Arrangement of Lamellar Microdomains of Block Copolymer Confined

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Arrangement of Lamellar Microdomains of Block Copolymer Confined in Hemispherical Cavities Having Two Controlled Interfaces Dagam Lee,† Myung-Hyun Kim,‡ Dusik Bae,† Gumhye Jeon,† Mooseong Kim,† Jongheon Kwak,† So Jung Park,‡ Jaeup U. Kim,*,‡ and Jin Kon Kim*,† †

National Creative Research Initiative Center for Smart Block Copolymers, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, Kyungbuk 790-784, Republic of Korea ‡ School of Mechanical and Advanced Materials Engineering, Ulsan National Institute of Science and Technology, Ulsan 689-798, Republic of Korea S Supporting Information *

ABSTRACT: We studied the arrangement of lamellar microdomains of polystyrene-block-poly(methyl methacrylate) copolymer (PS-b-PMMA) confined in hemispherical cavities prepared by anodic aluminum oxide template. The cavities have two controlled interfaces. One is the cavity wall grafted with three different brushes (two selective brushes made of PMMA-OH and PS-OH and one neutral brush made of PSran-PMMA-OH). The other is the top surface of the cavity covered with films of PMMA and PS homopolymers and PSran-PMMA copolymer. When the cover film was selective to one of block components, parallel lamellae were formed at the top of the cavity. On the other hand, the lamellar microdomain arrangement at the middle and bottom parts of the cavity highly depended on the nature of brushes on the cavity wall: concentric (onion-like) lamellae for a selective brush and unspecific complex structure for a neutral brush. Once parallel lamellae and concentric lamellae meet together, isolated spheres consisting of one block are formed. When the cavity wall is modified by the neutral brush and the cover layer is selective to one of the blocks, stacked lamellae are observed. However, when the cover film is neutral to both block chains, concentric lamellae or bicontinuous lamellae are formed, depending on the nature of the brushes on the cavity wall. We also performed numerical mean field calculations based on the well-established self-consistent field theory, from which we were able to reproduce the experimental results.

1. INTRODUCTION Block copolymers (BCPs) are composed of two or more chemically distinct homopolymer blocks linked covalently. Owing to the thermodynamic incompatibility and chain connectivity between the blocks, BCPs can self-assemble into various microdomains such as lamellae, gyroids, cylinders, and spheres. In general, the microdomain structures are mainly controlled by the volume fraction f of one block, the degree of polymerization N, and the Flory−Huggins interaction parameter χ.1−5 The microdomains between constituent blocks are separated in size of 10−100 nm. The self-assembled nanostructures can be used as etching mask for nanolithography, fabrication of nanoporous membranes, electronic memory devices, optical materials, templates to energy-harvest metallic or inorganic materials, photonic crystals, and carriers of drug delivery.6−14 Confinement of the BCPs under certain geometries can offer new methods to develop unique morphologies that cannot be obtained in bulk or thin film without confinement.15−18 Confinement geometry can be classified into 1-,19−21 2-,22−25 and 3-dimensions.26−32 The most important parameter to © 2014 American Chemical Society

control lamellar morphology in the confinement is the commensurability between the lamellar domain spacing in bulk L0 and the specific distance D of 1-D confinement. Additionally, the curvature of the cylindrical or spherical geometry influences the lamellar morphology for 2-D and 3-D confinements. Also, affinity between surface of confining geometry and block copolymer is important to control the morphologies. To achieve novel and complex morphologies, various affinities should be introduced on a given confining geometry. However, only one affinity was applied in previous literatures. Recently, we investigated the arrangement of lamellae of polystyrene-block-poly(methyl methacrylate) copolymer (PS-bPMMA) confined in hemispherical cavities.33 The inner wall of the cavity was modified by three different polymer brushes (PSOH, PMMA-OH, and PS-ran-PMMA-OH), but the top surface of the cavity directly contacted air. When the inner surface has Received: April 14, 2014 Revised: May 22, 2014 Published: June 3, 2014 3997

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Table 1. Molecular Characteristics of Polymers Employed in This Study polymer

lot. no (supplier)

Mn (×103 g mol−1)

Mw/Mn

L0a (nm)

f PSb

SML-98 SML-320 PS-OH PMMA-OH PS-ran-PMMA-OH PS PMMA PS-ran-PMMA

P2355 (Polymer Source) P5543 (Polymer Source) P9022 (Polymer Source) P1719 (Polymer Source) P6469A (Polymer Source) P10672 (Polymer Source) 182265 (Aldrich Chemical) P9128F (Polymer Source)

98 320 21 35 7 980 996 68

1.13 1.09 1.06 1.06 1.48 1.16 1.5 1.6

47.3 100.0

0.54 0.53

0.57

0.54

a

L0 is the lamellar domain spacing in bulk measured by small-angle X-ray scattering (see Figure S1 in Supporting Information). bf PS is the volume fraction of PS block.

Figure 1. Scheme for sample preparation. (a) Hemispherical AAO template prepared by two-step anodization and pore widening process. (b) Grafting of three polymer chains (PMMA-OH, PS-OH, PS-ran-PMMA-OH) on cavity wall. (c) PS-b-PMMAs are spin-coated on AAO template. (d, e) Placing three different polymer films (PMMA, PS, PS-ran-PMMA) as the cover layer on the top of the cavity: (d) separation of each polymer film from silicon wafer by using HF and (e) transfer onto AAO template containing PS-b-PMMA.

strong selectivity to one of the blocks, concentric (onion-like) lamellae replicating the boundary of spherical cavity are formed, while complex layered structures are observed for neutral brush to PS and PMMA chains. Also, the degree of commensurability did not affect much the arrangement of the lamellae compared with the surface selectivity at the cavity wall. A hemispherical cavity has another surface, namely, the top of the cavity contacting air. The selectivity at the top surface of the cavity is easily controlled by placing a cover layer of various polymer films on the cavity. The interface between the cover layer and the block copolymer chains located at the top of the cavity is flat, while a curved interface exists between the grafted layer and block copolymer chains at the cavity wall. When these two interfaces are controlled independently, the alignment of lamellar microdomains can be significantly changed, and thus new kinds of morphology are expected. To achieve this objective, the hemispherical cavities were prepared by anodic aluminum oxide (AAO) template, and the inner wall of the AAO was modified by three kinds of polymer brushes (PS-OH, PMMA-OH, and PS-ran-PMMA-OH). The affinity at the top surface was also modified by covering three different polymer films (PS, PMMA, PS-ran-PMMA) on the top of the cavity inside which PS-b-PMMAs were located. When the cover film and cavity wall were selective to one of block components, parallel lamellae and concentric lamellae met together and isolated spheres consisting of one block were formed. On the other hand, stacked lamellae are observed when

the cavity walls are modified by the neutral brush and the cover layer is selective to one of the blocks. Stacked lamellae in hemispherical cavity have not been reported in the literature. This is possible because two interfaces are independently controlled. Also, concentric lamellae or bicontinuous lamellae were also formed, depending on the nature of the brushes on the cavity wall, when the cover film was neutral to both block chains. Finally, the experimental results were compared with the simulations based on self-consistent field theory (SCFT).34,35 This novel morphology found in this study could be used as surface-enhanced Raman scattering (SERS), Fano resonance, and anisotropic optical properties. Symmetry breaking is essential to obtain Fano resonances because the plasmon hybridization resulting from multiple dipole moments is observed for asymmetric nanostructured materials.36,37 The hemispherical geometry has both flat and curved surfaces, providing broken symmetry to generate plasmon hybridization, which could not be obtained by fully spherical shaped geometry.

2. EXPERIMENT AND SIMULATION Materials and Sample Preparation. The molecular characteristics of two different symmetric PS-b-PMMAs (SML-98 and SML320), hydroxyl end-functionalized PS, PMMA, and PS-ran-PMMA (SMMA-1), and two homopolymers of PS and PMMA and PS-ranPMMA (SMMA-2) are given in Table 1. For the random brush at the inner cavity, we used the lower molecular weight SMMA-1, while the 3998

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block copolymer and a homopolymer, 50 mesh points were used in the polymer length direction. The Neumann boundary condition was used for all boundaries to correctly represent the polymer/air interface, while minimizing the unit cell size.

higher molecular weight SMMA-2 was employed for the cover layer to provide neutral condition at the top of the cavity. To prevent possible diffusion of homopolymer chains in the cover layer to the block copolymer chains inside the cavity, we used very high molecular weights of PS (980 000) and PMMA (996 000). The hemispherical cavities were prepared according to our previous paper.33 The height and diameter at the top of the hemispherical cavity were 220 and 400 nm, respectively (Figure S2 in Supporting Information). The cavity walls were modified by PS-OH, PMMAOH, and PS-ran-PMMA-OH solutions by spin-coating and annealing at 180 °C under vacuum for 5 days. Nongrafted polymer chains were completely removed by rinsing with toluene. Then, block copolymers in toluene solution (3.8 wt % for SML-98 and 2.8 wt % for SML-320) were spin-coated. Finally, PMMA and PS films with thickness of 200 nm and PS-ran-PMMA film with thickness of 1 μm on silicon wafers were transferred onto the top of the cavities and annealed at 170 °C for 5 days in vacuum, followed by quenching to room temperature, as schematically shown in Figure 1. Characterization. The cross-sectional image of the block copolymer inside the hemispherical cavity was investigated by transmission electron microscope (TEM: S-7600: Hitachi Ltd.) at an accelerated voltage of 80 kV. To prepare TEM sample, epoxy resin was dropped on the cavities containing block copolymers and heated to 60 °C for 12 h for curing. The aluminum layer below the AAO template was removed by using CuCl2 solution, and the AAO template was removed by using 0.5 M sodium hydroxide for 90 min, followed by embedding epoxy resin again. Ultrathin sections were prepared by using a Leica Ultracut Microtome equipped with diamond knife.38 All samples were stained with ruthenium tetraoxide (RuO4), a selective staining agent for PS.39 Theory and Numerical Implementation. The alignment of lamellar microdomains of AB block copolymer with a volume fraction of A block having 0.5 (A and B corresponds to PMMA and PS, respectively) was calculated by the SCFT method with some modifications given below. AB block copolymer is modeled as flexible Gaussian chains composed of N segments with statistical segment length a. The Flory−Huggins interaction parameter χij represents the interaction strength between i and j segments. In this study, we set χABN = 35 to represent well-segregated lamellar microdomains in a hemispherical cavity with a size D = 4.0L0. When these parameters are used, the domain spacing (L0) of lamellar phase in the bulk is given by L0 = 1.95aN1/2. Throughout this paper, we used this theoretically calculated period (100 nm) for SML-320. To describe the preference of the top surface layer to each block component, we introduced two new parameters, ηA and ηB. The top surface property in the theoretical model is controlled by imposing delta function-like surface interaction, and the two parameters, ηA and ηB, represent the delta function strength. The sign of the parameters is chosen such that if ηA is positive, there exists favorable interaction of the cover film to A component. To exclude the BCP chains from the outside of the cavity, C homopolymer with the same segment number N and statistical segment length a is introduced.40 Outside of the cavity, C homopolymer is fixed by wexternal(r⃗) in the following self-consistency equations:

3. RESULTS AND DISCUSSION Figure 2 gives cross-sectional TEM images of SML-320 (left panels) and SML-98 (right panels) confined inside hemisphere

Figure 2. Cross-sectional TEM images of SML-320 (left panel) and SML-98 (right panel) confined at hemispherical cavities with PMMAselective top cover and three different brush layers: (a, b) PMMA-OH brush, (c, d) PS-OH brush, and (e, f) PS-ran-PMMA-OH brush. The scale bar in all images is 100 nm. A schematic of the cross-sectional images is shown in the inset of each figure. Red and blue colors represent PMMA and PS microdomains, respectively.

wA( r )⃗ = − 2ηA δ(z)aN1/2 + χAB NϕB( r )⃗ + χAC NϕC( r ⃗) + ξ( r ⃗)

cavities with PMMA cover layer and three different brushes on the cavity wall. A schematic of the cross-sectional images is shown in the inset of each figure. Since the radius of the cavity was 200 nm, R/L0 for SML-320 and SML-98 are 2 and 4.25, respectively. The PS microdomains seem to be bigger than the PMMA microdomains because beam irradiation during TEM damaged more PMMA chains than PS chains which were stained by RuO4.43,44 For the PMMA−OH brush, the combined morphology of parallel lamellae and concentric lamellae was obtained, as shown in Figure 2a,b. Parallel lamellae seen near the top cover result from the top layer of PMMA film. On the other hand, the

wB( r )⃗ = − 2ηBδ(z)aN1/2 + χAB NϕA ( r )⃗ + χBC NϕC( r ⃗) + ξ( r ⃗) wC( r )⃗ = χAC NϕA ( r )⃗ + χBC NϕB( r )⃗ + ξ( r ⃗) + w external( r ⃗) where ϕi(r)⃗ is the segment density of i type polymers, ξ(r)⃗ is the pressure field, and wi(r⃗) is the field acting on them. As mentioned above, the preference of the top surface to each type of material was assigned by setting either ηA or ηB parameter as 0.4. The simulation box consisted of a 3-dimensional 64 × 64 × 64 grid, and the modified diffusion equations for the partition functions of block copolymers and homopolymers were solved using the SCFT with pseudospectral method modified for this research.41,42 For both 3999

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was observed, except that the top layer of the cavity was now PS lamellae (see Figure S4 in the Supporting Information). Figure 3 gives cross-sectional TEM images of SML-320 (left panels) and SML-98 (right panels) confined inside hemi-

concentric lamellae are formed by the selectivity of hemispherical wall to block copolymer chains. For R/L0 = 2 (Figure 2a), parallel lamellae and concentric lamellae are merged into one PS layer. Interestingly, an isolated PMMA sphere was clearly seen inside the PS layer, while one layer of concentric PMMA and flat PMMA lamellae was formed due to the selectivity of both the wall and the top of the cavity. At R/L0 = 4.25 (Figure 2b), the alignment of lamellae is similar to that of Figure 2a, except that more PS and PMMA lamellar layers are formed. For both R/L0 = 2 and 4.25, when parallel and concentric lamellae met together, PMMA chains in concentric lamellar layer easily merged to those in the parallel lamellar layer without chain perturbation. On the other hand, because PS chains in the concentric lamellar layer do not like PMMA chains, PS microdomain became spherical to reduce the unfavorable contact with PMMA chains. For the PS-OH brush, the lamellae inside the cavity are formed by two different types of lamellae: parallel lamellae from the top and concentric lamellae from the cavity wall (Figure 2c,d). Comparing the results in Figures 2a and 2b, the PS layer starts from the cavity wall. Thus, this concentric lamellar layer should be combined with parallel lamellar layer resulting from the top for R/L0 = 2 (Figure 2c). Thus, one PS sphere which is surrounded by another PMMA layer is observed. To understand the formation of interesting morphology such as isolated spheres inside the concentric lamellae, we consider the junction regions where parallel lamellae and concentric lamellae meet,45 as schematically shown in Figure S3 of the Supporting Information. When there is no top layer, the innermost PS chains (blue color) can be formed as hemispherical shape. However, the confinement under the top layer forced to form parallel lamellae. In this situation, unfavorable contact between PS and PMMA should occur near the junction. To minimize the interfacial tension, hemispherical PS should be transformed into spherical shape. Then, the upper part of PS spheres would in turn push PMMA chains in parallel lamellae, which makes some wavy shaped, not straight, lamellar microdomains of PMMA. The number of concentric lamellae is less than expected from the values of R/L0 because the parallel lamellae near the top cover occupy the upper part of the hemispherical cavity, which decreases effective cavity size for concentric lamellae. As seen in Figure 2a, the number of concentric lamellae is 3 even though it should be 5 without the upper parallel lamella. When R/L0 is increased, the effect of confinement should decrease, but we could still observe some regions containing spherical shaped island. For the neutral brush of the cavity wall, we observed stacked (or parallel) lamellae (Figure 2c,f). This is ascribed to the top cover layer, which is strongly selective to PMMA chains of PSb-PMMA. PMMA lamella is located at the top surface contacting PMMA homopolymer followed by PS and PMMA lamellae alternately. On the other hand, since the wall is neutral for both PS and PMMA blocks, there is no need to form concentric lamellae, and the formation of stacked lamellae is preferred. The stacked lamellae, however, are not perfectly lateral but bent slightly because of the curvature of the AAO wall. We also investigated the alignment of lamellar microdomains of SML-320 and SML-98 confined inside hemispherical cavities with PS cover layer and three different brushes on the cavity wall. Lamellar microdomain alignment very similar to Figure 2

Figure 3. Cross-sectional TEM images of SML-320 (left panel) and SML-98 (right panel) confined at hemispherical cavities with neutral top cover and three different brush layers: (a, b) PMMA-OH brush, (c, d) PS-OH brush, and (e, f) PS-ran-PMMA-OH brush. The scale bar in all images is 100 nm. A schematic of the cross-sectional images is shown in the inset of each figure. Red and blue colors represent PMMA and PS microdomains, respectively.

spherical cavities with PS-ran-PMMA cover layer and three different brushes on the cavity wall. When the cavity wall has selectivity to one of the blocks, alternating concentric lamellae are observed while the outermost layer is the selective block to the grafted brush (Figure 3a−d). The results indicated that the morphologies were affected greatly by the surface selectivity. At R/L0 = 2, the number of concentric layers is 4 as shown in Figures 3a and 3c, although perfectly concentric lamellae are not observed. When R/L0 increases to 4.25, the number of concentric layer increases to 9, as shown in Figures 3b and 3d. The imperfectness of the concentric lamellae as shown in Figures 3a and 3c is attributed to the strong confinement effect at smaller R/L0. Figures 3e and 3f are the resulting morphologies inside the hemispherical cavities when both interfaces have no selectivity. Because two blocks do not have any preferential interaction, 4000

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Figure 4. Microdomain alignment of AB block copolymer (PS-b-PMMA) confined within a hemispherical cavity calculated by the SCFT pseudospectral method. The polymer-grafted cavity walls are modeled as C homopolymer (green color). By controlling their interaction with A and B segments, the walls are covered by three different polymer brushes: PMMA-OH (top panel), PS-OH (middle panel), and PS-ran-PMMA-OH (bottom panel). Also, the top surface preference is given as a delta function-like potential with ηA and ηB, representing the delta function strength. The red and blue colors represent PMMA and PS blocks, respectively.

When the cavity is covered by PS-OH brush, we set χCAN = 35 and χBCN = 15, and the observed morphologies are very similar to those in the cavity covered by PMMA-OH, except that the outmost cavity wall is PS layer (Figure 4d−f). One can easily check that Figures 4a and 4e are essentially the same except for the coloring, and the same relation holds true for Figures 4b and 4d and Figures 4c and 4f. As was the case with experiments, the most interesting case is when the cavity wall is covered by PS-ran-PMMA-OH brush. To represent this case, we set χCAN = 35 and χBCN = 35, and the results are shown in Figure 4g−i. When the top surface has a preference for one type of segment, staked lamellae are observed (Figure 4g,h), while bicontinuous structures having interconnected lateral and vertical lamellae are observed for a neutral top layer (Figure 4i). The formation of bicontinuous morphology could be explained as follows. The cavity has a bowl shape, and it is obvious that the lamellae must be distorted at the bottom parts of the cavity even if they are perpendicular to the upper surface. The distortion of lamellae imposes additional entropic penalty, which may provide driving force toward bicontinuous domain formation. The morphology as shown in Figure 4i was obtained by using random fields as an input. In SCFT methodology, it is practically impossible to prove if such a morphology is in a true equilibrium, but we used many other inputs to create competing morphologies and found this one most favorable. Thus, it is highly plausible that the bicontinuous morphology is at least very close to the free energy minimum, and it is not surprising that experiments did find a similar morphology. For a better visualization of the bicontinuous morphology, its 3D structure is demonstrated in Figure S6 of the Supporting Information. Even though studies on block copolymers in hemispherical cavity are rare, there are relatively richer theoretical16,46,47 and experimental27−30 studies for the fully spherical cavities (3-D confinement). When the spherical cavity wall is completely preferential to one block, concentric (onion-like) lamellae are usually observed, while a wall with zero or weak selectivity may

they make stable bicontinuous structures where lateral and vertical lamellae are interconnected. The stable bicontinuous structures are verified by the same TEM image even when the sample was cut at various directions (Figures S5 in the Supporting Information). Also, 3-D simulation clearly demonstrated the bicontinuous structure (Figure S6 in the Supporting Information). Thus, we consider that the morphology given in Figures 3e and 3f represents the thermal equilibrium. This is because the sample was annealed for a long time (5 days at 170 °C). The domain spacing of the bicontinuous morphologies is almost the same as that in bulk. However, it would be a little larger (or smaller) in some regions to form a stable structure. All of the morphologies given in Figure 3 are almost the same as those confined inside the hemispherical cavities without the cover layer (i.e., the top of the cavity directly contacts air).33 The morphologies obtained in this experiment were compared with predictions based on the SCFT calculation. Figure 4 summarizes the result by showing the morphology behavior at a few given parameter values. In the figure, the red and blue colors represent PMMA and PS, respectively. For better visualization, the C homopolymer filled vessel that determines the hemispherical cavity shape is shown in green. When the cavity is covered by the PMMA-OH brush, we set χCAN = 15 and χBCN = 35 to represent the preference of cavity wall for PMMA. In this case, the arrangement of lamellae highly depends on the top surface interaction (Figure 4a−c). If the top surface prefers one type of segment (Figure 4a,b), a parallel lamellar structure is observed on the top surface due to the preferential surface interaction, and concentric lamellae are observed at the bottom of the cavity due to the inner wall brush which prefers one type of segment. The combined structure of parallel lamellae and concentric lamellae is consistent with experimental results (see Figure 2a,d). On the other hand, when the top layer becomes neutral, concentric lamellae with outermost PMMA shell is observed until they meet the top layer, as shown in Figure 4c. 4001

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produce stacked lamellae or more complicated structures such as tennis ball, mushroom, or screwlike structures, which can be generally called bicontinuous morphologies. A detailed one-to-one comparison of the two cases would be beyond the scope of this study, but it is worthwhile to mention a few general differences in morphologies confined between hemispherical and full spherical cavities. (i) Concentric lamellae always have a reflecting symmetry at the top surface of the cavity, and thus they are the dominant morphology for both systems as long as the cavity wall is preferential. (ii) For the fully spherical cavity, the stacked lamellae can orient in any direction, but for the hemispherical cavity, they can have a reflecting symmetry only when they align parallel to the top surface. Moreover, the preferential top surface can enhance the lamellar alignment; thus, we have a strong control for the creation and orientation of the stacked lamellae. (iii) Many of the bicontinuous structures observed in the fully spherical cavities do not have the proper reflecting symmetry, and they are strongly suppressed in hemispherical cavities. Our simulation results show that only morphologies whose AB boundary is perpendicular to the top surface can survive when the top surface is not preferential. This is the main reason why we did not observe screwlike structures reported in 3-D confinement.

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (J.K.K.). *E-mail [email protected] (J.U.K.). Author Contributions

D.L. and M.-H.K. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Creative Research Initiative Program supported by the National Research Foundation of Korea (NRF) and NRF grant (No. 2012R1A1A2043633) funded by Korean government (MEST). Small-angle X-ray scattering was performed at PLS beamline supported by POSCO and NRF.

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4. CONCLUSIONS We investigated the microdomain structures of symmetric PSb-PMMA confined at hemispherical cavities having two interfaces: top cover layer and inner cavity wall. The interfacial activity at the top cover layer was controlled by three different polymers (PS, PMMA, and PS-ran-PMMA), while that at the cavity wall was modified by three different brushes (PMMAOH, PS-OH, and PS-ran-PMMA-OH). When the cover film was selective for one type of segment, parallel lamellae were formed at the cover interface irrespective of brushes. However, inside the cavity, concentric lamellae were observed for selective brush, while unspecific bicontinuous structure was found for the neutral brush and neutral top surface. The combination of parallel lamellae and concentric lamellae allows us to observe various interesting domain shapes such as isolated spheres. For the first time, we observed stacked lamellae when selective cover layer and neutral brush were combined. Novel morphologies, which were not previously reported in the literature, were obtained in this work because we have the freedom of controlling two interfaces independently. We expect that the novel microdomain structures obtained in this study could be used as surface-enhanced Raman scattering (SERS), Fano resonance, and anisotropic optical properties because symmetry breaking is very essential. The hemispherical shape combined flat and curved surface would provide broken symmetry to generate plasmon hybridization, which could not be obtained by fully spherical shaped geometry.

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ASSOCIATED CONTENT

S Supporting Information *

SAXS profiles of PS-b-PMMAs, preparation of hemispherical cavities, characterization of PS-b-PMMAs confined at hemispherical cavities with PS-selective cover layer and three different brushes. This material is available free of charge via the Internet at http://pubs.acs.org. 4002

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dx.doi.org/10.1021/ma500761e | Macromolecules 2014, 47, 3997−4003