Polymeric Janus Particles with Hierarchical Structures

May 30, 2014 - Synthesis of polymeric Janus colloidal particles must be well-adapted to get particles with precise control of their various structural...
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Polymeric Janus Particles with Hierarchical Structures Renhua Deng,† Shanqin Liu,† Fuxin Liang,*,‡ Ke Wang,† Jintao Zhu,*,† and Zhenzhong Yang*,‡ †

Key Laboratory for Large-Format Battery Materials and System of the Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China ‡ State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ABSTRACT: Janus colloidal particles with hierarchical structures are generated by phase separation of diblock copolymer polystyrene-block-poly(4-vinylpyridine) (PS-bP4VP) and homopolymer poly(methyl methacrylate) (PMMA) binary blends in confined geometry. The dependence of their morphology on the copolymer composition, solvent selectivity, particle size, and polymer/aqueous solution interfacial property was investigated. By varying the particle/ aqueous solution interfacial property alternately, the Janus particles exhibited a reversible morphological transformation under solvent-adsorption annealing process. In addition, by introducing 3-n-pentadecyphenol (PDP) which can hydrogen bond with P4VP to form supramolecules, the structure of the Janus particles can be well tuned. Furthermore, due to the complexation of pyridine unit with Au precursor, composite Janus particles with Au nanoparticles selectively incorporated in P4VP microdomains can be easily manipulated.

1. INTRODUCTION Polymeric Janus particles with two incompatible sides of different chemical composition and/or surface properties have gained growing interest.1−5 The interesting features of polymeric Janus particles are attributed to the soft nature of the polymer chains and their tunable asymmetric structure, which allows controllable physicochemical properties. For example, polymeric Janus particles are flexible and may respond to multiple stimuli, such as solvent, pH, or temperature.6,7 In selective media, polymeric Janus particles can self-assemble into complex superstructures in a controlled manner.8−10 They can also be designed to have hydrophilic and hydrophobic performances on each side, serving as solid surfactant.11−13 Thus, it is important to develop simple and effective methods for the controllable preparation of polymeric Janus particles with well-defined shape, structure, and composition. Synthesis of polymeric Janus colloidal particles must be welladapted to get particles with precise control of their various structural/physical/chemical properties. Among routes available today for the preparation of polymeric Janus colloidal particles,14−19 one of the most simple and feasible pathways would be the internal phase separation of polymeric blends, which is based on the immiscible nature of the blends.20,21 Typically, phase separation of A/B homopolymers blends in emulsion droplets was investigated.22−24 The morphology of the particles is mainly determined by the interactions between the two polymers with oil/water interface. When the interfacial free energies between both polymers and the interface are similar, Janus structure is formed in the interior of the solidified A/B blended particles.25 © XXXX American Chemical Society

Unlike Janus colloidal particles of homopolymer/homopolymer blends, few researches about block copolymer/homopolymer Janus colloidal particles have been reported so far.26−28 Under 3D confinement, block copolymer can self-assemble into unique nanostructures through micorphase separation,29−35 and the nanostructures can be well controlled by varying molecular composition, confinement effect, and boundary interaction.36−38 Also, introduction of a homopolymer to the copolymer particles can be used to control the self-assembled morphology. Previous reports usually focused on the blending of diblock copolymer AB and homopolymer A. In the AB/A system, the location of A in the particles depends on the molecular weight ratio of A to A-block of AB.28,39 When the molecular weight of A is lower than that of A-block, A is solubilized in the A-block microdomains, which is similar to the increase of the volume fraction of A-block, resulting in the phase transition of the particles.30,40 In contrast, A will macrophase separate with AB, which makes it possible for the generation of Janus particles.26,27 However, when AB/A was confined in the emulsion droplets, particles with asymmetric internal structure but single surface composition rather than Janus particles were formed as a result of the incompatible interfacial energies and incomplete macrophase separation.39 Thus, the idea of blending an incompatible homopolymer C with AB copolymer should be more reliable for the formation of Janus particles. Recently, the phase separation behavior of Received: February 12, 2014 Revised: May 14, 2014

A

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evaporation method.31 PS-b-P4VP and PMMA were first dissolved in organic solvent (e.g., chloroform) separately. The concentration of both PS-b-P4VP and PMMA in the solvent was 0.3 wt %. Then, the solutions were mixed together with desired ratio, followed by stirring overnight to form homogeneous solution. Subsequently, 0.1 mL of the PS-b-P4VP/PMMA solution was emulsified with 1.0 mL of PVA aqueous solution (0.3 wt %) by using a membrane-extrusion emulsification.42 Organic solvent was then allowed to slowly evaporate for 48 h at 30 °C. After complete removal of the organic solvent, the particles were separated by centrifugation (14 000 rpm, 6 min) to remove the PVA. Solvent-Adsorption Annealing.37 500 μL of the polymer particle aqueous suspension (or PVA aqueous solution suspension) with particle content of ∼0.4 mg/mL in a small vial (5 mL) was placed inside a large vial (25 mL) with 1.0 mL of the chloroform at 30 °C for 8 h. Afterward, the inner small vial was taken out to the ambient atmosphere to release the absorbed chloroform for 12 h at 30 °C. Finally, the particles were separated by centrifugation (14 000 rpm, 6 min) to remove the PVA, if any. Preparation of PS-b-P4VP(PDP)x/PMMA Supramolecular Janus Particles. The supramolecular Janus particles were prepared by dissolving PS-b-P4VP, PDP, and PMMA in chloroform at a concentration of 0.3 wt % separately. Then, the PS-b-P4VP and PDP solutions were mixed together with the desired ratio of P4VP/PDP, followed by stirring overnight to form PS-b-P4VP(PDP)x supramolecules (the subscript x represents the ratio of PDP to 4VP units). Afterward, the supramolecules and PMMA solutions were mixed together with the desired ratio, followed by stirring overnight to form homogeneous solution. Then, similar strategies were performed to prepare polymer particles, as described above. Preparation of PS-b-P4VP(Au)/PMMA Composite Janus Particles. The composite Janus particles were prepared by dissolving PS-b-P4VP, HAuCl4·4H2O, and PMMA in toluene at a concentration of 0.3 wt %. Then, the PS-b-P4VP and HAuCl4·4H2O solutions were mixed together with desired ratio, followed by stirring overnight to form PS-b-P4VP(Au) complex. Afterward, the resulting complex solution and PMMA solution were mixed together with the desired ratio. Similar strategies were performed to prepare polymer particles, as described above. After centrifugation, the PS-b-P4VP(Au)/PMMA particles were redispersed in 1 mL of water, and 20 μL of freshly prepared NaBH4 solution (0.5 mg/mL) was added dropwise while stirring, resulting in the formation of Au NPs inside the P4VP microdomains. The resulting composite particles were separated by centrifugation at 14 000 rpm for 6 min. Characterization. The internal structure of the particles was observed using Tecnai G2 20 TEM (FEI Co., Netherlands) or JEM1011 TEM (JEOL Ltd., Japan). Before TEM characterization, the samples were stained with iodine vapor for 2 h. SEM images were recorded using S-4800 (JEOL Ltd., Japan) operated at an acceleration voltage of 10−15 kV. Size and size distribution of the polymer particles were measured by dynamic light scattering (DLS, Malvern Zetasizer Nano ZS90).

binary mixtures of AB and C confined in spherical nanopores was investigated by computer simulation, and novel Janus particles with various shapes and internal structures are successfully predicted.41 In experiment, it is desirable to develop methods for the separation of AB/C blends, which is expected to offer new and exciting opportunities for the generation of Janus particles with hierarchical nanostructures. Herein, we demonstrate a facile and effective approach toward Janus colloidal particles with hierarchical structures by 3D confined self-assembly of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) and poly(methyl methacrylate) (PMMA) binary blends (AB/C system). Janus colloidal particles are prepared by the emulsion-solvent evaporation method, in which solvent evaporation leads to simultaneous macrophase separation of the blends and microphase separation of the block copolymer within the confined space (Scheme 1). Scheme 1. Illustration Showing 3D Confined Self-Assembly of PS-b-P4VP/PMMA or PS-b-P4VP(PDP)x/PMMA via the Emulsion-Solvent Evaporation Methoda

a

Their structures can be controlled by tuning the composition of PS-bP4VP, interfacial properties of the particle/aqueous solution, and supramolecular property.

The internal morphology of the particles is controlled by the copolymer composition, solvent selectivity, particle/aqueous solution interfacial interaction, particle size, and supramolecular strategy. Moreover, reversible morphological transformation of the Janus particles is demonstrated by solvent-adsorption annealing in aqueous suspension. In addition, composite Janus particles with Au nanoparticles (NPs) selectively incorporated in the P4VP domains are prepared by the absorption of Au precursor to P4VP blocks, followed by the reduction of the precursor in situ.

3. RESULTS AND DISCUSSION Formation and Structure Control of Janus Particles. Typically, chloroform solution containing PS9.8K-b-P4VP10K (mass ratio of P4VP, f P4VP = 0.5) and PMMA21.5K blends was emulsified with PVA aqueous solution through membraneextrusion emulsification approach.41 Solvent evaporation induces the microphase separation of PS-b-P4VP and the macrophase separation of the polymer blends in emulsion droplets. Pine-cone-shaped particles were obtained after complete removal of chloroform from the emulsion droplets. Because of I2 vapor staining for P4VP, the corresponding microdomains became dark in the TEM image (Figure 1a). Clearly, one side of the particle is a cone of PS-b-P4VP, while the other side is an approximate hemisphere of PMMA. Generation of this Janus structure is due to the similar Flory−

2. EXPERIMENTAL METHODS Materials. Diblock copolymers PS9.8K-b-P4VP10K (Mw/Mn = 1.08), PS12K-b-P4VP3.2K (Mw/Mn = 1.05), PS51K-b-P4VP18K (Mw/Mn = 1.15), PS20.5K-b-P4VP36K (Mw/Mn = 1.08), PS17K-b-P4VP49K (Mw/Mn = 1.05), and homopolymer PMMA21.5K (Mw/Mn = 1.08) were purchased from Polymer Source, Inc. 3-n-Pentadecyphenol (PDP, purity: 98 wt %) and poly(vinyl alcohol) (PVA, Mw: 13K−23K g/mol, 87−89% hydrolyzed) were purchased from Aldrich. Chloroauric acid (HAuCl4·4H2O), chloroform, and toluene were purchased from Beijing Chemical Works. trans-1,2-C2H2Cl2 was purchased from Tokyo Chemical Industry Co. All of the materials were used after receiving without further purification. Preparation of PS-b-P4VP/PMMA Janus Particles. The polymeric Janus particles were prepared by the emulsion-solvent B

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including initial polymer concentration, surfactant concentration, number of extrusion passes, and membrane pore size.45 Balance of the Janus particles can be easily tuned by simply varying the weight ratio of copolymer/homopolymer (C/H ratio). For example, by increasing the C/H ratio from 1:1 to 2:1, the PS9.8K-b-P4VP10K domains with stacked nanodisks in the Janus particles will increase correspondingly (Figure 1d). Nanostructures of the Janus particles can be tailored by varying f P4VP of PS-b-P4VP while keeping the C/H ratio the same. Interestingly, the changes in the nanostructure will also induce the shape variation of the overall particles. Thus, it is expected that PS-b-P4VP/PMMA Janus particles with varied morphologies can be prepared by systemically changing f P4VP. Taking PS12K-b-P4VP3.2K ( f P4VP = 0.21) as an example, spherical Janus particles with P4VP cylindrical internal structures can be obtained when blending with PMMA (Figure 2a). PS-b-P4VP forms a partial spherical capped domain

Figure 1. (a) TEM image of PS9.8K-b-P4VP10K/PMMA21.5K Janus particles with C/H ratio of 1:1. (b) Illustration of the Janus particles based on the TEM image: the lower right green arrow show a P4VP monolayer on the tip, and the red arrow shows a PS monolayer at the copolymer−homopolymer interface. (c) DLS result showing size distribution of the polymer particles. (d) TEM image of PS9.8K-bP4VP10K/PMMA21.5K Janus particles with C/H ratio of 2:1. In TEM images, P4VP domains become black after staining with I2 vapor while the PS and PMMA domains keep gray. Inset in (a) is the cartoon showing the structure of the Janus particles: yellow, red, and green represent PMMA, PS, and P4VP, respectively.

Huggins solubility parameters (δ) of each block of PS-b-P4VP with PMMA (δ for PS, P4VP, and PMMA is 18.6, ∼22, and 19.4 MPa1/2, respectively).43 Formation of the unique stacked nanodisks in the PS-b-P4VP cone can be attributed to the comparable interactions of the two blocks with the PVA aqueous solution at the interface, similar to that of neat PS-bP4VP ellipsoid particle in our previous report.13,37 In the current system, PMMA occupies part space of the particle, so the PS-b-P4VP can only occupy the rest space to form an incomplete ellipsoid (e.g., cone). The apex of the cone is covered with P4VP chains as the outermost layer (as indicated by the lower right green arrow in Figure 1b) due to the lower interfacial energy of P4VP, while the bottom composition of the cone, which is next to PMMA side, should be PS block (as marked by the red arrow in Figure 1b). We note that it is pretty hard to distinguish PS from PMMA directly in the TEM image due to their similar contrast. Presumably, PS locates at the copolymer/homopolymer interface since the thickness of the P4VP layer (indicated as the higher right green arrow in Figure 1b), which is nearest to the PMMA side, is the same as that of middle P4VP bilayers, indicating that this layer is also consisted of P4VP bilayer. Thus, there should be a PS monolayer between this P4VP bilayer and the PMMA side, whose thickness is about half of the middle PS bilayer.44 This assumption can also be understandable according to the similarity−intermiscibility theory of δcompared with P4VP, the δ value of PS is closer to that of PMMA. The size and size distribution of the particles were measured by DLS (Figure 1c), which can be tuned by varying experimental parameters,

Figure 2. TEM images of PS-b-P4VP/PMMA21.5K Janus particles (C/ H ratio of 1:1) prepared by using PS-b-P4VP with different compositions: (a) PS12K-b-P4VP3.2K; (b) PS51K-b-P4VP18K; (c) PS20.5K-b-P4VP36K; (d) PS17K-b-P4VP49K. Insets in (a, b) are the cartoons showing the structure of the Janus particles: yellow, red, and green represent PMMA, PS, and P4VP, respectively.

occupying a peripheral region of the particle, while PMMA occupies the rest of the space. In the copolymer cap, the P4VP form confined hexagonal cylindrical domains in PS matrix. PSb-P4VP block copolymers with increased f P4VP were systematically investigated, as shown in Figures 2b−d. When bulk cylinder-forming block copolymer PS51K-b-P4VP18K ( f P4VP = 0.26) was employed, Janus particles with P4VP spherical microdomains were generated (Figure 2b). This result is consistent with that of the neat copolymer system.31 In bulk, P4VP tends to form a continuous phase and PS forms a dispersed phase while the P4VP chain length is longer than PS. Differently, in our case, P4VP preserves in the form of dispersed spherical microdomains when PS20.5K-b-P4VP36K ( f P4VP = 0.64) with long P4VP chains (Figure 2c) was confined in 3D space. Furthermore, even when bulk reversed cylinderforming block copolymer PS17K-b-P4VP49K (f P4VP = 0.74) with C

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is usually evaluated by D/L0: the size of the confined space (D) and the periodicity of phase separation (L0). Generally, the degree of confinement can be categorized as the weak confinement (D/L0 > 3−4) and strong confinement (D/L0 < 2).46 In the weak confinement, the confinement effect plays less influence on the phase morphology, which usually can be ignored when D/L0 > 4.30,47 In our aforementioned cases (Figures 1−3), the confinement is not accounted since weak confinement is mostly satisfied. Compared with the weak confinement, unique frustrated phases may emerge under strong confinement.47−49 In order to create strong confinement, PS-b-P4VP/PMMA Janus particles with smaller size (∼60−200 nm, Figure 4a) were prepared by decreasing the initial concentration of the polymer solution to 1 mg/mL as well as increasing PVA concentration in the aqueous phase to 5 mg/mL. Figures 4b−d show the unique PS17K-b-P4VP49K/ PMMA Janus particles with different morphologies due to the strong confinement. The dependence of particles morphology on their size is summarized in Figure 4e. Here, D is the diameter of PS-b-P4VP part (blue two-way arrows in Figures 4b−d) rather than the whole particle, while L0 is 58 nm (the sum of thickness of PS and P4VP layers). When D/L0 < 0.8, PS-b-P4VP self-assemble into Janus structure with PS microdomain near the PMMA part, resulting in structured Janus nanoparticles with P4VP/PS/PMMA three-layers (Figure 4b). When the value of D/L0 approaches 1.0, an additional PS microdomain appears at the end of P4VP microdomain, making the whole particle a PS/P4VP/PS/PMMA four-section structure (Figure 4c). In these particles, the phase-separated PS/P4VP/PS structure can be considered as just right a lamellar period of PS-b-P4VP, which corresponds to the value of D/L0 = 1.0. Increasing the value of D/L0 to ∼1.5, two or three PS nubbly microdomains embedded at the end of P4VP are found (Figure 4e). When the value of D/L0 is further increased to ∼2.0, a new PS layer is found in the middle of P4VP microdomain (compared with the case of D/L0 = 1.0), making the whole particle a PS/P4VP/PS/P4VP/PS/PMMA six-section structure (Figure 4d). In this case, the PS/P4VP/ PS/P4VP/PS can be considered as just two lamellar periods of

longer P4VP chains was used, P4VP lamellar phase were observed in their Janus particles (Figure 2d), in which the thickness of P4VP and PS layers is 40 and 18 nm, respectively. Shape and structure of the Janus particles can also be tuned by varying the solvent selectivity for each block. Chloroform (δ = 19 MPa1/2) is a good solvent for both blocks of PS9.8K-bP4VP10K; thus, lamellar structures form due to the similar volume ratio of PS and P4VP. On the other hand, toluene (δ = 18.2 MPa1/2), a good solvent for PS block but a poor solvent for P4VP block, will induce the formation of PS-b-P4VP core− corona micelles in the solution, resulting in the formation of spherical internal structures. As expected, spherical P4VP microdomains rather than lamellar ones were formed in the copolymer side of the Janus particles when toluene was employed as solvent for PS9.8K-b-P4VP10K/PMMA particles formation (Figure 3a). Similarly, when trans-1,2-C2H2Cl2 (δ =

Figure 3. TEM images of PS9.8K-b-P4VP10K/PMMA21.5K Janus particles (C/H ratio of 1:1) prepared by selective solvents: (a) toluene and (b) trans-1,2-C2H2Cl2.

18.4 MPa1/2), another selective solvent for PS block, is adopted for the formation of the polymer particles, Janus particles with similar spherical P4VP microdomains dispersed in PS matrix were also obtained (Figure 3b). The degree of confinement is another key factor in determining particle morphology of block copolymer, which

Figure 4. (a) DLS result showing size distribution of the PS17K-b-P4VP49K/PMMA21.5K small particles. (b−d) Representative TEM images of these small Janus particles. (e) Diagram showing the dependence of particles morphology on their size. The inset cartoons in (b−e) showing the structure of the Janus particles: yellow, red, and green represent PMMA, PS, and P4VP, respectively. D

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similar to neat PS9.8K-b-P4VP10K particles.37 The transition is due to the removal of PVA from the aqueous media, which will induce preferential P4VP/aqueous solution interfacial interaction. A layer of P4VP occupies the external surface of the swollen particle in order to reduce the interfacial energy. The lamellar phase starts to rearrange along the outermost P4VP layer, and the concentric shells-composed terrace structure with PS and P4VP alternant is achieved. To minimize the surface free energy, PMMA locates in the peripheral regions of terrace structure to form a cap, triggering the formation of integral spherical shape of the whole particle. When the incomplete onion-like particles were treated with chloroform in PVA aqueous solution, they can change back to their initial pinecone-shaped Janus morphology (Figures 5c,d). Therefore, the structural transformation of the Janus polymer particles during solvent adsorption annealing is reversible. These polymeric Janus particles with switchable structures as a response to the environment can be potentially used for sensing, detection, and smart materials fabrication. Tuning Internal Structure of Janus Particles by Supramolecular Strategy. Introduction of PDP to P4VP domain of the copolymer is effective in tuning structure of the block polymer particles.31,32 The phenol group in PDP can form hydrogen bonding with P4VP, leading to the formation of PS-b-P4VP(PDP)x comb-like supramolecules. By combining with PDP, the effective weight fraction of P4VP component is increased. For PS12K-b-P4VP3.2K(PDP)1.0, f P4VP(PDP) increases to 0.51, which is similar to the symmetric block copolymer of PS9.8K-b-P4VP10K. After blending with PMMA in the droplets, Janus particles with PMMA hemisphere and PS-b-P4VP(PDP) onion-like hemisphere formed (Figure 6a). Clearly, addition of PDP can induce the phase transition from cylinder to lamellar morphology. Although similar effective weight fraction of P4VP

PS-b-P4VP. Compared with the case of D/L0 = 1.5, when the value of D/L0 approaches 2.5, a new PS layer is also increased in the middle of P4VP microdomain (Figure 4e). The phase behavior of PS-b-P4VP in the current system can be easily understood. First, periodic structures can be formed when D matches L0 (D/L0: 1.0, 2.0); otherwise, irregular structures may emerge (D/L0: 1.5, 2.5). Second, when the value of D/L0 increases from 1.0 to 2.0 (or from 1.5 to 2.5), it corresponds to the addition of one lamellar period in the particles. Morphological Transformation of Janus Particles by Solvent-Adsorption Annealing. Solvent-adsorption annealing in suspension has proved to be an effective method to induce the structural transformation of block copolymer colloidal particles.37,50 In our previous report, we demonstrated the reversible transformation between onion-like and pupa-like particles of PS-b-P4VP by varying the particle/aqueous solution interfacial interaction.37 In the current system, an additional confinement factor induced by PMMA exists in the polymer blend particles during solvent-adsorption annealing, which will offer new opportunities to investigate the phase behavior of the copolymer under the special 3D confinement. Taking the Janus particles of Figure 1a as an example, after solvent annealing treatment with chloroform in water, both the shape and structure of the particles have completely changed. The orientation of PS-b-P4VP layers changes from parallel structures (Figure 1a) to concentric structures (Figure 5a), and the pine-cone-shaped Janus particles transform into spherical ones (Figure 5b). Because of the existence of PMMA, the PS-b-P4VP can only form an incomplete onionlike morphology or fan-shaped structures. A closer inspection revealed that the outmost layer of the copolymer side is P4VP,

Figure 5. PS9.8K-b-P4VP10K/PMMA21.5K Janus particles with switchable shape and internal structure induced by solvent-adsorption annealing: (a) TEM and (b) SEM images of Janus particles which are derived from (Figure 1a) after chloroform annealing in water; (c) TEM and (d) SEM images of Janus particles which are derived from (a) after chloroform annealing in PVA aqueous solution. Insets in (a, c) are the cartoons showing the structure of the Janus particles: yellow, red, and green represent PMMA, PS, and P4VP, respectively.

Figure 6. TEM images of supramolecules/homopolymer Janus particles (C/H ratio of 1:1) with different composition: (a) PS12K-bP4VP 3.2K (PDP) 1.0 /PMMA 21.5K ; (b) PS 9.8K -b-P4VP 10K (PDP) 0.2 / PMMA21.5K; (c, d) PS9.8K-b-P4VP10K(PDP)0.5/PMMA21.5K. Insets in (a, c) are the cartoons showing the structure of the Janus particles: yellow, red, and green represent PMMA, PS, and P4VP, respectively. E

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domains were achieved and same solvent (chloroform) was employed for the generation, the PS12K-b-P4VP3.2K(PDP)1.0/ PMMA particles are different from the PS9.8K-b-P4VP10K/ PMMA Janus particles (Figure 1a) in both shape and internal structure. The main reason lies in the interfacial energy reduction of P4VP domains after combining with PDP. For the onion-like hemisphere, the P4VP(PDP) occupies the outmost layer. For symmetric block copolymer PS9.8K-b-P4VP10K, a small amount of PDP (x = 0.2, f P4VP(PDP) = 0.62) addition can induce the internal morphological transition from parallel stacking lamellar structure to stacking curved cup structure (Figure 6b). Meanwhile, the shape evolves from cone to stacked cups. Similarly, this transition is attributed to the enhanced interaction of P4VP(PDP) with the emulsion droplet interface. Such similar morphological evolution has been observed in the neat PS-b-P4VP(PDP)x system.32 The stacked cups here are similar but slightly different from the above terrace cups in Figure 5a, and the PMMA portion is more inclined to form a hemisphere rather than a cap. This particular structure of PS9.8K-b-P4VP10K(PDP)0.2/PMMA particles can be considered as the intermediate state of PS9.8K-b-P4VP10K/PMMA particles (Figure 1a) and PS12K-b-P4VP3.2K(PDP)1.0/PMMA particles (Figure 6a). Further increase of PDP content cannot induce the formation of the onion-like hemisphere because the increased f P4VP(PDP) will induce the phase transition. With the increase of PDP to x = 0.5 ( f P4VP(PDP) = 0.69), the lamellar phase transforms into cylindrical phase, and curved PS cylindrical structures are generated in the supramolecular hemisphere (Figures 6c,d). On the basis of the above-mentioned results (Figures 2 and 6), we can further tune the shape and internal microstructure of the Janus particles by changing the P4VP ratio of PS-b-P4VP and the content of PDP. In the absence of PDP, the phase behavior of PS-b-P4VP is not consistent with that of the bulk phase except for the symmetric one PS9.8-b-P4VP10. Especially, when the P4VP block is longer than PS, the inconsistency becomes more remarkable (Figure 2). In comparison, the effect of PDP addition on the phase behavior of PS-b-P4VP(PDP)x in bulk is consistent with that in the Janus particles. With the increase of x (e.g., f P4VP+PDP) by adding more PDP, the separated microphase evolves from cylindrical, to lamellar, to inverted cylindrical phase (Figure 6). Composite Janus Particles Incorporated with Au NPs. Composite materials, especially composites of polymeric Janus particles with functional inorganic NPs, are attractive because they can potentially exhibit promising new properties, such as electronics, catalysis, and magnetic response.51 Compared with homopolymer, NPs can be introduced into specific microdomains of self-assembled copolymer in a regular and orderly fashion.37,52 It is thus more interesting to prepare composite Janus particles with NPs in copolymer microdomains. PS9.8K-bP4VP10K/PMMA Janus particles incorporated with Au NPs can be formed (Figure 7a) by adding HAuCl4·4H2O into toluene solution containing polymer blends during particle preparation, followed by in situ reduction.53 Because of the complexation between the Au and P4VP block, Au NPs are selectivity located in P4VP microdomains. No Au NPs can be observed in the PMMA portion (Figure 7b), which further confirmed the Janus structure of the particles. This technique can also be applied to the formation of composite Janus particles with other inorganic NPs (e.g., Pd or Pt NPs) for broad applications.32,54

Figure 7. TEM images of PS9.8K-b-P4VP10K(Au)/PMMA21.5K Janus particles: (a) low magnification and (b) high magnification. Inset in (a) is the cartoon showing the structure of the composite Janus particle: yellow, red, green, and blue represent PMMA, PS, P4VP, and Au NP, respectively. Dotted circles in (b) illustrate the contour of the hybrid Janus particles.

4. CONCLUSIONS In summary, we have demonstrated the preparation of Janus colloidal particles with hierarchical nanostructures by phase separation of PS-b-P4VP and PMMA binary blends. We show that morphologies of the Janus particles can be tuned by varying copolymer composition, blending ratio, solvent selectivity, and particle size. Interestingly, the Janus particles exhibited a reversible morphological transformation under solvent-adsorption annealing process by varying the particle/ aqueous solution interfacial property. In addition, by introducing PDP to the P4VP domains, morphology of the particles can be well tailored. Furthermore, composite Janus particles with Au NPs selectively incorporated in P4VP domains can be easily obtained via coassembly of Au precursor with PS-b-P4VP/PMMA. Unique hemispherical block copolymer particles with hierarchical structures and tunable overall shapes can also be generated by selective removal of the homopolymer portion. This work not only provides a simple and robust route for the formation of hierarchically structured copolymer/homopolymer Janus colloidal particles but also presents the 3D confined self-assembly and transformation of diblock copolymer in the existence of an immiscible homopolymer as an extra parameter experimentally.

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.Z. acknowledges finance support by 973 Program of China (2012CB821500) and NSFC (51173056). J.Z. also thanks HUST analytical and Testing Center for the EM measurements. Z.Y. acknowledges finance support by 973 Program of China (2012CB933200) and NSFC (51233007).

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REFERENCES

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dx.doi.org/10.1021/ma500331w | Macromolecules XXXX, XXX, XXX−XXX