macromol.7b02601 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Schematic Illustration of the Synthesis of (P2VP)2−PS Copolymers

THF and n-hexane, respectively, and stored at −30 °C in ampules equipped with break-seals. P2VP-b-PS (Mn = 70.0 kg/mol for P2VP block and Mn = 48.5 kg/mol for PS block, PDI = 1.13) was purchased from Polymer Source, Inc. and used as received. Synthesis of A2B Branched Copolymer. Scheme 1 illustrates the synthetic process of a (P2VP)2−PS copolymer. The polymerization of S (45.9 mmol) was carried out using s-BuLi (0.11 mmol) in tetrahydrofuran (THF) at −78 °C for 0.5 h in an all-glass apparatus equipped with break-seals under high vacuum. TBSOM-DPE (0.27 mmol) was then added to the living PS solution, and the reaction was continued for 0.5 h. The polymerization solution was terminated with methanol (MeOH) and poured into a large excess of MeOH to obtain ω-functionalized PS (PS-Si2). Then, PS-Si2 (3.75 g, 0.174 mmol for 3tert-butyldimethylsilyloxymethylphenyl (TBSOMP) group) was dissolved in a mixed solvent of CHCl3 (80 mL) and CH3CN (20 mL) under nitrogen. LiBr (0.73 g, 8.35 mmol) and (CH3)3SiCl (1.34 mL, 10.6 mmol) were added to the solution, and the reaction mixture was stirred at 40 °C for 24 h. After quenching the reaction solution with water (5 mL), the solution was washed with water and dried over magnesium sulfate (MgSO4). The synthesized PS with benzyl bromide groups (PS-Br2) was purified by reprecipitation twice from THF into MeOH and freeze-dried from its benzene solution (3.1 g, 83%). The polymerization of 2VP (18.5 mmol) was performed using Ph2CHK (0.059 mmol) in THF at −78 °C for 0.5 h in an all-glass apparatus equipped with break-seals under high vacuum. PS-Br2 (0.016 mmol) was then added to the living P2VP solution at −78 °C, and the reaction mixture was allowed to stand in THF at −40 °C for 24 h. After quenching with MeOH, the mixture was stirred vigorously with a large excess of MeOH for 24 h and filtered to remove P2VP used in excess. By repeating this purification method, (P2VP)2−PS was isolated in 75% yield. The resulting products at each step were characterized by 1H NMR and size exclusion chromatography (SEC) as shown in Figures S1−S3 (Supporting Information). The volume fraction of PS (ΦPS) was calculated as 0.44 from the density of PS (1.04 g cm−3) and that of P2VP (1.14 g cm−3)35 in combination with the molecular weights of the respective blocks. Preparation of A2B Branched Copolymer Thin Films under Solvent Vapor Annealing (SVA). 1.5 wt % solutions of (P2VP)2− PS and P2VP-b-PS in toluene were spin-coated onto plasma-cleaned silicon substrates, where the film thickness was controlled by adjusting the speed of spin-coating between 2000 and 5000 rpm. The initial film thickness on the silicon substrate was confirmed by ellipsometry (model LSE, Gaertner Scientific Corp.). SVA was performed in a sealed jar (volume of the jar = 46.5 cm3, surface area of solvent = 12.6 cm2) at room temperature according to the method described previously.36 During SVA, the swelling ratio (SR), which is defined as

of star-branched polymers forcing a greater interfacial curvature. However, little attention has been given to the morphologies of A2B-type copolymers in thin films.28−33 In this case, it should be noted that interfacial interactions at substrate/polymer and polymer/air interfaces and the commensurability between the domain spacing of the BCP (L0) and the film thickness are important in controlling the orientation and stability of A2B morphologies. In this work, we synthesized an A2B-type copolymer consisting of two poly(2-vinylpyridine) (P2VP) arms and one PS arm ((P2VP)2−PS) and investigated its morphologies both in thin films and in the bulk. These morphologies were compared to those of the corresponding linear, diblock copolymer to gain insight into the influence of chain topology. While the configurational constraints on the polymer chains clearly give rise to changes in the morphology, it also gives rise to a change in the orientation of the microdomains in thin films. Solvent vapor annealing (SVA) was used to develop the morphology in thin films as a function of the ratio of the mixed solvents and annealing time. Using a substrate with a faceted surface topography to guide the self-assembly of the copolymers, long-range alignment of the microdomains was achieved.

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EXPERIMENTAL SECTION

Materials. Styrene (S, Aldrich, ≥99%) was distilled over calcium hydride (CaH2) in vacuo and further distilled from di-n-butylmagnesium on a vacuum line. 2-Vinylpyridine (2VP, Aldrich, 97%) was stirred over potassium hydroxide (KOH). After filtration of KOH, 2VP was distilled over CaH2 on a vacuum line. Chloroform (CHCl3, Aldrich, anhydrous, ≥99%), acetonitrile (CH3CN, Aldrich, anhydrous, 99.8%), and sec-butyllithium (s-BuLi, 1.4 M in cyclohexane, Aldrich) were used as received. Diphenylmethylpotassium (Ph2CHK) was synthesized by the reaction of 1.5 molar excess of diphenylmethane with potassium naphthalenide in dry tetrahydrofuran (THF) at room temperature for 3 days. THF was distilled from its sodium naphthalenide solution on a vacuum line. n-Hexane was distilled from its 1,1-diphenylhexyllithium solution on a vacuum line. Lithium bromide (LiBr, Aldrich, ≥99%) was dried under a high vacuum at 100 °C for 24 h. Chlorotrimethylsilane ((CH3)3SiCl, Aldrich, ≥98%) was distilled from CaH2 under nitrogen. 1,1-Bis(3-tert-butyldimethylsilyloxymethylphenyl)ethylene (TBSOM-DPE) was synthesized according to prior work.34 The monomers and initiator were diluted with B

DOI: 10.1021/acs.macromol.7b02601 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules the swollen film thickness divided by the initial film thickness, was monitored using the reflectometer (F20-UV, FILMETRICS Inc.). Fabrication of Faceted Sapphire Substrates. Faceted sapphire substrates were produced as described previously.37 Specifically, by cutting a single crystal of sapphire along the M-plane, a miscut single crystal surface was obtained. The miscut sapphire was annealed at 1400 °C in air for 24 h, leading to a corrugated or sawtoothed topography with a facet pitch of 84 nm and amplitude of 10 nm over the entire surface of the sapphire. Characterization. 1H NMR was measured on a Varian Mercury 500 instrument using CDCl3. Molecular weight and molecular weight distribution were determined by SEC in THF containing 2% trimethylamine (TEA) at 40 °C with flow rate of 1.0 mL/min using a Polymer Laboratories GPC-120 SEC equipped with a Precision Detector PD2040 (two angle static light scattering), Precision Detector PD2000DLS (dynamic light scattering), Viscotek 220 differential viscometer, and a Polymer Laboratories refractometer. The column set installed in this instrument consists of the following four columns: Polymer Laboratories PLgel; 7.5 × 300 mm; 10 μm; 500, 1 × 104, 1 × 106, and 1 × 107 Å. Bulk samples for small-angle X-ray scattering (SAXS) were thermally annealed at 180 °C under vacuum for 2 days. The SAXS was measured by a Ganesha SAXS-LAB using Cu Kα radiation (λ = 0.154 nm). The sample-to-detector distance and beam area were 1041 mm and 0.04 mm2, respectively. For cross-sectional transmission electron microscopy (TEM) measurement of thin films, gold was sputter-coated on top of the films after thermal annealing. Subsequently, it was put on an epoxy mold and then cured for 12 h at 60 °C. This sample was microtomed into 50 nm thick sections using a diamond knife at room temperature (Leica Ultracut Microtome). Selective staining of P2VP domains was accomplished by exposure to iodine vapor for 2−4 h. TEM measurements were conducted using a JEOL 2000FX TEM operated at an accelerating voltage of 200 kV. Grazing incidence small-angle X-ray scattering (GISAXS) was measured at the Advanced Light Source (ALS) Beamline 7.3.3 at the Lawrence Berkeley National Laboratory.38 The energy of the incident beam was 10 keV corresponding to a wavelength of 0.124 nm. The angle of incidence was 0.14°. The surface morphologies of (P2VP)2−PS thin films were investigated by scanning force microscopy (SFM, Dimension 3100, Digital Instruments) operated in the tapping mode. The SFM tip has a pyramidal shape with a nominal radius of curvature of ∼6 nm at the tip apex.

(P2VP)2−PS copolymer. The SEC curve of the product obtained from the linking reaction showed two distinguishable peaks corresponding the (P2VP)2−PS and the P2VP used in excess, showing that the reaction was performed without any undesirable side reactions (Figure S1d). The unreacted P2VP was removed by vigorously stirring the (P2VP)2−PS and the P2VP mixture with a large excess of MeOH and subsequent filtering to isolate (P2VP)2−PS, which shows a single SEC profile with narrow Mw/Mn (Figure S1e). Moreover, the 1H NMR of (P2VP)2−PS indicates that all benzyl bromide functionalities took part in the linking reaction with living P2VP (Figure S3). Consequently, (P2VP)2−PS copolymers with controlled chain structure were successfully synthesized under the reaction conditions employed here. The morphology of (P2VP)2−PS copolymer in the bulk state after thermal annealing was investigated using small-angle X-ray scattering (SAXS), as shown in Figure 1a. Multiple reflections

Figure 1. Bulk morphology of (P2VP)2−PS copolymer. (a) SAXS intensity profile. (b) TEM image after annealing at 180 °C for 2 days. The scale bar in the image is 100 nm.

are observed at scattering vector (q) ratios of 1:√3:√4:√7:√9:√12, which is the characteristic of hexagonally packed cylindrical microdomains with L0 = 2π/q* of 38.0 nm, where q* is the first-order reflection. In Figure 1b, the TEM image of (P2VP)2−PS also shows PS cylindrical microdomains with the axis of cylinders being parallel and normal to the plane of the TEM image, where P2VP microdomains correspond to darker areas due to the selective staining of P2VP by iodine. From the TEM image, it is evident that (P2VP)2−PS copolymer with a nearly symmetric volume fraction of components (ΦPS = 0.44) showed hexagonally packed cylindrical microdomains of PS. However, it should be noted that for linear PS-b-P2VP with almost the same composition, the self-assembled microdomains showed a lamellar morphology.39 This can be attributed to the difference in the constraints place on the configurations of the blocks for the A2B and AB BCPs, as argued theoretically15−20 and shown experimentally.21−27 As shown in Figure 2, the morphology map of A2B copolymers in the strong-segregation regime is presented as a function of composition and the asymmetry parameter, ε = (nA/nB)(lA/lB)1/2, where ni is the number of arms and li = Vi/ Ri2; Vi is the molecular volume and Ri is the radius of gyration of the respective chains.15 It should be noted that the ordinate in the phase map for AB BCPs is χN. The asymmetry parameter represents the effects, not only of the interactions between each block but also the chain architecture. The Ri of an unperturbed chain is calculated by Ri = b(N/6)1/2, where b is the average statistical segment length. The value of b is 0.68 nm for both the PS and P2VP blocks.40 Consequently, ε for the (P2VP)2−PS copolymer used in this study was calculated to be

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RESULTS AND DISCUSSION Scheme 1 shows the synthetic strategy used to prepare welldefined (P2VP)2−PS copolymers. First, PS-Si2 was synthesized by a living anionic polymerization of S initiated with s-BuLi, followed by the reaction with TBSOM-DPE. The treatment of PS-Si2 with a 1:1 mixture of LiBr and (CH3)3SiCl was then used to obtain PS-Br2. The synthetic results of PS-Si2 and PSBr2 are summarized in Table S1. The Mn values of both polymers, determined by SEC, agreed well with those calculated. The SEC curves are narrow (Mw/Mn = 1.04 for both polymers) and almost identical in shape and eluent time (Figure S1a,b). The transformation of TBSOMP groups into benzyl bromide groups was confirmed by 1H NMR analysis. As shown in Figure S2, after the transformation reaction, the peaks for methyl protons of the tert-butyldimethylsilyloxy groups (0.91 and 0.05 ppm) and the benzyl methylene protons (4.61 ppm) disappeared, while the peak corresponding to methylene protons of the benzyl bromide groups appeared at 4.36 ppm. In addition, the peaks for PS were maintained even after the transformation reaction, suggesting good stability of PS under such transformation reaction conditions. These results indicate that PS-Br2 was precisely synthesized. The reaction of benzyl bromide groups of PS-Br2 with excess of the living P2VP was carried out at −40 °C for 24 h to synthesize well-defined C

DOI: 10.1021/acs.macromol.7b02601 Macromolecules XXXX, XXX, XXX−XXX

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polymer/air interface, due to the lower surface energy of PS. As in the bulk, the cylindrical microdomain of (P2VP)2−PS can be attributed to the configurational constraints of the PS and P2VP chains that are joined at one point blocks and the imposed interface curvature, as argued theoretically.15−20 The 2-dimensional GISAXS profile from a thin film of (P2VP)2−PS (Figure S4) showed two strong reflections with the ratio of 1:2, indicating a hexagonal packing of microdomains with the cylinders {10} plane oriented parallel to the substrate.41,42 An underlying isotropic ring of scattering indicates that a random orientation of grains of the microdomains in the plane of the film,41,43 corresponding to the TEM results. To compare the evolution of the surface morphology of (P2VP)2−PS with that of P2VP-b-PS by SVA, films were prepared with thicknesses of L0 ((P2VP)2−PS: 38.0 nm; P2VPb-PS: 65.0 nm). Figures 4a and 4g show SFM phase images of

Figure 2. Phase diagram of A2B copolymers calculated by Milner.15 The marked point with the black star represents (P2VP)2−PS copolymer with ε = 1.91 and ΦPS = 0.44 used in this study. ΦB (PS) is the volume fraction of the PS block, and ε is the asymmetric parameter. sphB = B spheres, cylB = B cylinders, bicB = B bicontinuous, lam = lamellar, bicA = A bicontinuous, cylA = A cylinders, and sphA = A spheres.

1.91. Thus, theoretically, (P2VP)2−PS copolymers would be expected to self-assemble into hexagonally packed cylindrical microdomains of the PS, at ΦPS = 0.44 and b = 0.68, as indicated by the black star in Figure 2. This theoretical prediction is consistent with the morphology of (P2VP)2−PS copolymers observed by SAXS (Figure 1a) and TEM (Figure 1b) in the current study. In thin films, the morphology of (P2VP)2−PS (ΦPS = 0.44) was compared to the morphology of linear PS-b-P2VP containing the similar volume ratio of the two blocks (ΦPS = 0.43). The TEM images in Figure 3 show the cross-sectional

Figure 3. TEM image of thin films from (P2VP)2-PS and P2VP-b-PS copolymer. Cross-sectional morphologies of (a) (P2VP)2-PS branched copolymer film, (b) P2VP-b-PS diblock copolymer film after annealing at 180 °C for 2 days. Scale bar inset, 100 nm. Figure 4. SFM images (phase mode) for thin films of (a−f) (P2VP)2− PS copolymer and (g−l) P2VP-b-PS diblock copolymer: (a, g) asspun, (b, h) pure chloroform (C), (c, i) chloroform/ethanol (90/10), (d, j) chloroform/ethanol (70/30), (e, k) chloroform/ethanol (50/ 50), and (f, l) pure ethanol (E). All solvent compositions are expressed in volume percentage (v/v), and the size of images is 2 × 2 μm.

morphologies of the films annealed at 180 °C for 2 days. The dark line on top of the film is a layer of sputtered gold to avoid the penetration of the epoxy into the copolymer film and to mark the polymer/air interface. A connected PS cylindrical microdomain morphology oriented parallel to the substrate was obtained from (P2VP)2−PS, arising from the different orientations of the cylindrical microdomains in the plane of the film (Figure 3a). PS-b-P2VP showed a lamellar microdomain morphology oriented parallel to the substrate, as would be expected (Figure 3b). For both the (P2VP)2−PS and the PS-b-P2VP, a thin wetting layer of P2VP (dark layer) at the substrate was observed due to the preferential interaction of P2VP with the SiO2 substrate and a PS layer was located at the

the as-spun (P2VP) 2 −PS and P2VP-b-PS thin films, respectively. For the (P2VP)2−PS, since the toluene is a poor solvent for P2VP and a good solvent for PS, the as-spun films were expected to show the formation of a micellar structure, with P2VP cores and PS corona.44 However, the film showed no distinct features, indicating that either micelles were not formed. P2VP-b-PS (Figure 4g), on the other hand, showed D

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come into contact with the substrate more readily and interact with and anchor to the substrate.47 This, coupled with the strong immiscibility between the PS and P2VP blocks, results in a lateral microphase separation. Thus, the surface area covered by the P2VP blocks for two P2VP arms is larger for (P2VP)2− PS than P2VP-b-PS, even though the volume of P2VP blocks in (P2VP)2−PS is essentially identical to that in P2VP-b-PS as illustrated in Figure 5.

circular, presumably spherical domains, characteristic of the formation of micelles. This difference can be explained by theoretical arguments where the ordered regime of A2B with a selective solvent for the B is very narrow.19 When the two A arms are forced to reside on the concave side of the interface, the laterally crowded arms are more stretched and form looser and less ordered microstructures. Consequently, it is far more difficult for the multiarmed (A2B) copolymers to form spherical or micellar structure under poor solvent conditions for P2VP (the A block). For SVA, mixtures of chloroform and ethanol having different compositions were used. Chloroform is a good solvent for both blocks,45 while ethanol is a good solvent for the P2VP block and a nonsolvent for PS.46 In the SFM images, the brighter and darker regions correspond to P2VP and PS microdomains, respectively. After SVA with pure chloroform, (P2VP)2−PS showed complex morphologies consisting of cylindrical and spherical structures (Figure 4b). In the case of P2VP-b-PS under the same SVA condition, lamellar microdomains formed (Figure 4h). By adding 10 vol % of ethanol to the chloroform, the complex morphologies observed in (P2VP)2−PS were transformed into cylindrical microdomains oriented parallel to the film surface (Figure 4c). This arises from the selective swelling of the P2VP with ethanol, resulting in an effective volume fraction of the swollen P2VP block that can induce cylindrical microdomains. This suggests that the morphology of solvent-vapor annealed (P2VP)2−PS in pure chloroform, a nonselective solvent, be bicontinuous or at the phase boundary between lamellar and cylindrical phases. Under SVA with a 90/10 chloroform/ethanol mixture, the correlation length of lamellar microdomains in P2VP-b-PS decreased (Figure 4i), relative to that achieved under SVA with pure chloroform, due to the imbalance in the volume fractions of the swollen domains. With increasing ethanol content, the morphologies of the (P2VP)2−PS film changed from long to shorter cylindrical structures (Figure 4d), so that a mixture of short cylindrical and spherical structures was observed at 50 vol % of ethanol (Figure 4e). The morphologies of P2VP-b-PS were converted from lamellar microdomains oriented normal to the surface to cylindrical microdomains oriented normal to the film surface or even spherical domains by increasing the amount of ethanol to 50 vol % (Figure 4j,k). For the pure ethanol case, the (P2VP)2−PS film showed wormlike structures due to the selective swelling of P2VP microdomains (Figure 4f). In the same system of P2VP-b-PS, small circular domains are obtained by decreasing the size of the PS cylinders as a result of the selective swelling of P2VP microdomains (Figure 4l). These results indicate that the chain architecture of the A2B shifts the morphology map to a higher volume fraction of the single chain B. The lateral crowding of the two P2VP chains causes an increased chain stretching, promoting interface curvature providing more volume for the P2VP chains. We turn now to the orientation of the microdomains with respect to the substrate surface. The P2VP-b-PS films show an orientation of the microdomain normal to the film surface, while a parallel orientation is found for the (P2VP)2−PS films. This may arise from the morphologies resulting from the spincoating process prior to SVA. For the as-spun films, the micellar structures were not found at (P2VP)2−PS in contrast to P2VPb-PS. This suggests that the P2VP blocks in P2VP-b-PS do not anchor to the substrate as effectively as those in (P2VP)2−PS due to micelle formation and the inability of the P2VP chains to interact with the substrate. However, for the (P2VP)2−PS, the absence of any ordered structure allows the P2VP chains to

Figure 5. Schematic illustration of (a) (P2VP)2−PS and (b) P2VP-bPS copolymer in thin film. The component of two arms (P2VP, blue) in branched copolymer has larger area than in linear diblock copolymer.

Figure 6 shows the time evolution of morphologies of the (P2VP)2−PS thin film exposed to a saturated vapor mixture of chloroform/ethanol (90/10 vol %). The initial thickness of the film was L0 (38.0 nm). As seen in Figures 6a−c, during the first 120 s of exposure to the mixed solvent vapor (SR increases from 1 to 2.1), a relatively disordered initial morphology gradually develops into a morphology where cylindrical microdomains of swollen PS oriented parallel to the substrate, where the more polar and chloroform-swollen P2VP forms the corona that preferentially interacts with the substrate. Subsequently, at 150−180 s (SR = 2.3−2.4), the formation of the cylindrical microdomains oriented parallel to the substrate was complete. With increased swelling time (240 s), the swelling ratio increases to 2.7, and the morphology was found to become more disordered. This stands counter to logic, where one would expect the ordering to be continually improved. However, given the finite size of the substrate, the continuous change in the repeat period of the copolymer, and the continuous change in the films thickness, it is highly likely that the lateral constraints on the film force a reorganization of the swollen (P2VP)2−PS normal to the film surface which is coupled with the commensurability between the swollen domains and the film thickness, resulting in a more disordered morphology. It is important to control the lateral ordering and orientation of BCP microdomains for further applications, such as semiconductor and data storage devices.2 The directed selfassembly (DSA) of (P2VP)2−PS films was investigated using sapphire faceted substrates, where the sawtooth topography has the pitch of 84 nm and the amplitude of 10 nm (Figure 7a). As shown in Figure 7b, the SVA of L0 thick (P2VP)2−PS films with chloroform/ethanol (90/10) resulted in highly aligned line patterns of the microdomains oriented parallel to the ridge direction of the faceted substrate over macroscopic length scales. The guiding mechanism is attributed to the incommensurability between the pitch and L0 and the triangular geometry of the facet that forces the line patterns of (P2VP)2− PS microdomains to orient parallel to the ridge direction.48

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CONCLUSION We synthesized a (P2VP)2−PS copolymer and studied its morphologies in thin films. The curved interface arising from E

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Figure 6. SFM images (phase mode) of (P2VP)2−PS exposed to chloroform/ethanol (90/10) solvent vapor for various times: (a) 60, (b) 90, (c) 120, (d) 150, (e) 180, and (f) 240 s. The size of all images is 2 × 2 μm.

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AUTHOR INFORMATION

Corresponding Author

*(T.P.R.) E-mail [email protected], Tel (413) 5771516, Fax (413) 577-1510. ORCID

Zhiwei Sun: 0000-0002-8932-3716 Thomas P. Russell: 0000-0001-6384-5826 Author Contributions

H.K. and B.-G.K. contributed equally to this work. Notes

The authors declare no competing financial interest.

Figure 7. (a) SFM image (height mode) of the faceted sapphire substrate. (b) SFM image (phase mode) of (P2VP)2−PS copolymer thin film on the faceted substrate. Inset in (b) is the magnified image of cylindrical microdomains. Scale bar in inset: 200 nm. The size of each image is 4 × 4 μm.

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ACKNOWLEDGMENTS This research was supported by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy. H.K., D.M.Y., and T.P.R. were supported by the Air Force Offices of Scientific Research under Contract 16RT1602 and the Army Research Office under Contract W911NF-17-1-0003. J.C. acknowledges Samsung Scholarship from the Samsung Foundation for financial support. This research used resources of the Advanced Light Source, which is a DOE Office of Science User Facility under Contract DE-AC02-05CH11231.

the lateral crowding of two arms induces a shift in the morphology map toward the minor component relative to the equilibrium phase behavior. The morphology of (P2VP)2−PS in thin film showed mixed orientations of PS cylinders rather than a lamellar microdomain morphology, as observed in a linear diblock BCP. With SVA, a parallel orientation of the microdomains was observed for the (P2VP)2−PS, whereas PSb-P2VP showed a perpendicular orientation of the microdomains. This parallel orientation of the microdomains results from the very strong preferential adsorption of the P2VP chains to the substrate. We also have achieved highly aligned line patterns of (P2VP)2−PS microdomains with lateral order using sapphire faceted substrates, which could be potentially used for the fabrication of metallic line patterns when metal salts are loaded in P2VP blocks.

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REFERENCES

(1) Park, M.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Adamson, D. H. Block Copolymer Lithography: Periodic Arrays of ∼ 1011 Holes in 1 Square Centimeter. Science 1997, 276, 1401−1404. (2) Kim, H.-C.; Park, S.-M.; Hinsberg, W. D. Block Copolymer Based Nanostructures: Materials, Processes, and Applications to Electronics. Chem. Rev. 2010, 110, 146−177. (3) Koo, K.; Ahn, H.; Kim, S.-W.; Ryu, D. Y.; Russell, T. P. Directed Self-Assembly of Block Copolymers in the Extreme: Guiding Microdomains from the Small to the Large. Soft Matter 2013, 9, 9059−9071. (4) Bates, C. M.; Maher, M. J.; Janes, D. W.; Ellison, C. J.; Willson, C. G. Block Copolymer Lithography. Macromolecules 2014, 47, 2−12.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02601. SEC, 1H NMR curves, and GISAXS data (PDF) F

DOI: 10.1021/acs.macromol.7b02601 Macromolecules XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.macromol.7b02601 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.7b02601 Macromolecules XXXX, XXX, XXX−XXX