Morphological Evolution of Block Copolymer Particles: Effect of Solvent Evaporation Rate on Particle Shape and Morphology Jae Man Shin,†,⊥ YongJoo Kim,‡,⊥ Hongseok Yun,† Gi-Ra Yi,§ and Bumjoon J. Kim*,† †
Department of Chemical and Biomolecular Engineering and ‡KAIST Institute for NanoCentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea § School of Chemical Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea S Supporting Information *
ABSTRACT: Shape and morphology of polymeric particles are of great importance in controlling their optical properties or self-assembly into unusual superstructures. Confinement of block copolymers (BCPs) in evaporative emulsions affords particles with diverse structures, including prolate ellipsoids, onion-like spheres, oblate ellipsoids, and others. Herein, we report that the evaporation rate of solvent from emulsions encapsulating symmetric polystyrene-b-polybutadiene (PS-b-PB) determines the shape and internal nanostructure of micron-sized BCP particles. A distinct morphological transition from the ellipsoids with striped lamellae to the onion-like spheres was observed with decreasing evaporation rate. Experiments and dissipative particle dynamics (DPD) simulations showed that the evaporation rate affected the organization of BCPs at the particle surface, which determined the final shape and internal nanostructure of the particles. Differences in the solvent diffusion rates in PS and PB at rapid evaporation rates induced alignment of both domains perpendicular to the particle surface, resulting in ellipsoids with axial lamellar stripes. Slower evaporation rates provided sufficient time for BCP organization into onion-like structures with PB as the outermost layer, owing to the preferential interaction of PB with the surroundings. BCP molecular weight was found to influence the critical evaporation rate corresponding to the morphological transition from ellipsoid to onion-like particles, as well as the ellipsoid aspect ratio. DPD simulations produced morphologies similar to those obtained from experiments and thus elucidated the mechanism and driving forces responsible for the evaporationinduced assembly of BCPs into particles with well-defined shapes and morphologies. KEYWORDS: solvent evaporation rate, block copolymer, emulsion, particle shape, monodisperse particles, dissipative particle dynamics, ellipsoid particles
S
and/or improper for large-scale production. Recently, a rather simple method using self-assembly of block copolymers (BCPs) confined in emulsion droplets, induced by evaporation of solvent from the interior phase of the emulsion, was described for the preparation of ellipsoid-shaped and convex lens-shaped particles by tuning the interfacial interactions between the BCP particles and the surrounding aqueous solution.25−31 One strategy for controlling these interfacial interactions is to use a mixture of surfactants that have selective interactions with each BCP domain. When the surfactants at the interface exhibit nonselective or minimal preferential interaction with both blocks, confined geometry does not affect internal BCP
hape-anisotropic particles have drawn significant attention as building blocks for colloidal superstuctures.1−4 The anisotropic shape gives rise to unusual physical responses, such as interparticle capillary interactions, light− matter interactions, rheological behaviors under flow, and collective behaviors under external fields.5−15 On the sub-100 nm scale, there has been significant progress in controlling the shapes of inorganic nanoparticles, including rods, tetrapods, and ellipsoids.16−20 However, it remains challenging to develop anisotropic soft polymeric particles by solution processing, particularly in the size range of 100 nm to several micrometers. Conventional methods for the fabrication of nonspherical polymeric particles are based on mechanical deformation, which relies on stretching of spherical particles in a sacrificial matrix or deformation of spherical particles that are swollen with solvent.21−24 However, these methods are either expensive © 2017 American Chemical Society
Received: December 12, 2016 Accepted: February 6, 2017 Published: February 6, 2017 2133
DOI: 10.1021/acsnano.6b08342 ACS Nano 2017, 11, 2133−2142
Article
www.acsnano.org
Article
ACS Nano
Scheme 1. Schematic Illustration of the SPG Membrane Emulsification System for Generation of Monodisperse PS-b-PB Particles with Evaporation-Rate-Dependent Morphology
distribution of the membrane pores, monodisperse emulsion droplets containing PS-b-PB were produced with a coefficient of variation (CV) of the mean particle diameter of approximately 10%, similar to the previously reported value.39 Two different membranes with pore sizes of 0.5 and 1.1 μm were used for the study. After emulsification, removal of toluene at room temperature resulted in the formation of nanostructured PS-b-PB particles in aqueous solution. The evaporation rate of toluene was varied systematically by changing the surface area available for toluene evaporation into air (Aemul/air). In these experiments, an equal amount of emulsion was stirred in differently sized containers: a 250 mL beaker (Aemul/air = 44.2 cm2), a 100 mL beaker (Aemul/air = 26.4 cm2), a 20 mL vial (Aemul/air = 2.3 cm2), and a 20 mL vial closed with a cap with a small hole in it (Aemul/air = 0.1 cm2) to afford different emulsion/air evaporation areas. Conditions for membrane emulsification are described in detail in the Experimental Section. A narrow size distribution of emulsion droplets is critical for precise analysis of the evaporation-rate-dependent morphological transitions of individual particles. For example, polydisperse droplets can result in uneven radial solvent gradients across the particle among droplets in the same batch due to the differences in the size of the droplets. Ensuring consistency of the evaporation of toluene from all droplets in a given batch allows quantitative analysis of the solvent removal process. Although previous studies reported the evaporation process of solvents from droplets, the observations were based on polydisperse emulsions.40−44 In the present work, gas chromatography was used to measure the amount of residual toluene in the emulsion at different evaporation times for quantitative determination of evaporation rate. The volume of individual BCP-containing toluene droplets (VD) was plotted as a function of evaporation time, as shown in Figure S1. These data were fitted with the relationship, VD(t) = (V0/ND)e−φt, yielding the φ (volumetric loss rate of toluene per volume of droplet in units of h−1) for each of the evaporation conditions from different Aemul/air values: φ = 4.07 h−1 (Aemul/air = 44.2 cm2), 0.26 h−1 (Aemul/air = 26.4 cm2), 0.08 h−1 (Aemul/air = 2.3 cm2), and 0.03 h−1 (Aemul/air = 0.1 cm2). Detailed derivation of this relationship is provided in the Experimental Section and in Figure S1.
nanostructure, but rather the internal structure affects the particle shape to minimize the free energy penalty associated with bending of BCP chains. Therefore, design of a surfactant− BCP pair has been the main focus of research to date. Nevertheless, investigation of kinetic effects (i.e., evaporation rate of solvent) on the formation of BCP particles from emulsions has been limited. In the case of microphase separation of solution-cast BCPs in the bulk or in thin films, solvent evaporation rate has been reported to influence both the ordering and orientation of the microphase-separated domains.32−38 Herein, we report a strategy for tuning the shape and internal morphology of polystyrene-b-polybutadiene (PS-bPB) particles, in which the evaporation rate of toluene was found to be a critical parameter in determining the final structure and shape of the particles. Prolate ellipsoids with axial lamellar stripes were formed at a relatively fast evaporation rate, whereas onion-like particles consisting of concentric lamellae with an outermost layer of PB were formed at a slow evaporation rate. The evaporation rate was systematically controlled and quantitatively determined by gas chromatography measurements to study the effect of evaporation rate on the morphology of the BCP particles. Importantly, these measurements were performed on monodisperse emulsion droplets produced by a cross-flow membrane emulsification technique to ensure constant toluene concentration gradients in all of the particles in each batch. The formation mechanisms of ellipsoid and onion particles were investigated by observation of the time-dependent evolution of their morphologies. In addition, dissipative particle dynamics (DPD) simulations were performed to elucidate the effects of solvent evaporation kinetics on the formation mechanism and resulting shape and morphology of BCP particles.
RESULTS AND DISCUSSION BCP particles were produced by controlled evaporation of toluene-in-water emulsion droplets containing PS-b-PB. Three different lamellae-forming BCPs, PS34k-b-PB25k, PS67k-b-PB75k, and PS112k-b-PB104k, were used in this study. A toluene solution containing 10 mg/mL of PS-b-PB was emulsified by passage through a Shirasu Porous Glass (SPG) membrane into an aqueous solution containing 5 mg/mL of sodium dodecyl sulfate (SDS) surfactant (Scheme 1). Due to the narrow size 2134
DOI: 10.1021/acsnano.6b08342 ACS Nano 2017, 11, 2133−2142
Article
ACS Nano
Figure 1. TEM images of PS112k-b-PB104k particles prepared at different evaporation rates: (a) φ = 4.07 h−1, (b) φ = 0.26 h−1, (c) φ = 0.08 h−1, and (d) φ = 0.03 h−1. PB domains appear dark due to OsO4 staining. The SPG membrane with a pore size of 0.5 μm was used for emulsification.
Figure 2. TEM images of PS-b-PB particles with various molecular weights (PS34k-b-PB25k, PS67k-b-PB75k, and PS112k-b-PB104k) formed at different evaporation conditions (φ) following emulsification with a 0.5 μm SPG membrane. Bar charts show the percent frequency of each particle morphology, indicated by different colors: disordered (red), ellipsoid (blue), intermediate (orange), and onion (green). The PB domains appear dark due to OsO4 staining.
disordered and ellipsoid morphologies was observed (Figure 1a). A disordered morphology was expected for the fastest evaporation due to the insufficient time for BCP to assemble into ordered nanostructures. As evaporation rate decreased to φ = 0.26 h−1, ellipsoids with well-ordered BCP domains were observed (Figure 1b). At φ = 0.08 h−1, particles with different
The organization of the BCPs within the particles was investigated for various evaporation rates by transmission electron microscopy (TEM). The TEM images in Figure 1 show a dramatic transition in the morphology of PS112k-bPB104k BCP particles as a function of evaporation rate. At the highest evaporation rate (φ = 4.07 h−1), the coexistence of 2135
DOI: 10.1021/acsnano.6b08342 ACS Nano 2017, 11, 2133−2142
Article
ACS Nano
Figure 3. (a) Droplet volume (VD) plotted as a function of time (squares) fitted with VD(t) = (V0/ND)e−φt (black line) upon evaporation of toluene from emulsions, yielding φ = 0.26 h−1. The emulsions were formed using an SPG membrane with a pore size of 1.1 μm. TEM images of PS112k-b-PB104k BCP particles acquired at evaporation times of (b) 6 h, (c) 9 h, and (d) 15 h. (e) Scheme showing the morphological evolution from emulsion droplets to ellipsoids.
particles (ellipsoid or onion) is determined by the competition between the interactions of the two different blocks with the surrounding solution (continuous phase), the interactions between the two blocks, and the entropic penalty associated with bending the BCPs within the particle.28 While the interaction of the two blocks with the surrounding is independent of BCP molecular weight, the bending energy penalty associated with the formation of lamellae near a curved surface is greater for BCP with higher molecular weight. Thus, ellipsoids with sections of low curvature surfaces are thermodynamically favored for higher molecular weight BCPs, consistent with our observations in Figure 2. Finally, at the slowest evaporation rate (φ = 0.03 h−1), only onion particles were observed irrespective of BCP molecular weight. These results demonstrate control of evaporation rate from emulsions encapsulating BCPs as a simple, yet powerful approach for tuning particle structure and shape. From a thermodynamic perspective, the interfacial interaction between the BCPs and the surrounding solution plays a major role in determining morphological transition between the ellipsoid and the onion particles for symmetric BCPs with same molecular weights.25−28,45−47 When one of the blocks (PS or PB) has a strong preference for the surrounding solution, onion-like particles having the block with the stronger preference for the surrounding solution as the outermost layer are formed to minimize interfacial energy. Therefore, emulsions stabilized with a single type of surfactant typically produce onion-like particles since most surfactants selectively interact with one of the blocks.48−51 In our system, a single surfactant (SDS) was used, and onion-like particles were produced at a slow evaporation rate. However, when the evaporation rate was increased, only ellipsoid particles were obtained even with SDS alone. Therefore, we suggest that an additional driving force, apart from the thermodynamic interfacial interaction, induced the formation of ellipsoids rather than onion-like particles. According to a previous report by Libera et al.,36 the orientation of cylindrical domains of PS-bPB-b-PS triblock copolymers in thin films was significantly affected by the solvent evaporation rate. Cylinders aligned perpendicularly to the film surface were formed under fast evaporation conditions, even though cylinders parallel to the
morphologies, categorized into three major groups: ellipsoid, onion, and intermediate structures, started to appear (Figure 1c). The slowest evaporation rate (φ = 0.03 h−1) afforded onion-like BCP particles with a half-period of PB at the outermost layer (Figure 1d). Overall, there appeared to be critical φ values for the formation of ellipsoid particles (φ = 0.26 h−1) and onion particles (φ = 0.03 h−1). Notably, the orientation of the PS-b-PB lamellae at the particle surface was found to depend significantly on the evaporation rate. The lamellar PS and PB domains were observed to align normal to the particle surface at high φ (0.26 h−1), whereas the lamellae were oriented parallel to the surface at low φ (0.03 h−1). Figure 2 shows the phase behavior of BCP particles as functions of φ and BCP molecular weight. For all three BCPs with different molecular weights, the particle shape and morphology changed dramatically with φ. The frequency of particle morphologies (disordered, ellipsoid, intermediate, and onion) observed for a given molecular weight BCP and φ are summarized in bar charts by counting more than 200 particles prepared using each set of conditions. At the highest evaporation rate (φ = 4.07 h−1), disordered and ellipsoid morphologies coexisted independent of BCP molecular weight (Figure S2). At φ = 0.26 h−1, predominantly ellipsoids were observed with different aspect ratios depending on BCP molecular weight. Particles prepared from the smallest BCP (PS34k-b-PB25k) had slightly deformed spherical shapes with an aspect ratio of 1.04. However, the aspect ratio of particles prepared from higher molecular weight BCPs, PS67k-b-PB75k and PS112k-b-PB104k, increased significantly to 1.39 and 1.51, respectively, forming elongated particles. The higher aspect ratios observed for particles formed from higher molecular weight BCPs are due to less energetic penalty for chain stretching of higher molecular weight BCPs.26,28 At an intermediate evaporation rate (φ = 0.08 h−1), the particle morphology distribution showed a clear molecular weight dependence. For the lowest molecular weight BCP, PS34k-bPB25k, the number fractions of the ellipsoids and onion-like particles were almost equal; however, the higher molecular weight BCPs, PS67k-b-PB75k and PS112k-b-PB104k, predominantly formed ellipsoids (Figure 2). When lamellae-forming BCPs are confined within emulsion droplets, the shape of the resulting 2136
DOI: 10.1021/acsnano.6b08342 ACS Nano 2017, 11, 2133−2142
Article
ACS Nano film surface were thermodynamically favored and formed under slower evaporation rate. The formation of cylinders aligned perpendicular to the surface was attributed to the significantly different diffusion rates of the solvent molecules in the PS and PB domains because perpendicularly aligned cylinders were expected to allow rapid diffusion of solvent from the film to the surface through the PB domains. To gain a deeper insight into the role of evaporation rate on the morphology of the resulting BCP particles, the structural evolution of the BCP particles (PS112k-b-PB104k) was monitored at different times during evaporation at different rates (φ = 0.26 and 0.03 h −1 ), which produced ellipsoid and onion morphologies, respectively. Initially, in both cases, BCP chains were disordered inside the emulsion droplets, as toluene mediated unfavorable enthalpic interactions between the PS and PB blocks. As the toluene evaporated, a radial concentration gradient was established within the particle with the lowest concentration of toluene at the particle surface. Eventually, the toluene concentration at the surface became insufficient to screen the unfavorable enthalpic interaction between PS and PB blocks, leading to microphase separation. Consequently, the nucleation and phase separation of the BCP into ordered domains occurs at the interface between the droplet and the surrounding aqueous solution, followed by the propagation of BCP ordering into the particle center upon additional evaporation. Therefore, BCP orientation at the emulsion surface is a key factor in determining the final shape and morphology of the BCP particle formed upon evaporation of toluene. Figure 3 shows the morphological evolution of PS112k-bPB104k BCP particles during the evaporation of toluene at a fast evaporation rate (φ = 0.26 h−1), where ellipsoid particles were formed. To precisely observe the morphology of the BCP particles by TEM at designated time points after the start of toluene evaporation, the emulsion samples were vitrified by treating with OsO4 aqueous solution immediately after removal from the containers to minimize undesired morphological changes of the BCP particles during the TEM sample preparation and measurements.52−54 After 6 h, a sufficient amount of toluene still remained within the BCP particle such that BCP phase separation was only observed near the surface (Figure 3b). Notably, both PS and PB blocks were exposed at the particle surface, producing striped lamellar phases. Evaporation for an additional 3 h resulted in the formation of ellipsoid particles with distinct PS and PB domains (Figure 3c). It is worth noting that both poles of the ellipsoid were covered with PB domains, suggesting slight selectivity of the surrounding solution for PB domains (γPB/surr < γPS/surr). Interestingly, at this stage of the assembly (t = 9 h), the PB domains were much thicker than the PS domains, particularly near the center of the ellipsoid (Figure 3c, inset), where the domain sizes of PB and PS were measured to be 57 and 21 nm, respectively. Moreover, in a single PB stripe, the PB domain size near the surface was smaller (45 nm) than that at the center (57 nm). In contrast, the PS domain showed an inverse trend, with a larger domain size at the surface (34 nm) than at the center (21 nm). Therefore, while the overall domain spacing remained relatively constant through the stripes, the relative sizes of the PS and PB domains varied significantly from the surface to the center of the ellipsoid. Evaporation for an additional 6 h (t = 15 h) removed additional toluene from the center of the ellipsoids, and the PB domains decreased to 35 nm (Figure 3d). The similar domain sizes of PS and PB
domains at both the center and the surface of the ellipsoids (∼33−35 nm) suggest the complete removal of toluene from the particle. Overall, the domain spacing of the PS-b-PB in the ellipsoids decreased from 78 nm after 9 h of evaporation to 67 nm after 15 h of evaporation. These observations indicated that the PB domains were selectively swollen by toluene and that the swelling of the PB domains was highest at the center of the particles. Thus, during evaporation, the BCP concentration at the center was the lowest, as evidenced by the smaller PS domain size (21 nm), at the particle center after 9 h of evaporation than after 15 h (33 nm). These results explained how toluene diffusion can define the morphology of particles achieved upon evaporation of toluene from BCP-containing emulsions. It is well-known that the evaporation of solvent from a polymer matrix is dependent on its diffusion rate in the matrix and its partition in each of the polymer components.55 In PS-b-PB, toluene diffuses much faster in PB than in PS at room temperature due to the higher free volume in the PB domains resulting from much lower glass transition temperature (Tg) of PB than PS. In addition, toluene has a slightly higher partition coefficient in PB than PS. The partition coefficient (K) and diffusion coefficient (D) of toluene in PB and PS are KPB = 194, KPS = 183, DPB = 2.67 × 10−6 cm2/ s, and DPS = 7.37 × 10−12 cm2/s.36,56 Therefore, toluene molecules preferentially escape through PB domains rather than through PS domains. Furthermore, it was reported that solvent mobility is dependent on the tortuosity associated with the BCP morphology and orientation.56 For example, lamellae oriented parallel to the particle surface should impede toluene diffusion from the particle center to the surface, due to continuous PS layers parallel to the surface. On the other hand, lamellae oriented perpendicularly to the particle surface provide effective pathways for toluene to diffuse through the PB domains to the surface. Accordingly, once the perpendicular lamella was achieved at the particle surface, toluene molecules preferred to evaporate throughout the PB domains, which resulted in selective swelling of the PB domains (Figure 3c). Furthermore, the radial gradient of toluene produced varying PB domain sizes along the radial position in the particle. Eventually, the perpendicular lamellar morphology formed at the particle surface propagated to the center along the toluene concentration gradient (Figure 3d). Therefore, we concluded that the evaporation direction of toluene after nucleation of BCP microphase separation was anisotropic, where toluene transport occurred predominantly through the low-curvature boundary of the ellipsoids. After evaporation for 15 h, the size of the PB domains was reduced to ∼33−35 nm from the ∼45− 57 nm (t = 9 h) (Figure 3e). To further understand the morphological development of the ellipsoid during the evaporation process, the particle shape evolution from spheres to ellipsoids was evaluated quantitatively by measuring the lengths of the major axis (L), minor axis (S), and the aspect ratio (L/S) of the particles from TEM images at different evaporation times. After evaporation for 6 h, most particles maintained their spherical shapes with an average diameter of 1.01 ± 0.11 μm. After evaporation for 9 h, significantly elongated particles were observed, accompanied by significant swelling of the PB domains. The average values of L, S, and L/S at 9 h were L = 1.73 ± 0.23 μm, S = 0.78 ± 0.07 μm, and L/S = 2.22. After 15 h, it was evident from Figure 3d that the degree of particle stretching was reduced to yield L = 1.45 ± 0.15 μm, S = 0.79 ± 0.07 μm, and L/S = 1.84. This reduction in the L/S was primarily caused by the decrease of the length of 2137
DOI: 10.1021/acsnano.6b08342 ACS Nano 2017, 11, 2133−2142
Article
ACS Nano
Figure 4. VD plotted as a function of time (squares) fitted with VD(t) = (V0/ND)e−φt (black line) upon evaporation of toluene from emulsions, yielding φ = 0.03 h−1. The emulsions were formed using an SPG membrane with a pore size of 1.1 μm. TEM images of PS112k-b-PB104k BCP particles acquired at evaporation times of (b) 48 h, (c) 72 h, and (d) 84 h. (e) Scheme showing a morphological evolution from the emulsion droplets to onion-like particles.
the major axis, L, due to the reduction of the PB domain size upon further removal of toluene. The diffusion of BCP chains in ordered structures is known to be significantly limited, particularly in the direction perpendicular to the domain interface.57−60 Overall, the variation of the domain size and the degree of particle elongation strongly suggest the selective evaporation of toluene molecules through the PB domains. Additionally, the evolution of particle morphology was examined as a function of evaporation time for BCPs with different molecular weight (PS34k-b-PB25k), and similar trends with respect to BCP domain size and particle shape were observed (Figure S3). Figure 4 shows the morphological evolution of BCPcontaining emulsions into onion-like particles at a slow evaporation rate (φ = 0.03 h−1). The overall ordering process of the BCPs was much slower than that at φ = 0.26 h−1. Incomplete BCP phase separation was observed in most areas inside the particle even after evaporation for 48 h, likely due to a significant amount of remaining toluene. The particles having concentric lamellae oriented parallel to the surface were predominantly observed (Figure 4b). At longer evaporation times (72 h), BCP ordering near the particle core was still not observed; however, all of the particles contained concentric lamellae oriented parallel to the surface with PB as the outermost layer (Figure 4c). In order to observe the swelling of the BCPs by toluene in the onion particles for comparison with that observed in the ellipsoid particles, the domain sizes of PS and PB were measured where they were visible, namely, in the outermost 3 or 4 layers. Interestingly, the domain sizes of PS and PB were 30 and 32 nm irrespective of the radial position in the particle, suggesting that there was no selective swelling of the PB domains and a small radial solvent gradient. Further removal of toluene induced the propagation of BCP ordering into the particle core, and complete onion particles were observed after evaporation for 84 h (Figure 4d). Thus, it was concluded that the evolution of emulsions into onion particles proceeded by an entirely different mechanism than that leading to the formation of ellipsoid particles. With less kinetic constraint at the slower evaporation rate, thermodynamic
effects (i.e., interfacial interaction between BCP and surrounding) were allowed to exert a controlling influence on particle morphology. Given sufficient time for PS-b-PB chains to reach an energetically stable state, preferential wetting of PB domains by the surrounding solution (γPB/surr < γPS/surr) led to the formation of onion particles with PB as the outermost layer. Consequently, the lamellae were oriented parallel to the droplet surface, eventually producing spherical onion particles. DPD simulations were performed in an attempt to explain these experimental observations. The BCP-containing toluenein-water emulsion was modeled with four types of beads representing PS, PB, the surrounding aqueous solution, and toluene. A single BCP chain was constructed from an equal number of coarse-grained, linearly sequenced PS and PB beads, where interactions between neighboring beads were modeled as springs. For simulation of PS-b-PB with different molecular weights, the total number of PS and PB beads (N) was varied. For example, to model PS34k-b-PB25k, N was set to 8 and the relative interaction parameters, APS−PS = APB−PB and APS−PB to 25.0 and 40.0, respectively, to match the expected χN value of 36.7 for PS34k-b-PB25k.61 The number of PS and PB beads was then increased to N = 12 and to N = 16 to model PS-b-PB BCPs with higher molecular weights (PS67k-b-PB75k, PS112k-bPB104k). The interaction parameter between the surrounding aqueous solution and the PS chains APS‑sur was set to 50.0, and the interaction parameter between the surrounding aqueous solution and the PB chains APB‑sur was set to 47.5. Slightly smaller values of APB‑sur relative to APS‑sur were set to capture the preferential wetting of PB domains by the surrounding aqueous solution. Also, we note that APS‑sur and APB‑sur were set to higher values than the other interaction parameters to prevent diffusion of PS-b-PB out of the particle to the surrounding solution, which is accomplished experimentally by the SDS surfactant. The interactions between toluene and each block, Atol‑PS and Atol‑PB, were set to 30.0 and 25.0, respectively, to reflect the higher diffusion and partition coefficients of toluene in the PB domain than those in the PS domain. Importantly, to simulate different evaporation rates, the interaction parameter between the toluene and the surrounding aqueous solution 2138
DOI: 10.1021/acsnano.6b08342 ACS Nano 2017, 11, 2133−2142
Article
ACS Nano
Figure 5. Snapshots of DPD simulations of transformations of BCP-containing emulsions into particles at (a) high φ (Atol‑sur = 17.5) and (b) low φ (Atol‑sur = 22.5) as a function of evaporation time (DPD time steps). The blue dots and red dots represent PS beads and PB beads, respectively. The beads representing toluene and the surrounding aqueous solution were omitted in this figure for enhanced visibility of BCP particles.
(Atol‑sur) was varied from 15.0 to 25.0. The evaporation of toluene from the emulsion droplet started to occur in the simulations when Atol‑sur < 25.0. As the Atol‑sur value was decreased further from 25.0, the relative attraction force between toluene and surrounding solution decreased, which allowed simulation of faster evaporation rate (higher φ). DPD simulations shown in Figure 5 successfully reproduce experimentally observed morphological evolution of BCP particles using N = 8 (PS34k-b-PB25k) at two different toluene evaporation rates (high φ, Atol‑sur = 17.5; low φ, Atol‑sur = 22.5). In the initial configuration, the BCP chains were distributed randomly within toluene-in-water emulsion droplets. In the simulation performed using a lower Atol‑sur interaction parameter, there was a stronger tendency for the toluene to evaporate, which led to partial alignment of the lamellae perpendicular to the particle surface after 5 × 104 time steps. Further evaporation of toluene, after 105 time steps, resulted in ellipsoids with lamellae stacked perpendicular to the long axis, as shown in Figure 5a. In contrast, the simulation conducted at a higher Atol‑sur value exhibited different morphological evolution of the BCP particles. The longer toluene residence time allowed the formation of a PB domain as the outermost layer of the particle due to the preferential interaction between PB and the surrounding aqueous solution, as shown in Figure 5b after 1.5 × 105 time steps. At this stage of the simulation, the morphology near the particle center was not fully developed, as demonstrated in the cross-sectional image. After 3 × 105 DPD time steps, complete onion particles were formed, as shown in both the surface and cross-sectional images in Figure 5b. Higher molecular weight BCPs (N = 12 and 16) were simulated with Atol−sur ranging from 15.0 to 25.0 to study the effect of the BCP molecular weight and evaporation rate on BCP particle morphology. For a direct comparison with the experimental results in Figure 2, a phase diagram was constructed as a function of N and Atol‑sur, as shown in Figure 6. Consistent with our experimental results, ellipsoids formed more frequently than onion particles at a faster evaporation
Figure 6. Phase diagram of BCP particle morphologies as functions of the BCP molecular weight and evaporation rate. The blue squares represent ellipsoids, the red squares represent onions, and the pink squares represent intermediate morphologies. Representative DPD snapshots of particles with ellipsoid, intermediate, and onion morphologies are given in the phase diagram. In the snapshots, blue and red colors represent PS beads and PB beads, respectively. For visualization purposes, only the PB beads are shown for particles with intermediate morphologies.
rate. Additionally, similarly as in experiments, intermediate morphology was observed in simulations between the onionand ellipsoid-forming regions. The simulations successfully captured the effect of BCP molecular weight on the critical φ value required for the formation of ellipsoids or onions. As molecular weight increased, the critical value of φ for the formation of onions shifted to lower values (higher Atol‑sur) due to the higher chain bending penalty for longer BCP chains. Thus, the simulations provided explanations for frequency distribution of experimentally observed morphologies in Figure 2 resulting from the different evaporation rate and BCP 2139
DOI: 10.1021/acsnano.6b08342 ACS Nano 2017, 11, 2133−2142
Article
ACS Nano
to be 858 and 390 nm for particles produced from membranes with pore diameters of 1.1 and 0.5 μm, respectively. The average diameters of the BCP particles measured from TEM images were 996 ± 93 nm for particles generated from the 1.1 μm membrane and 429 ± 42 nm for those from the 0.5 μm membrane, which showed reasonable agreement with the estimated values. Control and Analysis of Solvent Evaporation Rate from the Emulsion Droplets. After emulsification, the emulsion was separated into four different containers (20 mL per container). The evaporation rate was varied by changing the contact area between the emulsion and air (Aemul/air) while stirring continuously in different containers: a 250 mL beaker (Aemul/air = 44.2 cm2), a 100 mL beaker (Aemul/air = 26.4 cm2), a 20 mL vial (Aemul/air = 2.3 cm2), and a 20 mL vial closed with a cap that had small hole in it (Aemul/air = 0.1 cm2). The dimensions of each container were 75 mm inner diameter and 90 mm height for the 250 mL beaker, 58 mm inner diameter and 65 mm height for the 100 mL beaker, and 17 mm inner diameter and 50 mm height for the 20 mL vial. We used a bar-type magnetic stirrer having a length that matches the diameter of each beaker, and the stirring speed was fixed to 250 rpm. This was sufficiently fast enough to prevent creaming of toluene droplets at the surface. Also, the temperature of the hot plate was fixed to approximately 30 °C while evaporating toluene by stirring. To determine the evaporaion rate of toluene from each of the four containers, the amount of residual toluene in the emulsion at different evaporation times was measured by gas chromatography using pxylene as an internal standard.42 At the initial stage of the evaporation of toluene, the volume of the emulsion droplet (toluene + BCP) was assumed to be identical to the volume of toluene due to the low concentration of BCP in toluene, 10 mg/mL or around 1 vol %. Therefore, the number of droplets (ND) was calculated using the 1 3 following relationship: ND = V0/ 6 πddroplet , where V0 is total volume of the initial droplets (approximately identical to the total amount of toluene prior to evaporation). When a membrane with a pore size of 0.5 μm was used for emulsification, V0 = 1 mL and ddroplet = 1.8 μm were substituted into the equation for ND presented above to obtain ND = 3.27 × 1011. The volume of individual droplets (VD) was plotted as a function of evaporation time (Figure S1). Assuming that evaporation of toluene is proportional to the volume of the droplets, the data were fitted with the relation VD(t) = (V0/ND)e−φt, where φ was defined as the rate of the volumetric loss of toluene per volume of droplet in h−1. This fitting yielded φ values corresponding to 4.07, 0.26, 0.08, and 0.03 h−1 for four different evaporation conditions with Aemul/air = 44.2, 26.4, 2.3, and 0.1 cm2, respectively. Characterization. TEM (JEOL 2000FX) was used to observe the shape and morphology of the particles. The particle dispersion was stained with an aqueous solution of 0.2 wt % OsO4, which selectively stains PB. After staining, the samples were washed with DI water to remove excess surfactant by repeated centrifugation at 12 000 rpm. The samples were prepared by drop-casting the particle suspensions onto TEM grids coated with 20−30 nm thick carbon and dried in air. Aliquots of the particle suspension were drawn at designated evaporation times for morhpological evaluation by TEM and immediately cross-linked with OsO4 aqueous solution to prevent further mophological changes during sample preparation. Simulation Method. BCP droplets were simulated using a dissipative particle dynamics method originally developed by Groot and Warren.62 The former model of a two-component BCP system was expanded into a four-component system including PS, PB, toluene, and the surrounding aqueous solution (water + SDS).63−66 A coarse-grained model of a single chain of PS-b-PB BCP with N = 8 (the number of beads in a single chain) contained 4 PS beads and 4 PB beads linearly connected by soft springs with a spring constant of 4. The relative interaction parameters between the polymers were set to APS−PS = APB−PB = 25.0 and APS−PB = 40.0 for χN = 36.7 to match the χN value of PS34k-b-PB25k.61 Additionally, longer chains with N = 12 (6 PS beads and 6 PB beads) and N = 16 (8 PS beads and 8 PB beads) were simulated to investigate the effect of BCP chain length on particle morphology. BCP chains (3860) were combined with 30 880 toluene beads for shorter BCP (N = 8), whereas fewer polymer chains were
molecular weights. However, the increase in the aspect ratio of the ellipsoids with BCP molecular weight was not as pronounced as that observed experimentally. That is, as N increased from 8 to 16, the particles elongated only slightly. This discrepancy is attributed to the smaller droplet sizes employed in the simulations to minimize computational cost.
CONCLUSIONS A simple yet powerful method for controlling the shape and internal morphology of PS-b-PB particles was developed. Decreasing the evaporation rate induced a transition of the resulting morphology from the ellipsoids to the onion-like particles. The uniform size of the droplets produced by SPG membrane emulsification allowed reliable determination of the φ value and correlation to the morphology of the resulting BCP particles. The mechanism of evaporation-dependent formation of ellipsoid or onion particles was investigated by monitoring morphological evolution at different evaporation times. Orientation of BCP chains at the particle surface was found to be critical in determining the final structure of the BCP particles. At high evaporation rates, the large difference between toluene diffusion rates in PS and PB enabled perpendicular orientation of lamellae to the particle surface, resulting in the ellipsoid with axially stacked lamellae. In contrast, low evaporation rates generated onion particles with concentric lamellae. Moreover, both the critical φ values for morphological transition and the aspect ratios of the resulting ellipsoids were dependent on BCP molecular weight, which was captured by the DPD simulations that were consistent with experimental results. Future work will address applications of anisotropic BCP particles with well-controlled sizes and aspect ratios. EXPERIMENTAL SECTION Materials. Three different symmetric poly(styrene-b-1,4-butadiene) (PS-b-PB) BCPs were purchased from Polymer Source, Inc.: (1) PS34k-b-PB25k (subscripts indicate the number-average molecular weight (Mn) of each block), polydispersity index (PDI): 1.20), (2) PS67k-b-PB75k (PDI: 1.08), and (3) PS112k-b-PB104k (PDI: 1.06). SDS was purchased from Sigma-Aldrich. Toluene and p-xylene were purchased from Samchun Chemical and used as received. Preparation of PS-b-PB Colloidal Particles by Cross-Flow Membrane Emulsification. A solution containing 10 mg/mL of PSb-PB (PS34k-b-PB25k, PS67k-b-PB75k, or PS112k-b-PB104k) in toluene (4 mL) was emulsified in a continuouse phase containing 5 mg/mL SDS in deionized (DI) water (80 mL) using an SPG membrane device.39 Monodisperse emulsion droplets of PS-b-PB were generated by passing the organic phase through the SPG membrane. The subsequent removal of toluene at room temperature resulted in the monodisperse PS-b-PB particles. The final diameter of the BCP particles after removal of toluene (dBCP) is determined by the initial concentration of BCP in toluene and the membrane pore size (dpore), as described previously.39 Assuming that the final particles were perfect spheres with no residual solvent, the following equation can be derived: ddroplet =
3
⎛ ρBCP ⎞ ⎜ ⎟ dBCP ⎝ c BCP ⎠
(1)
where ddroplet is the diameter of the initial droplets, ρBCP is the density of the BCP, and cBCP is the concentration of BCP in the dispersed phase. In this work, two different membranes, with pore sizes of 1.1 and 0.5 μm, were used for emulsification. The concentration cBCP was fixed as 10 mg/mL, and ρBCP was 0.97 g/mL. Under these processing conditions, the ddroplet value was 3.6 times larger than the pore size of the membrane,39 allowing empirical estimation of the BCP particle size (dBCP) as dBCP = 0.78dpore. Accordingly, the dBCP values were calculated 2140
DOI: 10.1021/acsnano.6b08342 ACS Nano 2017, 11, 2133−2142
Article
ACS Nano combined with approximately 30 880 toluene beads for longer BCPs (i.e., 2573 BCP chains with 30 884 toluene beads for N = 12, and 1930 BCP chains with 30 880 toluene beads for N = 16) to yield a constant number of beads in each simulation. For all BCP chain lengths, 130 240 beads representing the surrounding aqueous solution were used to provide sufficient space for BCP assembly within the emulsion droplets without finite size effects. The size of the simulation box was set as 40 × 40 × 40 to have 3 as the density of beads, and the periodic boundary condition was applied in all three directions.64 For the initial configuration, the BCPs were dissolved in toluene within a spherical droplet surrounded by water molecules. From the initial configuration, 106 DPD time steps were run for each set of conditions (i.e., different BCP molecular weights and evaporation rate). To reduce expensive computation time, tiny encryption algorithm random number generation was adopted for the GPU platform.67
(8) Doshi, N.; Mitragotri, S. Needle-Shaped Polymeric Particles Induce Transient Disruption of Cell Membranes. J. R. Soc., Interface 2010, 7, S403−S410. (9) Yablonovitch, E.; Gmitter, T. J.; Leung, K. M. Photonic Band Structure: The Face-Centered Cubic Case Employing Nonspherical Atoms. Phys. Rev. Lett. 1991, 67, 2295−2298. (10) Yunker, P. J.; Still, T.; Lohr, M. A.; Yodh, A. G. Suppression of the Coffee-Ring Effect by Shape-Dependent Capillary Interactions. Nature 2011, 476, 308−311. (11) Madivala, B.; Fransaer, J.; Vermant, J. Self-Assembly and Rheology of Ellipsoidal Particles at Interfaces. Langmuir 2009, 25, 2718−2728. (12) Asano, S.; Yamamoto, G. Light Scattering by a Spheroidal Particle. Appl. Opt. 1975, 14, 29−49. (13) Velikov, K. P.; Van Dillen, T.; Polman, A.; Van Blaaderen, A. Photonic Crystals of Shape-Anisotropic Colloidal Particles. Appl. Phys. Lett. 2002, 81, 838−840. (14) Kim, M. P.; Kang, D. J.; Jung, D. W.; Kannan, A. G.; Kim, K. H.; Ku, K. H.; Jang, S. G.; Chae, W. S.; Yi, G. R.; Kim, B. J. GoldDecorated Block Copolymer Microspheres with Controlled Surface Nanostructures. ACS Nano 2012, 6, 2750−2757. (15) Ku, K. H.; Kim, Y.; Yi, G.-R.; Jung, Y. S.; Kim, B. J. Soft Patchy Particles of Block Copolymers from Interface-Engineered Emulsions. ACS Nano 2015, 9, 11333−11341. (16) Chen, S.; Wang, Z. L.; Ballato, J.; Foulger, S. H.; Carroll, D. L. Monopod, Bipod, Tripod, and Tetrapod Gold Nanocrystals. J. Am. Chem. Soc. 2003, 125, 16186−16187. (17) Glotzer, S. C.; Solomon, M. J. Anisotropy of Building Blocks and Their Assembly into Complex Structures. Nat. Mater. 2007, 6, 557−562. (18) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Shape Control of CdSe Nanocrystals. Nature 2000, 404, 59−61. (19) Sun, Y.; Xia, Y. Shape-Controlled Synthesis of Gold and Silver Nanoparticles. Science 2002, 298, 2176−2179. (20) Johnson, P. M.; van Kats, C. M.; van Blaaderen, A. Synthesis of Colloidal Silica Dumbbells. Langmuir 2005, 21, 11510−11517. (21) Champion, J. A.; Katare, Y. K.; Mitragotri, S. Making Polymeric Micro- and Nanoparticles of Complex Shapes. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 11901−11904. (22) Lele, P. P.; Furst, E. M. Assemble-and-Stretch Method for Creating Two- and Three-Dimensional Structures of Anisotropic Particles. Langmuir 2009, 25, 8875−8878. (23) 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. (24) Mei, S.; Wang, L.; Feng, X.; Jin, Z. Swelling of Block Copolymer Nanoparticles: A Process Combining Deformation and Phase Separation. Langmuir 2013, 29, 4640−4646. (25) Deng, R.; Liang, F.; Li, W.; Yang, Z.; Zhu, J. Reversible Transformation of Nanostructured Polymer Particles. Macromolecules 2013, 46, 7012−7017. (26) Jang, S. G.; Audus, D. J.; Klinger, D.; Krogstad, D. V.; Kim, B. J.; Cameron, A.; Kim, S. W.; Delaney, K. T.; Hur, S. M.; Killops, K. L.; Fredrickson, G. H.; Kramer, E. J.; Hawker, C. J. Striped, Ellipsoidal Particles by Controlled Assembly of Diblock Copolymers. J. Am. Chem. Soc. 2013, 135, 6649−6657. (27) Jeon, S. J.; Yi, G. R.; Yang, S. M. Cooperative Assembly of Block Copolymers with Deformable Interfaces: Toward Nanostructured Particles. Adv. Mater. 2008, 20, 4103−4108. (28) Klinger, D.; Wang, C. X.; Connal, L. A.; Audus, D. J.; Jang, S. G.; Kraemer, S.; Killops, K. L.; Fredrickson, G. H.; Kramer, E. J.; Hawker, C. J. A Facile Synthesis of Dynamic, Shape-Changing Polymer Particles. Angew. Chem., Int. Ed. 2014, 53, 7018−7022. (29) Ku, K. H.; Shin, J. M.; Kim, M. P.; Lee, C. H.; Seo, M. K.; Yi, G. R.; Jang, S. G.; Kim, B. J. Size-Controlled Nanoparticle-Guided Assembly of Block Copolymers for Convex Lens-Shaped Particles. J. Am. Chem. Soc. 2014, 136, 9982−9989.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b08342. Additional TEM images and analysis (PDF)
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. ORCID
Hongseok Yun: 0000-0003-0497-6185 Bumjoon J. Kim: 0000-0001-7783-9689 Author Contributions ⊥
J.M.S. and Y.J.K. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This research was supported by National Research Foundation Grants (2012M3A6A7055540), funded by the Korean Government. This work was supported by the KETEP and the MOTIE of the Republic of Korea (No. 20163030013620, 20163010012470). We acknowledge support from the Research Projects of the KAIST-KUSTAR. We thank Dr. Rachel Letteri for helpful discussions. REFERENCES (1) Sacanna, S.; Korpics, M.; Rodriguez, K.; Colón-Meléndez, L.; Kim, S.-H.; Pine, D. J.; Yi, G.-R. Shaping Colloids for Self-Assembly. Nat. Commun. 2013, 4, 1688. (2) Wang, Y.; Wang, Y.; Zheng, X.; Yi, G. R.; Sacanna, S.; Pine, D. J.; Weck, M. Three-Dimensional Lock and Key Colloids. J. Am. Chem. Soc. 2014, 136, 6866−6869. (3) Gröschel, A. H.; Walther, A.; Löbling, T. I.; Schacher, F. H.; Schmalz, H.; Müller, A. H. E. Guided Hierarchical Co-Assembly of Soft Patchy Nanoparticles. Nature 2013, 503, 247−251. (4) Gröschel, A. H.; Schacher, F. H.; Schmalz, H.; Borisov, O. V.; Zhulina, E. B.; Walther, A.; Müller, A. H. E. Precise Hierarchical SelfAssembly of Multicompartment Micelles. Nat. Commun. 2012, 3, 710. (5) Han, Y.; Alsayed, A. M.; Nobili, M.; Zhang, J.; Lubensky, T. C.; Yodh, A. G. Brownian Motion of an Ellipsoid. Science 2006, 314, 626− 630. (6) Man, W.; Donev, A.; Stillinger, F. H.; Sullivan, M. T.; Russel, W. B.; Heeger, D.; Inati, S.; Torquato, S.; Chaikin, P. M. Experiments on Random Packings of Ellipsoids. Phys. Rev. Lett. 2005, 94, 198001. (7) Donev, A.; Cisse, I.; Sachs, D.; Variano, E. A.; Stillinger, F. H.; Connelly, R.; Torquato, S.; Chaikin, P. M. Improving the Density of Jammed Disordered Packings Using Ellipsoids. Science 2004, 303, 990−993. 2141
DOI: 10.1021/acsnano.6b08342 ACS Nano 2017, 11, 2133−2142
Article
ACS Nano (30) Ku, K. H.; Yang, H.; Shin, J. M.; Kim, B. J. Aspect Ratio Effect of Nanorod Surfactants on the Shape and Internal Morphology of Block Copolymer Particles. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 188−192. (31) Yang, H.; Ku, K. H.; Shin, J. M.; Lee, J.; Park, C. H.; Cho, H. H.; Jang, S. G.; Kim, B. J. Engineering the Shape of Block Copolymer Particles by Surface-Modulated Graphene Quantum Dots. Chem. Mater. 2016, 28, 830−837. (32) Kim, S. H.; Misner, M. J.; Xu, T.; Kimura, M.; Russell, T. P. Highly Oriented and Ordered Arrays from Block Copolymers via Solvent Evaporation. Adv. Mater. 2004, 16, 226−231. (33) Phillip, W. A.; Hillmyer, M. A.; Cussler, E. L. Cylinder Orientation Mechanism in Block Copolymer Thin Films upon Solvent Evaporation. Macromolecules 2010, 43, 7763−7770. (34) Paradiso, S. P.; Delaney, K. T.; García-Cervera, C. J.; Ceniceros, H. D.; Fredrickson, G. H. Block Copolymer Self Assembly during Rapid Solvent Evaporation: Insights into Cylinder Growth and Stability. ACS Macro Lett. 2014, 3, 16−20. (35) Park, S. C.; Kim, B. J.; Hawker, C. J.; Kramer, E. J.; Bang, J.; Ha, J. S. Controlled Ordering of Block Copolymer Thin Films by the Addition of Hydrophilic Nanoparticles. Macromolecules 2007, 40, 8119−8124. (36) Kim, G.; Libera, M. Morphological Development in SolventCast Polystyrene - Polybutadiene - Polystyrene (SBS) Triblock Copolymer Thin Films. Macromolecules 1998, 31, 2569−2577. (37) Park, S.; Wang, J. Y.; Kim, B.; Chen, W.; Russell, T. P. SolventInduced Transition from Micelles in Solution to Cylindrical Microdomains in Diblock Copolymer Thin Films. Macromolecules 2007, 40, 9059−9063. (38) Phillip, W. A.; O'Neill, B.; Rodwogin, M.; Hillmyer, M. A.; Cussler, E. L. Self-Assembled Block Copolymer Thin Films as Water Filtration Membranes. ACS Appl. Mater. Interfaces 2010, 2, 847−853. (39) Shin, J. M.; Kim, M. P.; Yang, H.; Ku, K. H.; Jang, S. G.; Youm, K. H.; Yi, G. R.; Kim, B. J. Monodispserse Nanostructured Spheres of Block Copolymers and Nanoparticles via Cross-Flow Membrane Emulsification. Chem. Mater. 2015, 27, 6314−6321. (40) Wang, J.; Schwendeman, S. P. Mechanisms of Solvent Evaporation Encapsulation Processes: Prediction of Solvent Evaporation Rate. J. Pharm. Sci. 1999, 88, 1090−1099. (41) Staff, R. H.; Schaeffel, D.; Turshatov, A.; Donadio, D.; Butt, H. J.; Landfester, K.; Koynov, K.; Crespy, D. Particle Formation in the Emulsion-Solvent Evaporation Process. Small 2013, 9, 3514−3522. (42) Okubo, M.; Saito, N.; Takekoh, R.; Kobayashi, H. Morphology of Polystyrene/polystyrene-Block-Poly(methyl Methacrylate)/poly(methyl Methacrylate) Composite Particles. Polymer 2005, 46, 1151−1156. (43) Fryd, M. M.; Mason, T. G. Time-Dependent Nanoemulsion Droplet Size Reduction by Evaporative Ripening. J. Phys. Chem. Lett. 2010, 1, 3349−3353. (44) Zhao, Y.; Berger, R.; Landfester, K.; Crespy, D. Polymer Patchy Colloids with Sticky Patches. Polym. Chem. 2014, 5, 365. (45) Yan, N.; Liu, H.; Zhu, Y.; Jiang, W.; Dong, Z. Entropy-Driven Hierarchical Nanostructures from Cooperative Self-Assembly of Gold Nanoparticles/Block Copolymers under Three-Dimensional Confinement. Macromolecules 2015, 48, 5980−5987. (46) Deng, R.; Liang, F.; Zhou, P.; Zhang, C.; Qu, X.; Wang, Q.; Li, J.; Zhu, J.; Yang, Z. Janus Nanodisc of Diblock Copolymers. Adv. Mater. 2014, 26, 4469−4472. (47) Yan, N.; Zhu, Y.; Jiang, W. Self-Assembly of AB Diblock Copolymer Confined in a Soft Nano-Droplet: A Combination Study by Monte Carlo Simulation and Experiment. J. Phys. Chem. B 2016, 120, 12023−12029. (48) Robb, M. J.; Connal, L. A.; Lee, B. F.; Lynd, N. A.; Hawker, C. J. Functional Block Copolymer Nanoparticles: Toward the next Generation of Delivery Vehicles. Polym. Chem. 2012, 3, 1618−1628. (49) Jeon, S. J.; Yi, G. R.; Koo, C. M.; Yang, S. M. Nanostructures inside Colloidal Particles of Block Copolymer/homopolymer Blends. Macromolecules 2007, 40, 8430−8439.
(50) Li, L.; Matsunaga, K.; Zhu, J.; Higuchi, T.; Yabu, H.; Shimomura, M.; Jinnai, H.; Hayward, R. C.; Russell, T. P. SolventDriven Evolution of Block Copolymer Morphology under 3D Confinement. Macromolecules 2010, 43, 7807−7812. (51) Saito, N.; Takekoh, R.; Nakatsuru, R.; Okubo, M. Effect of Stabilizer on Formation Of “onionlike” multilayered PolystyreneBlock-Poly(methyl Methacrylate) Particles. Langmuir 2007, 23, 5978− 5983. (52) Higuchi, T.; Shimomura, M.; Yabu, H. Reorientation of Microphase-Separated Structures in Water Suspended Block Copolymer Nanoparticles through Microwave Annealing. Macromolecules 2013, 46, 4064−4068. (53) 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. (54) Higuchi, T.; Tajima, A.; Motoyoshi, K.; Yabu, H.; Shimomura, M. Frustrated Phases of Block Copolymers in Nanoparticles. Angew. Chem., Int. Ed. 2008, 47, 8044−8046. (55) Ho, R. M.; Tseng, W. H.; Fan, H. W.; Chiang, Y. W.; Lin, C. C.; Ko, B. T.; Huang, B. H. Solvent-Induced Microdomain Orientation in Polystyrene-B-Poly(l-Lactide) Diblock Copolymer Thin Films for Nanopatterning. Polymer 2005, 46, 9362−9377. (56) Faridi, N.; Duda, J. L.; Hadj-Romdhane, I. Unsteady-State Diffusion in Block Copolymers with Lamellar Domains. Ind. Eng. Chem. Res. 1995, 34, 3556−3567. (57) Yokoyama, H. Diffusion of Block Copolymers. Mater. Sci. Eng., R 2006, 53, 199−248. (58) Bates, C. M.; Seshimo, T.; Maher, M. J.; Durand, W. J.; Cushen, J. D.; Dean, L. M.; Blachut, G.; Ellison, C. J.; Willson, C. G. PolaritySwitching Top Coats Enable Orientation of Sub-10-Nm Block Copolymer Domains. Science 2012, 338, 775−779. (59) Yokoyama, H.; Kramer, E. J. Self-Diffusion of Asymmetric Diblock Copolymers with a Spherical Domain Structure. Macromolecules 1998, 31, 7871−7876. (60) Yokoyama, H.; Kramer, E. J.; Rafailovich, M. H.; Sokolov, J.; Schwarz, S. a. Structure and Diffusion of Asymmetric Diblock Copolymers in Thin Films: A Dynamic Secondary Ion Mass Spectrometry Study. Macromolecules 1998, 31, 8826−8830. (61) Men’shikov, E. A.; Bol’shakova, A. V.; Yaminskii, I. V. Determination of the Flory-Huggins Parameter for a Pair of Polymer Units from AFM Data for Thin Films of Block Copolymers. Prot. Met. Phys. Chem. Surf. 2009, 45, 295−299. (62) Groot, R. D.; Warren, P. B. Dissipative Particle Dynamics: Bridging the Gap between Atomistic and Mesoscopic Simulation. J. Chem. Phys. 1997, 107, 4423. (63) Groot, R. D.; Madden, T. J. Dynamic Simulation of Diblock Copolymer Microphase Separation. J. Chem. Phys. 1998, 108, 8713− 8724. (64) Gavrilov, A. A.; Kudryavtsev, Y. V.; Chertovich, A. V. Phase Diagrams of Block Copolymer Melts by Dissipative Particle Dynamics Simulations. J. Chem. Phys. 2013, 139, 224901. (65) Gavrilov, A. A.; Kudryavtsev, Y. V.; Khalatur, P. G.; Chertovich, A. V. Microphase Separation in Regular and Random Copolymer Melts by DPD Simulations. Chem. Phys. Lett. 2011, 503, 277−282. (66) Berezkin, A. V.; Papadakis, C. M.; Potemkin, I. I. Vertical Domain Orientation in Cylinder-Forming Diblock Copolymer Films upon Solvent Vapor Annealing. Macromolecules 2016, 49, 415−424. (67) Phillips, C. L.; Anderson, J. A.; Glotzer, S. C. Pseudo-Random Number Generation for Brownian Dynamics and Dissipative Particle Dynamics Simulations on GPU Devices. J. Comput. Phys. 2011, 230, 7191−7201.
2142
DOI: 10.1021/acsnano.6b08342 ACS Nano 2017, 11, 2133−2142
