Monodipserse Nanostructured Spheres of Block Copolymers and

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Monodipserse Nanostructured Spheres of Block Copolymers and Nanoparticles via Cross-flow Membrane Emulsification Jae Man Shin, Minsoo P Kim, Hyunseung Yang, Kang Hee Ku, Se Gyu Jang, Kyung Ho Youm, Gi-Ra Yi, and Bumjoon J. Kim Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b02020 • Publication Date (Web): 19 Aug 2015 Downloaded from http://pubs.acs.org on August 20, 2015

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Chemistry of Materials

Monodipserse Nanostructured Spheres of Block Copolymers and Nanoparticles via Cross-flow Membrane Emulsification Jae Man Shin1†, Minsoo P. Kim1,2†, Hyunseung Yang1, Kang Hee Ku1, Se Gyu Jang3, Kyung Ho Youm4, Gi-Ra Yi*,2, and Bumjoon J. Kim*,1

1

Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science

and Technology (KAIST), Daejeon, 305-701 Republic of Korea 2

School of Chemical Engineering, Sungkyunkwan University, Suwon, 440-746 Republic of

Korea 3

Soft Innovative Materials Research Center, Korea Institute of Science and Technology (KIST),

Jeonbuk, Republic of Korea 4

Department of Engineering Chemistry, Chungbuk National University, Cheongju, Chungbuk

362-763, Republic of Korea

*E-mail: [email protected] (B. J. K.), [email protected] (G.R.Y.)

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ABSTRACT Monodispersed colloidal particles of polystyrene-b-polybutadiene (PS-b-PB) block copolymers (BCPs) were successfully prepared, in which uniform emulsion containing BCPs were firstly generated by cross-flow membrane emulsification using tubular Shirasu porous glass (SPG) membrane and then unique internal nanostructures were developed by controlled evaporation of emulsions. The diameter of those BCP particles could be controlled from 200 nm to 5 µm by tuning the pore diameter of the membrane. With symmetric BCPs, onion-like nanostructures inside particles were formed by slow evaporation of emulsion. Coiled-cylinders in the BCP particles were also developed by adding homopolymers, in which the assembled BCP structure is strongly dependent on the particle size, demonstrating the importance of our membrane method in generating monodispersed BCP particles. Further investigation of process parameters showed that for a given pore diameter, the operation pressure (P) and surfactant concentration were critical parameters for narrow size distribution of the particles. When the ratio of the operation pressure to the critical pressure (P/Pc) was less than 4.33, uniform emulsions were produced with a sufficient amount of sodium dodecyl sulfate surfactants in the continuous phase. In addition, uniformly-sized, hierarchically structured particles of BCPs and nanoparticles (NPs) were produced, in which oleylamine coated, 3-nm sized Au NPs were incorporated selectively into the PB domains inside the particles.

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INTRODUCTION The self-assembly of BCPs confined in a small droplet can generate hierarchical structured particles with tunable particle shapes, internal structures, and surface properties,1 thus giving rise to potential applications in particle-based technologies such as photonic bandgap materials,2–4 optochemical sensing devices,5 and catalytic supports.6 Their hierarchical structures can be enriched and controlled by tuning the degree of confinement and the interfacial interactions between the BCP and the surrounding media.7–9 Interestingly, using the BCP particles, diverse non-spherical shapes can be induced by generating anisotropic interfacial interactions or surfactant distribution at the particle surface, which can relieve the large free energy penalty for bending lamellar or cylindrical BCPs during morphology evolution.8,10 Recently, ellipsoid- and convex lens-shaped BCP particles were fabricated using a mixture of surfactants that had selective interaction with each block of the BCP domains at the particle surfaces.8,11–14 Due to their well-defined shape and internal nanostructure, these particles potentially will be useful as optical lens,13 sensors,15,16 high performance electrodes17 and catalysts,18 and dielectric resonators.19 For implementing the particles into the practical applications mentioned above, it is necessary and essential to be able to produce monodispersed micron- or submicron-sized BCP particles on large production scale because their properties depend decisively on particle size.20 Unfortunately, most emulsifications for producing BCP particles to date have been achieved by ultrasonication or homogenization, both of which result in very broad distributions of particle sizes. Since variation in particle sizes can induce different degree of confinement effects and affect the internal BCP structure significantly, BCP particles with different internal morphologies 3



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are typically obtained in one batch of the conventional process so that the overall properties of samples cannot be reproduced. The microfluidic technique can produce uniform polymer particles, but it has yet to be used for generating the sub-micrometer scale particles or the largescale production of the particles.21 Further, most of the studies of BCPs using microfluidics have been limited to BCP micelles.22,23 Cross-flow membrane emulsification has emerged as a promising technique for the largescale production of particles with uniform size, ranging from hundreds of nanometers to tens of micrometers.24,25 In particular, tubular Shirasu porous glass (SPG) membranes have been used for such cross-flow membrane emulsification because they are chemically stable and their pore size can be tuned.26 The number of pores on the surface of the SPG membrane per unit crosssectional area is in the range of 109 to 1013 /m2, which indicates that droplets are produced at much higher throughput than by any other techniques.27 In addition, the processes for the continuous production of polymer particles can be easily achieved by using hollow, tubularshaped SPG membrane. Based on these findings, it would be highly advantageous to use SPG membrane emulsification for the large-scale production of uniformly-sized BCP particles with well-controlled nanostructure. However, most work related to SPG membrane emulsification has been limited to the production of the emulsion droplets that contain either homopolymers28–30 or monomers for in-situ polymerization.31–33 Recently, Okubo et al. demonstrated monodispersed micrometer-sized BCP particles of polystyrene-b-poly(methylmethacrylate) using membrane emulsification with a pore diameter (dpore=1.4 µm).34 However, it is still challenging to produce the uniform BCP particles with small sizes, in particular for submicron-sized particles, using SPG membrane emulsification. The confinement effect on the BCP structure within the particle 4


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will be greatly amplified in submicron-sized particles, resulting in the interesting structural evolution of the BCP particles. Also, the operation parameters should be systematically investigated to determine the BCP particle size and its size distribution in the SPG membrane emulsification. And, the evaporation rate of the solvent during emulsification should be controlled to develop well-ordered, reproducible BCP structure, which barely has been achieved in the previous, open cross-flow membrane-emulsification device.33 Herein, we demonstrate the successful preparation of monodispersed BCP particles having various diameters from 200 nm to 5 µm, using the SPG membrane emulsification method. The internal BCP morphology inside the particles was consistent among particles due to the generation of uniform-sized BCP particles. We were able to achieve morphological consistency among particles of poly(styrene-b-1,4-butadiene) (PS-b-PB) for both the onion-like and coiledcylindrical morphologies, which were controlled by varying the volume fraction of each block with the addition of homopolymers. Furthermore, we examined the critical parameters, including membrane pore size, operation pressure, and surfactant concentration to determine their effects on the particle size and particle size distribution. In particular, operation pressure should be optimized in order to produce monodispersed BCP particles. Finally, we succeeded in preparing uniform hybrid particles with hierarchically controlled structures of the BCPs and Au nanoparticles (NPs), which is the first demonstration of the production of monodispersed, submicrometer sized BCP-hybrid particles.

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EXPERIMENTAL SECTION Preparation of BCP particles using membrane emulsification Solution of poly(styrene-b-1,4-butadiene) (PS-b-PB, the number average molecular weight (Mn) of PS block = 112 kg/mol, Mn of PB block = 104 kg/mol, polydispersity index (PDI) = 1.06 purchased from Polymer Source Inc.) in toluene (0.01 g/ml) was prepared as a disperse phase. Water (100 ml) containing different amount of sodium dodecyl sulfate (SDS, Sigma Aldrich) ranging from 0.1 to 2.0 wt% was used as a continuous phase to form toluene-in-water emulsion. With the help of nitrogen pressure, disperse phase (6 ml) passed through the SPG membrane pores at various pressures. As shown in Scheme 1a, droplets formed at the pore mouth were detached by shear force coming from stirring cell operated at 280 rpm, forming monodispersed toluene-in-water emulsion droplets. Subsequently, toluene was evaporated slowly by stirring at room temperature for 24 h. The sample was washed with deionized (DI) water to remove the large excess of remaining surfactants by repeated centrifugations performed at 13,000 rpm for 10 min. The BCP particles were redispersed in DI water and used for further characterization. For preparation of BCP particles containing Au NPs, 6 ml of toluene containing 0.01 g/ml of PS-bPB and 0.004 g/ml of Au NPs was used as a disperse phase, following the same procedure.

Synthesis of 3-nm oleylamine-capped Au NPs The 3-nm Au NPs capped with oleylamine ligand were prepared according to the reference.35 An orange precursor solution of 1,2,3,4-tetrahydronaphthalene (tetralin, Sigma Aldrich, 10 ml), oleylamine (Sigma Aldrich, 10 ml), and HAuCl4·3H2O (Sigma Aldrich, 0.1 g) was prepared at 6


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room temperature and magnetically stirred under nitrogen purge for 10 min. A solution containing 0.5 mmol of tert-butylamine-borane complex (reducing agent, Sigma Aldrich), tetralin (1 ml), and oleylamine (1 ml) was mixed and injected into the precursor solution. The reduction was initiated instantaneously, and the color of the solution changed from orange to deep purple within 5 s. The mixture was allowed to react at room temperature for 1 h until acetone (60 ml) was added to precipitate the Au NPs. The Au NPs were collected by centrifugation (10,000 rpm, 10 min), washed with acetone, and redispersed in toluene.

Characterizations Field-emission scanning electron microscopy (FE-SEM) (Nova230), optical microscopy (Nikon, Eclipse 80i) and transmission electron microscopy (TEM) (JEOL 2000FX) were used to observe the surfaces and the internal structures of the particles. For FE-SEM measurements, the samples were prepared by drop-casting BCP particle suspensions onto silicon wafers, which were then dried and sputtered with platinum. To investigate the morphology of the particles by TEM, the samples were prepared by drop-casting the particle suspensions onto TEM grids coated with a 20-30 nm thick carbon, followed by drying in air. The prepared samples were exposed to OsO4 vapor to selectively stain the PB domains of PS-b-PB polymers. To investigate the internal structures of the BCP particles by cross-sectional TEM, the samples were prepared by dropcasting the particle suspensions onto an epoxy film and drying the treated film. Then, the dried samples were exposed to OsO4 vapor to selectively crosslink the PB domains of PS-b-PB. Then, the epoxy-supported films were cured in an oven at 60 °C for 24 h. The epoxy-supported films were then microtomed with a diamond knife at room temperature into 50-nm slices. 7



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RESULTS AND DISCUSSION

Scheme 1. (a) Schematic illustration of the SPG membrane emulsification process. Parameters to control the particle size and its distribution during the emulsification process: (b) membrane pore size; (c) operation pressure

Scheme 1 presents the illustration of the cross-flow membrane emulsification using tubular SPG membranes. A solution containing 0.01 g/ml of PS-b-PB (Mn,PS = 112 kg/mol, Mn,PB = 104 kg/mol, PDI = 1.06) in toluene was extruded through the SPG membrane under a given pressure (P) into an aqueous solution that contained SDS surfactant. Then, emulsion droplets were generated due to the shear force applied by the stirrer in the continuous phase. SDS was rapidly adsorbed at the interface of the generated toluene/water droplets to stabilize the emulsion droplets. Subsequently, slow evaporation of toluene from the droplets resulted in the production of the PS-b-PB BCP particles with internal nanostructures. 8


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Compared to other devices,36,37 our membrane emulsification system was improved distinctively in two aspects. First, SPG membrane with a hollow tubular structure that was designed to endure high P was used instead of disk-type membrane. The tube-type membrane can endure high internal pressure up to 300 kPa and external pressure up to 500 kPa, which is much higher than the pressure limit in disk-type membrane. The tube-type membrane is more advantageous for our purpose to prepare particles as small as 200 nm, because higher pressure is required for producing smaller droplets. Also, the tube-type membrane is more suitable for the continuous production of particles. Second, as shown in Scheme 1, the SPG membrane device is equipped with a cap on the tank containing the continuous phase, which enables the control of the solvent evaporation rate during emulsification. Precise control in the solvent evaporation rate through the SPG membrane device is crucial to developing the self-assembled structure of the BCP particles close to their equilibrium morphology, thus resulting in particles with well-defined, homogeneous internal nanostructure. With this device, we were able to investigate systematically the process parameters required to control the size distribution and the internal nanostructure of BCP particles. In particular, we focused on the pore diameter of the membrane, the operation pressure and the concentration of the surfactant.

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Figure 1. SEM images of PS-b-PB colloidal particles prepared using membranes with different pore sizes: (a) 5.1 µm; (b) 1.1 µm; (c) 0.5 µm; (d) 0.2 µm. The average diameters of the final particles were measured to be (a) 4.366 ± 0.370 µm, (b) 0.996 ± 0.093 µm, (c) 0.429 ± 0.042 µm, and (d) 0.201 ± 0.020 µm, respectively. The inset Figures are cross-sectional ((a), (b)) and topview ((c), (d)) TEM images of the particles. The scale bars in the insets of Figures (a), (b), (c) and (d) are 1000, 200, 100, and 100 nm, respectively.

Since the final size of BCP particles is determined by the generated droplet size of emulsion for given concentration of BCPs,28 the pore diameter in the membrane is the most critical factor in controlling the diameters of the BCP particles. As shown in SEM images of Figure 1, monodispersed PS-b-PB particles were successfully prepared from various membranes with different mean pore sizes of (a) 5.1, (b) 1.1, (c) 0.5, and (d) 0.2 µm. The average diameters of BCP particles after solvent evaporation were found to be 4.366 ± 0.370, 0.996 ± 0.093, 0.429 10


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± 0.042, and 0.201 ± 0.020 µm, respectively All of the particles showed a narrow distribution of sizes, as indicated by the fact that the coefficient of variation (CV) values were less than 10%. The insets in Figure 1 present the TEM images of the BCP particles with different diameters. Onion-like internal morphology of symmetric PS-b-PB within the monodispersed particles was observed irrespective to the particle diameter, and the number of BCP layers decreased as the particle diameter was reduced. Due to the slow evaporation of organic solvent, the condition was sufficient to develop the internal BCP structure close to its equilibrium state. To better understand the formation of BCP particles from the membrane emulsification, we investigated the relationship between the size of membrane pore (dpore), the size of emulsion droplet (ddroplet) and the size of the final BCP particle (dBCP). Taking the membrane with dpore of 5.1 µm (5.1 µm membrane) as a representative, we observed the emulsion droplets before solvent evaporation and the BCP particles after complete removal of the solvent. Figure S1 shows that the emulsion droplets had average diameter of 18.365 µm, which was approximately 3.6 times larger than the dpore value.24–27 Next, we estimated the relationship between ddroplet and dBCP using Equation [1] and compared it with our experimental observation. We assumed that the final BCP particles are perfect spheres solely composed of polymers with no residual solvent:

= ( ) [1]

where ρBCP is the density of BCP (g/ml), and cBCP is the concentration of BCP in the disperse phase (g/ml). Since the densities of the polymers in PS-b-PB were 1.05 g/ml (PS) and 0.89 g/ml (PB), and the weight fraction of PS in our PS-b-PB BCP was 0.52, the ρBCP was 0.97 g/ml. With the cBCP of 0.01 g/ml, Equation [1] becomes ddroplet= 4.6dBCP. Because we obtained ddroplet= 11



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4.5dBCP from the measured values of ddroplet= 18.365 µm and dBCP= 4.366 µm, the experimental result was in excellent agreement with the calculated value. Interestingly, the relationship in Equation [1] suggested that the size of the BCP particles can be also controlled by varying cBCP. In this regard, we compared the diameter of the BCP particles produced at different cBCP values but using the same 5.1 µm membrane. Figure S2 shows the SEM images of the BCP particles produced at different cBCP conditions of (a) 0.1 g/ml, (b) 0.01 g/ml, and (c) 0.001 g/ml. The average diameters of BCP particles (dBCP) were 6.890 ± 0.603 µm, 4.366 ± 0.370 µm, and 1.679 ± 0.172 µm, respectively, with a narrow size distribution, producing a relationship of ∝ 21 , which was consistent with Equation [1] and other literature. Therefore, the size of the

BCP particles can be controlled not only by varying the pore size of the membrane, but also by tuning the cBCP values.

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Figure 2. TEM images of PS-b-PB (ϕPS = 0.48) and PS-b-PB/hPS (ϕPS = 0.73) BCP particles using membrane emulsification: (a) PS-b-PB particles from the 0.5 µm membrane; (b) PS-bPB/hPS blend particles from the 0.5 µm membrane; (c) PS-b-PB particles from the 1.1 µm membrane; (d) PS-b-PB/hPS particles from the 1.1 µm membrane. The inset figures are crosssectional TEM images. And the scale bars in the insets of (b) and (d) are 100 and 200 nm, respectively. The PB domain appears dark due to OsO4 staining. The SDS concentration was fixed at 0.1 wt%.

Spatial confinement of BCP in the emulsion droplets provides an efficient route to manipulate self-assembled nanostructures. To demonstrate the versatility of our membrane emulsification method in producing different structured BCP particles, we attempted to produce a series of the monodispersed BCP particles with different PS volume fractions (ϕPS) using two 13



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different 0.5 and 1.1 µm membranes. To enhance the contrast between the two different BCP domains in the TEM images, the PB domains were stained selectively with OsO4 vapor. As shown in Figures 2(a) and (c), symmetric PS-b-PB (ϕPS = 0.48) within the particles produced the onion-structured particles. Upon the addition of low-molecular weight PS homopolymers (hPS, Mn= 10 kg/mol), the hPSs tend to be positioned in the PS domains of PS-b-PB, resulting in an increase of the ϕPS value.38 Therefore, the addition of hPS led to a transition from the onionlike morphology to coiled, cylindrical morphology (Figures 2(b) and (d)). These morphological variations of the particles can be understood by considering the confinement effect with the ratio of the particle diameter (D) to the feature spacing (L). The L values were measured from TEM images of onion-like structured particles using image processing software. First, in the case of the onion-like structure, the number of PB layers increased in proportional to the particle size. For example, in Figure 2(a), the particles from the 0.5 µm membrane (D/L~ 6.0 contained 3.5 layers of PB domain, while those from the 1.1µm membrane (D/L~ 14.1) in Figure 2(c) contained 7.5 layers of PB domain. The versatility of our control over the number of PB layers can be further extended by using different Mn of PS-b-PB (Mn= 112k-104k, 67k-75k, and 20k22k), which produced variations in the L values (Figure S2, Table S1). As the Mn of the PS-bPB decreased, the D/L value and the number of PB layers increased within the particles. Using the 1.1 µm membrane, the number of PB layers increased from 7.5 to 8.5 to 17.5 for 112k-104k, 67k-75k, and 20k-22k PS-b-PB, respectively. Similarly, when we used the 0.5 µm membrane, the number of PB layers increased from 3.5 to 4.5 to 7.5. Therefore, the degree of confinement and the resulting BCP morphology in the particles can be effectively tuned by controlling the D and L values. Compared to the case at ϕPS = 0.48, the cylindrical morphology at ϕPS = 0.73 underwent more dramatic changes with the changes in the D/L values because of their large 14


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bending penalty under confinement.39 For example, the recent simulation work reported a generation of various structures, including single helix, stacked toroids, circular helices, and stacked hoops, for cylindrical BCPs in the confinement.40 Figure 2(b) shows the two shells of the helical PB cylinders with PB sphere in the core for the small particles (D/L~ 6.0) produced from the 0.5 µm membrane. The shell structure resembles that of “double helices” morphology, which was theoretically reported by Chi et al.40 In contrast, the particles from the 1.1 µm membrane in Figure 2(d) (D/L~14.1) showed hexagonally packed hoops and helices of the PB phases as the confinement effect on the BCP morphology became less dominant.39–41 This structure was similar to that of “stacked hoops and circular helices” that was reported by Jeon et al., although some distortion might have occurred during the sample preparation (i.e., microtoming process).38 This morphological transition of BCPs as a function of the particle size highlighted the importance of our membrane emulsification that produced the uniform sizes of the particles. Because of the narrow size distribution of the particles, the internal nanostructure of the particles was consistent throughout the sample in each batch. These hierarchical structures from self-assembled BCPs are potentially useful in creating multi-functional devices and carriers,42,43 and their potential can be greatly amplified when morphological homogeneity is achieved in BCP particles. For example, the layer-controlled onion-like structures are considered to be accurate time-release drug-delivery systems by progressive degradation or diffusion through layers.44,45

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Figure 3. (a) Effects of P/Pc values on the particle size and its CV. The CV values are denoted by error bars; (b) SEM images of BCP particles corresponding to the data points in the plot (a). Scale bars are 2 µm. The BCP particles from the 1.1, 0.5 and 0.2 µm membranes were obtained using SDS concentration of 0.1 wt%, while the higher SDS concentration of 0.5 wt% was used for the 5.1 µm membrane.

To ensure narrow size distribution, systematic study on process parameters was essential. The operation pressure (P) is one of the crucial parameters that affect the particle size and the size distribution in the membrane emulsification. A minimal pressure, known as the critical pressure (Pc), is required for membrane emulsification, and it can be estimated by following equation:46 =

4

[2] 16


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where is the interfacial tension between the emulsion droplet and the water continuous phase, is the contact angle between the disperse phase and the membrane surface, and is the mean pore size of the membrane. Below the Pc, the dispersed phase cannot pass through the pores, thus no emulsion droplets can be formed. Thus, the Pc values could be experimentally determined for different membrane pore sizes and surfactant concentrations. Also, we compared the measured Pc values with the values calculated from Equation [2], and they showed good agreement (Figure S4). Figure 3(a) shows that when the value of P/Pc was within the range of 1.3-2.3, the particles attained a narrow size distribution in which the CV values of the BCP particles were less than 10%. However, when the P/Pc was increased to higher than 3.3, a significant increase in the CV values was observed. Interestingly, particles produced from smaller membrane pore sizes (i.e., 0.2 and 0.5 µm) had a broad size distribution (CV = 32 and 24%, respectively), while the particles from the membranes with pore sizes of 1.1 and 5.1 µm still retained their narrow size distribution. When P/Pc was increased to 4.3, the size distribution became very broad for all cases, as shown in the right column of Figure 3(b), which could be ascribed to the coalescence between the adjacent droplets at the membrane surface. According to Darcy’s law,25,47 higher P condition during the emulsification increased the flux of the disperse phase, resulting in increased number of activated pores. Therefore, the resulting droplets formed at the surface of the membrane were more likely to coalesce with adjacent droplets, as shown in Scheme S1.46 This was also because, if P is too high, the formation of the droplets at the membrane surface can occur much faster than the adsorption of the surfactants on the surface of the droplets. In addition, the flux of the disperse phase may pass through the membrane pores as a jet-stream, resulting in production of the emulsion droplets with broad size distributions.48

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Therefore, P/Pc values between 1.3 and 2.3 could provide optimum conditions for production of uniform PS-b-PB particles. Table S2 summarizes the particle diameters and their distributions depending on various P values.

Figure 4. (a) Effect of surfactant concentration on the size and distribution of the BCP particles; (b) Effect of surfactant concentration on the Pc value; (c) SEM images of the corresponding data points in plot of (a). Scale bars are 2 µm. P/Pc was kept at 1.33

Figures 4(a) and (c) show BCP particles prepared at different SDS concentrations ranging from 0.1 to 2.0 wt% at fixed P/Pc value of 1.33. Table S3 summarizes the BCP particle 18


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diameters and their distributions produced at different SDS concentrations. Below 0.1 wt% of SDS concentration, the particles were not well-generated in our system, but monodispersed BCP particles were obtained when the SDS concentration became over 0.1 wt%. In particular, in the case of the membrane with 5.1 µm pore size, a higher critical concentration of SDS surfactants is required to produce the monodispersed particles. This is probably because the larger droplets had higher instantaneous interfacial area and require longer droplet formation time, resulting in higher probability for coalescences to occur at the membrane pore.49,50 A similar trend was observed in the literature, where the critical concentration of SDS required for production of monodispersed emulsions increased with larger pore size.51 Pc value was also affected by the concentration of the SDS surfactants. As shown in Figure 4(b), the value of Pc decreased abruptly at 0.5 wt% and then became saturated at higher concentration of SDS, which was in good accordance with Equation [2]. With fixed value, the parameter to determine the Pc value is the interfacial tension () between the emulsion droplet and the water continuous phase. As the concentration of the surfactant increases, the value decreases, resulting in the reduction of the Pc value. In particular, the value decreased abruptly when SDS was added (low SDS concentration regime), and the value became saturated near the critical micelle concentration (0.236 wt%) of the SDS surfactants.52

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Figure 5. TEM images of PS-b-PB/Au NP hybrid particles prepared from 0.5 ㎛ membrane. Inset figure is a high magnification view of the cross-section of single particle.

BCPs can serve as excellent template that allows hierarchical supramolecular control over the spatial location of the nanoscale objects, providing interesting properties for various applications such as BCP nanocomposites, inks for display, photonic bandgap materials, high contrast bio-imaging and fluorescent sensors.15,53–60 In particular, if the BCP-hybrid particles have uniform size and internal structure with controlled organization of the NPs, their collective properties including optical and electrical properties can be precisely tuned to optimize their synergistic effects in the above applications. To explore the potential of membrane emulsification in the production of monodispersed hybrid particles, we fabricated Au NPincorporated BCP hybrid particles. We first prepared 3 nm size oleylamine-capped Au NPs (Figure S6). Then, a mixture of PS-b-PB solution and Au NPs was emulsified using 0.5 µm membrane followed by slow evaporation of toluene. Figure 5 presents the monodispersed hybrid colloidal particles with the size of ~ 400 ± 25 nm. The internal nanostructures of the particles 20


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were consistent for each sample without any variation of morphology. Since the oleylamine ligands on the surface of the Au NP preferentially interact with the PB chains than the PS chains,61 they were selectively incorporated within the PB domain, and this segregation was confirmed by cross-sectional TEM image of the inset in Figure 5. The optical properties of these BCP particles with alternately-assembled Au layers can be further controlled by tuning the density of Au NPs in the layers, the number of Au layers or the distance between the Au layers by varying the molecular weights or the volume fractions of the BCP templates.

CONCLUSIONS We described the successful preparation of monodispersed BCP particles ranging from 200 nm to 5 µm in diameter using SPG membrane emulsification. Monodispersed BCP particles were produced by adjusting the operation pressure, the mean pore size of the membrane, and the surfactant concentration. The internal morphologies of BCP particles with onion-like layers (ϕPS = 0.48) and coiled-cylinders (ϕPS = 0.73) were precisely tailored by addition of homopolymer. Interestingly, the uniformity of their morphology accompanied by monodispersity of particle size were clearly demonstrated. We also showed the morphological transition of BCP particles (ϕPS = 0.73) from “spherical core with double helical shell” to “hexagonally packed hoops and helices” as a function of the particle size, highlighting the importance of controlling the monodispersity of the particle size. Further extension to the preparation of uniform BCP/NP composite particles by the selective incorporation of NPs in the PB domains was successfully achieved. These uniform hybrid particles would be explored extensively as potential key materials in several 21



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future applications such as spherical dielectric resonators, multifunctional biomedical probes, broadband scattering, and self-assembled metamaterials.62

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ASSOCIATED CONTENT Supporting Information. Information of particle size and distribution depending on pressure and surfactant concentration. TEM images of Au nanoparticle. This material is available free of charge via the internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author [email protected]; [email protected] Author Contribution †

These authors contributed equally.

ACKNOWLEDGMENT This research was supported by the Korea Research Foundation Grant, funded by the Korean Government

(2012R1A1A2A10041283,

2012M1A2A2671746,

2014R1A2A2A01006628).

Authors also acknowledge the KAIST-KUSTAR project for the financial support.

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Table of Contents

Monodipserse Nanostructured Spheres of Block Copolymers and Nanoparticles via Cross-flow Membrane Emulsification Jae Man Shin1†, Minsoo P. Kim1,2†, Hyunseung Yang1, Kang Hee Ku1, Se Gyu Jang3, Kyung Ho Youm4, Gi-Ra Yi*,2, and Bumjoon J. Kim*,1

*E-mail: [email protected] (B. J. K.), [email protected] (G- R. Y.)

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Monodipserse Nanostructured Spheres of Block Copolymers and

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