Swelling of Block Copolymer Nanoparticles: A Process Combining

Mar 18, 2013 - Kang Hee Ku , Jae Man Shin , Minsoo P. Kim , Chun-Ho Lee , Min-Kyo Seo , Gi-Ra Yi , Se Gyu Jang , and Bumjoon J. Kim. Journal of the ...
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Article pubs.acs.org/Langmuir

Swelling of Block Copolymer Nanoparticles: A Process Combining Deformation and Phase Separation Shilin Mei,† Lu Wang,† Xunda Feng,‡ and Zhaoxia Jin*,† †

Department of Chemistry, Renmin University of China, Beijing 100872, P. R. China Max Planck Institute for Dynamics and Self-Organization (MPIDS), Am Fassberg 17, 37077 Goettingen, Germany

‡

S Supporting Information *

ABSTRACT: Swelling of block copolymers is a complex process in which deformation and microphase separation couple together. Here we demonstrated that nanoparticles of block copolymers and polymer composites which have a large variety of phase separation patterns and different shapes can be generated through swelling process. Particularly, we focused on the swelling process of lamellae-forming diblock copolymer nanoparticles and first observed the formation of terrace edges in diblock copolymer nanoparticles as a metastable microstructure in swelling. Moreover, the trace amount of swelling solvent shows a significant influence on the shape of polymer nanoparticles, leading to block copolymer nanodisks and snowman-like composite nanoparticles.

1. INTRODUCTION Shape is an important feature for polymer nanoparticles which influences their properties significantly. Several studies have clarified that the interaction of polymer nanoparticles with cells1 and the rate of phagocytosis2 of polymer particles are impacted by their shapes. Flow effects and shear forces in blood circulation decreased the ability of macrophages to uptake these particles with high aspect ratio, thus resulting in extraordinarily long blood circulation times.3 So nonspherical nanoparticles with large aspect ratio are expected to be good candidates for long-circulating vehicles that can deliver anticancer drugs to tumors.4 How to generate polymer nanoparticles with different shapes is a big challenge for researchers. Recently, many efforts have been devoted to the fabrication of nonspherical polymer particles. Mitragotri et al. demonstrated two simple approaches to generate a diverse range of particle shapes.5 In their fabrication methods, polystyrene particles were first dispersed in poly(vinyl alcohol) film, and then they were softened by solvent liquefying or heating. Finally, PVA films containing polystyrene particles were stretched in different manners to generate deformed particles with a wide range of geometries including worms, elliptical disks, and barrels. DeSimore’s group developed the PRINT method to fabricate particles with a variety of shapes and sizes.6 By controlling the directionality of phase separations in seeded polymerization techniques, Weitz et al. produced the nonspherical colloidal particles with tunable shapes.7 Polymer disks are also obtained by transformation of polymer spheres under magnetic stirring.8 On the other hand, microphase separation of block copolymer enables the patterned morphology and distinguished chemical compositions in single particle, providing possibilities to integrate different functions in single particle device.9,10 To better © 2013 American Chemical Society

understand and capitalize on the role of shape and chemical composition in particle’s functions, we focus on the nonspherical nanoparticles composed of block copolymers or polymer composites which are scarcely studied. In our previous study, we have developed a novel strategy to produce polymer nanospheres through the swelling-induced deformation of polymer nanocylinders generated by Rayleigh instability of polymer nanotubes inside AAO channels.9 We noticed that deformation of swollen block copolymer nanocylinders dispersed in aqueous solution is a complex process in which many experimental parameters may have influences on their morphologies. The final products are the results of the coupling effect of deformation and phase separation. Here we reported our detailed studies on the cooperation of deformation and phase separation of polymer nanoparticles. We highlighted the influences of swelling solvents and the composition of block copolymers (BCPs) and polymer composites. We noticed that the deformation of lamellaeforming block copolymer presents a process with peculiar multistages, in which BCP nanoparticles show terrace edges. This is the first report of the terrace edge appeared in lamellaeforming BCP nanoparticles. Previous investigations of terrace edge are only in lamellae-forming BCP thin film. Besides, we also showed an experimental detailtrace amount of solvent in particle suspension results in great change of nanoparticles’ shape, generating disk-shaped BCP nanoparticles and snowman-shaped polymer composite nanoparticles. The deep study of deformation and phase separation of polymer nanoblocks Received: January 29, 2013 Revised: March 18, 2013 Published: March 18, 2013 4640

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Scanning electron microscopy (SEM, JEOL 7401) was conducted at an accelerating voltage of 5 kV. The samples were coated with a thin layer of gold before SEM characterization. TEM studies were conducted with a Hitachi TEM (TEM, H-7650B) at an accelerating voltage of 100 kV. A droplet of polymer nanoparticle suspension was placed onto copper grid for TEM analysis. The samples of PS-b-P2VP nanospheres, PS/PMMA nanorods with periodic encapsulated holes, PS/PMMA nanoparticles, and PS-b-PB/PMMA nanoparticles were stained by I2 vapor (at 60 °C) or OsO4 separately to enhance the phase contract before TEM observation.

not only helps us develop new methods to produce particles with large variety of shapes and patterns but also clarifies some complex phenomena in the swelling process.

2. EXPERIMENTAL SECTION Polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) with different molecular weights (PS27700-b-P2VP4300, Mw/Mn = 1.04, volume fraction of P2VP f P2VP ∼ 0.13; PS23600-b-P2VP10400, Mw/Mn = 1.04, f P2VP ∼ 0.31; and PS40500-b-P2VP40000, Mw/Mn = 1.10, f P2VP ∼ 0.50), polystyrene-bpolybutadiene (PS190600-b-PB97000, 1,2-addition, Mw/Mn = 1.09), and poly(methyl methacrylate) (PMMA, Mw = 163 500, Mw/Mn = 1.09) were the products of Polymer Source Inc. and used as received. Polystyrene (Mw = 25 000, Mw/Mn = 1.06) was the product of Alfa Aesar. Analytical toluene, dichloromethane (CH2Cl2), m-xylene, ethanol, and sodium hydroxide were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd., and used as received. Poly(ethylene glycol) monododecyl ether (C12E6) was obtained from Tokyo Chemical Industry Co., Ltd. Deionized water (Millipore Q; >18 MΩ cm) was used for all aqueous solutions. Anodic aluminum oxide (AAO) membrane (Anodic 13, 0.2 μm) was the product of Whatman Ltd. The membranes were thoroughly rinsed by organic solvents and deionized water and then annealed at 150 °C for 1 h in vacuum before use. In general, clean AAO membrane was first immersed in PS-b-P2VP/ CCl4 (80 mg/mL) solution for 1 h, and it was dried at ambient condition and then dried in vacuo. The AAO membrane with BCP nanotubes were heated at 180 °C for 20 min. Because of Rayleigh instability of polymer nanotubes in AAO channels, polymer nanorods with periodically encapsulated holes were generated. AAO membrane was then removed in 5 wt % NaOH aqueous solution, and the released polymer nanorods were picked up and thoroughly washed by DI water. Then the aqueous suspension of polymer nanorods were chopped to separated nanoblocks under ultrasonication and used for swelling.9 To identify the influence of different solvents to swelling products, we have compared three different BCPs (PS27700-b-P2VP4300, PS23600-bP2VP10400, PS40500-b-P2VP40000) swollen by two different solvents: toluene and CH2Cl2. In general, the above obtained polymer nanoblocks were dispersed in 4.5 mL of C12E6 aqueous solution (the concentration of C12E6 is 1.4 mg/mL) under stirring. Swelling solvent (0.5 mL) was dropped into this aqueous suspension under sonication. The emulsion was kept at room temperature for 48 h to swell nanoblocks. Then the emulsion was heated to evaporate swelling solvents. The suspension containing swollen BCP nanoparticles was centrifuged at 10 000 rpm for 10 min, and the precipitate was collected. Finally the obtained BCP nanoparticles were washed by DI water several times to remove C12E6. We selected PS/PMMA as the model system for polymer composite. To produce double-layer PS/PMMA polymer nanotubes, AAO membrane was first dipped in PMMA/CHCl3 solution (50 or 40 mg/mL). Upon solvent evaporation, the dried AAO membrane with PMMA nanotubes was soaked in PS/CHCl3 solution (20 or 40 mg/ mL) to deposit the second PS layer on existing PMMA nanotubes. The AAO membrane with PS/PMMA double-layer polymer nanotubes was heated at 190 °C for 20 min, and the nanotubes performed Rayleigh instability also. The following steps, such as dissolving AAO membrane in NaOH aqueous solution, ultrasonication, and swelling nanoblocks in toluene, were conducted as ordinary polymer samples. The PS-b-PB/PMMA composite particles were also produced by using the above-mentioned method. The concentrations of PS-b-PB and PMMA in CHCl3 were both 30 mg/mL. To elucidate the influence of trace amount of swelling solvents, the nanoparticles (∼0.8 mg) produced by the above-mentioned method were redispersed in aqueous solution (8 mL); different amounts of toluene (1, 2, or 3 μL) were dropped in this particle suspension under ultrasonication. After setting for 5 h, the suspension was centrifuged at 10 000 rpm for 30 min. A summarized scheme of all these fabrication methods of various polymer nanoparticles is presented in Scheme S1.

3. RESULTS AND DISCUSSION Influence of Properties of Swelling Solvents. Block copolymers can assemble to periodically ordered patterns not only in bulk but also in their nanostructures. Our previous study showed that the nanospheres of PS23600-b-P2VP10400 presented the ordered cylindrical pattern on their surface after swelling by toluene.9 To elucidate the influence of solvent property to the phase separation pattern of obtained BCP nanoparticles, we first compared the swelling products of three BCP samples with different volume fraction of P2VP. On the basis of volume fraction of P2VP blocks, the three samples show different patterns in their bulks: S1 (PS27700-b-P2VP4300) is sphere, S2 (PS23600-b-P2VP10400) is cylinder, and S3 (PS40500b-P2VP40000) is lamellae. At room temperature, the solubility parameters δ (MPa1/2) of toluene and CH2Cl2 are 18.3 and 20.2,11 respectively. For two blocks of PS-b-P2VP, their δ (MPa1/2) are 18.5 (PS) and 20.4 (P2VP).12 On the basis of this data, we knew that CH2Cl2 is preferential solvent to P2VP blocks. Selective solvents are known to drive order−order transitions.13 Theoretical study indicated that for a symmetric BCP in the presence of a slightly selective solvent the progress follows the order lamellae → cylindrical → sphere → micelles → disorder; in contrast, for asymmetric copolymers, the order is from sphere to lamellae.13 Such different solubilities of toluene and CH2Cl2 may have impact on the surface pattern of obtained BCP nanospheres. Figure 1 shows TEM images of three samples swollen by different solvents. For S1, the array of dark areas which is the P2VP block presents clear difference in Figures 1a and 1b due to the different solubility parameters. S2 shows most significant change of phase separation pattern of obtained nanospheres: nanoparticles swollen by toluene give perfect hexagonal arranged lattice, while those swollen by CH2Cl2 show onion-like lamellar phase separation pattern. For S3, the nanospheres swollen by toluene generate onion-like pattern, and the others swollen by CH2Cl2 change to strawberry-like. We interpreted such changes as a result of the volume fraction of P2VP blocks changes by preferential solvent (CH2Cl2) in swelling. When the BCP nanoparticles are swollen by CH2Cl2, the absorption of CH2Cl2 increases the volume fraction of P2VP phase, inducing a shift of the phase separation pattern. The most typical case is that the cylinderforming S2 changing to lamellar-forming pattern which is an order−order transition driven by the shift of P2VP phase as volume fraction of P2VP blocks increasing. A similar morphology shift from cylinder to lamellae was also reported by other studies recently.14 The morphology of S3 swollen by CH2Cl2 gives rise to an unusual morphology in which PS blocks are surrounded by P2VP blocks that may not be an equilibrium phase in bulk. In these experiences, we observed that the transformation of S3 from nanoblocks to nanospheres in swelling by toluene was slower than the other two samples (S1 and S2). We felt curious to this progress and characterized it step by step. Figure 2 4641

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blocks have slightly disordered morphology, after swelling for 12 h, they change to an alternating appearance of bright layer (PS) and dark layer (P2VP) pattern (Figure 2b). The transformation of phase separation pattern looks faster than their deformation. Then the elongated swelling period leads to a deformation of these BCP nanoparticles, from cylinder to sphere. It is interesting to find that the deformation progress of symmetric BCP nanoblocks gradually passes through a shellshaped stage (Figure 2c,d), and finally they return to a symmetric round shape (Figure 2f). Why do not these nanoblocks take a symmetric pathway to change to sphere? In other words, why cannot we obtain a particle with stripped pattern from these stripped nanoblocks (Figure 2b)? We suppose it maybe because of the polar environment (water) which is preferred to P2VP block. Previous studies have shown that the lamellar PS-b-PI nanoparticles have alternating stripe morphology, and they can be further transformed to nanodisks in aqueous solution10,15 because both two blocks, PS and PI, are nonpolar. However, because the onion-like structure is the thermodynamically stable structure for PS-b-PI in water, after annealing at higher temperature, the stacked lamellae will change to onion.16 In our PS-b-P2VP case, P2VP is polarpreferred, so it must segregate to the outmost layer to lower the interface tension between nanosphere and water; thus, the nanoparticle should have violent transformation to generate the final onion-like nanospheres. Maybe because the swelling liquefies PS-b-P2VP nanoparticle to directly form the most stable microstructure, we have not found the BCP nanoparticles with staked-lamellae microstructure which are usually obtained at 20 °C in lamellae-forming PS-b-PI nanoparticles.16 It is worth noting that terrace edges appeared in these shellshaped nanoparticles (Figure 2d). Such terrace-like edges have been observed in the thin film of lamellar block copolymer.17−19 On the other hand, the formation of terraces in lamellae-forming diblock copolymer thin film,20,21 or supramolecular complex of PS-b-P4VP with pentadecylphenol,22 has been studied in more complex systems in which the coupling and competition of dewetting and microphase separation are interwoven.20 Yi and Yang have observed lamellae PS-b-PB nanoparticles with terrace-like morphology.23 They interpreted the formation of such unique morphology is due to the special interfacial effect generated by mixed surfactant. Our observation

Figure 1. TEM images of three different PS-b-P2VP nanospheres swollen by toluene and CH2Cl2, respectively. PS27700-b-P2VP4300 (S1): (a, b); PS23600-b-P2VP10400 (S2): (c, d), PS40500-b-P2VP40000 (S3): (e, f).

shows all these TEM images corresponding to different stages in swelling, from original nanoblocks just released from AAO membrane after Rayleigh instability to the different swollen products during varied swelling periods. The original nano-

Figure 2. TEM images of the multistage presented in the deformation process of PS40500-b-P2VP40000 in swelling. All samples have stained by I2 vapor before observation. P2VP shows darker than PS block. (a) The BCP nanoblocks after Rayleigh instability show a slightly disordered phase separation pattern. (b) BCP nanoblocks swollen 12 h by toluene show a stripe pattern. (c) BCP nanoparticles with shell shape after 20 h swollen by toluene. (d) The shell-shaped BCP nanoparticles after 30 h swelling. (e) BCP nanospheres after 48 h swelling. (f) BCP nanospheres after 72 h swelling. 4642

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is the first report of terrace edges of these lamellae-forming PSb-P2VP nanoparticles. Because these nanoparticles were swollen by toluene and then solidified to TEM observation, the particles shape may reflect their original shape as oil droplets in aqueous solution. On the basis of these images (Figure 2c−e), we can draw a simple illustration for this deformation (see TOC figure): one end of cylinder shrinks to form the core, while the other end expands to form the outmost layer. In this period, droplets of BCP nanoparticles have terrace edges. And the original stacked lamellae bend to accommodate the shape transformation. Finally, BCP nanoparticles with onion-like microstructure formed. Such nice coevolution archives the deformation from cylinder to sphere for a lamellar BCP nanoparticle. Dalnoki-Veress et al. have studied the morphology change of thin film of lamellar-forming diblock copolymer using self-consistent-field theory, and they observed that selective surface to one of the polymer components is a critical point.18 We believe that the affinity of aqueous environment to P2VP block of diblock copolymer PS-bP2VP, similar to the selective substrate in lamellar BCP thin film case,18 is responsible for it. After elongated swollen time the BCP nanoparticle finally goes to smooth round shape as ordinary droplets, indicating the shell-shaped nanoparticle with terrace edges is a metastable structure, which is similar to the previous report. 20 In the further study, a theoretical investigation of this complicated process presented in our experiments will be conducted. And that may deepen our knowledge on the cooperation of phase separation and deformation in swelling. Influence of Trace Swelling Solvents to the Shape of Polymer Nanoparticles. On the other hand, we also observed that trace amount of swelling solvents may impact the shape of final products significantly. To clarify it, we added small amount of toluene (1−3 μL) to the obtained BCP nanoparticles suspension and let it reswell for 5 h. After that, these nanoparticles were centrifuged at 10000 rpm for 30 min. Figure 3 shows the SEM images of nanoparticles with different amounts of toluene: 1 μL (Figure 3a), 2 μL (Figure 3b), and 3 μL (Figure 3c). In these two samples (Figure 3b,c) the shape of nanoparticles changes to disk. We suppose that nanoparticles adsorbed trace amount of swelling solvent would become slightly softer than before, so the strong centrifugal force in centrifugation could lead them to deform. Recently, the fabrication strategy of discoidal or rodlike polymeric microparticles is generally based on the deformation process,8,24,25 in which crucial step is to soften polymer particles that was conducted either by heating them at a temperature above the glass transition temperature or liquefying them by a given amount of good solvent. To these liquefied polymer microparticles, shear force generated by magnetic stirring8 or by impeller in emulsification25,26 can drive the deformation of soft polymer particles. In our experiments, the readdition of trace amount of swelling solvent also softens the BCP nanoparticles and makes them deformable in the following centrifuge process. Such a deformation step also can couple with the selective swelling of BCP nanoparticles. If we add a small amount of ethanol which is a selective solvent to P2VP in aqueous solution, and then disperse BCP nanoparticles in ethanol/water solution, the reswelling of BCP nanoparticles will generate BCP nanodisks with clear surface pattern. Figure 4 presents the BCP nanodisks with surface pattern. Because of the existing ethanol, P2VP is swollen more severely than that in pure toluene; the collapse of P2VP chains after solvent evaporation results in

Figure 3. Images of disk-shaped BCP nanoparticles obtained by deformation by shear force in centrifugation at 10 000 rpm. These BCP nanoparticles were liquefied by different amounts of toluene: (a) 1 μL, (b) 2 μL, (c) 3 μL. (d) TEM image of these BCP nanodisks obtained by adding 2 μL of toluene in aqueous suspension of BCP nanospheres.

Figure 4. SEM images of disk-shaped nanoparticles with clear surface pattern produced by (1) adding 2 μL of toluene and 0.5 mL of ethanol in aqueous suspension of BCP nanospheres (8 mL), (2) swelling for 5 h at room temperature, and (3) centrifuging at 10 000 rpm for 10 min.

indention of P2VP region, leading to their surface pattern becoming prominent. Such disk-shaped BCP nanoparticles with clear surface pattern could be used as a template to direct the decoration of inorganic compounds. Fabrication of Polymer Composite Nanoparticles through Swelling. Composite colloidal particles with special microstructure have broad application in industry as special additives in coating or latex.27,28 Most of these composite colloidal particles are generated by seeded emulsion polymerization29 or seeded suspension polymerization.30 Recently, Okubo et al. presented a generation method of PMMA/PS composite particles by solvent evaporation.31 On the other hand, the anisotropic composite particles, for example, Janus composite particles, show great performance in colloidal selfassembly.32,33 Based on our swelling method, the PS/PMMA composite nanoparticles are also obtained (Figure 5). The PS/ PMMA composite nanotubes were first fabricated in AAO channels by coating PMMA thin layer and PS layer separately. PMMA layer adheres to AAO membrane surface because of its 4643

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composite colloidal has core−shell microstructure in which PS is the core part and PMMA is the shell (Figure 5d,e). Because of the hydrolysis of PMMA by NaOH solution while dissolving AAO membrane, the obtained nanoparticles are connected together by PMMA bridges (Figure 5c). Interestingly, when the sample was scraped gently by blade, the outmost PMMA layer was broken and the inner part of PS was uncovered (Figure 5d). Then we tested the influence of trace amount solvent to the shape of composite colloidal particles. 2 μL of toluene was added to the PS/PMMA core−shell nanoparticles aqueous suspension, and the suspension was set 5 h for swelling. Then the suspension was centrifuged at 10 000 rpm for 30 min. Snowman-shaped composite nanoparticles were obtained (Figure 6). And the variation of the composition in PMMA/PS composite can influence the size of composite nanoparticles, but they all keep the snowman shape (Figure S1). Another composite particle (PS-b-PB/PMMA) also shows snowman shape (Figure 6d−f). Both these two composite particles change to Janus ones in which PS or PS-b-PB segregates from PMMA and forms hemisphere (Figure 6c,f). We knew that PMMA shell covered PS core in their original composite colloidals (Figure 5), but PMMA and PS are immiscible, and they are not connected by strong interaction such as cross-linking, so these two parts in composite colloidal have a tendency to depart. The reswelling and strong shear force in centrifugation may accelerate the separating process, resulting in snowman-shaped composite nanoparticles. Such a separation of polymer composite particles has also been reported by Okubo et al. recently.37 But maybe the amount of toluene added in PS/PMMA particle suspension is too small to dissolve these composite particles; we still have not observed the cleavage of PS/PMMA composites after centrifugation. Because the size ratios of PS (hydrophobic part) and PMMA (hydrophilic part) are directly related to their weight ratios of original PS/PMMA composite nanotubes, the Janus balance of these composite particles can be tuned easily. Our further study will focus on adjusting the Janus balance of these composite particles and exploring their applications.

Figure 5. Core−shell polymer composite nanoparticles generated through swelling process. (a) SEM image of PS/PMMA double layers nanotubes after Rayleigh instability. (b) TEM image of PS/PMMA nanorods with encapsulated pores obtained from Rayleigh instability. The inset illustration in (b) presents the formation process of core− shell PS/PMMA composite particles. Because of the strong interaction between PMMA and AAO surface, PMMA thin layer was always covered AAO membrane in this process, PS layer was transformed to uncontinuous blocks covered by PMMA layers due to its dewetting from AAO surface. (c, d) SEM images of the core−shell PS/PMMA composite nanospheres. After gently scratching nanoparticles by blade, the shell layer of PMMA was broken and uncovered the core part composed of PS (d). (e) TEM image of PS/PMMA composite particle.

4. CONCLUSIONS In this study, we carefully studied the synergistic effect of deformation and phase separation in the swelling of diblock copolymer or polymer composite nanoparticles. We observed that several experimental parameters can influence the morphology and shape of the obtained BCP nanoparticles: The changing of the properties of swelling solvents significantly changes the morphology of BCP nanoparticles; the trace amount of swelling solvents can liquefy the BCP nanoparticles and let them become nanodisks under the strong shear force in centrifugation; for polymer composite nanoparticles, the deformation of PS/PMMA nanoparticles couples with the separation of two immiscible polymer parts, resulting in snowman-shaped composite nanoparticles. Particularly, the most important progress is that we reveal the complex procedure in swelling of lamellae-forming BCP nanoparticles. We first discovered the lamellae-forming BCP nanoparticles with terrace edges, which have been recognized only in lamellae-forming BCP thin film in previous researches. Our studies presented that swelling process of block copolymers and polymer composites provides us not only a useful strategy to generate nanoparticles with various morphologies and shapes but also a special platform to understand what will happen

strong affinity to AAO surface.34 The double layer polymer composite nanotubes were annealed to induce Rayleigh instability. Because of the difference affinity of PMMA and PS with AAO surface, PS as nonwetting component will segregate to the central part of obtained nanorods, as highlighted by the yellow line, while PMMA layer covers the PS parts as highlighted by red line in Figure 5a,b. Such special core-segmented nanostructures are similar to the assembly nanostructures of poly(acrylic acid)-block-poly(methyl acrylate)-block-polystyrene35 and polystyrene-block-polybutadieneblock-poly(2-vinylpyridine).36 After swelling, the produced 4644

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Figure 6. Images of snowman-shaped nanoparticles generated by reswelling composite particles in trace amount of solvent. (a−c) Toluene (2 μL) was added in 8 mL of core−shell PS/PMMA composite particle (PS 40 mg/mL and PMMA 40 mg/mL) aqueous suspension. After setting for 5 h to liquefy particles, the suspension was centrifuged at 10 000 rpm for 30 min. (a, b) SEM images, (c) TEM image. (d−f) Toluene (2 μL) was added in 8 mL of PS-b-PB/PMMA composite particle aqueous suspension. (d, e) SEM images; (f) TEM image. PS-b-PB segregated from PMMA to form Janus particles. Controlled Sizes and Shapes via the Mechanical Elongation of Master Templates. Langmuir 2011, 27, 524−528. (7) Kim, J.-W.; Larsen, R. J.; Weitz, D. A. Uniform Nonspherical Colloidal Particles with Tunable Shapes. Adv. Mater. 2007, 19, 2005− 2009. (8) Liu, B.; Wang, D. High-Throughput Transformation of Colloidal Polymer Spheres to Discs Simply via Magnetic Stirring of Their Dispersions. Langmuir 2012, 28, 6436−6440. (9) Mei, S. L.; Feng, X. D.; Jin, Z. X. Fabrication of Polymer Nanospheres Based on Rayleigh Instability in Capillary Channels. Macromolecules 2011, 44, 1615−1620. (10) Yabu, H.; Higuchi, T.; Shimomura, M. Unique Phase-Separation Structures of Block-Copolymer Nanoparticles. Adv. Mater. 2005, 17, 2062−2065. (11) Hansen, C. Hansen Solubility Parameters A User’s Handbook, 2nd ed.; CRC Press: Boca Raton, FL, 2007. (12) Lescanec, R. L.; Fetters, L. J.; Thomas, E. L. Assessing Homopolymers Distribution in ABC Triblock Copolymer/Homopolymers Blends through a Transition in Interfacial Geometry. Macromolecules 1998, 31, 1680−1685. (13) Huang, C.-I.; Lodge, T. P. Self-Consistent Calculations of Block Copolymer Solution Phase Behavior. Macromolecules 1998, 3, 3556− 3565. (14) Wadley, M. L.; Hsieh, I.-F.; Cavicchi, K. A.; Cheng, S. Z. D. Solvent Dependence of the Morphology of Spin-Coated Thin Films of Polydimethylsiloxane-Rich Polystyrene-block-Polydimethylsiloxane Copolymers. Macromolecules 2012, 45, 5538−5545. (15) Higuchi, T.; Tajima, A.; Motoyoshi, K.; Yabu, H.; Shimomura, M. Suprapolymer Structures from Nanostructured Polymer Particles. Angew. Chem., Int. Ed. 2009, 48, 5125−5128. (16) Higuchi, T.; Motoyoshi, K.; Sugimori, H.; Jinnai, H.; Yabu, H.; Shimomura, M. Phase Transition and Phase Transformation in Block Copolymer Nanoparticles. Macromol. Rapid Commun. 2010, 31, 1773−1778. (17) Croll, A. B.; Massa, M. V.; Matsen, M. W.; Dalnoki-Veress, K. Droplet Shape of an Anisotropic Liquid. Phys. Rev. Lett. 2006, 97, 204502. (18) Stasiak, P.; McGraw, J. D.; Dalnoki-Veress, K.; Matsen, M. W. Step Edges in Thin Films of Lamellar-Forming Diblock Copolymer. Macromolecules 2012, 45, 9531−9538. (19) Ausserre, D.; Raghunathan, V. A.; Maaloum, M. Static Wetting Behaviour of Diblock Copolymers. J. Phys. II 1993, 3, 1485−1496.

when the phase separation in nanostructures meets the deformation.

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ASSOCIATED CONTENT

S Supporting Information *

Fabrication methods summarized in Scheme S1; PS/PMMA snowman-shaped nanoparticles with different weight ratio presented in Figure S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the National Nature Science Foundation of China (Grants 21074149 and 51173201) for financial support.

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REFERENCES

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dx.doi.org/10.1021/la400390b | Langmuir 2013, 29, 4640−4646