Effective Morphology Control of Block Copolymers and Spreading

Letter pubs.acs.org/JPCL

Effective Morphology Control of Block Copolymers and Spreading Area-Dependent Phase Diagram at the Air/Water Interface Dong Hyup Kim and So Youn Kim* School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan 44919, Republic of Korea S Supporting Information *

ABSTRACT: Over the past several decades, tremendous efforts have been made to understand the fundamental physics of block copolymer (BCP) self-assembly in bulk or thin films, and this has led to the development of BCP-based bottom-up nanofabrication. BCPs also form periodic nanostructures at the air/water interface, which has potential application to ultrathin-film nanopatterning with molecular-level precision. Nonetheless, controlling the nanostructure formation at the air/water interface is restricted by the inherent parameters of BCPs; BCP morphology is determined by the hydrophilic-tohydrophobic block ratio. Here we show that controlling the spreading area of BCPs at the air/water interface can tune the shape and size of BCP structures, suggesting a new phase diagram of BCP structures as a function of the relative block fraction and spreading area. A neat polystyrene-b-poly(2-vinylpyridine), known to form a dot morphology, instead forms a strand or planar morphology when the spreading area is varied with Langmuir−Blodgett technique.

B

weight,27,40 solution concentration,27,33−37 solvent quality,17,25,31 temperature,32 and the pH of the subphase.18,20 Therefore, there is generally no morphology transition observed when the BCP block ratio is fixed. A few morphology transitions have been observed19,26,41 but only under certain conditions. Controlling the surface pressures19,26 or blending BCPs with other polymers19,41 could induce such transitions. Although these results are interesting, these transitions are incomplete41 or require complicated conditions such as crudely controlled area reduction with residual spreading solvent26 and changing polymer chemistry19,41 through blending. Here we show that simple control of the spreading area results in morphology transition, thus regulating the selfassembling nature of BCPs at the interface. We propose a general phase diagram of BCPs at the interface for the first time, to the best of our knowledge, where the relative block fraction and the spreading area are two parameters. When a solution of PS-b-P2VP 44−18.5 kg/mol (44−18.5k, VP 29.4%) in chloroform is dropped at the air/water interface, hydrophilic blocks (29.4 mol %) induce a dot morphology at the free air/water interface, which is consistent with the results of previous studies.21,27 Here VP is a mole fraction of P2VP block. We found, however, that the blocks showed a strand morphology, as spreading was confined to a smaller area (Figure 1); the spreading area was varied by spreading the solution onto a pre-compressed area. We postulated that the final morphology would depend on whether the space constraints were applied before the

lock copolymers (BCPs) are remarkable materials in nanoscience and are employed in many applications, especially in bottom-up nanofabrication.1 Many of these applications rely on the self-assembling nature of BCPs, which results in periodic nanostructures such as spheres, cylinders, and lamellas.2,3 Therefore, the fundamental chemistry and physics needed to control BCP self-assembly in bulk or thin films have been extensively studied,4−7 which led a complete phase diagram for diblock or triblock copolymers.6,7 The periodic structure of BCP self-assembly is determined by the Flory−Huggins interaction parameter (χ), the degree of polymerization, and the relative block ratio.6 BCPs can also form picturesque periodic structures at air/ water interfaces in ultrathin films. The Langmuir−Blodgett (LB) technique8−10 is often employed to facilitate interfacial self-assembly in ultrathin films, and early studies11−15 were dedicated to understanding the interfacial assembly of BCPs, focusing on the parameters that control the morphology. Wellknown amphiphilic BCP systems have been investigated11−41 such as polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP),17−21 PS-b-P2VP/other BCP blends,21 polystyrene-b-poly(ethylene oxide) (PS-b-PEO),11,14,15,29−37 polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA),31,38−41 polystyrene-b-poly(butyl methacrylate),13 polystyrene-b-poly(4-vinylpyridine) (PS-bP4VP),22−27 and a mixture of PS-b-P4VP/3-n-pentadecylphenol.22−27 These BCPs form dot (disks), strand (stripes/ ribbons/rods), and planar (large disks) morphologies at the air/water interface analogous to those obtained in thin films or in bulk. Similar to self-assembly in bulk, the assembling behavior at the interface is universally governed by the relative ratio of hydrophilic to hydrophobic blocks,11−15,27 whereas the detailed structures from self-assembly are affected by molecular © XXXX American Chemical Society

Received: February 27, 2017 Accepted: April 6, 2017 Published: April 6, 2017 1865

DOI: 10.1021/acs.jpclett.7b00471 J. Phys. Chem. Lett. 2017, 8, 1865−1871

Letter

The Journal of Physical Chemistry Letters

the sizes of the circular PS domains and the center-to-center distance of the dots decreased as the layer was compressed, as shown in the images from ▲7 to ▲3. An interesting order transition of the dot morphology was found, from a quasihexagonal array to quasi-square array, after the plateau of the π−A isotherm curve (▲1 and ▲2). The order transition buffered the increasing surface pressure, and thus this order transition was preferred as the layer was compressed. These results are consistent with the results of previous studies.23,24 The BCP interfacial assembly in the pre-area confinement (★) experiments was qualitatively and quantitatively different from that in the post-area confinement experiments. First, the hydrophobic PS domains formed larger dots with increased center-to-center distance (Figure 2c, ★5 and ★6). Second, the PS domains started to form strands as the spreading area was reduced (★4), whereas the surface pressure remained the same as that of the post-area confinement (▲4) at the plateau of the π−A isotherm curve. Third, a planar morphology is seen in the more reduced surface areas (★1 and ★2). The surface pressure no longer increased, presumably due to partial aggregation of the polymer chains that exceeded the Langmuir monolayer. The approximate film thicknesses were found to be 5−10 nm based on AFM height images. The interfacial assembly of the PS-b-P2VP films was also characterized by grazing incidence small-angle X-ray scattering (GISAXS) (Figure 3a). The detailed structure of the films, including the array types and center-to-center distance of the PS domains, were confirmed from peak assignments of the 1D line cuts of the GISAXS images (Figure S1). In the post-area confinement experiment, the center-to-center distance of the PS dots linearly decreased (▲3−▲7) as the barrier compressed the monolayer until the dots formed a square array (▲1−▲3) (Figure 3b). The peak became broader as the

Figure 1. AFM height images of PS-b-P2VP (44−18.5k) LB films made with 10 μL of a 1 mg/mL of chloroform solution obtained from (a) a free spreading area (confined to a 12 cm diameter Petri dish) and (b) an area confined to a 6 cm diameter Petri dish. Scale bar is 1 μm.

spreading solvent evaporation or after. Further systematic experiments with PS-b-P2VP (44−18.5k, 29.4%) were therefore performed to examine the constraint effect. With post-area confinement, the LB monolayer of the BCP solution was spread over the surface in the maximum trough area and compressed by barriers after 20 min of solvent evaporation. For pre-area confinement, the trough area was reduced first; then, the LB monolayer was spread over the surface of the water (Figure 2a). Figure 2b shows the obtained surface pressure−area (π−A) isotherm curve of PS-b-P2VP (44−18.5k). The blue triangles (▲) represent the LB films obtained by the conventional deposition process (post-area confinement) and the red stars (★) represent the LB films obtained by pre-area confinement. Atomic force microscopy (AFM) images of each film are shown in Figure 2c. In the post-area confinement experiments (▲), PS-b-P2VP formed dot morphology on the surface (▲7), and

Figure 2. (a) Schematic illustration of post-area confinement (first row, ▲) and pre-area confinement (second row, ★) experiments. (b) π−A isotherm curve of PS-b-P2VP (44−18.5k). (c) 2 × 2 μm AFM height images at each point in panel b (▲1−7, post-area confinement and ★1−6, prearea confinement). Scale bar is 400 nm. 1866


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Figure 3. (a) GISAXS 2D images of all PS-b-P2VP (44−18.5k) films in Figure 2. (b) Measured center-to-center distances of structures in each film (▲1−7, post-area confinement; ★3−6, pre-area confinement). (c) GISAXS 1D line cuts of ▲1−7 and ★5−6 from panel a. (Peaks are emphasized by multiplying q2 and I(q) (Kratky plot)). (d) Calculated correlation length from panel c.

When BCPs are spread on an air/water interface, the hydrophilic chains are absorbed onto the water surface to prevent contact between the hydrophobic chains and the water; the hydrophobic chains form a uniform planar morphology before solvent evaporation. Next, the hydrophobic domains start to partially dewet, forming a strand or dot morphology, as the area continuously increases during solvent spreading. The partial dewetting initially occurs by local density fluctuation of the nascent hydrophobic planar domains; then, the hydrophilic chains are exposed to air at the dewetting point, which causes additional dewetting until the hydrophilic chains are fully stretched. The dewetting of hydrophobic domains is attributed to the repulsive interactions of the PS with the hydrophilic surface (P2VP and water), while the chain stretching of the hydrophilic blocks brings additional dewetting forming different morphologies. The consumed elastic energy by the hydrophilic chain stretching is compensated by the interactions between hydrophilic monolayer and the water surface. The shorter the hydrophilic chains of the block, the less additional dewetting occurs during film spreading because of the finite size of the hydrophilic chains. If the hydrophilic chains are short enough, then even fully stretched hydrophilic chains will not result in additional dewetting of the hydrophobic chains. Thus PS-b-P2VP with a high hydrophilic fraction (greater than ∼28%) forms dot morphology, whereas PS-bP2VP with a low hydrophilic fraction (less than ∼15%) forms a planar morphology because of the limited chain stretching of the short hydrophilic blocks. (Figure S2) The degree of additional dewetting of hydrophobic domain is governed by the relative lateral areas between the hydrophobic domains and the hydrophilic domains.27

spreading area decreased (Figure 3c), and the correlation length, calculated based on the Scherrer equation42−46 (see Supporting Information), linearly decreased with the spreading area, implying that the compressing barrier after solvent evaporation disturbed the order of the array (Figure 3d). In the pre-area confinement experiments, the sizes of the dots in the hexagonal arrays were much larger than those in the post-area confinement experiments, with longer center-tocenter distances of ∼130 nm (★5−★6) (Figure 3b). As the spreading area decreased, the morphology transitioned to a strand morphology, where the center-to-center distance of the strands suddenly decreased to 102.5 nm (★4); the morphology later transitioned to a planar morphology (Figure 3b). Thus one can effectively tune the self-assembling morphology by controlling the spreading area. Furthermore, the ordering quality of post-area confinement is better than that of pre-area confinement, with enhanced correlation length for the same spreading area (Figure 3d). Moffitt and coworkers suggested a dewetting mechanism for the morphology formation process of interfacial assembly.36,47 Furthermore, Bazuin and coworkers introduced the concept of relative lateral areas of the two blocks as the principle of interfacial assembly, originated from the competition between hydrophilic and hydrophobic blocks; a hydrophilic block tends to maximize its surface area, whereas a hydrophobic block tends to minimize it.27 In the present study, morphology transitions are explained using a modified dewetting mechanism, which combines the dewetting mechanism36,47 with the concept of relative lateral areas27 of the two blocks. The dewetting mechanism proposed in the present study is shown schematically in the first row of Figure 4 and Figure S2. 1867


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Figure 4. Schematic illustration of dewetting process for BCP with high hydrophilic block ratio in various spreading areas. Red arrows indicate the stretching of hydrophilic chains and yellow arrows indicate the dewetting of hydrophobic chains.

forces, solvent evaporation, vitrification or entanglement of hydrophobic block, and diffusion of the polymers. They showed that the initial concentration of the BCP solution is important because a concentrated solution can restrict polymer spreading and thus kinetically trap the hydrophilic chain stretching. For example, PS-b-PEO at a fixed block fraction formed a dot morphology at a moderate BCP concentration but a strand morphology at much higher concentrations. Similarly, a few morphology changes at very high concentration solution spreading were reported.33,34,36 Here the final morphologies were varied because rapid solvent evaporation kinetically prevented further chain stretching of hydrophilic blocks and further dewetting of the hydrophobic blocks; the morphology transition occurred when the number of BCP chains per unit area at the air/water interface increased, as observed in the present study. Nonetheless, chain stretching and uniform film formation were not easily controlled in this method. We also calculated the free energy of an ultrathin block copolymer film based on the study by Potemkin and coworkers,50 considering the interfacial and elastic free energy. We compared the free energy of ▲3 (▲5) and ★3 (★5) of Figure 2 according to the disk or stripe models.50 The approximated free energy of ★3 (★5) was smaller than that of ▲3 (▲5), which implies that the strand (large dots) morphology is more thermodynamically favored than compressed dots (small dots), supporting the order transitions (Figure S4). The detailed calculation is provided in the Supporting Information. In addition, we found that the morphology transition was universal, regardless of the molecular weight of BCP. In Figure S5, the same experiments were performed with two other BCPs having different molecular weights but similar relative block fractions, PS-b-P2VP (30−12.5k, VP 29.6%) and (180−77k, VP 29.8%). Regardless of size differences, all BCPs with similar block fractions exhibited the same morphology transitions: dotto-strand/planar morphology transitions in pre-area confine-

We hypothesized that pre-area confinement traps the spreading of the BCP solution mechanically, thus preventing further stretching of the hydrophilic chains and reducing the lateral area occupied by hydrophilic domain. Therefore, it suppresses the additional dewetting of the hydrophobic domains; the morphology cannot be developed but can be frozen in an early stage of dewetting process. The overall morphology transition mechanism is depicted in Figure 4. Controlling the spreading area by pre-area confinement can effectively suppress the chain stretching of hydrophilic blocks. In a confined area, even long hydrophilic chains cannot be stretched; instead, they remain coiled. These coiled hydrophilic chains cannot facilitate the additional dewetting of hydrophobic domains in a confined area (third−fifth rows in Figure 4), similar to what occurs with short hydrophilic chains (second or third row in Figure S2). Subsequent solvent evaporation freezes the morphologies. Therefore, all three morphologies (dot, strand, and planar, ★1−★6; Figure 2c) were formed at a fixed hydrophilic chain length by controlling the stretching of the hydrophilic blocks. These results correspond to those of a recent study, where hydrophilic chains were coiled when PS-bPMMA formed strands at the air/water interface with a boundary block fraction between that of strands and dots.41 When the spreading area was reduced little (second row in Figure 4), the BCPs formed dots larger than those from postarea confinement. This is because the presence of fewer large dots is more advantageous for hydrophilic chain stretching than the presence of many smaller dots. The proportion of the area occupied by the hydrophilic chains was greater in pre-area confinement (★ 5 −★ 6 ) than in post-area confinement (▲5−▲6), as shown in Figure S3. There have been some theoretical approaches35,48−51 discussing the thermodynamics of ultrathin BCP films at the interfaces based on the free-energy calculation of BCP structures. Hosoi and coworkers35 simulated a PS-b-PEO LB system based on a mathematical model considering Marangoni 1868


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chains is suppressed because of the limited area, further dewetting is prevented and the morphology is determined by the degree of the area confinement. Moreover, we believe that occasionally reported19,26,33−36,41 morphological changes can be integrated with this spreading-area controlled dewetting phenomenon. To the best of our knowledge, we suggested new phase diagram of interfacial BCP assembly for the first time, in which the spreading area is an important variable. Therefore, one can obtain various morphologies even with a neat BCP at a fixed block fraction. We believe our study provides a new understanding of interfacial assembly and offers potential applications of BCP ultrathin films.

ment. The hexagonal-to-square array order transition of the dots was also observed in post-area confinement. Furthermore, when the spreading area was normalized with chain length, there was a universal threshold at each transition, such that all three BCPs showed dot-to-strand morphology transitions at 5 Å2 per repeat unit (3 Å2 for strand-to-planar transition) (Figure S5c). The molecular-weight-independent morphology transition implies that there is an intrinsic generality for morphology transitions based on their block fractions. Thus we further examined the spreading area-dependent morphology transition of various PS-b-P2VPs with varying hydrophilic ratios: 55−8k, 88−18k, 30−8.5k, 32.5−12k, 44−18.5k, 40−18k, 50.9−29.1k, and 66−44k (full information is in Table S1). On the basis of the hydrophilic block fractions of the BCPs, we created a spreading area-dependent phase diagram of the three basic morphologies (dot, strand, and planar) with two variables: the relative block fraction and the spreading area (Figure 5). (All π−A isotherm curves and AFM images are given in Figure S6.)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b00471. Experimental section, AFM and SEM images, GISAXS 1D profiles, Calculation details, and π−A isotherm curves (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Tel: +82 52 217 2558. ORCID

So Youn Kim: 0000-0003-0066-8839 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) funded by the Ministry of Education (NRF2016R1D1A1B03930627). GISAXS experiments were performed at the 6D UNIST-PAL Beamline of the Pohang Accelerator Laboratory.

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Figure 5. Apparent phase diagram of interfacial self-assembly with varying hydrophilic block ratio and spreading areas. Dot, strand, and planar morphologies are indicated. PS-b-P2VP 55−8k, 88−18k, 30− 8.5k, 32.5−12k, 44−18.5k, 40−18k, 50.9−29.1k, and 66−44k were employed. The spreading area was normalized with the number of total repeat units of each BCP.

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Interestingly, this phase diagram resembles the half cut of the BCP phase diagram in bulk.6,7 Furthermore, we found that the perforated planar morphology formed in the boundary between the strand and planar morphology in the phase diagram of the LB monolayer (Figure S7), similar to the gyroid morphology between lamellar and cylinder in the BCP bulk phase diagram. The perforated planar morphology formed in the intermediate stage of the transition from planar to strand morphology during the dewetting process (fourth row in Figure 4). In conclusion, we show that the self-assembling morphology of a neat amphiphilic BCP at the air/water interface is not fixed by a fixed relative block ratio; instead, it can be effectively modified when the spreading area is controlled. The morphology transition is explained in terms of the modified dewetting mechanism based on the concept of relative lateral areas27 of the two blocks. When chain stretching of hydrophilic 1869


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Effective Morphology Control of Block Copolymers and Spreading

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