Blending Mechanism of PS-b-PEO and PS Homopolymer at the Air

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Blending Mechanism of PS‑b‑PEO and PS Homopolymer at the Air/ Water Interface and Their Morphological Control Baekmin Q. Kim,† Yunji Jung,‡ Myungeun Seo,*,‡,§ and Siyoung Q. Choi*,† †

Department of Chemical and Biomolecular Engineering, ‡Graduate School of Nanoscience and Technology, and §Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Korea

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S Supporting Information *

ABSTRACT: We report a blending mechanism of polystyrene-b-poly(ethylene oxide) (PS-b-PEO) and PS homopolymer (homoPS) at the air/water interface. Our blending mechanism is completely different from the well-known “wet−dry brush theory” for bulk blends; regardless of the size of homoPS, the domain size increased and the morphology changed without macrophase separation, whereas the homoPS of small molecular weight (MW) leads to a transition after blending into the block copolymer domains, and the large MW homoPS is phase-separated in bulk. The difference in blending mechanism at the interface is attributed to adsorption kinetics at a water/spreading solvent interface. Upon spreading, PS-b-PEO is rapidly adsorbed to the water/spreading solvent interface and forms domain first, and then homoPS accumulates on them as the solvent completely evaporates. On the basis of our proposed mechanism, we demonstrate that rapid PS-b-PEO adsorption is crucial to determine the final morphology of the blends. We additionally found that spreading preformed self-assemblies of the blends slowed down the adsorption, causing them to behave similar to bulk blends, following the “wet−dry brush theory”. This new mechanism provides useful information for various block copolymerhomopolymer blending systems with large fluid/fluid interfaces such as emulsions and foams.

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INTRODUCTION The self-assembly of block copolymers has attracted significant attention over the last few decades in the field of nanotechnology because of their ability to form diverse nanostructures.1,2 Accordingly, they have been used for numerous applications, from membrane science to electronic devices.3−6 The morphology and domain sizes of nanostructures formed by the block copolymers are typically controlled by the overall molecular weight (MW) and composition.2,7 However, laborious synthesis is required to achieve desired nanostructures with a specific size and morphology. Adding homopolymers to the block copolymer can be an attractive alternative approach because the homopolymer can selectively occupy the corresponding microdomain of the block copolymer, changing the domain size and morphology.8−14 It is related to the competition between enthalpy and entropy of the system, and α (MW ratio of the homopolymer to the corresponding block of the block copolymer) is usually used as a criterion to classify phase separation. When the MW of the homopolymer is comparable to or less than that of the corresponding block in the block copolymer (α < 1) and the concentration of the homopolymer is small enough, the homopolymer becomes a part of the microdomain of the block copolymer, resulting in a gradual increase in domain size (enthalpic stabilization > entropic loss). A morphological transition, such as from cylindrical to lamellar morphology, eventually occurs when the accommodation capacity reaches a © XXXX American Chemical Society

limit to overcome the conformational entropic loss of the block copolymer. On the other hand, when the MW of the homopolymer is larger than that of the corresponding block in the block copolymer (α > 1), a macrophase separation of the homopolymer occurs without its infiltration because of the high conformational entropy penalty of the block copolymer. Both cases can be described by the “wet−dry brush theory”, where the small homopolymer case (α < 1) is called the “wet brush” and the large homopolymer case (α > 1) is called the “dry brush”.9,14 For ultrathin film fabrication with the block copolymer, an air/water interface system is thought to be suitable because the self-assembled block copolymer forms a monolayer at the interface. Additionally, the density of the self-assembly can be simply controlled by exploiting the compressible air/water interface.15 After pioneering studies by Zhu et al.,16−19 a considerable number of studies have reported on thin films of amphiphilic diblock copolymer at the air/water interface, including polystyrene-b-poly(ethylene oxide) (PS-bPEO),20−27 PS-b-poly(4-vinylpyridine) (PS-bP4VP),16−19,28−30 and PS-b-poly(2-vinylpyridine) (PS-bP2VP).31−33 The self-assembly of amphiphilic diblock copolymer at the air/water interface has several features that Received: June 29, 2018 Revised: August 8, 2018 Published: August 10, 2018 A

DOI: 10.1021/acs.langmuir.8b02192 Langmuir XXXX, XXX, XXX−XXX

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Langmuir Table 1. Characteristics of PS-b-PEO and PS Homopolymer Mn,PEO (g mol−1)

PS-b-PEO BCP20k BCP33k homoPS PS0.8k PS2.3k PS4.3k PS9k

Mn,PS (g mol−1) 846 2300 4300 9000

10 000 10 000 Đ 1.13 1.087 1.072 1.063

Mn,PS (g mol−1) 10 000 23 000 homoPS

Tg (°C) 25.1 68.8 82.5 95.7

PS21k PS36k PS66k

D̵

Mn,PS

1.16 1.28 (g mol−1)

21 600 36 100 66 500

wPS 0.50 0.70 Tg (°C)

Đ 1.048 1.043 1.113

98.7 99.1 100.3

wPS as the overall PS mass over the total polymer mass

are different than microphase separation in bulk and in thin films on a solid substrate: (1) the free energy of the system is complicated because the affinity of each block to air and water must be considered, in addition to any incompatibility between two blocks and entropic contributions; (2) the polymers at the fluid/fluid interface have higher mobility than in solid substrates because of relatively small drag from the subphase. For amphiphilic block copolymer, the aggregated hydrophobic blocks float on water and are surrounded by hydrophilic blocks residing at the air/water interface to minimize the interfacial contact between water or hydrophilic blocks. On the basis of this interfacial energy minimization, circular (dot), cylindrical (including spaghetti), and planar (continent) morphologies are mainly observed in the monolayers,16−19,21−24,27−29,31−33 whereas different morphologies such as “ring” or “chain” have additionally been found.25 Two factors that determine those morphologies are the relative block ratio and spreading condition,16−19,21−25,27−29,32 but other factors such as temperature,28 pH of subphase,31 and spreading area33 should also be considered. Until now, a mechanism for the various morphological formation has been explained mainly by spreading solvent dewetting26 and the differences in the relative lateral area of the blocks.30 Meanwhile, few studies have been performed on the blends of amphiphilic diblock copolymer and the homopolymer at the air/water interface. Wen et al. have investigated the blends of PS-b-P2VP diblock copolymer and PS homopolymer (homoPS) at the air/water interface, and they observed that the size of the circular PS domain increased when the amount of homoPS was small and that the morphology of the PS domains changed when the homoPS content increased further.34,35 More importantly, they showed that two homoPS of different molar masses (one corresponds to the “wet brush” and the other corresponds to the “dry brush”9,14) provided similar results: both homoPS increased the circular PS domain size and changed the morphology in the same way. They simply stated that an unstable homoPS at the air/water interface would be stabilized by the diblock copolymer. However, this observation cannot be explained by the “wet−dry brush theory” for blends in bulk,9,14 and thus the blending mechanism at the air/water interface is still elusive. In this article, we propose a blending mechanism for an amphiphilic diblock copolymer and homopolymers at the air/ water interface using PS-b-PEO and homoPS. Our proposed blending mechanism at the air/water interface is completely different from that in bulk. We show that PS-b-PEO preferentially forms domains first because of the better affinity to water and then the homoPS increases the size of the domains by simply settling on them as the solvent evaporates. To examine this proposed mechanism, we constructed the phase diagrams of the PS-b-PEO and homoPS blends at the air/water interface as a function of two parameters, wPS and α:

(w

PS

=

mPS mtotal

) and α as the ratio of number-average molar mass

(

(Mn) of homoPS to PS Mn in PS-b-PEO α =

M n,homoPS M n,PS in PS ‐ b ‐ PEO

),

respectively. wPS that represents the relative PS:PEO ratio of the blends was chosen as an analogy to the block ratio that determines morphology in PS-b-PEO, and α that means the size compatibility of homoPS to PS in PS-b-PEO was selected, considering the “wet−dry brush theory” [wet brush (α < 1) and dry brush (α > 1)]. On the basis of the results, we found that PS-b-PEO mainly determines the morphology of the blends, which agrees well with previously suggested mechanisms based on solvent dewetting26 and differences in the relative lateral area of the blocks30 as well as our proposed mechanism. Regardless of the molar mass of the PS in the block copolymer, the size of domains increased as wPS increased, as expected from our suggested blending mechanism and previous studies. In addition, we show that changing the adsorption kinetics allows different morphologies to be obtained, by spreading the blends as self-assembled. When the size of the self-assembled blends is large enough, adsorption of the PS-b-PEO to the air/water interface can be delayed. We found that such blends seem to follow the “wet− dry brush theory”,9,14 as the domain size increases for the small homoPS but macrophase separation occurs for the large homoPS.

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EXPERIMENTAL SECTION

Materials. A series of PS-b-PEO and two homoPS with different Mn (36 100 and 66 500 g mol−1) were synthesized by reversible addition−fragmentation chain-transfer polymerization36−39 and used in experiments with the five homoPS (Mn = 846, 2300, 4300, 9000, and 21 600 g mol−1) purchased from Sigma-Aldrich (St. Louis, MO). S-1-Dodecyl-S′-(R,R′-dimethyl-R″-acetic acid) trithiocarbonate was synthesized following the literature procedure and used as the chaintransfer agent (CTA).40 The PEO macro-CTA (PEO-CTA) was synthesized by following the literature procedure,41 which includes esterification of CTA with hydroxyl-terminated PEO with an Mn of 10 kg mol−1 purchased from Sigma-Aldrich. PS-b-PEO was synthesized by the bulk polymerization of styrene (S) in the presence of PEOCTA at 120 °C and precipitating the polymerization mixture in diethyl ether.42 Using the same procedure, homoPS was also synthesized by bulk polymerization of S in the presence of CTA at 120 °C and precipitating the polymerization mixture in methanol. The composition and Mn of the synthesized PS-b-PEO were determined by 1H nuclear magnetic resonance spectroscopy performed on a Bruker AVANCE 400 MHz spectrometer (Billerica, MA) using deuterated chloroform purchased from Euriso-Top (SaintAubin, France) as a solvent. The Mn of all homoPS was estimated by size-exclusion chromatography (SEC) analysis based on linear PS standards (EasiCal) purchased from Agilent Technologies (Santa Clara, CA). SEC was performed in chloroform at 35 °C on an Agilent 1260 Infinity system equipped with a refractive index detector and B

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interface was measured by the pendant drop method at 25 °C. Chloroform droplets that contained the polymers were first introduced in the bulk water phase at the end of a glass syringe (0.705SNR 50 μL, Hamilton). PS-b-PEO was dissolved in chloroform for 1 mg/mL similar to the spreading solution, and PS4.3k was dissolved in 500 mg/mL to observe adsorption at the much higher concentration. Images of droplets were captured at a rate of 10 images per minute by a charge-coupled device camera (WAT-902H, Watec), and then analyzed to extract information about the interfacial tension. More details about the pendant drop method are explained by Stauffer et al.43 Self-Assembly of the PS-b-PEO and homoPS Blends in Solution. BCP33k and homoPS (PS4k, PS66k) were dissolved in a 1:10 mixture of chloroform (HPLC Plus grade, Sigma-Aldrich) and methanol (anhydrous 99.8%, Sigma-Aldrich). Methanol was chosen because it is a poor solvent for PS and good solvent for PEO at the same time. Dynamic light scattering (DLS) measurements were performed on the polymer solution using a Brookhaven 90Plus/BIMAS particle size analyzer (Holtsville, NY) at a wavelength of 658 nm with a scattering angle of 90°.

three PLgel 10 μm MIXED-B columns in series with a molar mass range 500−10 000 000 g mol−1, and the dispersity (D̵ ) of all of the polymers was also calculated by SEC analysis using linear PS standards. Additionally, differential scanning calorimetry (DSC) was performed on a TA instruments DSC Q20 (New Castle, DE) using a scan rate of 10 °C/min under N2 atmosphere. Thermograms displayed in the Supporting Information (Figure S1) were recorded during the second heating cycle under nitrogen atmosphere with a ramp rate of 10 °C/min. Glass transition temperature (Tg) was determined as the midpoint of the transition and indicated within the plot. Characterization data of the polymers used in this study are summarized in Table 1. Spreading Solution Preparation. A chloroform (HPLC Plus Grade, Sigma-Aldrich) solution of PS-b-PEO or a mixture of PS-bPEO with homoPS was prepared with a 1 mg/mL concentration of PS-b-PEO. Concentration of PS-b-PEO was kept constant to prevent a possible morphological change of the PS-b-PEO at the air/water interface, which could be caused by a change in the spreading concentration.23,25,27 For the mixture, wPS and α were adjusted by varying the amount and Mn of homoPS in the solution, respectively. More detailed recipe is presented in the Supporting Information (page S-3). All of the solutions were stored in glass vials capped with Teflon-coated caps and sealed with parafilms, and then they were placed in a refrigerator. To allow for equilibrium, all of the solutions were made at least 24 h prior to the experiments. Langmuir Isotherms. A Wilhelmy plate tensiometer (RIEGLER and Kirstein GmbH) using filter paper and a customized Langmuir trough made of Teflon were used to measure surface pressure and surface area, respectively. The total surface area of the trough was 120 × 245 mm2, and the depth of the trough was 20 mm. The Langmuir trough was washed with acetone (Extra pure, OCI Company Ltd.), ethanol (Extra pure, OCI Company Ltd.), and ultrapure deionized water (Merck Milipore, 18.2 MΩ·cm) and then filled with 500 mL of ultrapure deionized water before the polymer solution was spread. The temperature of the water was maintained at 25 ± 0.5 °C by ambient atmosphere. In all of the experiments, 7−15 μL of the spreading solution was carefully spread on the air/water interface by small drops (ca. 3−4 μL) using a syringe (10 μL Gastight, Hamilton). After 15 min of waiting to allow chloroform evaporation and polymer film stabilization, compression was started at a constant rate of 6 mm/ min. While the two moving barriers of the trough symmetrically compressed the polymer films at the air/water interface, the values from the tensiometer and the trough were read in a customized LabVIEW program in real time. Langmuir−Schaefer Films and Atomic Force Microscopy. The Langmuir−Schaefer films were prepared similar to the description for the Langmuir isotherm measurement. A mica substrate (highest grade mica disk 9.9 mm, Ted Pella Inc.) with a glass supporting fixture (diameter = 20 mm, height = 15 mm) was submerged in water before compression, and the other procedures were the same. The polymer films at the air/water interface were compressed to a surface pressure of 8 mN/m, where dense and nonoverlapping PS domains could be observed. After waiting for 3 min to allow polymer film stabilization, the water outside of the barrier region was sucked out at a rate of 10 mL/min by an aspirator from the bottom. The polymer films were deposited on the submerged mica substrate as the surface of the water went down. An NX-10 (Park Systems) atomic force microscopy (AFM) equipped with etched silicon cantilevers NCHR (Nano World, resonance frequency 250−390 kHz and force constant 21−78 N/m) was used to characterize the morphology of the polymer films in tapping mode. At least five images of different spots including hundreds of domains were obtained for reproducibility and statistical analysis. The scan areas were usually from 2 μm × 2 μm to 8 μm × 8 μm, and the scan rate was 0.2−1.0 Hz. Flattening and height measurement of images were conducted using the image processing program XEI (Park Systems), and the binarization tools in MATLAB and Image J were used for width measurement. PS-b-PEO and homoPS Adsorption Test. PS-b-PEO (BCP33k, BCP20k) and homoPS (PS4.3k) adsorption to the chloroform/water

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RESULTS AND DISCUSSION Langmuir Isotherms of the BCP33k and homoPS Blends. Figure 1 presents the Langmuir isotherms of the

Figure 1. Langmuir isotherms of the BCP33k (PEO10k + PS23k) and homoPS of various Mn blends. Both polymers were dissolved in chloroform together with BCP33k concentration of 1 mg/mL and then spread on the air/water interface. (a) wPS = 0.80 and (b) wPS = 0.85.

BCP33k and homoPS of various Mn blends at the air/water interface when the PS mass fraction wPS = 0.80 and wPS = 0.85, respectively. The Langmuir isotherms for the BCP20k and homoPS blends are presented in the Supporting Information (Figure S2). We controlled the area per block copolymer molecule to focus on the effect of added homoPS. On the basis of previous research studies,20−23,25,27 Langmuir isotherms of PS-b-PEO exhibit three different regimes: (1) at low surface concentrations, PEO blocks stretching vertically into the water subphase are compressed first, showing higher compressibility C

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Langmuir (pancake regime); (2) at moderate concentrations, the wider projected area of the PEO blocks compared to hydrophobic PS blocks leads to the first-order phase transition because of the solubilization of the PEO blocks44 (plateau regime); (3) at high concentrations, PS blocks begin to be compressed, showing their lower compressibility (condensed regime). As the size of the PS blocks increases, the plateau regime can be shortened or disappear because large PS domains start to be compressed in the middle of plateau regime or pancake regime.20,22,27 In Figure 1, the isotherm of BCP33k shows that the pancake regime appears at surface pressures lower than 10 mN/m, followed by the condensed regime at higher than 10 mN/m without the plateau regime. For the BCP33k and various homoPS blends, the condensed regime starts at the larger area per block copolymer molecule simply because of the increase in wPS. This shift is correlated with the size increase of the PS domains according to wPS. Interestingly, except for the very low and high αs, the shift is similar for all Mn of homoPS as long as wPS remains the same. The exception seems to be attributed to the morphological differences because area per molecules is different depending on the morphology in spite of the same amount of PS.27 The morphological differences along the two variables, π and α, will be discussed in the next section. Phase Diagrams of the PS-b-PEO and homoPS Blends at the Air/Water Interface. To characterize the films in the blends at the air/water interface, the PS-b-PEO and homoPS of various Mn blends at the air/water interface were deposited on hydrophilic mica substrates by the Langmuir−Schaefer method for AFM analysis. The deposition was conducted at a surface pressure of 8 mN/m to obtain images of dense but not overlapping PS domains before the condensed regime begins. The AFM images of the BCP33k (wPS ≈ 0.70) and BCP20k (wPS ≈ 0.50) are shown in Figure 2. The bright areas represent

Figure 3. Phase diagram of the BCP33k (PEO10k−PS23k) and homoPS of various Mn blends at the air/water interface with two variables, wPS and α. Both BCP33k and homoPS were spread on the air/water interface from the prepared spreading solution and then deposited at 8 mN/m. The morphological transition from circular to cylindrical to planar structure is observed as wPS and α increase, and the new morphology is observed at a low α. The diagram is divided into four morphology regimes: new (white); circular and cylindrical (light gray); spaghetti (gray); planar (dark gray) morphology, and the scale is 8 μm × 8 μm except for the circular and cylindrical regime (4 μm × 4 μm).

planar structure was observed as α and wPS increased, which is consistent with the typical trend of PS-b-PEO associated with the increase in PS block ratio and spreading concentration.23−25,27 However, for the BCP20k and homoPS blends, cylindrical morphology was not observed, and only circular to planar morphology transition instead was observed despite the large Mn of homoPS (Figure 4). To rigorously confirm the

Figure 2. Morphology of PS-b-PEO diblock polymer at the air/water interface obtained from AFM. The polymers were spread by 1 mg/mL chloroform solution and then deposited and observed at a surface pressure of 8 mN/m. (a) BCP33k (PEO10k−PS23k) and (b) BCP20k (PEO10k−PS10k).

Figure 4. Phase diagram of the BCP20k (PEO10k−PS10k) and homoPS blends at the air/water interface with two variables, wPS and α. Both BCP20k and homoPS were spread on the air/water interface from the prepared spreading solution and then deposited at 8 mN/m. Morphological transition from circular to planar structure without cylindrical morphology is observed as wPS and α increase, and new morphology is seen at low α. The diagram is divided into three regimes: new (white); circular (light gray); planar (dark gray), and the scale is 4 μm × 4 μm except for the circular regime (2 μm × 2 μm).

PS domains whose height is taller than the light brown PEO. In Figure 2a, it is difficult to capture the height difference of the PEO and substrate by AFM because the PS domains are relatively tall compared to the PEO on the substrate. We mainly observed a circular morphology with a few short cylinders for both BCP33k and BCP20k, which is in good agreement with the previous studies.21−25,27 Figure 3 summarizes the phase diagram from the AFM images of the BCP33k and various homoPS blends at the air/ water interface. We controlled two parameters: α determined by Mn of the homoPS and wPS determined by the amount of homoPS. Overall, a transition from circular to cylindrical and

absence of the cylindrical morphology, we scanned the BCP20k and PS4.3k blends from wPS = 0.91 to wPS = 0.93 and still could not find any cylindrical morphology (Figure S3). We suspect that PS entanglement might be important for the blends as it is for PS-b-PEO.23−27 The PS block in PS-bPEO (BCP33k), larger than the PS entanglement MW (Me, approximately 13.3k−19.1k),45−47 showed cylindrical morD

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Figure 5. Schematic illustration of our suggested blending mechanism for the PS-b-PEO and homoPS blends at the air/water interface. Both polymers are spread on the air/water interface at the same time from the prepared spreading solution. As solvent evaporates, amphiphilic PS-b-PEO is adsorbed to the interface and then forms monolayers, whereas homoPS remains in solvent. After that, preformed domains are formed by the selfassembly of PS-b-PEO with solvent dewetting. At last, homoPS settles on the preformed domains as the solvent evaporates completely.

phology in the blends, whereas the BCP20k did not. In addition, we should note that the morphology transitions can be controlled by the amount and Mn of homoPS because of the spreadability change: as α and wPS increase, homoPS increases the viscosity of the spreading solution, resulting in the difference in spreadability. Eventually, the high viscosity of the spreading solution leads to the planar morphology because of the lack of spreadability. Moreover, when α is very small, in both the BCP33k and BCP20k blends with very low molar mass homoPS (PS0.8k), a “melt-like” morphology was discovered. It seems that PS0.8k acts as a plasticizer and makes domains behave as a liquidlike phase because of the low Tg. The details about this morphology are presented in the Supporting Information (Figure S4). Likewise, the morphologies can be different depending on α, and this explains the exception in the previous Langmuir isotherm section. We speculate that the liquidlike phase in PS domains causes the lower compressibility regime to start at the smaller area per block copolymer molecules in the Langmuir isotherm (α = 0.037 in Figure 1b). More elaborate explanation on the compressibility is provided in the next section. Blending Mechanism of the PS-b-PEO and homoPS Blends at the Air/Water Interface. Interestingly, the morphology transitions described in the previous section could not be explained by the “wet−dry brush theory”; the transition and the domain size increase are only expected at α < 1, and phase separation should occur when α > 1.9,14 However, the transition was observed regardless of α, and the macrophase separation of PS-b-PEO and homoPS did not occur even at high α (α ≫ 1) in our blends system. On the basis of this observation, we suggest a blending mechanism based on the different adsorption capability of PS-b-PEO and homoPS at the air/water interface, as depicted in Figure 5. Upon spreading the blends at the air/water interface, we found that the amphiphilic PS-b-PEO was immediately adsorbed to the interface, whereas hydrophobic homoPS remained in the solvent until all of the solvent evaporated. Figure 6 shows the interfacial tension measurements by the pendant-drop method, and the tension rapidly decreases with the adsorption of PS-b-PEO to the chloroform/water interface. Both BCP33k and BCP20k completed their adsorption within 3 s and reduced interfacial tension from 31 mN/m down to 20 mN/m. We used 1 mg/mL of block copolymer/chloroform solution which was the same concentration of spreading solution used in our blends system. However, PS4.3k did not reduce interfacial tension at all, even after a longer period of time for solvent evaporation, and even at a much higher concentration (500 mg/mL), thus proving that it is not

Figure 6. Interfacial tension measurement by pendant-drop method for PS-b-PEO and PS4.3k at the chloroform/water interface. Both BCP33k (PEO10k−PS23k, green triangle) and BCP20k (PEO10k− PS10k, blue circle) decrease the interfacial tension down to ∼20 mN/ m, whereas PS4.3k does not decrease interfacial tension (∼31.5 mN/ m). The inset is for the very early stage of adsorption for BCP33k and BCP20k to the interface. It took at least 3 s for a drop to become large enough to be measured.

adsorbed to the interface. Therefore, in the blends, only PS-bPEO form monolayers at the air/water interface at an early stage of the self-assembly. The PS-b-PEO monolayers in the blends, at the air/water interface, preferentially form domains by solvent dewetting as the solvent evaporates in the intermediate state, which is the same mechanism that forms domains in the block copolymer at the air/water interface,26,30 and then homoPS would increase the size of the preformed domains by settling on them as the solvent evaporates completely. Because hydrophobic homoPS prefers the PS domains to PEO blocks or water, they selectively land on the preformed PS domains. Therefore, it is PS-b-PEO that forms the domains and eventually determines the final morphology. This applies to the cylindrical morphology as well; PS-b-PEO forms the cylindrical domains first, and then the homoPS increases the size (Figure 5). Again, the morphology of the blends at the air/water interface is determined by the fast PS-b-PEO adsorption and slow homoPS adsorption, and homoPS increases the size of the domains preformed by PS-b-PEO. Moreover, homoPS could induce the phase transition from circular to cylindrical morphology by simply changing the viscosity of the spreading solution, similar to other studies of the block copolymer at the air/water interface,23−25,27 thus changing the morphology of the preformed PS-b-PEO domains before homoPS is deposited. E

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the well-blended state: the inner area is a PS block of PS-bPEO and the outer area is homoPS (the rightmost picture in Figure 5). Size Analysis of the BCP20k and homoPS Blends at the Air/Water Interface. The size of domains in the blends also supports our proposed mechanism. To analyze the phase diagrams of the blends in detail, we performed a size analysis of the domains in the circular morphology. The size analysis of the BCP33k and various homoPS blends is presented in the Supporting Information (Figure S5). The height and diameter of the domains were measured by AFM (see details in the Experimental Section). We observed various heights and diameters for the circular domains depending on the amount and Mn of homoPS, and both average height and diameter are presented in Figure 8, respectively. In the size analysis, at least

To support our proposed blending mechanism further, we looked into the compressibility of the PS domains in the PS-bPEO and low Mn homoPS blends at the air/water interface. We chose the low Mn homoPS because it has a higher compressibility than other higher Mn homoPS. Figure 7a

Figure 7. Langmuir isotherm (a) and compressibility for PS regime (b) of the BCP20k (PEO10k−PS10k) and PS0.8k blends (▲) compared to other polymers and blends at the air/water interface. PS compressibility (β) was calculated from the PS regime in (a), 1 dA β = − A × d π (A: trough area and π: surface pressure).

presents the Langmuir isotherm of the BCP20k and PS0.8k blends (black full triangle) at wPS = 0.90 including other polymers and blends systems. The BCP20k and PS0.8k blends have two different PS regimes different from the BCP20k and other homoPS (α = 0.23−3.61) blends (black full circle), probably because of the high compressibility of PS0.8k. We calculated the compressibility (Figure 7b), compressibility 1 dA (β) = − A × dπ , where A is the trough area and π is surface pressure, from the isotherm results. We should note that we plotted the compressibility only for PS. PS0.8k (red empty triangle) exhibits higher compressibility (0.1−0.15 m/mN), which is likely because of the low glass-transition temperature, whereas BCP20k (black line) with Mn of PS block is ∼10 000 g mol−1 and PS10k (Mn of PS ≈ 10 000 g mol−1, blue empty circle) show an order of magnitude lower compressibility (0.01−0.02 m/mN). In the “wet−dry brush theory”,9,14 thermodynamic blends of BCP20k and PS0.8k are supposed to have a compressibility between that of BCP20k and PS0.8k because the blends of PS10k and PS0.8k (green empty inverted triangle) show at the air/water interface. However, the BCP20k and PS0.8k blends show two different compressibilities: a high compressibility regime similar to PS0.8k is observed, followed by a separated and relatively low compressibility similar to BCP20k. Two distinct compressibilities imply that the PS domains consist of two areas rather than

Figure 8. Plots of size measurement on the circular domains in the BCP20k (PEO10k−PS10k) and homoPS blends at the air/water interface: (a) height and (b) diameter. Blue squares and red circles represent when wPS = 0.90 and 0.80, respectively, and black triangles indicate BCP20k without the addition of homoPS.

five images from different spots and hundreds of domains were used for statistical accuracy and reproducibility. In Figure 8a, the average height of BCP20k alone is less than 2 nm (1.61 ± 0.41 nm), and it increases to 7.51 ± 1.51 nm at wPS = 0.80 and 11.63 ± 0.76 nm at wPS = 0.90 after the addition of homoPS, indicating that height could be controlled by the amount of added homoPS. As expected, the increased average diameter of the domains is not affected by α, as can be seen in Figure 8b. The domain size is only relevant to the amount of homoPS that settles on the preformed PS-b-PEO domains, according to our proposed mechanism. The diameter, 30.5 ± 5.9 nm for BCP20k, was increased by wPS, 79.22 ± 5.07 nm at wPS = 0.80 and 99.47 ± 3.26 nm at wPS = 0.90. Therefore, in addition to F

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is added, but macrophase separation might have occurred for PS66k. This is in good agreement with previous studies.8−14 Figure 10 summarizes the morphology that results from spreading the self-assembled BCP33k and homoPS (PS4.3k,

the morphology control discussed earlier, domain size is controllable depending on wPS. Spreading of homoPS after PS-b-PEO Domain Formation at the Air/Water Interface. To confirm the importance of the preformed domains in determining the final morphology of the blends at the air/water interface, we spread PS-b-PEO first to form domains and then spread homoPS. First, BCP33k was spread and compressed to 8 mN/m, followed by the spread of PS4.3k from the same concentration of chloroform solution (1 mg/mL). Figure 9 shows the

Figure 10. Morphology of the BCP33k (PEO10k−PS23k) and homoPS (PS4.3k, PS66k) blends at wPS = 0.80 when they were spread on the air/water interface as self-assembled from the 1:10 mixture of chloroform and methanol. (a) BCP33k + PS4.3k and (b) BCP33k + PS66k.

PS66k) blends in solution on the air/water interface at wPS = 0.80. Unfortunately, we were not able to perform the direct adsorption experiments using the pendant-drop method because the solvent where PS-b-PEO and homoPS selfassemble is not completely immiscible with water. When these self-assembled blends in solution were spread on the air/ water interface, the BCP33k and PS4.3k blends exhibited a morphology (Figure 10a) similar to the phase diagram (Figure 4), whereas the BCP33k and PS66k blends exhibited macrophase separation (Figure 10b) that is completely different from the simple spreading without self-assembly in solution. The self-assembled BCP33k and PS66k blends in solution might slow down the adsorption to the interface until all of the solvent evaporates and then are simply deposited as macrodomains without sequentially following our proposed mechanism. This suggests that PS-b-PEO adsorption to the interface is crucial, and controlling the adsorption kinetics allows different morphologies to be obtained.

Figure 9. Morphology of the BCP33k (PEO10k−PS23k) and PS4.3k blends at the air/water interface when wPS = 0.85 (BCP33k is spread first followed by the later-spread of PS4.3k).

morphology of the BCP33k and PS4.3k blends at wPS = 0.85 from the sequential and separate spreading. We found that circular domains were formed first from BCP33k, and then the later-spread PS4.3k increased the size of the circular domains without morphology transition. Therefore, the later-spread homoPS on the preformed domains simply increases the size of the domains, as our proposed mechanism is suggested. For the case of the BCP33k and homoPS blends, when both polymers are spread at the same time, homoPS increases the viscosity of the spreading solvent, which preferentially forms the cylindrical domains of BCP33k, and homoPS settles on the domains later. Adsorption Control through Self-Assembly in Solution to Achieve Different Morphologies. The adsorption of PS-b-PEO to the air/water interface is a key factor in our blending mechanism; therefore, we tried to control the adsorption kinetics through self-assembly in solution. We expected that it would take far longer time for the selfassembled PS-b-PEO in solution to get adsorbed to the interface because they are much bigger in size, so that it takes more time to diffuse and reach the interface. It also takes additional time for the self-assembled PS-b-PEO in solution to disassemble and reassemble at the interface.48,49 We selfassembled the polymer in the 1:10 mixture of chloroform and methanol. We chose methanol as it is a selective solvent for each block, that is, a poor solvent for PS and good solvent for PEO. The DLS analysis is summarized in the Supporting Information (Figure S6, page S-9). The size of BCP33k selfassembly in the chloroform and methanol solution was ∼72.6 nm, which is much bigger than a single polymer (it is expected to be less than 20 nm in good solvent50) with a slight size increase to 117.0 nm, following the addition of PS4.3k and significant increase to 27470.3 nm, with the addition of PS66k (Figure S6). This size difference shows that MW of the homoPS plays a key role in determining the self-assembled structure in solution. Before spreading on the interface, the BCP33k self-assembly in solution is likely to swell when PS4.3k

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CONCLUSIONS We performed a systematic study of PS-b-PEO and homoPS blends at the air/water interface using two PS-b-PEO (BCP20k, BCP33k) with different PS block sizes and various homoPS of different Mn. We constructed the phase diagrams of the blends and then found that the size of the PS blocks in PSb-PEO tended to determine the morphology of the blends. The diagrams show the circular to cylindrical to planar morphology transition as wPS and α increase for the blends of BCP33k whose PS block size is larger than PS Me, whereas cylindrical morphology is absent for the blends of BCP20k. The domain sizes of both blends were increased by wPS. On the basis of the phase diagrams of the blends, we suggested a blending mechanism, which is different from the “wet−dry brush theory”9,14 employed for blends in bulk. In our blending mechanism, the PS-b-PEO forms domains first at the air/water interface, which eventually determines the final morphology. After PS-b-PEO has self-assembled at the air/water interface, homoPS that are still in the spreading solution will be added to the preformed PS domains, thus increasing the size of the domains. While PS-b-PEO determines the final morphology by its fast self-assembly, homoPS rather controls the morphology G

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and Improved Lithography by Directed Block Copolymer Assembly. Science 2008, 321, 936−939. (4) Rodwogin, M. D.; Spanjers, C. S.; Leighton, C.; Hillmyer, M. A. Polylactide−Poly(dimethylsiloxane)−Polylactide Triblock Copolymers as Multifunctional Materials for Nanolithographic Applications. ACS Nano 2010, 4, 725−732. (5) Kelley, E. G.; Albert, J. N. L.; Sullivan, M. O.; Epps, T. H., III Stimuli-Responsive Copolymer Solution and Surface Assemblies for Biomedical Applications. Chem. Soc. Rev. 2013, 42, 7057−7071. (6) Segalman, R. A.; McCulloch, B.; Kirmayer, S.; Urban, J. J. Block Copolymers for Organic Optoelectronics. Macromolecules 2009, 42, 9205−9216. (7) Hamley, I. W. Nanostructure Fabrication Using Block Copolymers. Nanotechnology 2003, 14, R39−R54. (8) Whitmore, M. D.; Smith, T. W. Swelling of Copolymer Micelles by Added Homopolymer. Macromolecules 1994, 27, 4673−4683. (9) Koizumi, S.; Hasegawa, H.; Hashimoto, T. Ordered Structures of Block Copolymer/Homopolymer Mixtures. 5. Interplay of Macroand Microphase Transitions. Macromolecules 1994, 27, 6532−6540. (10) Torikai, N.; Takabayashi, N.; Noda, I.; Koizumi, S.; Morii, Y.; Matsushita, Y. Lamellar Domain Spacings of Diblock Copolymer/ Homopolymer Blends and Conformations of Block Chains in Their Microdomains. Macromolecules 1997, 30, 5698−5703. (11) Smith, M. D.; Green, P. F.; Saunders, R. Anisochemical Homopolymer/Diblock Copolymer Thin Film Blends. Macromolecules 1999, 32, 8392−8398. (12) Orso, K. A.; Green, P. F. Phase Behavior of Thin Film Blends of Block Copolymers and Homopolymers: Changes in Domain Dimensions. Macromolecules 1999, 32, 1087−1092. (13) Kim, S. H.; Misner, M. J.; Russell, T. P. Solvent-Induced Ordering in Thin Film Diblock Copolymer/Homopolymer Mixtures. Adv. Mater. 2004, 16, 2119−2123. (14) Luo, C.; Han, X.; Gao, Y.; Liu, H.; Hu, Y. Crystallization behavior of ″wet brush″ and ″dry brush″ blends of PS-b-PEO-b-PS/hPEO. J. Appl. Polym. Sci. 2009, 113, 907−915. (15) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (16) Zhu, J.; Eisenberg, A.; Lennox, R. B. Interfacial Behavior of Block Polyelectrolytes. 1. Evidence for Novel Surface Micelle Formation. J. Am. Chem. Soc. 1991, 113, 5583−5588. (17) Zhu, J.; Lennox, R. B.; Eisenberg, A. Interfacial Behavior of Block Polyelectrolytes. 2. Aggregation Numbers of Surface Micelles. Langmuir 1991, 7, 1579−1584. (18) Zhu, J.; Eisenberg, A.; Lennox, R. B. Interfacial Behavior of Block Polyelectrolytes. 5. Effect of Varying Block Lengths on the Properties of Surface Micelles. Macromolecules 1992, 25, 6547−6555. (19) Zhu, J.; Lennox, R. B.; Eisenberg, A. Interfacial behavior of block polyelectrolytes. 4. Polymorphism of (quasi) two-dimensional micelles. J. Phys. Chem. 1992, 96, 4727−4730. (20) Fauré, M. C.; Bassereau, P.; Lee, L. T.; Menelle, A.; Lheveder, C. Phase Transitions in Monolayers of PS−PEO Copolymer at the Air−Water Interface. Macromolecules 1999, 32, 8538−8550. (21) Cox, J. K.; Yu, K.; Constantine, B.; Eisenberg, A.; Lennox, R. B. Polystyrene−Poly(ethylene oxide) Diblock Copolymers Form WellDefined Surface Aggregates at the Air/Water Interface. Langmuir 1999, 15, 7714−7718. (22) Baker, S. M.; Leach, K. A.; Devereaux, C. E.; Gragson, D. E. Controlled Patterning of Diblock Copolymers by Monolayer Langmuir−Blodgett Deposition. Macromolecules 2000, 33, 5432− 5436. (23) Devereaux, C. A.; Baker, S. M. Surface Features in Langmuir− Blodgett Monolayers of Predominantly Hydrophobic Poly(styrene)− Poly(ethylene oxide) Diblock Copolymer. Macromolecules 2002, 35, 1921−1927. (24) Hosoi, A. E.; Kogan, D.; Devereaux, C. E.; Bernoff, A. J.; Baker, S. M. Two-Dimensional Self-Assembly in Diblock Copolymers. Phys. Rev. Lett. 2005, 95, 037801. (25) Cheyne, R. B.; Moffitt, M. G. Novel Two-Dimensional ″Ring and Chain″ Morphologies in Langmuir−Blodgett Monolayers of PS-

transition of the blends by changing the viscosity of the spreading solution, similar to other studies of the block copolymer at the air/water interface.23−25,27 The change in the domain size and the morphology transition successfully obtained in the blends can replace the synthesis of various block copolymers for the desired films. Moreover, because the domain size is rarely affected by Mn of homoPS (even no macrophase separation when α ≫ 1), we can easily control the domain size regardless of the size of homoPS. Our proposed mechanism explains well the observed morphology of the PSb-PEO and homoPS blends at the air/water interface. This type of mechanism can be also exploited for other blending systems with large fluid/fluid interfaces such as emulsions and foams.51 Moreover, it might be applied to typical thin films on solid substrates as well in cases where one of the blocks prefers to adsorb on the solid substrate, thus providing an additional route for controlling the block copolymer thin-film morphology.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b02192. Thermograms of various homoPS, recipe for preparing the PS-b-PEO and homoPS mixtures, Langmuir isotherms of the BCP20k and various homoPS blends, AFM images and size analysis of the BCP20k and PS4.3k blends at various wPS, AFM images and height information for the BCP20k and PS0.8k blends, size analysis on the circular and cylindrical domains in the BCP33k and homoPS blends, DLS analysis for the selfassembly of BCP33k and homoPS (PS4.3k and PS66k) in the 1:10 mixture of chloroform and methanol, and estimation of the effective diameter from DLS analysis (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.S.). *E-mail: [email protected] (S.Q.C.). ORCID

Baekmin Q. Kim: 0000-0002-6231-9741 Myungeun Seo: 0000-0002-5218-3502 Siyoung Q. Choi: 0000-0002-6020-3091 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF-2015R1C1A1A01054180).

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REFERENCES

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