Soft Colloidal Molecules with Tunable Geometry by 3D Confined

Aug 6, 2015 - Yuqing Wu , Ke Wang , Haiying Tan , Jiangping Xu , and Jintao Zhu. Langmuir ... Nan Yan , Yan Zhang , Yun He , Yutian Zhu , and Wei Jian...
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Soft Colloidal Molecules with Tunable Geometry by 3D Confined Assembly of Block Copolymers Renhua Deng,†,‡ Hui Li,§ Fuxin Liang,‡ Jintao Zhu,*,† Baohui Li,*,§ Xiaolin Xie,† and Zhenzhong Yang*,‡ †

Key Laboratory for Large-Format Battery Materials and System of the Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China ‡ State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China § School of Physics and Key Laboratory of Functional Polymer Materials of the Ministry of Education, Nankai University, Tianjin 300071, China S Supporting Information *

ABSTRACT: We present with experiments and computer simulations that colloidal molecules with tunable geometry can be generated through 3D confined assembly of diblock copolymers. This unique self-assembly can be attributed to the slight solvent selectivity, nearly neutral confined interface, deformable soft confinement space, and strong confinement degree. We show that the symmetric geometry of the colloidal molecules originates from the free energy minimization. Moreover, these colloidal molecules with soft nature and directional interaction can further self-assemble into hierarchical superstructures without any modification. We anticipate that these new findings are helpful to extend the scope of our knowledge for the diblock copolymer self-assembly, and the colloidal molecules with new composition and performance will bring new opportunities to this emerging field. assembly or disassembly.15,16 Their patches are covalently linked to the core. Stable superstructures can be generated from these particles without any modification. Therefore, synthesis of colloidal molecules from BCPs is more attractive and urgently expected to bring new development opportunities for this field. Three-dimensional (3D) confined self-assembly of BCPs has been extensively employed to form soft colloid particles with controllable internal structures.17−20 Although nonspherical colloid particles have been achieved through 3D soft confined assembly,18 there is no report on the formation of colloidal molecules in this means. The main challenge is how to create bulges on the particle surface. Recently, we have successfully achieved patchy particles of BCPs. Yet, they are raspberry-like particles rather than colloidal molecules due to the multipatches on their surface.21 Herein, we present that colloidal molecules with tunable geometry can be generated through 3D confined assembly of polystyrene-block-poly(4-vinylpyridine) (PS-bP4VP) at strong confinement degree. Experiments and computer simulations are combined to elucidate the formation mechanism. More interestingly, these colloidal molecules with soft nature and directional interaction can further self-assemble into hierarchical superstructures.

1. INTRODUCTION Anisotropic colloidal particles with tunable geometry and composition can mimic artificial giant atoms or molecules to self-assemble into hierarchical superstructures, which are of great interest in chemistry, physics, and material science.1−3 Especially, those particles with shapes resembling space-filling models of molecules and specific directional interactions are termed as colloidal molecules which can exhibit complex behavior similar to molecules or even beyond.4 Colloidal molecules and their self-assembled superstructures may help to explore new materials with custom-made properties. It is important to develop effective approaches to synthesize colloidal molecules with different size, structure, and composition.5 Previously reported strategies can be mainly categorized into clustering of spherical particles and seed growth.6−11 Generally, the resulting colloidal molecules are usually submicron or larger, and they are “hard” and “static” building blocks, which can only stack into simple superstructures determined by their geometry.12 Usually, the cumulate superstructures from these colloidal molecules are rather weak. Directional bonds are rendered after further selective modification of the colloidal molecules in order to strengthen the superstructures.13 In comparison, colloid particles of organic supramolecules are intrinsically adaptive to their surrounding physicochemical environment.14 Particularly, block copolymer (BCP) based patchy particles can act as “soft” and “dynamic” building units, whose size are easily tunable at nanoscale, offering attractive feature for further self© 2015 American Chemical Society

Received: June 10, 2015 Revised: July 26, 2015 Published: August 6, 2015 5855

DOI: 10.1021/acs.macromol.5b01261 Macromolecules 2015, 48, 5855−5860

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Figure 1. (a, b) Transmission electron microscopy (TEM) images of PS110K-b-P4VP107K colloidal molecules with increased patches before and after selective growth of Au NPs within P4VP domains. (c) Scanning electron microscopy (SEM) images of the colloidal molecules. The scale bars in (a), (b), and (c) are applied to the following images in the same line. (d, e) Illustrative structures of the colloidal molecules and the analogous molecules. a clean silicon wafer, and the solvent was allowed to evaporate, followed by coating a thin layer of Pt on the samples. Simulation Model and Method. The simulations were performed on a model system which was embedded in a simple cubic lattice of volume V = LX × LY × LZ. To be consistent with the experiments, the model system contains four components: symmetric diblock copolymers A6B6, organic (oil) molecules (O), water molecules (W), and surfactant (C). Each copolymer molecule is a chain of 6 A- and 6 B-monomers (beads), and each O, W, or C molecule is composed of only one monomer. Each monomer occupies one lattice site, and two monomers cannot occupy the same site. The bond length of a copolymer molecule is set to be 1 and √2 lattice spacing, and thus each site has 18 nearest neighbors. The starting configuration is generated in the following way. A spherical pore with diameter D is constructed from a box with LX = LY = LZ = D + 24. The pore includes those lattice sites whose distance to the pore center is less than D/2. Initially, the copolymers and oil molecules are confined inside the spherical pore while W and C are confined outside the pore, and C must be one of the nearest neighbors of a monomer or an oil molecule. The total monomer concentration in the oil solvent is c0 = 15%. The trial moves are the same as those used in our previous studies.22,23 Only the nearest-neighbor interactions are considered. The repulsive interaction between the A and B monomers is modeled by a parameter εAB = 12.0, which indicates that the copolymer is in the strong segregation region. The organic solvent is chosen as selective to A-block with εAO = −6.0 and εBO = 1, while water and C monomer are chosen as slightly selective to B-block with εAW = 4.0, εBW = 3.0, and εBC = −1.0. Water is a good solvent for the C monomers while incompatible with organic phase, and thus we set εCW = −2.0 and εOW = 4.0. All other interaction parameters without mention are set to be zero. All the parameters are in the unit of kBT, where kB is the Boltzmann constant and T is temperature. To simulate the organic solvent evaporation, a number of organic solvent molecules are changed to water molecules after performing a given number of Monte Carlo steps (MCS). An organic solvent is selected randomly; whether it is changed to a water molecule depends on its position. If the selected organic solvent is a nearest neighbor of a C molecule or a water molecule, it is changed to a water molecule with

2. EXPERIMENTAL SECTION Materials. Diblock copolymers PS110K-b-P4VP107K (Mw/Mn = 1.15), PS20K-b-P4VP17K (Mw/Mn = 1.08), PS22K-b-P4VP22K (Mw/Mn = 1.15), PS20.5K-b-P4VP36K (Mw/Mn = 1.08), and PS51K-b-P4VP18K (Mw/Mn = 1.15) were purchased from Polymer Source, Inc. PVA (Mw = 13K−23K g/mol, 87−89% hydrolyzed) was purchased from Aldrich. Chloroauric acid (HAuCl4·4H2O) and chloroform were purchased from Beijing Chemical Works. All of the materials were used as received without further purification. Preparation of Colloidal Molecules. Colloidal molecules were prepared by emulsion droplet confined self-assembly of PS110K-bP4VP107K. Typically, PS-b-P4VP was dissolved in chloroform at a concentration of 1.0 mg/mL. Subsequently, 0.1 mL of the solution was emulsified with 1.0 mL of PVA aqueous solution (5 mg/mL) through membrane-extrusion emulsification.21 Initial emulsion droplet size can be tuned by manipulating the extrusion pass times and membrane pore size. The resultant emulsion was collected in a 10 mL small vial to allow the evaporation of chloroform slowly for 24 h at 30 °C. The resulting polymer particles were first separated by centrifugation (12 000 rpm, 3 min) to remove bigger particles with raspberry-like stucture,21 and the supernatant was then separated by centrifugation (16 000 rpm, 8 min) to remove PVA. Incorporation of Au NPs into P4VP Domains. The formed colloidal molecules were dispersed in an aqueous solution of HAuCl4· 4H2O (1 mL, 0.5 mg/mL) to allow a preferential absorption of Au precursor within the P4VP bulges for 12 h. The resulting composite particles were separated by centrifugation (16 000 rpm, 8 min) and redispersed in deionized water. Au NPs were in situ formed in P4VP domains after electron irradiation under TEM investigation. Characterization. TEM investigation was performed in a brightfield mode on JEM-1011 TEM (JEOL Ltd., Japan) operated at an acceleration voltage of 100 kV. The samples were prepared by placing one drop of the sample suspension (∼5 μL) onto carbon-coated copper grids, followed by evaporation of the solvent at room temperature. For selective staining of P4VP, the TEM specimens were exposed to I2 vapor for 2 h. SEM images were recorded using S4800 (JEOL) operated at an acceleration voltage of 15 kV. To prepare the sample for SEM, the particles aqueous dispersion was dispersed on 5856

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Macromolecules a probability of 100%; otherwise, the probability is 15%, which is used to ensure a fixed evaporation rate. As water is repulsive to both organic solvent and BCPs, it will be repelled out of the droplet in the following MCS. Each MCS is defined as the average time taken for all the lattice sites to be visited for an attempted move. Starting from the initial state, MC simulations are performed on a series of states with increasing polymer concentration due to the organic solvent evaporation.

MP4VP = 10K−17K, no micellization phenomenon is observed while segmented particles with a lamellar structure form.25,26 Thus, PS110K-b-P4VP107K is chosen to generate the colloidal molecules. In comparison, if the solvent is miscible with both blocks of the chosen diblock copolymer, the block length has no significant effect on the internal structure of the particles. For example, when chloroform was employed as the solvent for symmetric PS-b-P2VP even with MP2VP = 20K−97K (the χS−P value of chloroform−P2VP is 0.42), segmented particles are always obtained.27 On the other hand, if the solvent is poor for P4VP, colloidal molecules cannot be observed either. For instance, when chloroform was replaced by toluene, a poor solvent for P4VP, PS110K-b-P4VP107K cannot be completely dissolved but form micelles with PS shell and P4VP core. As a result, core−shell particles or particles with multicompartmental internal structures are obtained (Figure S4). The interfacial property of the oil/water plays another important role in forming the colloidal molecules. Coexistence of PS and P4VP on the particle surface is attributed to the surfactant PVA, which creates a nearly neutral oil/water interface for PS and P4VP.25 Presumably, the interface is not exactly neutral but slightly selective with P4VP, which is beneficial for the formation of P4VP bulges. This can be deduced from the segmented particles formed by PS20K-bP4VP17K in PVA aqueous solution, whose two poles are always occupied by P4VP (Figure S5a). If the interface is exactly neutral, the two blocks are equal to occupy the two poles. This conjecture has been confirmed by our simulation results (Figure S5b,c).28 Meanwhile, deformable soft confinement space and strong confinement degree are two requirements for the formation of colloidal molecules. Based on the above discussion, the formation process of colloidal molecules could be divided into the following stages. PS110K-b-P4VP107K first formed micelles with P4VP-core and PS-shell in emulsion droplet due to the solvent selectivity. Small micelles coalesced into bigger ones to reach the equilibrium state. Then, evaporation of chloroform induced shrinkage of the droplets, leading to confined stacking of the micelles and solidification of the droplet. PS-blocks are more extended than P4VP-blocks in droplets during solvent evaporation, and PS-blocks shrink more remarkably after complete solvent removal. At the particle surface, shrinkage of PS-blocks facilitates the formation of P4VP-bulges. Meanwhile, formation of P4VP-bulges can increase hydrophilic areas of the particle surface to favor the decrease of interfacial free energy. At the equilibrium state, bulges tend to adopt symmetric packing at the particle surface similar to colloidal cluster to minimize the second moment of the mass distribution.29 Computer Simulation of Assembly of Diblock Copolymers into Colloidal Molecules. Previously, we have investigated the phase behavior of symmetric diblock copolymer A6B6 inside spherical nanopores by simulations.30 No colloidal molecules were obtained due to the nondeformable 3D hard confinement. In this study, Monte Carlo simulations are performed to reveal the formation process of colloidal molecules by 3D soft confined self-assembly of A6B6. We choose interaction parameters close to those in the experiment. A large A−B repulsion (εAB = 12) is set to simulate the large molecular weight of the BCP. Specifically, the interfacial selectivity for B-block (P4VP) is described by setting εAC = 0 and εBC = −1, and the slight selectivity of chloroform to A block (PS) is selected as εAO = −6.0 and εBO = 1. In this case,

3. RESULTS AND DISCUSSION Formation of Colloidal Molecules from Diblock Copolymers. Colloidal molecules are synthesized through 3D soft confined self-assembly of PS110K-b-P4VP107K using the emulsion solvent-evaporation route21 with poly(vinyl alcohol) (PVA) as the surfactant. Representative colloidal molecules are displayed in Figure 1, where PS and P4VP domains can be distinctly discerned by TEM images after selectively staining P4VP domains with I2 vapor (Figure 1a). In this case, the gray parts represent PS domains while the darker areas are assigned to P4VP domains. P4VP domains can be further functionalized through a selective growth of other species for example Au nanoparticles (NPs) by an in situ route (Figure 1b). The overall shape of the colloidal molecules is further confirmed by SEM images (Figure 1c). Bulges are regularly protruded from the particle surface and covalently bonded to the PS core. Obviously, the number of bulges (n) or valence of the colloidal molecules increases as a function of particle size. AB colloid particles (n = 1) are analogous to HCl molecules, and linear AB2 colloid particles (n = 2) mimic BeCl2 molecules. Similarly, trigonal planar (n = 3), tetrahedral (n = 4), and square pyramid (n = 5) colloid particles correspond to BF3, CH4, and IF5, respectively. It is shown that solvent selectivity and interfacial property of the emulsion droplet play key roles in forming the colloidal molecules from PS110K-b-P4VP107K. Flory−Huggins interaction parameters of solvent−polymer pairs (χS−P) are shown in Table 1. According to the Flory−Huggins criterion where complete Table 1. Characteristic of Polymers and Organic Solvents24 polymer/ solvent

solubility parameter δ (MPa1/2)

molar volume Vs (cm3/mol)

χS−PS

χS−P4VP

PS P4VP chloroform toluene

18.6 22.2 19.0 18.2

80.7 105.7

0.35 0.35

0.67 1.01

solvent−polymer miscibility is expected when χS−P < 0.5, we chose chloroform as the oil phase. Clearly, chloroform is a slightly selective solvent for PS, since χS−P of chloroform-PS is 0.35, while that of chloroform-P4VP is 0.67. The slight selectivity of chloroform to PS block has been proved by a series of micellization experiments (Figures S1−S3 in the Supporting Information). Initially, PS110K-b-P4VP107K is completely dissoluble in chloroform. When the polymer/ chloroform solution contacts with PVA aqueous solution or water, PS110K-b-P4VP107K starts to form core/shell micelles during diffusion of water into the polymer/chloroform solution. P4VP should form the cores of the micelles which eventually evolve into bulges on the particle surface. Notably, this selfassembly of PS110K-b-P4VP107K induced by water in slightly selective solvent is one of the key factors in forming the colloidal molecules. We have further found that the length of P4VP block should be large enough to form micelles (Figure S3b). When the P4VP block is not long enough, for example, 5857

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Figure 2. Colloidal molecules as a function of De/L0 by computer simulation. Interaction parameters are εAB = 12, εAO = −6, εBO = 1, εAC = 0, and εBC = −1. Color scheme: A (purple) and B (blue).

colloidal molecules rather than segmented particles are always obtained. Representative morphologies as a function of the confinement degree De/L0 are presented in Figure 2, where De is the effective diameter, corresponding to diameter when all the polymers in the droplet are solidified to form a compact particle, and L0 is the period in the bulk phase. De can be calculated from eq 1:

De =

3

c0 × D0 ρ

(1)

where D0 is the diameter of the initial droplet, c0 is the initial concentration of polymer, and ρ is the density of the final polymer particle (here we set ρ = 1). Clearly, De/L0 can be tuned by varying c0 and/or D0. Particularly, at a given c0, De/L0 is proportional to D0. Generally, colloidal molecules will form under strong confinement with De/L0 < 2.0 (Figure 2). The number of bulges on colloidal molecules increases with the increase of De/L0. At De/L0 ≤ 0.8, AB type of colloidal molecule is obtained. With the increase of De/L0 to 0.85−1.14, AB2 type colloidal molecule with two bulges on the two ends of the particle is observed. Further increase of De/L0 to 1.16−1.31, 1.37−1.65, 1.77−1.82, and 1.88−1.94 gives rise to 3, 4, 5, and 6 bulges, respectively. When De/L0 = 2.0, particles with multibulges are obtained. These results agree well with the above experimental ones. Therefore, morphology of the colloidal molecules can be easily tuned by varying D0, and we can easily deduce number of bulges on a particle for any droplets. To gain a better understanding of the formation mechanism of colloidal molecules, the morphological evolution of an AB3 colloidal molecules is presented in Figure 3. The BCP morphology in the droplet evolves with increasing BCP concentration (c) and time t (in the unit of 104 Monte Carlo steps: MCS). The relationship between c and t can be written as c0 c= c 1 − 1 − ρ0 rt

(

r=

)

N − Ns′ 1 = s tm 401Ns

Figure 3. Evolution process of an AB3 colloidal molecule with BCP concentration (c) variation in droplets during solvent evaporation. Interaction parameters and color scheme are the same as those in Figure 2.

coalesce into big ones. For example, they coalesce into four Baggregates at t = 300 × 104 MCS (c = 40.3%) and then turn into three B-aggregates at t = 340 × 104 MCS (c = 51.5%), which finally evolve into three bulges at t = 380 × 104 MCS (c = 71.4%), while A-blocks form a continuous phase. The Bbulges tend to locate at the interface symmetrically. When the organic solvent O was nearly completely removed, an AB3 colloidal molecule forms (t = 401 × 104 MCS, c = 90.7%). When D0 is large enough (e.g., De/L0 = 2.0), B-blocks not only form bulges at the particle surface but also form a core located in the center of the particle (Figure S6). These results agree well with the experimental ones. Fundamentally, the number of bulges (n) is the result of the free energy minimization to form an equilibrium structure. The free energy for each particle contains contributions from three sources: (1) interfacial energy between A and B domains, (2) surface energy between particle and surrounding media, and (3) entropy cost of stretching chains in the segregated morphologies. Notably, the equilibrium morphology of the BCPs particles reflects the competition among these three contributions. Interfacial and surface energies are proportional to the corresponding areas. Presumably, the bulge shape is semispherical which only contribute to the surface energy, while the interfacial and surface areas are both qualitatively proportional to nRP2, where RP is the radius of each semispherical bulge. Considering that the chain stretching of the bulge-forming block (B) may differ significantly in particles with a different number of bulges whereas that of A-block should differ slightly, we only consider the contribution from Bblock in estimating the entropy. B-blocks are confined in n

(2)

(3)

where r is the evaporation rate (the relative volume changes of solvent per 104 MCS), tm is the time of complete evaporation of the organic solvent, and Ns and Ns′ are the initial numbers of solvent and the remaining ones in the droplet after 401 × 104 MCS, respectively. Taking the formation process of an AB3 colloidal molecule as an example (Figure 3), when t = 4 × 104 MCS, at a relatively low polymer concentration of c = 15.1%, Ablocks are homogeneously dissolved in the droplet while Bblocks form small aggregates with different sizes. With the evaporation of organic solvent O, the small B-aggregates 5858

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Macromolecules semispheres, which leads to a conformational entropy reduction of ΔS ∝ (RP − R0)2. On the basis of these estimations, we can estimate the free energy per particle with n bulges as Fn ∝ αnRP 2 + β′ΔST = nRP 2 + β(RP − R 0)2

(4)

where α and β are constants and R0 is the radius of the B-block domain in the equilibrium state. In the bulk, R0 = L0/4, while in a colloidal molecule, it is deduced that R0 > L0/4 because Bblock and C-monomers are compatible in our study. A comparison of the free energies (Fn) for particles with different number of bulges in Figure 4 (R0 = L0/3, β = 7)

Figure 5. TEM images and corresponding illustration of superstructures of self-assembled PS110K-b-P4VP107K colloidal molecules (a− c) and (d) the 1D superstructure with the periodic P4VP domains incorporated with Au NPs. The scale bar in (a) is applied to (b−d).

only when their two bulges were modified with specific molecules.31 Yet, when the two ends of AB2 units are polymer chains, they can directly self-assemble without modification.32,33 In this study, we employed a solvent absorption method34 to trigger the “polymerization” of AB2 colloidal molecules. The “polymerization” is initiated by Brownian motion. Hydrophilic P4VP bulges between colloidal molecules can easily contact together, while the hydrophobic PS cores are separated by water. During evaporation and diffusion of chloroform molecules into water for 2 h at 30 °C, some chloroform can be absorbed by the colloidal molecules to initiate the “polymerization” of P4VP bulges by swelling and rearranging of the polymer segments. AB2-type colloidal molecules tend to form 1D superstructures, so-called linear “BCPs” (Figure 5a,b).35 Moreover, “copolymerization” of AB2 with AB3 type colloidal molecules can result in the formation of “star copolymers”, taking an AB3-type colloidal molecule as node (Figure 5c). More complex superstructures can be obtained when different types of colloidal molecules are mixed (Figure S8). Furthermore, periodic P4VP domains of self-assembled 1D superstructure can be functionalized with Au NPs (Figure 5d) to form periodic materials with built-in functionalities.35,36

Figure 4. Variations of the free energy (Fn) as a function of De/L0 for particles with different number of bulges when R0 = L0/3 and β = 7.

confirms that the particles shown in Figure 2 are equilibrium structures with minimized free energy. For example, when De/ L0 < 0.82, the value of curve F1 is less than that of other free energy curves. Free energy curves with different values of R0 and β have the same trend as those shown in Figure 4 while the positions of the crossover points between curves with n = i to i + 1 (i = 1−4) depend upon the values of R0 and β. Moreover, evaporation rate plays another important role in the morphology of colloidal molecules. At the given D0, the solvent evaporation time (tm) decreases with increasing evaporation rate. At slow evaporation rate, most of the particles are thermodynamically stable state (Figure S7a), while at fast evaporation rate, polymers do not gain enough time to reach their equilibrium state, and a series of nonequilibrium structures are frozen. For example, increasing evaporation rate may induce the increase of bulge numbers (labeled by blue circles) or yield bulges that are nonuniform in size (labeled by yellow circles) on a particle (Figure S7b). This observation is also confirmed by computer simulations (Figure S7c,d). Hierarchical Assembly of Colloidal Molecules. These BCP-based colloidal molecules are soft units with directional interactions, and their P4VP-bulges are covalently bonded to the PS-core. These soft and dynamic colloidal molecules can “polymerize” into hierarchical assemblies without any modification on P4VP-bulges (Figure 5) because polymer chains can self-assemble between building units. In previous reports, hard AB2 colloidal molecules can “polymerize” into “BCPs”

4. CONCLUSION We have demonstrated a facile yet robust approach to achieve soft and dynamic colloidal molecules via 3D confined selfassembly of diblock copolymer. Colloidal molecules with different geometries are available depending on the confinement degree. Combination of experiments and simulation analysis reveals the mechanism of this unique self-assembly, which depends on four crucial factors, i.e., slightly selective solvent, nearly neutral interfacial interaction, strong confinement degree, and soft confinement space. Size and number of the bulges in the colloidal molecules rely on the confinement degree and solvent evaporation rate. These new findings of colloidal molecules are helpful to extend the scope of our knowledge for the diblock copolymer self-assembly. The colloidal molecules can further self-assemble into superstructures, and functional species can be selectively incorporated into the particles. This facile and effective synthetic strategy allows access to colloidal molecules with new 5859

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composition and performance, providing new opportunities for applications of this unique material in photonics, electronics, and sensors.

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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.macromol.5b01261. Additional figures showing the experimental and simulation results of the colloidal molecules (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (J.Z.). *E-mail [email protected] (B.L.). *E-mail [email protected] (Z.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.Z. acknowledges founding from MOST of China (2012CB821500) and NSFC (51173056 and 91127046). Z.Y. acknowledges support by MOST of China (2012CB933200) and NSFC (51233007 and 51173191). B.L. acknowledges NSFC (20925414 and 91227121).

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REFERENCES

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DOI: 10.1021/acs.macromol.5b01261 Macromolecules 2015, 48, 5855−5860