Extending Dynamic Range of Block Copolymer Ordering with

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Extending Dynamic Range of Block Copolymer Ordering with Rotational Cold Zone Annealing (RCZA) and Ionic Liquids Changhuai Ye, Yan Sun, Alamgir Karim, and Bryan D. Vogt* Department of Polymer Engineering, University of Akron, Akron, Ohio 44325, United States S Supporting Information *

ABSTRACT: Scalable and low-cost methods to align and orient block copolymer (BCP) films and membranes are critical for their adaptation for nonlithographic applications. Cold zone annealing (CZA) can align BCP microdomains and is scalable via roll-to-roll (R2R) manufacturing. However, the efficacy of orientation by CZA is strongly dependent on the thermal zone velocity (Vcza). Optimization of this rate can be time-consuming and tedious. To address this shortcoming, we report rotational or radial CZA (RCZA) that provides a combinatorial approach to efficiently determine how linear Vcza rate impacts microdomain orientation. RCZA rapidly identifies the optimal CZA velocities for perpendicular orientation of cylinders in polystyrene-block-poly(methyl methacrylate) films that previously required tens of measurements [Macromolecules 2012, 45, 7107], demonstrated here with much finer velocity resolution using three overlapping radial regimes. Notably, the efficacy of CZA for perpendicular alignment rapidly decays for Vcza > 10 μm/s. To overcome this limitation, the addition of 2 wt % 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide sufficiently alters the surface tension and segmental relaxations via reduced viscosity to increase the processing window for perpendicular cylinders, approximately 75% at Vcza ≈ 330 μm/s, approaching R2R speeds. Further increasing ionic liquid content to 5 wt % leads to mostly parallel orientation due to surface wetting. Ionic liquids can dramatically increase BCP processing speeds for applications, such as membranes, and RCZA can efficiently map out the optimal processing parameters.

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Translation of thermal gradients provides an external field to align BCP domains over long length scales,14−17 but these methods generally lead to parallel cylindrical morphologies. However, a sharp temperature gradient, ∼45 °C/mm, with maximum temperature less than the order-to-disorder temperature (ODT) of the BCP enables fabrication of perpendicular oriented cylinders in both thin and thick films of polystyreneblock-poly(methyl methacrylate), PS-b-PMMA.18 This process is termed cold zone annealing (CZA). In this case, the fraction of perpendicular oriented cylinders is strongly dependent on the velocity of the thermal gradient (Vcza) varying from 7% to 97% over a narrow Vcza window from 0.1 to 15 μm/s.18 This sensitivity of the morphology on Vcza has been reported for other systems: mesoporous carbon films,19 semicrystalline polymer films,20 and crystalline conjugated oligomers.21 A strong dependency of morphology on Vcza can be problematic for commercialization as it leads to narrow processing windows limiting throughput, and small variations can lead to large changes in material properties. One route to overcome challenges in identification of a processing window is through combinatorial and high throughput methods.22−24 These techniques have successfully identified phase diagrams and structure−property relationships

INTRODUCTION Block copolymers (BCPs) self-assemble into ordered nanostructures that provide promise for applications such as membrane separations,1,2 templated nanostructures,3,4 and organic optoelectronics.5 In many applications, the performance of the BCP depends strongly on the details of the ordering, such as orientation of the nanostructure. For example, perpendicular orientation of cylindrical BCPs relative to the membrane surface is desired for separations6 and electrolytic transport.7 BCP processing strongly influences the morphology as the ordering of BCPs is commonly kinetically limited with short-range orientation of its domains.8 Moreover, perpendicular alignment of the nanostructure is generally thought to require a chemically modified substrate9 and/or top coating10 with a neutral surface to prevent preferential wetting by one segment at interfaces that would promote parallel orientation. Similarly, solvent vapor annealing (SVA) can produce highly ordered, perpendicularly oriented cylindrical nanostructures in BCP films.11,12 However, the ability to scale SVA for commercial production is limited by its batch processing nature and environmental and health concerns associated with volatile organic vapors. Alternatively, external fields, such as electric13 and magnetic fields,7 can provide forces necessary to overcome wetting preferences to obtain perpendicular orientation, but these processes are limited by contrast in magnetic or electrical susceptibility between the segments. © 2015 American Chemical Society

Received: September 27, 2015 Published: October 13, 2015 7567

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Figure 1. Schematic of the RCZA setup. The substrate is mounted to a frame that is suspended vertically above the hot wire and cold blocks. The sample is rotated across the hot wire, which produces a gradient in Vcza across the film for the CZA process.

quickly and efficiently.25 Prior combinatorial studies associated with BCP films have focused on film thickness,26 molar mass of the BCP,27 SVA solvent,28 and surface energy.29,30 These studies have provided rapid insight into the influence of these variables on the morphology of BCP films. For perpendicular alignment of BCP domains in films and membranes, the surface wetting characteristics are critical; e.g., neutral wetting promotes perpendicular alignment.31 For SVA, the use of selective solvents can act to tune the surface energy of the segments to promote perpendicular orientation.28,32 One route to avoid vapor concerns associated with SVA is through the use of nonvolatile additives. Ionic liquids (ILs) are liquids at > 100 °C with near zero vapor pressure33 and a wide variety of available chemistries. BCP/IL mixtures have been examined for electrochemical devices.34,35 Addition of ionic liquid to a BCP changes the effective volume fractions and the interaction strength between segments, which can impact the nanostructure and domain sizes,35−37 but ILs have not thus far been used to explicitly tune surface wetting in BCPs. A low surface energy IL that is selective to the high surface energy segments of the BCP should provide a simple route to tune the wetting characteristics of BCP films and be fully compatible with CZA processing. Here we report on rotational cold zone annealing (RCZA), an alteration to CZA,18,38,39 to enable combinatorial experiments, rotating the sample through the temperature gradient leading to almost a decade in accessible Vcza in a single experiment. Because of the radial nature of the experiment, a continuous spectrum of linear velocities Vcza is naturally obtainable at constant angular (rotational) CZA speed ω, as Vcza = rω, where r is the radial distance from axis of rotation and ω = dθ/dt. Changing ω in steps allows for large overlapping ranges of Vcza using few experiments. We qualify the RCZA method using PS-b-PMMA thin films for direct comparison to a prior linear CZA study where the Vcza variation required multiple samples, with 3−5 discrete stepped-Vcza on a typical quartz slide.18 Subsequently, the impact of low concentrations of IL on morphology development during RCZA for a PS-bPMMA/IL system is examined. The window for obtaining perpendicular orientation of the domains for PS-b-PMMA using CZA is strongly dependent on the IL content, demonstrating that ILs can provide a simple and safe route to tune morphology in BCP films. This combinatorial approach via RCZA provides new opportunities to rapidly investigate

zone annealing for other systems for a variety of possible applications.

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EXPERIMENTAL SECTION

Materials. PS-b-PMMA (Mn = 82 kg/mol, f PMMA = 0.3, Đ = 1.21) was purchased from Polymer Source Inc. The IL, 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIm-TFSI, ≥97.0%), and tetrahydrofuran (THF, >99.0%, ACS reagent with 250 ppm BHT) were purchased from Sigma-Aldrich and used as received. Sulfuric acid (H2SO4, >51% acid, Avantor Performance Materials, Inc.) and hydrogen peroxide (H2O2, 30 wt % aqueous solution, EMD Chemicals Inc.) were used as received. Film Preparation. Quartz (2.5 cm × 7.5 cm × 0.65 mm, GM Associates) was used as the substrate for the BCP films and cleaned using piranha solution (H2SO4:H2O2 (30 wt % aqueous) = 7:3) at 90 °C for 30 min. The substrates were rinsed with deionized water several times and dried in flowing N2 prior to film coating. 3 wt % PS-bPMMA and EMIm-TFSI were dissolved in THF. The EMIm-TFSI added to the PS-b-PMMA solution was 0, 1, 2, 5, and 15 wt % based on the mass of PMMA. The films were coated on quartz substrates using flow coating from the THF solution to coat an area approximately 2.5 cm × 6.0 cm. The thickness of the films was measured using a variable angle spectroscopic ellipsometer (VASE, J.A. Woollam Co., Inc.) at incident angles of 65°, 70°, and 75° over the wavelength range from 300 to 1689 nm. The thickness of the BCP film was approximately 100 nm in all cases. Rotational Cold Zone Annealing (RCZA). The rotational cold zone annealing (RCZA) setup consists of a hot wire (1 mm nickel− chromium alloy wire in a ceramic shell with a 4 mm outside diameter) sandwiched by two cold blocks that are actively cooled by a circulating chiller (Thermoscientific Haake SC100 A 28). The substrate was heated from below by the hot wire with the heat transfer through the thickness of the quartz to generate the desired temperature profile at the BCP/substrate. This configuration yields a sharp temperature gradient on the top surface of the substrate as shown schematically in Figure 1. The temperature gradient width for the RCZA setup is affected by a series of experimental variables, such as the thermal conductivity of the substrate, the location of hot wire, the temperature of cold blocks, and the gap distance between the two cold blocks. The maximum temperature gradient for this RCZA setup is approximately 40 °C/mm, which was determined using an IR camera (875-1 infrared camera) focused at the surface of the quartz substrate. The quartz substrate coated with the BCP film was held in place by a metal frame that was mounted to the motorized continuous rotation stage (CR1Z7, with T-Cube DC Servo Controller). The substrate and metal frame were aligned parallel to the surface of cold blocks, and the gap between the metal frame and the top surface of cold blocks was less than 0.5 mm. The rotational center for the substrate was always set to be 15 mm from the end of quartz substrate lengthwise and centered across the width (Figure 1). This center point (axis of rotation) was 7568

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Figure 2. AFM micrographs of neat PS-b-PMMA thin films at varying Vcza using RCZA. All AFM micrographs are obtained using diDimension V. fixed approximately at the center of the hot wire. The BCP film was positioned facing upward with the heat from the wire conducting through the quartz to reach the film. To initiate an RCZA experiment, the sample was lowered approximately 0.5 mm from the hot wire (same position every time using a stop) with long axis of the sample perpendicular to the hot wire. Then the sample was rotated across the hot wire (either clockwise or counterclockwise) as shown in Figure 1. Rotation of the substrate across the temperature gradient yields Vcza that was proportional to the distance from the center of rotation. Morphological Characterization. Atomic force microscopy (AFM, diDimension V, Veeco and Dimension Icon, Veeco) was used to characterize the surface morphology of the PS-b-PMMA/IL films. Tapping mode was used, and the scan size was 2 μm × 2 μm with a scan rate of 1 Hz. The contrast of the phase image and peak force image for the same sample obtained by these two instruments is inversed. To examine known positions across the quartz substrates, the back of substrates was marked by parallel lines with a step size of 2 mm. This enabled simple mapping of the morphology at known radii from the rotational center using AFM. The fraction of perpendicularly oriented cylinders was determined from the AFM micrographs using ImageJ software. The AFM images were first “threshold” into black and white and the minority phase as analyzed for area. The fraction of perpendicular oriented cylinders was calculated as

temperature gradient, the region near the rotational center, R ≤ 2.5 mm, is continually heated above the glass transition temperature of both segments of the BCP due to the finite width of the thermal gradient (as illustrated by the dashed circle in Figure 3). This precludes the slowest regions that are nearest

fperpendicular =

area of perpendicular cylinders area of perpendicular cylinders + area of parallel cylinders

The process to determine the fraction of perpendicular oriented cylinders is detailed in the Supporting Information. This analysis follows the protocols previously described by Singh et al. where there was good agreement between the values measured by AFM analysis and GISAXS analysis.18

Figure 3. Temperature profile for RCZA with maximum temperature Tmax = 210 °C and schematic illustration of the annealing region and path of sample across the temperature profile that limits the use of RCZA near the rotation center.

RESULTS AND DISCUSSION Assessing CZA Processing Window for Domain Orientation in Neat PS-b-PMMA Using RCZA. Figure 2 illustrates the evolution in the surface morphology of PS-bPMMA films across the film by changing Vcza using RCZA. The continuous phase is PS with PMMA cylinders. In these cases, three different rotational speeds (ω = 0.324, 1.38, and 6.0°/ min) were utilized to assess Vcza from approximately 1.3 to 91 μm/s. As the film supporting substrate rotates across the

to the rotational axis from being examined. Likewise, the circular path of the sample across the temperature gradient for RCZA near the rotational center leads to an alteration in the processing history in comparison to the standard linear CZA.14 This circular path can be approximated as straight if R is much larger than the length of the effective annealing region. To characterize the effect of the circular path, especially at small rotational radius (R ∼ width of thermal gradient), three

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Macromolecules rotational velocities with overlapping Vcza ranges were characterized. The orientation of the cylindrical structure in the overlapped Vcza for different rotational speeds is similar when R > 6 mm. To avoid challenges associated with translation of RCZA results to production conditions for standard CZA, we have only examined regions of the film for 50 mm ≥ R ≥ 6 mm. The upper limit of R is dictated by the size of the substrate in order to avoid edge effects. The surface temperature profiles on the quartz substrate are utilized to assess the gradient generated during the RCZA process, from static state to highest rotational speed (30°/min) examined (Figure S5). At all rotational speeds except the fastest examined, the maximum temperature is essentially invariant of Vcza (up to approximately 100 μm/s). At rotation speed of 30°/min, the maximum temperature along the radius decreases almost linearly from approximately 210 to 180 °C due to insufficient time for heat transport. As shown in Figure 2, the morphology of PS-b-PMMA at low Vcza is a mixture of parallel and perpendicular cylinders. The fraction of perpendicular oriented cylinders is approximately 40% when Vcza is about 1.3 μm/s. In general, at low Vcza < 2 μm/s, the relaxation of polymer chains results in a transformation to parallel alignment due to the gradient of in-plane surface tension and surface wetting effects as the film passes too slowly over the gradient peak temperature, losing much of the induced perpendicular order on the increasing temperature gradient side.18 As Vcza increases, the fraction of perpendicular oriented cylinders gradually increases until a maximum in perpendicular fraction (∼80%) is obtained near 5 μm/s. The mechanism for optimal perpendicular order has been attributed to a coupling of the longest molecular relaxation time with the dynamic velocity of CZA.18 As Vcza further increases, the fraction of perpendicular oriented cylinders decreases. From these data, there appears to be an optimal low speed regime for perpendicular orientation of cylinders, Vcza between approximately 3.2 and 10 μm/s. This result obtained from these three experiments is consistent with the optimized linear CZA previously reported for this PS-b-PMMA, which involved an order of magnitude more experiments.18 Quantitative comparison of RCZA to prior reports for linear CZA yields differences, such as a larger fraction of perpendicular oriented cylinders (97%) obtained from linear CZA. However, the maximum temperature gradient for RCZA is 40 °C/mm, which is 5 °C/ mm smaller than linear CZA reported previously.18 A sharper temperature gradient leads to larger local gradient strain in the thin film and enhances the driving force for the perpendicular orientation. A more detailed summary analysis of AFM morphologies in Figure 4 shows that there is an intermediate Vcza ∼ 30 μm/s where the perpendicular orientation increases again but peaks at a lower fraction (∼55%). Between these two peaks, the perpendicular orientation drops down to ∼30% at Vcza ∼ 20 μm/s. We believe these twin peaks arise from different BCP molecular response mechanisms. The peak at the lower CZA speed has been explained previously due to the molecular relaxation coupling of the BCP with CZA speed. The peak at the higher CZA speed of ∼30 μm/s is attributed to the sharp temperature gradient at high speed appears like a step-height annealing to the as-cast film that induces an instantaneous (in seconds) rapid normal expansion of the film. Under oven annealing this rapid perpendicular expanded stressed film can relax with time on order of minutes and hours to an isotropic relaxation state.40 We have shown that this rapid thermal

Figure 4. Quantification of the fraction of perpendicular oriented cylindrical domains in neat PS-b-PMMA thin films as a function of Vcza using RCZA at three different rotational speeds. Observe the logarithmic X scale markers for Vcza to correlate with the double humped perpendicular orientation maxima.

expansion stress effect in oven annealing leads to modest perpendicular orientation (65%) at short times (∼5 min at 180 °C), followed by loss of perpendicular order to parallel orientation at long time (∼ hours) in our BCP system.22 The difference from rapid oven annealing and high speed CZA is that continued annealing of the BCP on the temperature gradient maintains the initially created perpendicular order by thermal expansion shock stress. While this is a high velocity “thermal shock” effect, there is nevertheless a required annealing time to induce order in the BCP film while under the perpendicularly applied thermal expansion stress field.41 Therefore, at even high speed of Vcza, the ordering is kinetically limited and the perpendicular order decreases. In fact, the net total ordering in the film that decreases at >30 μm/s is the way to understand the gradual decrease of perpendicular order at progressively higher velocities. The next section shows how to expand the high speed perpendicular ordering from its kinetically bound limits with addition of ionic liquids. Collectively, our data illustrate that at least qualitatively RCZA provides an efficient route to screen zone annealing conditions for the alignment of block copolymers that provides the optimized range to use for production with linear CZA. Ionic Liquid (IL) Additive Tunable Orientation of Cylindrical PS-b-PMMA with RCZA. Typically, ionic liquids are added to BCPs to enhance the ionic conductivity,7,42 but ionic liquids can significantly impact both processability and morphology of the BCP.35,37 EMIm-TFSI is miscible with PMMA segments but immiscible with PS segments.43 This selectivity and low surface tension associated with the EMImTFSI should effectively change the wetting characteristics of the PS-b-PMMA. Addition of only 1 wt % EMIm-TFSI to the BCP changes the fraction of perpendicularly oriented cylinders modestly in comparison to the neat BCP (see Supporting Information). The fraction of perpendicularly oriented cylinders of PS-b-PMMA at high Vcza is increased with the addition of 1 wt % EMIm-TFSI, but this still remains below 50%. Increasing the EMIm-TFSI concentration to 2 wt % dramatically influences the morphology developed with CZA as shown in Figure 5. At low Vcza (0.6 μm/s), there is a significant fraction of parallel cylinders (PS remains the matrix here. The shift in contrast in comparison to the other samples is due to the contrast difference between peak force and phase imaging on the two AFMs utilized.) This large parallel fraction at low Vcza is consistent with the relaxation of the segments at long times and the preferential parallel orientation of oven 7570

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Figure 5. AFM micrographs of PS-b-PMMA with 2 wt % IL thin films as a function of Vcza from RCZA. The perpendicular orientation persists at high Vcza. The AFM micrograph at Vcza= 0.6 μm/s is obtained using Dimension Icon, while the others are using diDimension V.

zone (3−10 μm/s) for neat PS-b-PMMA. Comparing the film with 5 wt % IL and that with 15 wt % IL demonstrates a general degradation in the fraction of perpendicularly oriented cylinders with increasing concentration of the IL in the film. Figure 8 quantifies the fraction of perpendicularly oriented cylinders for PS-b-PMMA thin films with 5 wt % IL and 15 wt % IL. This fraction is below 20% for low Vcza in both cases. At higher V cza , there is an increase in the fraction of perpendicularly oriented cylinders with a maximum approaching 50%, but this is still significantly less than that found for the film containing 2 wt % IL (Figure 6). These data show the preference for parallel oriented cylinders at high IL concentrations. Additionally, the IL acts to plasticize the PMMA segments thus promoting relaxation of the stresses that induce the perpendicular orientation using CZA.18 Finally, the fluorinated IL wets the top interface of the film due to its low surface energy. Preferential wetting of interfaces and decreased relaxation time for the BCP due to plasticization tend to promote parallel orientation of the cylinder domains, which is consistent with the AFM micrographs associated with 5 and 15 wt % EMIm-TFSI. Thus, it might be surprising that the CZA processing window for perpendicularly oriented cylinders is significantly increased when the PS-b-PMMA contains 2 wt % EMIm-TFSI. We believe that this low concentration can promote the perpendicular orientation at high Vcza by slight plasticization of the PMMA segments without significant wetting side effects, since the poor orientation in the neat PS-b-PMMA at high Vcza is attributed to poor mobility of the BCP that limits the ordering in short time frames.18 The order of magnitude increase in productivity for CZA to generate perpendicular orientation by the addition of 2 wt % IL could be extremely useful for the production of filtration membranes, especially as CZA is applicable to roll-to-roll manufacture.41

annealed samples (see Figure S4). There is an increased fraction of perpendicularly oriented cylinders at 3.2 and 10.4 μm/s, which then dramatically decreases at 17.7 μm/s. This behavior is consistent with neat PS-b-PMMA. However, upon increasing Vcza further, the fraction of perpendicularly oriented cylinders increases to around 75% and is nearly independent of Vcza from 45 to 330 μm/s as shown in Figure 6. This is

Figure 6. Distribution of perpendicularly oriented cylindrical domains of PS-b-PMMA with 2 wt % IL films as determined from four RCZA experiments. A significantly increased processing window at high Vcza for primarily perpendicularly oriented cylinders is obtained with the addition of only 2 wt % IL. Open symbols without error bars are based on 1−2 AFM images. Solid symbols with error bars examine a series of five or more AFM micrographs at each condition. The error bar is the standard deviation based on the analysis of the multiple images.

significantly different from neat PS-b-PMMA where a low fraction of perpendicularly oriented cylinders are found at high Vcza ( 42 μm/s). We attribute this increased processing region for perpendicularly oriented cylinders in PSb-PMMA/IL to plasticization of the PMMA domains by IL. This effectively speeds the ordering kinetics to allow less time for annealing and thus faster Vcza can be used to order the BCP film. Figure 7 shows how the morphology is impacted by higher concentrations of IL for a variety of Vcza using RCZA. At both 5 and 15 wt % IL, the fraction of parallel cylinders appears to be significantly greater even in the previously determined optimal

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CONCLUSION Here, we demonstrate a route to determine the impact of Vcza on morphology of zone annealing in combinatorial manner through rotational cold zone annealing (RCZA). This method provides a simple route to systematically vary Vcza continuously over nearly an order of magnitude in a single experiment. The 7571

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Figure 7. AFM micrographs of PS-b-PMMA films containing 5 and 15 wt % IL after processing with RCZA. A significant fraction of parallel cylinders are observed in both cases irrespective of Vcza. The AFM micrographs are obtained using diDimension V.

Figure 8. Fraction of perpendicularly oriented cylindrical domains of PS-b-PMMA films with 5 and 15 wt % IL using RCZA. The rotational speed used for PS-b-PMMA film with 15 wt % IL is 1.38°/min.

optimal Vcza for the perpendicular orientation of neat PS-bPMMA films is between 3.2 and 10 μm/s, obtained using three angular velocities with RCZA agrees with many single standard linear CZA. The successful demonstration of this method enables us to consider its application to rapid optimization of CZA conditions for more complex systems. As an example, we demonstrate the significant increase in the Vcza window for >70% perpendicularly oriented structure by the addition of 2 wt % of an ionic liquid, EMIm-TFSI. This small addition of ionic liquid enables an order of magnitude increase in the directed assembly of perpendicularly oriented domains that could be used as filtration membranes. However, the orientation of cylindrical structure is sensitive to the ionic liquid content with primarily parallel orientation at both 5 and 15 wt % EMImTFSI. The rapid screening potential of RCZA can enable new advances in the application of zone annealing.

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maps for various rotational speeds and the maximum temperature at various Vcza for RCZA examined; as-cast films of PS-bPMMA with various ionic liquid contents (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (B.D.V.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been partially supported by the National Science Foundation Division of Materials Research, Grant NSF DMR1006421. The authors thank Clinton Wiener for the useful discussions associated with the construction of the rotational cold zone annealing setup.

ASSOCIATED CONTENT

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02128. The process for calculating the fraction of perpendicularly oriented cylinders in BCP thin films; AFM micrographs and quantification of the fraction of perpendicular oriented cylindrical domains for PS-bPMMA thin films with 1 wt % IL at varying Vcza using RCZA; AFM micrograph of PS-b-PMMA thin film with 2 wt % IL oven annealed at 200 °C for 2 h; temperature

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DOI: 10.1021/acs.macromol.5b02128 Macromolecules 2015, 48, 7567−7573