Entropy-Driven Hierarchical Nanostructures from Cooperative Self

Article pubs.acs.org/Macromolecules

Entropy-Driven Hierarchical Nanostructures from Cooperative SelfAssembly of Gold Nanoparticles/Block Copolymers under ThreeDimensional Confinement Nan Yan,†,‡ Hongxia Liu,†,§ Yutian Zhu,*,† Wei Jiang,*,† and Zeyuan Dong∥ †

Downloaded by UNIV OF CAMBRIDGE on August 25, 2015 | http://pubs.acs.org Publication Date (Web): August 14, 2015 | doi: 10.1021/acs.macromol.5b01219

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China § College of Materials Science and Engineering, Jilin University, Changchun 130022, People’s Republic of China ∥ State Key Laboratory of Supramolecular Structure and Materials College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China S Supporting Information *

ABSTRACT: The cooperative self-assembly of polystyrene-b-poly(4-vinylpyridine) block copolymers (BCPs) and gold nanoparticles (AuNPs) confined within the emulsion droplets is studied by combining both the experiments and Monte Carlo simulations. The results indicate that the entropic interaction between the AuNPs and BCP domain is a critical parameter to dominate the spatial arrangement of AuNPs and the nanostructure of the hybrid nanoparticles, which can be utilized to design novel hierarchical hybrid nanoparticles. Based on this theoretical observation, a large number of unique Janus hybrid nanoparticles, including pupa-like nanoparticles with AuNPs concentrated at one pole of the particles, spherical nanoparticles with AuNPs enriched in a bulge on the sphere surface, and the gourd-like, clover-like, and four-leaf-clover-like nanoparticles from the further hierarchical assembly of small hybrid Janus nanoparticles, are fabricated via threedimensional (3D) confined self-assembly.

1. INTRODUCTION The incorporation of inorganic nanoparticles (NPs) into selfassembled block copolymers has been intensively studied in recent years since it can combine the inherent properties of these two building units into the new hybrid nanomaterials with desirable properties, which can be widely used in various applications such as sensors, catalysis, optics, electronic devices, and so forth.1−5 Up to now, a number of strategies have been developed to fabricate the hybrid BCPs/NPs nanomaterials, including coprecipitation method,6,7 heating−cooling processing,8 electrostatic interaction approach,9 directed supramolecular assembly,10 interfacial instabilities of emulsion droplets,11,12 and so forth. On the other hand, it has been reported that the neat BCPs can self-assemble into unique microphaseseparated nanostructures confined in emulsion droplets because the confinement effect can effectively break the symmetry of the structure, thus resulting in the nanostructures that are not available in the free systems.13−21 Thereafter, the cooperative self-assembly of NPs and BCPs confined in emulsion droplets has attracted increasing attention since the 3D confined selfassembly offers an efficient route for fabricating novel hybrid nanostructures.22−27 For example, Kim and co-workers proposed a powerful strategy to control the overall shape of the BCP particles by using size-controlled NP surfactants.23 Novel convex-lens-shaped (CL) hybrid BCP/AuNPs particles © 2015 American Chemical Society

with highly ordered and oriented nanoporous channels were fabricated through the combination of confined geometry and interfacial modulation. Jang et al. prepared a striped asymmetric nanoparticle by controlled assembly of diblock copolymers and AuNPs in the confined geometry.22 They observed that the addition of Au-based surfactant nanoparticles to colloidal particles containing lamella-forming polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) diblock copolymers can result in a dramatic transition from a traditional spherical, radial morphology to unique axially stacked lamella ellipsoids. So far, however, the practical application of the hybrid BCPs/ NPs nanomaterials is still limited because the properties of the hybrid materials depend not only on the intrinsic properties of individual components but also on the spatial arrangement of the NPs within the BCP substrate.28,29 Although the inorganic NPs have been successfully encapsulated into BCP microdomain, it is still a big challenge to precise control the spatial arrangement of NPs in the polymer matrix. Basically, both enthalpic and entropic interactions are involved during the incorporation of NPs into the polymer matrix. In general, enthalpic repulsion will be enforced because the NPs and BCPs Received: June 5, 2015 Revised: July 29, 2015 Published: August 14, 2015 5980

DOI: 10.1021/acs.macromol.5b01219 Macromolecules 2015, 48, 5980−5987

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Figure 1. TEM images of self-assembly of PS9.8k-b-P4VP10k (a) and cooperative self-assembly of AuS/PS9.8k-b-P4VP10k (b, c) under 3D confinement. The AuS in (b) and (c) are Au1.7S and Au3.5S, respectively. The volume fractions of AuS are 31.8 vol % in (b) and 32.2 vol % in (c). P4VP domain appears darker than PS domain due to staining with I2 vapor.

producing unique convex-lens-shaped BCP particles. Different from the previous work, the current study reveals that large AuNPs will be expelled to the outer layer of the affinitive polymer domains because of the strong entropic repulsion. However, since the BCPs and AuNPs are confined in one emulsion droplet, macrophase separation between BCPs and AuNPs is suppressed. Therefore, some novel hierarchical nanostructures, including asymmetric stacked-lamellae particles with AuNPs concentrated at one pole of the pupa-like particles, spherical nanoparticles with AuNPs enriched in a bulge onto the sphere surface, and the gourd-like, clover-like, and four-leafclover-like superparticles obtained by the hierarchical assembly of small Janus-like hybrid nanoparticles, were fabricated by controlling the entropic contribution during the 3D confined coassembly, i.e., by tuning the size of NPs and the length of the affinitive blocks. Compared to the previous works, the current study mainly demonstrates that entropic interaction can be effectively utilized to control the spatial distribution of NPs in BCPs, resulting in some unique hybrid nanostructures.

are normally incompatible with each other. In order to minimize the enthalpic repulsion, chemical modification of inorganic NPs with polymer brushes that favorably interact with one of the BCP blocks was widely used.29−32 For example, Kramer and co-workers demonstrated a simple procedure to incorporate gold nanoparticles (AuNPs) into the BCP film and precise control their location by the surface modification of the AuNPs.29 They found that PS-coated AuNPs can entirely locate within the PS domain of the PS-b-P2VP block copolymer while the mixtures of PS and P2VP ligands on the surface of AuNPs directed the AuNPs to the interface between the PS and P2VP domains, providing a versatile method to precise control the spatial distribution of NPs in polymer matrix. On the other hand, upon incorporation of NPs into BCP matrix, the polymer chains also experience conformational changes, resulting in entropy loss of the system.4,33−35 The entropic loss is mainly dominated by the ratio of the NP diameter to the size of the selected polymer domain, which can significantly affect the spatial arrangement of the NPs.36 The dependence of the spatial organization of NPs in BCP matrix on the NP size has been investigated by both simulations and experiments.10,33−37 For instance, Balazs and co-workers applied Monte Carlo simulation to investigate the influence of hard nanoparticles on the phase behavior of diblock copolymers.34 They found that when the size of the nanoparticles was comparable to the radius of gyration of the minority block, new superstructures can be generated. In addition, Xu and co-workers have demonstrated that the 3D spatial organization of nanoparticles in polymer thin film can be controlled by the size and the volume fraction of the nanoparticles.4 In spite of these developments in this field, however, precise control the spatial arrangement of NPs in the hybrid BCPs/NPs particles remains a challenge. Herein, the cooperative self-assembly of symmetric and asymmetric polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) block copolymers with PS/P4VP-coated AuNPs confined in emulsion droplets was studied by combining experiments and Monte Carlo simulations. For well incorporation of AuNPs into the affinitive polymer phase, the AuNPs with relatively small d/ L value (d is the diameter of AuNPs, whereas L is the dimension of the preferred polymer domain) were normally used to coassemble with BCPs in the previous works, which generally resulted in the hybrid nanostructures with AuNPs uniformly dispersed in the preferred domain.2,10 Recently, Jang and Kim reported a size-controlled nanoparticle-guided assembly of BCPs into a convex-lens-shaped particle.23 In that work, however, AuNPs mainly played a role of surfactant, which generated a balanced interfacial interaction between two different PS/P4VP domains of the BCP particles and water,

2. RESULTS AND DISCUSSION Figure 1a shows the transmission electron microscopy (TEM) image of the PS9.8k-b-P4VP10k ellipsoidal particles with stacked lamellar structure prepared by the 3D confined self-assembly within the emulsion droplets. This pupa-like structure has also been observed in the previous works when the interactions of the two blocks with the aqueous solution are comparable.38,39 In the current study, 100 μL of chloroform solution containing PS9.8k-b-P4VP10k (0.5 wt % in chloroform, the volume fraction of P4VP, f P4VP ≈ 50.5%) was emulsified by ultrasonication in 1.0 mL of deionized water containing PVA (12 mg/mL) as a surfactant, which played dual roles in both stabilizing the emulsion droplets and providing a nearly neutral interface for PS and P4VP blocks.38,39 Then, the chloroform diffused through the water phase and slowly evaporated, leading to the microphase separation of PS-b-P4VP within the emulsion droplets. After complete removal of chloroform from the emulsion droplets, glassy PS9.8k-b-P4VP10k particles with axially stacked lamellar nanostructure were obtained, as shown in Figure 1a. Moreover, it is found that all of the particles in Figure 1a adopt the ellipsoidal shape rather than the spherical shape. This is because that the anisotropic ellipsoidal shape has a lower curvature boundary than that of the spherical shape and thus can minimize the entropic penalty associated with bending of the polymer blocks.40 To unveil the phase-separated structures of the PS-b-P4VP nanoparticles, these particles were stained with iodine vapor (I2) which selectively stained the P4VP domain, leading to a darker appearance than the PS 5981


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Figure 2. Morphologies of the hybrid A6B6/AuNPs nanoparticle with NP diameters of 1 and 2. The polymer−solvent interactions are set as εAS = εBS = 2.0. εAB is fixed at 3.0 to mimic the incompatibility between A and B blocks. NPs are assumed to have chemical affinity with B monomers, i.e., εBN = 0, εAN = εAB. (a) Nanoparticles with a diameter of 1. (b) Nanoparticles with a diameter of 2. Green represents the A blocks, blue represents the B blocks, and yellow represents the NPs. Small amounts of the free NPs (not involved in the coassembly) are not shown in the images.

nanoparticles (IONs) were concentrated in the middle of the PB layers when d/L ∼ 0.5, whereas IONs aggregated into the clusters as d was comparable to L. In the current study, the average thickness of PS lamellae in neat pupa-like PS9.8k-bP4VP10k particles is ca. 13.21 nm, while the diameters of Au1.7S and Au3.5S are 6.0 and 10.4 nm (see the Supporting Information, Table S1), respectively. Thus, the d/L values are 0.45 and 0.79 for hybrid Au1.7S/PS9.8k-b-P4VP10k and Au3.5S/ PS9.8k-b-P4VP10k systems, respectively. Since d/L is lower for Au1.7S/PS9.8k-b-P4VP10k, the conformational entropy penalty for incorporating Au1.7S in PS domain is relatively low, which makes the Au1.7S arrange at the center of the PS domain. For large Au3.5S particles with a d/L value of 0.79, these particles tend to aggregate together to reduce the heavy loss of conformational entropy, which is consistent with the previous experiments and simulations.34,43 This observation indicates that the entropy contribution in the cooperative self-assembly of NPs and BCPs can be tuned by the size of NPs, which can be utilized to control the spatial arrangement of NPs in the hybrid nanoparticles. On the other hand, it has been proved that the computer simulation method, especially Monte Carlo simulation, is a powerful tool that can not only predict new structures but also explore the formation mechanism of the self-assembly of block copolymers.44,45 In the current study, we also applied Monte Carlo simulation method to mimic the cooperative selfassembly of BCPs and AuNPs under soft confinement. In emulsion-evaporation-induced self-assembly, the environment for the block copolymer is gradually changed from good solvent (organic solvent) to poor solvent (water) as the organic solvent is gradually evaporated. Therefore, this experimental process can be mimicked by gradually changing solution atmosphere from good solvents to poor solvents.41,46 In order to correspond with our experiments, the symmetric A6B6 diblock copolymer was considered in the simulations. To examine the effect of the size of the gold nanoparticle on the hybrid micelles, two types of nanoparticles with two different diameters, i.e., the diameters of 1 and 2, were introduced into the simulation system. For simplification, we use NP1 and NP2 to represent these two nanoparticles with the diameters of 1 and 2. More detailed information for the simulation model is presented in the Supporting Information (S2). Figures 2a and 2b show the morphologies of A6B6 block copolymer incorporated with NP1 and NP2, respectively. Under the soft confinement, the A6B6 and NPs are more like to coassemble into axially stacked lamellae structure when A and B blocks have the same

domain in TEM image. It is observed that the two poles of the ellipsoids are always formed by P4VP blocks, which is attributed to the slight selectivity of the solution for P4VP blocks.39,41 Since the nanostructured BCP particles could be used as the scaffolds to direct the arrangement of functional inorganic NPs, hydrophobic PS2k-coated AuNPs (i.e., Au1.7S and Au3.5S, 1.7 and 3.5 are the diameters of the cores of AuNPs, S represents PS ligands) were mixed with PS9.8k-b-P4VP10k/chloroform solution, and 100 μL of chloroform solution was emulsified in 1.0 mL of PVA aqueous solution (12 mg/mL) by ultrasonication. The cooperative self-assembly of AuNPs and PS9.8k-b-P4VP10k occurred during the evaporation of the emulsion droplets. Since the AuNPs are coated with PS ligands, it is expected that these particles will localize within the PS domain to reduce the enthalpic repulsion. After the evaporation of the emulsion solvent, hybrid nanoparticles are obtained. Figures 1b and 1c show the TEM images of hybrid nanoparticles of PS9.8k-b-P4VP10k incorporated with 31.8 vol % of Au1.7S (the volume fraction of Au1.7S to the sum of Au1.7S and PS blocks) and 32.2 vol % of Au3.5S (the volume fraction of Au3.5S to the sum of Au3.5S and PS blocks), respectively. As expected, it is observed that the PS2k-coated AuNPs (Au1.7S or Au3.5S) are entirely selectively located in the PS domain of the pupa-like particles. However, it is worth to note that the dispersion of AuNPs in PS domain depends on their sizes. For Au1.7S/PS9.8k-b-P4VP10k hybrid nanoparticles (Figure 1b), we can see that the introduced Au1.7S particles disperse uniformly and locate at the center of PS lamellae, while it is observed that the Au3.5S particles tend to aggregate together and disperse randomly in PS lamellae for Au3.5S/PS9.8k-b-P4VP10k hybrid nanoparticles (Figure 1c). To quantitatively analyze the effect of the size of NPs on their distribution in PS domain, d/L, i.e. the relative size between particle diameter and the preferred domain, is introduced, where d is the total diameter of the AuNP core and the ligand shell and L is the dimension of the selected domain. It has been proved that d/L is a key parameter for controlling the distribution of the nanoparticles in polymer matrix.36,42,43 For example, Thomas and co-workers have predicted that the nanoparticles will concentrate at the center of the lamellar domains when d/L > 0.3.36 Recently, Fu and coworkers investigated the influence of magnetic nanoparticle size on the particle dispersion and phase separation in styrene− butadiene−styrene block copolymer (SBS) matrix with a lamellar structure.43 It was observed that the iron oxide 5982


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favorable to expel the NPs to the surface of the polymer film when the NPs are larger than the interstitial sites of the BCP microdomains.4 They observed that small NPs can distribute uniformly within the polymer matrix, whereas large NPs located at the air/polymer interface. In our experiment, the chain length of PS9.8k-b-P4VP10k copolymer is relatively short, which cause a relatively higher conformational entropy loss for encapsulation of Au3.5S and Au1.7S, resulting in the selective location of Au3.5S and Au1.7S in the area close to the interface or at the interface. From both the above experiments and simulations, it is confirmed that the spatial distribution of AuNPs in hybrid PSb-P4VP/AuNPs nanoparticles can be controlled by tuning the entropic contribution. Since both the PS-b-P4VP and AuNPs are confined in the same emulsion droplet, the AuNPs cannot completely separate from PS-b-P4VP even when the diameter of the NPs is comparable to or larger than that of the affinitive polymer domain. In the following section, we examine the solvent-evaporation-induced cooperative self-assembly of P4VP2.5k-coated Au3.5NPs (Au3.5V, 3.5 is the diameter of the core of AuNPs, V represents P4VP ligands) with a series of PSb-P4VP block copolymers containing different P4VP contents. The choice of P4VP modified AuNPs is because that only the PS-b-P4VP block copolymers with P4VP as the minority component are available. When P4VP is the minority component, strong entropic repulsion can be utilized to create novel hierarchical hybrid nanoparticles. In Figures 4a and 4b, we show the TEM images of Au3.5V/PS9.8k-b-P4VP10k hybrid nanoparticles incorporated with 34.7 vol % of Au3.5V (the volume fraction of Au3.5V to the sum of Au3.5V and P4VP blocks) before and after staining by I2 vapor, respectively. In order to further observe the hierarchical structure, the magnified TEM image and the corresponding scanning

hydrophobicity with the solvent (i.e., εAS = εBS = 2.0). Since it is assumed that NPs have chemical affinity with B monomers (εBN = 0), NPs are selectively located in the B-lamellae, as shown in Figures 2a and 2b. Moreover, it is found that the small NP1 particles distribute more uniformly than the large NP2 in B lamellae. Significant entropy-driven aggregation of large NP2 particles in B lamella is observed in Figure 2b. When solid NPs are mixed into the polymer matrix, entropic deformation of the polymer chains up NPs will expel NPs to aggregate together to reduce the entropic penalty. As the particle size is increased, the entropic repulsion becomes more prominent. It has been reported that the nanoparticles will be expelled to the air− polymer or polymer−substrate interface to reduce the cost in conformational entropy of polymer chains when the diameter of nanoparticle is larger than the radius of gyration of polymer chain.47−49 To obtain the radius of gyration (Rg) of block copolymer with no NPs incorporated, we also mimic the selfassembly of neat A6B6 diblock copolymer under the same conditions. The diameters of NP1 and NP2 particles as well as the Rg of the affinitive B block in axially stacked lamellar A6B6 nanoparticles are listed in Table S2 (Supporting Information). Clearly, the Rg of B block is 1.782, which is larger than NP1 diameter (1) but smaller than NP2 diameter (2). Therefore, bigger NP2 particles will be expelled from B phase to polymer− solution interface and form significant aggregation at the surface domain of the hybrid A6B6 nanoparticles to reduce the cost in conformation entropy, as shown in Figure 2b. To further reveal the spatial arrangement of Au1.7S and Au3.5S particles inside the PS lamellae, the hybrid PS9.8k-bP4VP10k/AuS particles are partially disassembled by ethanol, which is a good solvent for P4VP blocks but a nonsolvent for PS blocks. Figures 3a and 3b show the hybrid nanodisks

Figure 3. TEM images of hybrid nanodisks disassembled from the hybrid PS9.8k-b-P4VP10k/Au1.7S (a) and PS9.8k-b-P4VP10k/Au3.5S (b) nanoparticles by ethanol.

obtained by disassembling the ellipsoidal stacked lamellar PS9.8k-b-P4VP10k/Au1.7S and PS9.8k-b-P4VP10k/Au3.5S hybrid nanoparticles with ethanol, respectively. It is found that a large number of Au1.7S nanoparticles distribute in the area near the border of the nanodisks (Figure 3a), while most of Au3.5S nanoparticles locate along the boundary of the nanodisks and arrange in a pattern of a necklace (Figure 3b). From both Figures 3a and 3b, we can see that AuNPs are more like to locate far away from the central part of the nanodisks no matter for Au1.7S or Au3.5S. This phenomenon is more evident for larger Au3.5S. The selective location of NPs at interface or close to the interface is attributed to the entropic repulsion arisen by the change in conformational entropy of the polymer chains upon NP incorporation. Xu et al. have reported that it is more

Figure 4. TEM images of the Au3.5V/PS9.8k-b-P4VP10k hybrid nanoparticles with 34.7 vol % volume fraction of Au3.5V before (a) and after (b) staining the P4VP domains with I2 vapor. The arrows in (b) point to the concentrated Au3.5V nanoparticles at one pole of the pupa-like nanoparticles. (c) Magnified image of the Au3.5V/PS9.8k-bP4VP10k hybrid nanoparticles and (d) the corresponding STEM image. 5983


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Macromolecules transmission electron microscopy (STEM) image of the PS9.8kb-P4VP10k/Au3.5V hybrid nanoparticles are shown in Figures 4c and 4d, respectively. From the images shown in Figure 4, we observe a unique Janus-like PS9.8k-b-P4VP10k/Au3.5V hybrid nanoparticle with hierarchical nanostructure. Interestingly, it is worth to note that nearly all of the Au3.5V particles are concentrated at one pole of the pupa-like nanoparticles, while the other part of the hybrid nanoparticles remains the typically original pupa-like structure. There are several reasons which contribute to the formation of the novel hierarchical Janus hybrid nanoparticles. First, the average thickness of P4VP lamellae in neat PS9.8k-b-P4VP10k pupa-like particles is ca. 12.53 nm, while the diameter of Au3.5V is about 11.0 nm (see the Supporting Information, Table S1). The value of d/L is as high as 0.88 for the Au3.5V/PS9.8k-b-P4VP10k system. In the previous study, it has been proved that NPs tend to concentrate together as the size of NPs is increased.34 The driven force of this aggregation is due to the huge conformational entropy loss of the polymer chains during the encapsulation of the large NPs. In the current study, macrophase separation between Au3.5V and PS-b-P4VP is limited because of the confinement effect. Therefore, the expelled Au3.5V can only aggregate at the surface of the BCP nanoparticles. Second, since the P4VP blocks have slight hydrophilicity and relatively strong polarity, two poles of the PS-b-P4VP ellipsoids are always covered by P4VP blocks.39,41 Therefore, Au3.5V nanoparticles are more likely to concentrate together at one pole of the hybrid nanoparticle to further reduce the entropic penalty. In addition, the entropic contribution can also be tuned by changing the AuNPs affinitive block length to control the spatial distribution of AuNPs as well as the nanostructure of the hybrid nanoparticles. When the size of AuNPs is comparable to the selected domain, i.e., when the entropic contribution is more prominent, some novel hybrid nanoparticles can be obtained. On the other hand, it was reported that asymmetric BCPs were the outstanding candidates for preparation novel nanostructures via the 3D confined self-assembly.50 Therefore, several asymmetric PS-b-P4VP block copolymers with long PS blocks but relatively short P4VP blocks were selected to coassemble with Au3.5V under 3D confinement. In Figure 5a, we present a TEM image of the neat PS35k-b-P4VP2.7k nanoparticles from the 3D confined self-assembly. To reveal the internal nanostructure, the particles were stained with I2 vapor. From Figure 5a, we can see that short P4VP blocks can only form some small dots uniformly distributing in the PS spheres. The average diameter of these P4VP dots is ca. 9.94 nm, which is slightly smaller than the particle size of Au3.5V (11.0 nm). It is interesting what happens when the AuNPs size is comparable to or even slightly larger than their affinitive phase domain. The resulted hybrid Au3.5V/PS35k-b-P4VP2.7k (the volume fraction of Au3.5V to the sum of Au3.5V and PS35kb-P4VP2.7k is 14.09 vol %) are shown in Figures 5b and 5c, which are the TEM images of the hybrid nanoparticles before and after staining with I2 vapor, respectively. It is observed that all of the Au3.5V particles are expelled together to form a bulge on the surface of the BCP nanoparticles, while the rest of the BCP nanoparticles remain the ordered lattice nanostructure. The novel structure of the PS35k-b-P4VP2.7k/Au3.5V Janus particles is confirmed by both TEM images (Figure 5b,c) and SEM image (Figure 5d). The formation of this interesting Janus hybrid particle is attributed to the strong entropic repulsion arisen by the change in the conformational entropy of the polymer chains upon the incorporation of NPs. For the PS35k-b-

Figure 5. (a) TEM image of neat PS35k-b-P4VP2.7k nanoparticles selfassembled under 3D confinement. (b) and (c) are the TEM images of Au3.5V/PS35k-b-P4VP2.7k hybrid nanoparticles before and after staining the P4VP domains with I2 vapor, respectively. (d) SEM image of Au3.5V/PS35k-b-P4VP2.7k hybrid nanoparticles. The volume fraction of Au3.5V to the sum of Au3.5V and PS35k-b-P4VP2.7k is 14.09 vol %.

P4VP2.7k block copolymer, P4VP is the minority domain and the volume fraction of P4VP is as low as 7.2%. The diameter of the introduced Au3.5V particles is about 11.0 nm, which is slightly larger than the size of P4VP dots (ca. 9.94 nm). During the evaporation of the emulsion droplets, the Au3.5V particles can neither distribute in P4VP domain due to the strong entropic repulsion nor locate in PS domain because of the strong enthalpic repulsion. As a result, the Au3.5V particles aggregate together at the surface of BCP nanoparticles, resulting in the formation of this particular hierarchical Janus nanostructure. To further examine the effect of the affinitive block length on the nanostructure of the hybrid nanoparticles, another asymmetric block copolymer, i.e. PS33k-b-P4VP8k (the volume fraction of P4VP is ca. 19.5%), was also used to fabricate the hybrid nanoparticles. Figure 6 shows a series of TEM images of the hybrid nanoparticles obtained by the cooperative selfassembly of Au3.5V and PS33k-b-P4VP8k under 3D soft confinement. The typically Janus hybrid nanoparticles, i.e., Au3.5V particles concentrate at one head of the PS33k-b-P4VP8k particles, while another head is the neat BCP, are observed in Figure 6a. In order to unveil the nanostructure of the Janus hybrid particles, the particles are then stained by I2 vapor, as shown in Figure S2 (Supporting Information). When the size of the neat BCP side is relatively large, P4VP dots can be observed in the PS domain, which is dependent on the strength of the confinement effect. More interestingly, these Janus particles can be performed as the building units to build up more complex nanostructures, such as gourd-like, clover-like, and four-leafclover-like nanoparticles, as shown in Figures 6b−d, respectively. To the best of our knowledge, these unique BCP/AuNPs hybrid nanoparticles via hierarchical self-assembly from small Janus hybrid nanoparticles have not been reported before. Because of the entropic repulsion imposed by polymer 5984


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Figure 6. TEM images of the novel hybrid nanoparticles fabricated by cooperative self-assembly of Au3.5V particles and PS33k-b-P4VP8k under 3D confinement. (a) Janus hybrid particles; (b)−(d) are the gourd-like hybrid particles, clover-like hybrid particles, and four-leaf-clover-like hybrid particles obtained from the hierarchical self-assembly of Janus particles shown in (a). The volume fraction of Au3.5V to the sum of Au3.5V and the P4VP domain is 41.54 vol %.


Table 1. Summary of the Unique Janus Hybrid Nanoparticles Fabricated via 3D Confined Cooperative Self-Assembly of PS-bP4VP and Au3.5V

only ca. 7.2%), the block copolymer tended to self-assemble into the ordered lattice nanostructures, while Au3.5V particles were expelled together to form a bulge on the surface of the BCP nanoparticles (Figure 5). For another asymmetric block copolymer, i.e. PS33k-b-P4VP8k ( f P4VP ∼ 19.5%), Au3.5V particles concentrated at one head of the PS33k-b-P4VP8k particles, resulting in the typically Janus hybrid nanoparticles under strong confinement (Figure 6a). Moreover, these Janus particles could be performed as the building units to build up more complex nanostructures, such as gourd-like, clover-like, and four-leaf clover-like nanoparticles, as shown in Figures 6b− d.

chains, Au3.5V particles are expelled from BCP domain to the polymer/solution interface and concentrate at one head of the BCP particles. The ubiquitous van der Waals (vdW) attraction between AuNPs may drag the small Janus particles together and head-to-head connect with each other in the gourd-like, clover-like, or four-leaf-clover-like shapes.51 On the other hand, since the Au3.5V particles are loosely aggregated together, the aggregates of the AuNPs on the Janus hybrid nanoparticles may be not solid, which can be fused together to form the more complex nanostructures shown in Figures 6b,c. According to the entropic repulsion between the Au3.5V and block copolymers, a series of unique Janus hybrid nanoparticles can be fabricated by 3D confined cooperative self-assembly of Au3.5V and PS-b-P4VP, as summarized in Table 1. Compared to the length of the P4VP blocks, the diameter of the Au3.5V particles (∼11.0 nm) is relatively large, and the particles are easily to be expelled from P4VP domain due to the huge conformational entropy loss of the polymer chains. On the other hand, since the Au3.5V and PS-b-P4VP are dissolved in the emulsion droplets, the confinement effect can limit the macrophase separation between the Au3.5V particles and block copolymers. When the symmetrical PS9.8k-b-P4VP10k block copolymer was selected to cooperatively self-assemble with Au3.5V, it was clear that PS9.8k-b-P4VP10k block copolymers tended to form pupa-like structures, as shown in Figure 1a. As a result, Au3.5V particles concentrated together at one pole of the pupa-like hybrid nanoparticles to reduce the entropic penalty (Figure 4). For the PS35k-b-P4VP2.7k block copolymer (f P4VP is

3. CONCLUSIONS In summary, both the experiments and Monte Carlo simulations confirm that small Au1.7S particles can distribute in the PS domain of the symmetric PS9.8k-b-P4VP10k block copolymer more uniformly, while large Au3.5S particles tend to aggregate together near by the PS/solution interface, which is caused by the strong entropic repulsion between Au3.5S particles and PS blocks. This observation indicates that the entropic repulsion between NPs and BCPs during 3D confined assembly can be tuned to design novel hybrid nanoparticles by changing the size of the introduced NPs or the dimension of the selected polymer domain. According to this strategy, some unique hybrid nanoparticles, including pupa-like nanoparticles with AuNPs concentrated at one pole of the particles, spherical nanoparticles with AuNPs enriched in a bulge onto the sphere 5985


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surface, and the gourd-like, clover-like, and four-leaf-clover-like nanoparticles from the further hierarchical assembly of the small hybrid Janus nanoparticles, are fabricated by the cooperative self-assembly of Au3.5V particles and PS-b-P4VP block copolymers under a 3D confined geometry.

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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.5b01219. Full experimental process; model and simulation method; and size analyses of the AuNPs used in this study (PDF)

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AUTHOR INFORMATION


Corresponding Authors

*E-mail: [email protected] (Y.Z.). *E-mail: [email protected] (W.J.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China for General Program (51373172), Major Program (51433009), and the Open Project of State Key Laboratory of Supramolecular Structure and Materials (sklssm2015030).

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REFERENCES

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Entropy-Driven Hierarchical Nanostructures from Cooperative Self

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