Unidirectional Alignment of Block Copolymer Films Induced by

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Unidirectional Alignment of Block Copolymer Films Induced by Expansion of a Permeable Elastomer during Solvent Vapor Annealing Zhe Qiang,† Longhe Zhang,† Gila E. Stein,‡ Kevin A. Cavicchi,*,† and Bryan D. Vogt*,† †

Department of Polymer Engineering, University of Akron, Akron, Ohio 44325, United States Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas 77204, United States

‡

S Supporting Information *

ABSTRACT: One challenge associated with the utilization of block copolymers in nanotechnology is the difficulties associated with alignment and orientation of the selfassembled nanostructure on macroscopic length scales. Here we demonstrate a simple method to generate unidirectional alignment of the cylindrical domains of polystyrene-blockpolyisoprene-block-polystyrene, SIS, based on a modification of the commonly utilized solvent vapor annealing (SVA) process. In this modification, cross-linked poly(dimethylsiloxane) (PDMS) is physically adhered to the SIS film during SVA; differential swelling of the PDMS and SIS produces a shear force to align the ordered domains of SIS in the areas covered by PDMS. This method is termed solvent vapor annealing with soft shear (SVA-SS). The alignment direction can be readily controlled by the shape and placement of the PDMS with the alignment angle equal to the diagonal across the rectangular PDMS pad due to a propagating deswelling front from directional drying of the PDMS by a dry air stream. Herman’s (second order) orientational parameter, S, can quantify the quality of the alignment over large areas with S > 0.94 obtainable using SVA-SS.

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state.17,18 However, it is difficult to achieve long-range order in some cases due to the sluggish dynamics associated with defect annihilation (especially for BCPs with small χ), intrinsically slow ordering kinetics for high-molecular-weight BCPs, or narrow temperature windows due to polymer thermal stability.14 Alternatively, another route to order BCP thin films is solvent vapor annealing (SVA),19 where sorption of solvent imparts mobility to the BCP segments with a substantial decrease in Tg.19,20 Additionally, the introduction of solvent vapor impacts segment preferentiality at the free surface of the film,21,22 generally decreases χN through screening of unfavorable interactions, and potentially affects the morphology due to the change of the effective volume fractions of the blocks with selective solvents.21,23,24 These properties enable SVA to tune nanostructures of BCP films by varying solvent choice,25 swelling thickness (solvent concentration in the films during SVA), and solvent removal rate by which morphologies are kinetically trapped.26,27 Typically, significant improvements in the long-range ordering of the BCP films can be achieved with SVA. However, these films still contain multiple ordered grains

INTRODUCTION The self-assembly of block copolymers (BCPs) provides a facile route to generate nanostructures without the need for expensive photolithographic processing. As such, BCPs have received significant attention for integration into numerous applications, including membranes for separations,1−4 a nextgeneration lithography technique for microelectronics,5,6 optical films including polarizers,7,8 and active layers in organic electronics.9 These applications take advantage of periodic BCP morphologies10,11 that are primarily determined by three major variables: the degree of polymerization (N), the interaction parameters (χ) between the different blocks, and the segmental volume fractions (f i) associated with the components in the BCP.12,13 However, commercial applications of BCPs to date have been limited due to difficulties in the spatial control of the ordered nanostructure, which is strongly dependent on processing.14 Moreover, many proposed applications require organization of the BCP in thin films, where the morphologies depend on many other factors such as the ratio of films thickness to domain spacing and surface energies (both substrate energies and free surface energies).15,16 In order to achieve modest ordering of BCPs, thermal annealing at temperatures higher than the glass transition temperature (Tg) of the segments provides chain mobility to enable rearrangements of BCP domains toward the equilibrium © 2014 American Chemical Society

Received: October 15, 2013 Revised: January 16, 2014 Published: January 28, 2014 1109

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that are not correlated with one another. One recent advance in SVA, termed raster SVA, provides a route to provide directionality to the ordered domains (vertical relative to the plane of the substrate) of the BCP through controlling local exposure with a moving stream of solvent vapor.28 As nanostructure orientation is a critical consideration for many of the aforementioned applications,29 numerous routes have been developed for alignment of self-assembled BCP films, typically through the application of an external field such as a magnetic30 or electric31 field, topological epitaxy,32 shear,33 zone casting,34 cold zone annealing (CZA),35 or patterned substrates.36,37 Although perfection in the pattern replication is the most critical component for block copolymers in most microelectronic applications,38 there are cases where a small density of defects is acceptable if the overall orientation is welldefined, most notably for optical applications.39 Recently, efforts have been made to utilize techniques in parallel to enhance alignment in BCP films. Building upon the initial work of Register and Chaikin for shear alignment of BCPs with a cross-linked elastomer pad,40 Karim and coworkers reported that capping of the BCP film with an elastomer and subsequent application of CZA can significantly increase the alignment of parallel cylinders with an angular spread of less than 5°.41 This high degree of alignment is a result of an induced shear field, self-generated from the elastomeric capping layer by thermal expansion, but this alignment was limited to the direction of the moving thermal zone. Similarly, graphoepitaxy, SVA, and thermal annealing have been combined by Ryu and co-workers to enable ordering and alignment of high-molecular-weight BCPs.42 Very recently, Jung and co-workers reported using a solvent-swollen polymer (PDMS) gel pad for aligning high-χ BCPs in an extremely short time on patterned substrates.43 However, these two methodologies for alignment require initial substrate patterning by photolithography to create topographical features in which the BCPs are ordered. This additional patterning step significantly increases the cost and time requirements for the alignment of BCPs. Here, a simple method to fabricate highly aligned BCP thin films is introduced based on SVA with a soft, flexible elastomeric capping layer that can be easily removed after SVA without damaging the BCP film. In this case, the capping elastomer swells and deswells with the BCP during SVA-SS (solvent vapor annealing-soft shear). Because of the swelling of the elastomer, the elastomer cap produces a soft-shear field for aligning the BCP domains during deswelling due to the directional drying of the PDMS from a dry air stream. We demonstrate the efficacy of this method using a commercially available polystyrene-block-polyisoprene-block-polystyrene, SIS, with a polydimethylsiloxane (PDMS) capping layer and toluene as the solvent for SVA-SS. Using this method, the hexagonal parallel cylindrical domains with high degree of alignment are formed with Herman’s orientational factor (S) greater than 0.89 for a wide range of film thickness from 98 nm to over 1000 nm as determined by grazing incidence small-angle X-ray scattering (GISAXS) and atomic force microscopy (AFM). Interestingly, the orientation of the alignment is controlled by the shape and placement of the elastomer, which provides a simple route to control the alignment direction and achieve macroscale orientation of BCPs without any significant change in processing in comparison to a standard SVA process.

Article

EXPERIMENTAL SECTION

Materials. Polystyrene-block-polyisoprene-block-polystyrene (SIS) copolymer (Kraton 1164P), containing 29 vol % polystyrene, was obtained from Kraton Polymers. The molecular weight of the SIS was estimated to be 129 000 Da from size exclusion chromatography in THF using polystyrene (PS) standards. It has been shown that use of PS standards can overestimate the molecular weight by a factor of 1.4.44 Taking this into consideration, the degree of segregation, χNdiblock, is estimated to be 93 at room temperature for the neat triblock copolymer where Ndiblock is equal to half the degree of polymerization of the triblock copolymer.45 For symmetric ABA triblocks (where both A blocks are the same length) and AB diblocks, the theoretical phase diagrams are nearly identical as a function of χNdiblock vs volume fraction.11 For the same composition as the triblock copolymer examined here, the order−disorder transition (ODT) of a diblock copolymer is estimated to be χNODT ≈ 20−30.45 Furthermore, the χNdiblock‑ODT is predicted to be slightly smaller for the triblock compared to it equivalent diblock due the reduction in the number of end segments. Thus, the SIS copolymer examined is not close to the ODT window. Poly(dimethylsiloxane), PDMS (Sylgard 184, Dow Corning), was prepared at 10:1 w/w ratio of base to curing agent with the components physically mixed by hand. The mixture was poured onto a flat glass plate. The PDMS was allowed to cure and degas at room temperature for 4 h, followed by an elevated temperature cure at 120 °C for 2 h. The cured PDMS was cut into slabs to act as soft capping layers for the SIS films. Toluene (ACS grade) was obtained from Sigma-Aldrich and used as received. Film Preparation and Processing. The (100)-oriented silicon wafers were obtained from University Wafer and were cleaned with ultraviolet-ozone (UVO, Jelight Company Inc., Model No. 42). The silicon wafers were cleaved into approximately 3 cm × 3 cm pieces to act as substrates for the SIS films. For film preparation, SIS was dissolved in toluene, and the solutions were spin-coated onto the silicon substrates. The thickness of the SIS films was varied from 40 nm to greater than 1 μm from variation in the concentration of the SIS solution. The films were then exposed to nearly saturated toluene vapor for solvent vapor annealing (SVA) to enhance the long-range ordering of the microphase-separated domains. PDMS slabs were laminated to the SIS films with sufficient time allowed to wet the SIS films prior to SVA. Control experiments were performed without the PDMS capping layer. For SVA, the vapor flow rate was controlled using mass flow controllers (MKS-146C-FF000-1) with the standard SVA conditions used here being 800 mL/min of saturated toluene vapor for 4 h, followed by drying of the films using air flowing at 25 mL/min for 1 h. These long processing times are selected to ensure that the SIS and PDMS are equilibrated during SVA. The PDMS was then carefully delaminated from the SIS films. For GISAXS measurements, the sections of the SIS film that was not covered by PDMS during SVA were removed by cleaving the silicon wafer to avoid scattering contributions from the unaligned regions. Characterization. The SIS films were characterized using variable angle spectroscopic ellipsometry (VASE, J.A. Woollam Co., M-2000) to determine the film thickness using a wavelength range from 246 to 1689 nm with angles of incidence of 65°, 70°, and 75°. The ellipsometric angles (Δ and Ψ) were fit using a recursive model consisting of the silicon substrate layer (containing an interface layer and a SiO2 layer) and a Cauchy layer to describe the polymeric films. Atomic force microscopy (AFM, Dimension ICON, Veeco) was used to investigate the surface morphology of the films after SVA by tapping mode at 0.5 Hz using PPP-NCC-50 tips (Nanosensors). Grazing incidence small-angle X-ray scattering (GISAXS) measurements were performed at the X9 beamline of National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL). An incident X-ray beam of energy of 13.5 keV (λ = 0.0918 nm) was used for all measurements. All samples were measured under vacuum (≈40 Pa) at multiple angles both below and above the critical angle. Scattering data were collected using a charged-coupled device (CCD) detector at a distance of 4.73 m. The data are presented in terms of the momentum 1110

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transfer vectors, qx and qz, which are associated with in-plane and outof-plane of the film momentum transfer, respectively.

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RESULTS AND DISCUSSION Figure 1 illustrates the SVA-SS process schematically. For a rectangular PDMS pad placed on top of the SIS film spun on a

Figure 2. (A) AFM phase image and (B) GISAXS patterns of 98 nm thick SIS film at ϕ = 30° exposed to toluene vapor SVA with an approximately 20 mm (length) × 10 mm (width) × 0.5 mm (thickness) PDMS slab. The inset FFT of the micrograph in (A) illustrates the orientation of the cylinders.

is covered with PDMS during the SVA process as determined by multiple AFM measurements randomly across the surface. During this SVA process, the SIS should not go through an order-to-disorder transition (ODT) due to its high molecular weight. From ellipsometry measurements, the swelling of the SIS films with toluene results in an expansion of the film by 33%. By use of scaling to an effective degree of segregation, (χNdiblock)eff = ϕ1.6 χNdiblock, where ϕ is the volume fraction of SIS, that has been demonstrated for SI-toluene,48 (χNdiblock)eff is estimated to be >45, which is significantly greater than the upper limit of χNdiblock = 20−30 for the ODT of SIS at 29 vol % polystyrene based on the SI phase diagram.11,45 To explain this alignment, we must consider the SVA-SS process where the PDMS is physically adhered to the surface of the SIS. The PDMS and SIS are highly swollen by the toluene vapor first, and then both polymers are deswelled when the toluene is removed during drying,28,49 which leads to the volumetric changes associated with swelling and deswelling. The lateral swelling of the PDMS induces a shear force at the PDMS−SIS interface. This effect is similar to what is observed in cold zone annealing with soft shear (CZA-SS) where shear force is obtained from the thermal expansion and contraction of PDMS.41 Similar to CZA-SS, the deswelling will move as a front due to the directionality associated with the gas stream used to dry the PDMS/SIS. The movement of material during SVA-SS can be optically observed with a trench formed at the edges of the PDMS slab (see Supporting Information Figure S3), which is attributed to the shear induced by the swelling/ deswelling process. This trench appears to form on deswelling as the PDMS contracts and pulls SIS. When examining the film after SVA-SS, there is a dark pattern directly tracing the outside of the PDMS pad, which upon examination is a trench in the film where the SIS thickness has been significantly reduced. While the AFM measurements only provide local surface information about the structure and alignment of the BCP, the GISAXS data as shown in Figure 2B provide a route to elucidate the global structure of the film at both the surface and the bulk of the film area that was under the PDMS layer during SVA-SS. One important factor for GISAXS on the aligned BCP films is the orientation of the sample relative to the incident beam. In order to define this orientation, we initially utilize the long axis of the rectangular PDMS cap used during the SVA-SS as the reference and the rotation angle, ϕ, is defined as the angle between the incident X-ray beam and this reference line; Figure 3 illustrates this orientation assignment pictorially. For the GISAXS pattern in Figure 2B, the incident X-ray beam is

Figure 1. Schematic of the swelling and deswelling process during SVA-SS that leads to shear alignment of the BCP from the expansion and contraction of the adhered PDMS pad.

silicon wafer, toluene vapor swells the PDMS isotropically with the strain being ε = ΔL/L0 = ΔW/W0, where ΔL = L − L0. This swelling (and subsequent deswelling) results in shear strains of γx = ΔL/h and γy = ΔW/h, where h is the film thickness. As the SIS is also highly swollen by the toluene, the mobility of the segments is increased, so the stress imposed by the swelling and deswelling of the PDMS should be transferred to the polymer and induce shear flow. Ideally, this flow would produce radially aligned BCP domains as the stress is isotropic, but the deswelling proceeds as a front due to the directionality of the air stream that is used to dry the film and PDMS pad. This produces a drying front47 as the PDMS deswells. In Figure 1, the L direction is oriented parallel to the hypothetical line connecting the inlet and outlet gas lines to the cell in which the films are exposed to the solvent. The change in lateral dimensions is visualized by using a viewing cell, which is ∼28% in L and ∼26% in W. The SIS utilized in this work exhibits a cylindrical morphology in the bulk with the matrix being polyisoprene (see Supporting Information Figure S1 for SAXS confirmation). When the SIS is capped with a cross-linked PDMS slab (approximately 20 mm in length × 10 mm in width × 0.5 mm in thickness) and exposed to standard SVA conditions using toluene vapor under flow, the cylindrical domains of the SIS film are aligned with a highly correlated stripe pattern persisting across the film (Figure 2A). The bright regions are attributed to the polystyrene domains due to the higher modulus of polystyrene relative to polyisoprene.22 Moreover, the analogous height image (see Supporting Information Figure S2) is smooth (height difference is less than 2 nm), illustrating the clean removal of the PDMS slab after the SVA-SS process. From the FFT of the AFM phase images as shown in the inset in Figure 2A, the alignment of the structure can be clearly seen by 2 orders of diffraction spots. The average cylinder-to-cylinder spacing is around 35 nm from the AFM micrograph, which is similar to the SIS films without PDMS capping. Thus, from the AFM micrographs, the PDMS confinement of the SIS film during SVA leads to preferential alignment with long-range ordering. This alignment is observed across the entire area that 1111

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with the cylinder-to-cylinder spacing obtained from AFM. Moreover, this mesostructure of SIS films after SVA with toluene vapor is consistent with previous reports.27,28 Previously, Register and co-workers reported significant increase in defects for BCP films when the thickness is greater than several layers using mechanical shear with PDMS,40 so the efficacy of this technique for aligning BCP films over a wider range thickness is examined. Figure 5 illustrates the surface morphologies of the SIS films from 40 nm (near monolayer) to 1030 nm (approximately 29 layers) thick annealed using the same dimension PDMS slab as for the film illustrated in Figure 2. In each case, the aligned structure is similar as shown in the AFM micrographs (Figure 5A−C). The alignment direction of the cylindrical nanostructure is the same for each of the thicknesses (micrographs are aligned to the initial orientation of the PDMS cap as shown in Figure 3A) from AFM; the vertical edges of these micrographs are parallel to the original long axis of the PDMS pad during SVA-SS. This consistency in orientation suggests that the alignment direction can be controlled by the PDMS. Moreover, the GISAXS patterns for these different thickness films at the same sample rotation angle (ϕ = 30°) illustrate the preservation of the aligned hexagonal cylindrical morphology readily observed among all three samples (Figure 5D−F). These GISAXS patterns have been fit to confirm the hexagonal packing and determine the lattice parameters (Supporting Information Figures S4−S6). For the monolayer film (Figure 5D), only the first-order peak with relatively weak intensity compared to the other two films is observed; this effect is likely due to the limited scattering volume for a monolayer. For the thicker films (312 and 1030 nm), the fifth-order peak can be observed clearly, indicating the highly ordered hexagonal cylindrical structures. There is some variability in the cylinder spacing from the GISAXS data (see Supporting Information Table S1), most notably for the 312 nm thick film; the cylinder spacing appears to increase to 40.5 nm relative to the 35 nm spacing typically observed, which is not consistent with cylinder spacing for other films. We attribute this increase in d-spacing to confinement of the film by the PDMS slab, which prevents formation of islands and holes associated with maintaining commensurability of the natural spacing of the BCP with the total film thickness. This variation in d-spacing under confinement is similar to nonequilibrium spacings associated with noncommensurate chemical38 or topographical50,51 patterns. However, this significant increase in the cylinder spacing is not observed in AFM for other films of similar thickness, which suggests significant sensitivity to the exact processing details. Additionally, there are defects in the ordered structure present in all of the films as observed in AFM micrographs, so the alignment by SVA-SS is not quite as great as can be achieved by mechanical shear,52−54 but this mechanical method does also include a significant concentration of defects when thicker multiple layer BCP films are considered. One issue with AFM measurement is that only the local alignment (smaller than 5 μm × 5 μm) can be examined. For further investigating the alignment of the macroscopic area of the film exposed to SVA-SS, rotational GISAXS as illustrated in Figure 3B is utilized with the scattering collected every 5° or 10° as the sample is rotated through the rotation angle, ϕ (in plane), to determine the orientational order parameter of these films. The GISAXS data for the 98 nm film at all rotation angles are included in the Supporting Information (Figure S7). Figure 6 illustrates the normalized scattering intensity of primary

Figure 3. Schematic of (A) the geometry of the PDMS pad during SVA-SS, which defines the rotation angle, ϕ, during (B) rotational GISAXS measurement with incident beam parallel to long axes of PDMS cap defined as ϕ = 0°.

nearly parallel to the cylinder axes at ϕ = 30° (theoretically the cylinder axes should be aligned to ϕ = 26.6°) as will be subsequently shown. Conversely, Figure 4 illustrates the morphology of SIS thin films after a standard SVA protocol using the same film

Figure 4. (A) AFM phase image and (B) GISAXS patterns of 98 nm thick SIS film exposed to toluene vapor without a PDMS cap. The inset FFT of the micrograph in (A) illustrates the isotropic properties of the cylindrical structures.

thickness and SVA conditions. The surface morphology determined from AFM exhibits the common characteristic fingerprint pattern associated with modest range order of parallel cylinders. The in-plane domain spacing for the cylindrical mesostructure is approximately 35 nm as determined by AFM. The isotropic properties of this mesostructure can be clearly ascertained from the FFT of the AFM micrographs as shown in the inset of Figure 4A; the FFT yields two isotropic rings, and the radius of first-order ring is associated with the average cylinder-to-cylinder spacing. This parallel cylinder mesostructure through the thickness of the film is confirmed by grazing incidence small-angle X-ray scattering (GISAXS) with clear diffraction peaks observed in qz (Figure 4B). From fits of the GISAXS data, the out-of-plane hexagonal symmetry (with out-of-plane unit cell thickness being 80% of perfect hexagonal packing) of the cylinders is confirmed. The primary diffraction peak is observed at qx= 0.177 nm−1, indicating an inplane d-spacing for SIS is around 35 nm, which is consistent 1112

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Figure 5. AFM phase images of SIS films for different thickness: (A) 40, (B) 312, and (C) 1030 nm. (D−F) After SVA using 20 mm (length) × 10 mm (width) × 0.5 mm (thickness) PDMS cap the GISAXS patterns correspond to the same films for (A−C) at the rotation angle ϕ = 30°.

can provide insight into the efficacy of the alignment at different processing conditions, it is useful to quantity the orientation through the Hermans orientational parameter (S) for the films, which is calculated as 3⟨cos2 ϕ⟩ − 1 2

S=

with 90 °

2

⟨cos ϕ⟩ =

∑ϕ = 0 ° I(ϕ) sin ϕ cos2 ϕ 90 °

∑ϕ = 0 ° I(ϕ) sin ϕ

where ϕ is the rotation angle and I(ϕ) is the intensity of the primary diffraction peak at ϕ from azimuthal scan. In this case, ϕ is offset to place the peak maximum at 0° as necessary to calculate S, since the preferred direction is ϕp = 31° from the Gaussian fit. The orientational factor of different thickness films is shown in the inset of Figure 6. For the thinnest film (40 nm), the orientation factor is 0.78, while the orientation of the thicker films is S ≥ 0.89 in all cases with the best alignment (S = 0.95) for the 98 nm thick film. The decreased orientational parameter for the thinnest film is attributed to the combination of reduced polymer mobility near the rigid substrate,55 which likely inhibits the reorganization of the BCP domains, and the large susceptibility of monolayers of BCP films to defect formation.56 There appears to be an optimum thickness for the alignment as there is a decrease in S for the thickest film in comparison to the other multilayer films. In order to better understand the origins of the well-defined directionality for the alignment of the BCP domains during SVA-SS, the confining PDMS overlayer geometry (shape and size) was systematically varied. Figure 7 illustrates that the orientation angle is strongly correlated with the aspect ratio of

Figure 6. Normalized primary peak intensity from the azimuthal scan using GISAXS that is fit to a Gaussian profile (R2 > 0.95) for SIS films at different thickness. Inset shows the orientational factor for the SIS films after SVA-SS as a function of film thickness.

diffraction peak as a function of the rotation angle for different thickness films. These data provide a quantitative measurement of the orientation from a Gaussian fit that yield the preferred direction, ϕp, and angular spread, Δϕfwhm. As the orientation is enhanced, Δϕfwhm decreases, so a sharp peak is associated with a highly oriented nanostructure as observed in Figure 6. The preferred direction (peak position) of the aligned hexagonal parallel cylinder is similar for the four different thicknesses (ϕp = 31 ± 5°). The uncertainty in the preferred direction is dominated by the manual alignment of the wafers on the rotation stage, which is estimated to be ±2°. The Δϕfwhm of 40 nm SIS film (near monolayer) is around 18°, while for the thicker films, the degree of alignment is similar at Δϕfwhm ≈ 14°. Although this qualitative analysis of the azimuthal scans 1113

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Figure 7. Influence of PDMS layer geometry and dimensions on BCP orientation. AFM phase images of SIS films exposed to toluene vapor with PDMS confinement are shown. The inset rectangle shows the relative PDMS dimensions, which correspond to PDMS pieces that are (A) 20 mm × 10 mm, (B) 10 mm × 20 mm, (C) 5 mm × 20 mm, (D) 20 mm × 20 mm, and (E) 20 mm × 5 mm. The size of scale bar is 500 nm in all micrographs. (F) The orientation angle is illustrated to be directly related to the PDMS stripe diagonal angle.

PDMS according to its length (LPDMS) and width (WPDMS). In all cases, the orientation angle (determined from the center of the area of the film capped by PDMS during SVA-SS) is more or less equal to the angle of the diagonal with respect to the length of PDMS cap across the rectangle. For example, the SIS film capped with a square PDMS piece (Figure 7D) is aligned at 41°, which is similar to the 45° diagonal across the square. The maximum disagreement between the diagonal and observed orientation is 9°, but this includes contributions from the uncertainty in placement of the PDMS during SVA and manual placement of the SIS film for the AFM measurement. These results are consistent with the proposed mechanism for the alignment of the nanostructure based on shear forces induced by expansion and contraction of the PDMS during SVA along with the directional drying front. In order to explain this alignment, the mechanics associated with swelling and deswelling of the PDMS is considered as shown in Figure 1. These strains from the swelling and deswelling lead to an overall shear alignment at the angle ϕ = tan−1(ΔW/ΔL) = tan−1(W0/L0) with respect to the x-direction (length), indicating the orientation angle is determined by the ratio of initial L0 and W0. One key aspect is that the corners associated with the rectangular shape of the PDMS breaks radial symmetry in the shear force to provide alignment across the entire SIS film area covered by the PDMS. Figure 7F illustrates the relationship between the alignment angle determined from AFM and the angle of the diagonal across the PDMS rectangle used during SVA. It should be noted that the strain associated with swelling the PDMS (∼30 wt % toluene expected at equilibrium44) is significantly greater than can be generated by the coefficient of thermal expansion (∼3−5%) that is used for alignment during CZA-SS. The diagonal selected for the cylinders initially appeared stochastic with both directions (slanted left or right) equally probable based on our experience with one edge of the PDMS aligned parallel to the gas flow direction such as described in Figure 1. However, if the sample

is purposefully aligned with the PDMS edge at a slight offset angle to the inlet, the selection of the diagonal direction can be reproducible. We attribute this effect to the initial deswelling of one corner of the PDMS (as expected from mass transfer). Even when aligned parallel to the gas inlet, there will be some bias due to imprecision in alignment by eye. Thus, one corner of the initially deswells and controls the orientation selection for the remainder of the film. This SVA-SS method provides a simple route to long-range orientation of the BCP nanostructures with control of the orientation angle through geometry of PDMS capping layer. To understand the importance of the radial anisotropy on the alignment of the SIS films during SVA-SS, the PDMS capping geometry on the film during SVA was changed to a circle (Figure 8), which lacks the corners found in the prior experiments (Figure 7). Interestingly, the nanostructure of the SIS film was aligned perpendicular to the outer edges of the circle after SVA-SS consistent with the expected shear, but these are divergent in a star-like morphology as shown in Figure 8A−H. The alignment of the SIS nanostructure is dependent on the local area examined for the SVA-SS with the circle PDMS cap. The swelling direction in this case should be isotropic radiating from the center of the circle. Near the edges of the circle PDMS, there is a local swelling orientation associated with the direction normal to the circumference. Figure 9 shows the angle of the alignment of the nanostructures as a function of the expected swelling direction (vector normal to the circumference) for the SIS film on the periphery of the circle. A linear relationship between the alignment angle and the location on the PDMS circle is obtained. However, at the center of the circle, a fingerprint morphology is observed (Figure 8I), which is similar to the typical surface structure for SIS with standard SVA (Figure 4A). This lack of alignment is due to an isotropic field induced by the circular PDMS pad near its center, which further illustrates that directional deswelling of 1114

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directional drying front is critical to the uniaxial alignment, but there may be some subtleties associated with the alignment depending on the details of the cell geometry utilized for the SVA process (see Figure S9 for a schematic with dimensions of the cell utilized in this work). In the future, detailed modeling of this process would be useful to tease out the underlying fundamental details and provide direction on how to better control the alignment through shape selection of the PDMS pad.

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CONCLUSIONS We demonstrate a simple route SVA-SS that enables fabricating highly oriented BCP films by solvent vapor annealing with an elastomer cap such as PDMS over a wide film thickness range (at least 1 μm). The differential swelling between the BCP films and elastomer induces a soft-shear field for aligning the domains of BCP. The alignment direction can be determined by the shape and placement of PDMS as the orientation angle is similar to the diagonal angle of the rectangular PDMS cap. The radial anisotropy alignment of BCP films can be achieved by using a circular PDMS pad, which is hard to produce by other alignment methods. Moreover, this technique can potentially be extended to most arbitrary BCP systems as long as the solvent can swell both BCPs for plasticization and segmental mobility and the elastomer to provide the shear strain for orientation.

Figure 8. AFM images of 98 nm SIS film confined with a specific circle PDMS during SVA are shown. (A−H) correspond to each spot on the film. (I) is the phase image of the center part of SIS film covered by PDMS, which experiences an isotropic strain field due to the symmetry.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Data including bulk SAXS profiles, optical micrographs of film near edge of PDMS cap after SVA-SS, AFM height images of the films after SVA-SS, GISAXS patterns at different rotating angle (from −90° to 90° by 10°) after SVA-SS, cylinder spacing for different thickness films determined from both AFM and GISAXS measurements, fits of the GISAXS profiles, AFM images of SVA-SS using a crystallization dish, and a schematic of the flow cell utilized. This material is available free of charge via the Internet at http://pubs.acs.org. Figure 9. Correlation between the angle associated with the circular PDMS and the angle of orientation of the cylindrical nanostructure.

Corresponding Authors

*Email [email protected] (K.A.C). *E-mail [email protected] (B.D.V.).

PDMS during SVA-SS provides the force requisite for longrange alignment of BCP nanostructures. The unaligned area in the center of the circle extends approximately 0.1 mm from the center; in this region the sum of the forces acting locally on the SIS chains is insufficient to produce alignment. However, a square PDMS pad that is approximately the same size as this unaligned area for circle produces uniaxial orientation (similar to Figure 7). At a given point, the strain should be similar, but the corners in the square break symmetry associated with the deswelling under directional dry air steam, and this leads to the highly aligned structures. If a highly cross-linked PDMS is utilized in a square, the orientation decreases, but there remains some directionality to the cylinders. Thus, the strain associated with the PDMS swelling is a critical factor for the alignment, but the shape of the PDMS determines how this strain is imparted to the SIS film and the global alignment. Moreover, without the directional drying associated with a flow system, a radial alignment pattern similar to that shown in Figure 8 is formed with a rectangular PDMS pad (see Figure S8). In this case, the SVA process is performed in a crystallization dish. Thus, the

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Kraton Polymers for donation of the SIS materials. The authors thank Jiachen Xue, Sarang Bhaway, Changhuai Ye, and Kevin Yager for their assistance with the GISAXS measurements. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-98CH10886.

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