ABC Triblock Copolymer Particles with Tunable Shape and Internal

Article pubs.acs.org/Macromolecules

ABC Triblock Copolymer Particles with Tunable Shape and Internal Structure through 3D Confined Assembly Jiangping Xu,†,‡ Ke Wang,† Jingyi Li,† Huamin Zhou,‡ Xiaolin Xie,† and Jintao Zhu*,† †

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 (HUST), Wuhan 430074, China ‡ State Key Laboratory of Materials Processing and Mold Technology, School of Materials Science and Engineering, HUST, Wuhan 430074, China S Supporting Information *

ABSTRACT: Here we present 3D confined assembly of polystyrene-b-polyisoprene-b-poly(2-vinylpyridine) (PS-b-PIb-P2VP) ABC triblock copolymers into particles with tunable shape and internal structures. Under weak confinement (i.e., ratio of the particle size to the periodicity dimension of the block copolymer D/L0 > 4), surfactants in the suspension show significant influence on the morphology of the particles. Unique structures, such as onion-, bud-, and pupa-like particles, can be obtained by tailoring properties of the surfactants. Both particle shape and internal structure can be reversibly tuned through pathway independent solvent vapor absorption annealing. While under strong confinement (e.g., D/L0 < 2), commensurability between D and L0 will dominate the structure of the particles. Moreover, these structured particles with cross-linkable PI domain can be selectively cross-linked and disassembled into isolated nano-objects. Janus nanodiscs with PS and P2VP chains at different sides can be obtained from pupa-like particles. Such nanodiscs can act as surfactants to stabilize oil/water emulsion droplets. This strategy, combining 3D confinement, selective cross-linking, and disassembly, is believed to be a promising approach for constructing structured particles and unique nano-objects.

1. INTRODUCTION Structured block copolymer (BCP) particles have been accounted as advanced materials in electronics, photonics, biomedicine, separation, and other fields due to their controllable morphology.1,2 For example, BCP particles with well-defined shape and internal structure can guide the deposition of inorganic nanomaterials for functional devices, of which the performance is significantly affected by arrangement of the nanomaterials.3,4 In addition, morphology of the particles plays a pronounced role in the particle−cell interaction, which is a major determinant for endocytosis and immune cell activation.5,6 As a consequence, precise control of the BCP particle morphology is of great importance. Threedimensional (3D) confinement, which can break the symmetry of a structure, has been applied as a powerful route to manipulate the morphology of the BCP particles.7 In general, placing BCPs in a limited spherical cavity introduces a number of factors, including the degree of structural frustration, confinement-induced entropy loss, and surface−polymer interactions.8 These factors, which are determined by the confined geometry and boundary selectivity, significantly influence the self-assembled morphologies. Strong selective interaction between the wall and the block results in a wetting layer of the preferred block at the confining boundary, which © XXXX American Chemical Society

may shift the morphology of the confined assembly to a different structure. Meanwhile, the confined morphology delicately depends on the commensurability between size of the confined space (D) and the periodicity dimension of the microphase separation (L0) of the BCPs. If D is commensurable with L0, the confined morphology maintains the symmetry and the period of the corresponding bulky phase. While incommensurability exists between the two parameters, frustration is composed on the polymer chains, which will be forced to stretch or compress. Behavior of the frustrated chain is entropically unfavorable, resulting in assembled morphologies that could relieve the frustration. Recently, many theoretical and experimental research works focused on the 3D confined assembly of AB diblock copolymer.7−23 Composition of the copolymers, surfactant property, homopolymers addition, and particle size dominate the morphology of the assemblies.17,24−28 Usually, triblock copolymers have been conducted to self-assemble into various micellar aggregates, such as helices, toroids, and giant segmented worms,29−34 in dilute solution. However, limited studies have focused on the 3D confined Received: February 16, 2015 Revised: March 26, 2015

A

DOI: 10.1021/acs.macromol.5b00335 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

bromide (CTAB, purity ≥99%) were purchased from Aldrich. All of the materials were used as received without further purification. 2.2. Preparation of Triblock Copolymer Particles. The emulsion-solvent evaporation method was applied to prepare the polymer particles. PS40K-b-PI33K-b-P2VP87K was first dissolved in chloroform at a concentration of 10 mg/mL. Subsequently, 0.1 mL of the polymer solution was emulsified with 1.0 mL aqueous solution of the surfactants (3 mg/mL) by using a hand-driven membraneextrusion emulsification13,47 or by ultrasonication. The surfactants aqueous solution consisted of PVA and CTAB with various weight ratios (R = PVA:CTAB). The resulting emulsions were diluted by adding another 0.5 mL of same surfactant solution. Chloroform was then allowed to slowly evaporate for 3 days at 30 °C. Finally, the particles were separated by centrifugation (14 000 rpm, 6 min) to remove the surfactants. 2.3. Solvent-Adsorption Annealing. In a typical annealing process, 0.05 mL of the polymer particle suspension was first centrifuged to remove the surfactant (for example, PVA). Then 0.5 mL of another surfactant solution (e.g., CTAB) was added to redisperse the particles. Afterward, the suspension with particle content of ∼0.1 mg/mL in a small vial (5 mL) was placed inside a large vial (25 mL) with 1.0 mL of chloroform at 30 °C for a certain period. Subsequently, the inner small vial was taken out to the ambient atmosphere to release the absorbed chloroform for 12 h at 30 °C.12,19 Finally, structure of the particles is kinetically frozen after removal of chloroform, and the particles were separated by centrifugation (14 000 rpm, 6 min) to remove the surfactants. 2.4. Cross-Linking and Disassembly of the Polymer Particles. The PS40K-b-PI33K-b-P2VP87K particles can be selectively disassembled into various nano-objects in selective solvent (ethanol) of P2VP because P2VP is major phase in the particles (volume fraction ∼52.5%).11 Typically, 0.1 mL suspension of the particles was centrifuged to remove the surfactants. Then, 0.5 mL of ethanol was added to redisperse the particles under ultrasonication. The suspension was stirred for 24 h to complete the disassembly process. On the other hand, middle block of the ABC triblock copolymer, i.e., PI block, can be cross-linked by OsO4.19 After cross-linking, the polymer particles can be disassembled into isolated nano-objects, in which the PS and P2VP chains will locate at different sides of the PI phase. Generally, 20 μL of OsO4 aqueous solution (2 wt %) was added to the particle suspension to cross-link PI phase overnight. After removing unreacted cross-linker, the particles were dried and then dispersed in chloroform to disassemble the particles. 2.5. Characterization. Internal structures of the polymer particles were investigated using FEI TecnaiG2 20 transmission electron microscope (TEM) operated at an accelerated voltage of 200 kV. Before TEM characterization, the samples were selectively stained with iodine vapor for 2 h (for P2VP block) or stained by OsO4 solution overnight (for PI block). The dried particles were embedded in epoxy resin (Electron Microscopy Sciences) and cured in an oven at 50 °C for 48 h. Thin cross sections with thickness of ∼70 nm were obtained by ultramicrotomy with a ultramicrotome (Leica, EM UC7). Shape and surface topology of the particle were characterized by a Sirion 200 scanning electron microscope (SEM). Stability of the emulsion droplets were investigated through optical microscope (Olympus, IX71).

assembly of ABC triblock copolymers.35,36 For example, Yu et al. investigated the confined assembly of ABC linear BCPs in nanopores by computer simulation. Patchy particles could be obtained by tailoring polymer composition, pore size, and wall selectivity.35 Because of the complicated interactions among the three blocks and the boundary, it is still difficult to precisely tailor the morphology of the ABC BCPs particles in a finite space.35−38 The 3D confinement exerted on the BCP chains results in polymer particles with novel and regular structures which cannot be obtained in bulk. Those structured particles can be selectively swollen or disassembled to produce mesoporous particles or isolated nano-objects.11,13,16,39 In the previous report,13 we have demonstrated that after selectively removing the hydrogen bonding agent (e.g., 3-pentadecylphenol, PDP) and swelling the P4VP phase of PS-b-P4VP (PDP) supramolecular particles, mesoporous particles with designable pore structure can be readily obtained. Moreover, chemical modification can be conducted for fixation of these novel structures. For example, Müller et al. have synthesized a series of Janus nanoparticles (NPs), such as Janus cylinder,40 sphere,41 and discs,42 by using ABC triblock copolymers with cross-linkable polybutadiene (PB) as the middle block. Such Janus nano-objects can be applied as building blocks for hierarchical self-assembly, stabilizer of immiscible fluids, supracolloidal dispersants for carbon nanotubes, and others.43−46 Notably, most of these Janus nano-objects are obtained by cross-linking and disassembling the bulky phase or thin film of the ABC triblock copolymers. Yet, the role of 3D confinement in such a selective cross-linkage and disassembly procedure of ABC triblock copolymer particles lacks deep understanding. As the confined effect can force frustration to the polymers, unique structures can be expected by controlling the interfacial interaction and confining size.21,22 Thus, unique nano-objects with designable structures, some of which are hard to be obtained by self-assembling in selective solvent or in bulk/film, can be readily synthesized. Herein, we demonstrate the 3D confined assembly of polystyrene-b-polyisoprene-b-poly(2-vinylpyridine) (PS-b-PI-bP2VP) ABC triblock copolymers through emulsion-solvent evaporation method. By carefully tuning the interfacial interaction (block preference of surfactants) and the confining size (diameter of the particle), shape and internal structure of the particles can be well tailored. A series of morphologies including onion-, pupa-, and bud-like particles are successfully obtained. The resulting structured particles can then be selectively disassembled to generate isolated nano-objects with well-defined structures. More interestingly, Janus nanodiscs with PS−PI−P2VP triple-layer structure are synthesized after selectively cross-linking PI phase in pupa-like particles, followed by direct disassembly of the pupa-like particles in chloroform. Such Janus nanodiscs can act as solid surfactant to stabilize oil-in-water emulsion droplets, and homopolymer PS microparticles can thus be fabricated by emulsion-solvent evaporation route with Janus nanodiscs acting as stabilizers.

3. RESULTS AND DISCUSSION The confined assembly of BCPs is mainly dominated by the boundary selectivity and the commensurability between D and L0.8 In sections 3.1 and 3.2, we will respectively discuss the influence of the surfactant property and the confined size on the shape and internal structure of the ABC triblock copolymer particles. 3.1. Effect of the Boundary Selectivity. The selectivity of boundary to copolymer plays a critical role in determining morphology of the BCPs in confined space.12,17,18,21,22,48,49 Here, two types of surfactants were employed to tailor the

2. EXPERIMENTAL SECTION 2.1. Materials. ABC triblock copolymer PS40K-b-PI33K-b-P2VP87K (the subscripts are the Mn of the blocks, Mw/Mn = 1.15) and homopolymer PS30K (Mw/Mn = 1.06) were purchased from Polymer Source, Inc., Canada. Poly(vinyl alcohol) (PVA, average Mw: 13K− 23K g mol−1, 87−89% hydrolyzed) and cetyltrimethylammonium B


Article

Macromolecules

Figure 1. TEM images of PS40K-b-PI33K-b-P4VP87K particles obtained at different weight ratios of PVA to CTAB (R): (a) onion-like particles, R = 1:0; (b) bud-like particles, R = 9:1; (c) pupa-like particles, R = 3:1; (d) coexistence of multistructures, R = 1:1; (e) reverse bud-like particles, R = 1:3; (f) reverse onion-like particles, R = 0:1. All of the samples are stained by I2 vapor before TEM investigation.

appear at the surface of the terrace-like tip, where the interface has no preference to any block of the copolymer. However, since PVA is majority in the surfactant mixtures, there are not enough CTAB molecules to occupy all the surface of PS and PI phases. Thus, a hemisphere of onion-like particle with P2VP at the outermost layer can be observed (inset in Figure 1b and Figure S2 in the Supporting Information). The preference of the boundary to the PS and PI phases can be enhanced by increasing the weight fraction of CTAB. Figure 1c shows the oval pupa-like particles with alternately stacked PS, PI, and P2VP layers obtained at R = 3:1. The boundary selectivity is neutral at this particular surfactant ratio. The shape of the particles changes from sphere to ellipsoid due to the commensurability between the preferred lamellar spacing and the finite emulsion droplet size.17 Further increasing the weight ratio of CTAB will pull more PS and PI to the surface, while relatively less P2VP chains contact with the boundary. Figure 1e shows the reverse bud-like particles consisted of a terrace tip with alternate layers and a reverse onion-like hemisphere with PS at the outermost layer. This morphology is similar to that shown in Figure 1b while the internal structure is reverse. From the above results, we can deduce that both shape and internal structure of the ABC triblock copolymer particles can be precisely tailored during the formation of the particles by changing surfactant property. Usually, morphology of the BCP particles is hard to be changed after their formation since the chain movement is severely restricted below the glass transition temperature of the BCP. As a result, it is important to find out a route to tailor the morphology of the particle with frozen chains. We employed solvent absorption annealing strategy to tailor the morphology of the ABC triblock copolymer particles.12 First, the surfactant in the suspension was removed, and the particles were then redispersed in another surfactant aqueous solution. After that, the suspension was annealed in chloroform vapor to enhance the movement of the polymer chains. As the interfacial interaction is changed by replacing different surfactants, the boundary selectivity will force the rearrangement of the chains

interfacial interaction of copolymer/water interface. PVA with hydroxyl as pendant group is attractive to P2VP block while CTAB with a long alkyl tail has a preference to PS and PI blocks.48,49 Figure 1 shows the TEM images of polymer particles prepared in the binary mixture of PVA and CTAB with different weight ratio R. Onion-like particles (diameter: D = 400−600 nm; periodicity dimension: L0 = 138 nm) can be obtained at R = 1:0 (Figure 1a). Clearly, the outermost darker layer is P2VP phase with I2 staining, due to the selectivity of PVA to P2VP. By staining the PI phase with OsO4 (see Figure S1 in the Supporting Information), we can find that the PI layer locates at the inward side of P2VP phase. Then, the PS phase appears at the inner layer adjacent to PI phase due to the sequential architecture of the triblock copolymer chains. As a result, all of the three blocks assemble into lamella structure (see the inset of Figure 1a and Figure S2 in the Supporting Information). Notably, the thicknesses of the dark phase (P2VP, ∼68 nm) and the gray phase (PS and PI, ∼70 nm) are almost the same due to the similar volume fraction of P2VP of 52.5 vol % compared to that of PS and PI (47.5 vol %). If neat CTAB is used as surfactant, reverse onion-like particles can be obtained. As CTAB prefers to wet PS and PI phase instead of P2VP phase, the PS layer locates at the surface of the particle. As shown in Figure 1f, the outermost layer is PS and the adjacent layer is PI, both of which look gray after staining the P2VP phase with I2 vapor. Clearly, the sequence of the layer is opposite to the onion-like particles when PVA is employed. Because of the selective wetting ability of PVA and CTAB, a binary mixture of them can be applied to precisely tailor the interfacial interaction of polymer/boundary interface. As shown in Figure 1b, bud-like particles with a terrace-like tip and an onion-like hemisphere form at R = 9:1. Although similar structures can be found for diblock copolymer particles,11,12,18,48 such a unique structure has not been constructed by confined assembly of ABC triblock copolymers. In the terrace-like tip, PS, PI, and P2VP phase stack alternately. The competition balance among the interactions of different surfactants to different blocks allows all of the three blocks to C


Article

Macromolecules

surfactant of PVA and CTAB (R = 3:1) was removed, and PVA aqueous solution was applied to redisperse the pupa-like particles (Figure 3a). After being annealed for 2 h, the P2VP

to accommodate the new interface. As a result, the shape and internal structure of the particles could be finely tailored. Taking the onion-like particles (Figure 1a) as an example, PVA was replaced by the mixed surfactant of R = 3:1, where no block selectivity occurred for the surfactant. The suspension was annealed in saturated chloroform vapor. The slightly dissolved chloroform in water will be enriched in the polymer particles to enhance the mobility of the polymer chains. Since the preference of the boundary has been changed to be neutral to the copolymer by surfactant replacement, the PS, PI, and P2VP blocks tend to move toward the interface. After annealing the onion-like particles (Figure 2a) for 4 h, bud-like particles

Figure 3. TEM images of the reversible transformation between pupaand onion-like particles upon solvent-absorption annealing starting from pupa-like particles. (a) Initial pupa-like particles. (b−d) Particles annealed for 2, 6, and 24 h after replacement of surfactant mixture (R = 3:1) by PVA. (e) Particles annealed for 4 h after replacing PVA in (d) by surfactant mixture with R = 3:1. After being annealed for 24 h, the onions in (d) will reverse to pupa-like particles in (a).

chains will first move to the interface (Figure 3b) due to the preference of PVA to P2VP phase. Then, one terminal of the pupa-like particles enlarges and bends toward another terminal, resulting in a particle with half pupa and half onion (Figure 3c and Figure S2e in the Supporting Information). A similar phenomenon was observed in our previous study for the reversible morphological transformation of diblock copolymer particles.12 Finally, after 24 h annealing, pupa-like particles transform to onion-like ones (Figure 3d). If we then exchanged PVA with mixed surfactants (R = 3:1) and annealed the onions in Figure 3d for 24 h, they would transform to pupa-like particles (Figure 3d to 3e, then to 3a). Furthermore, we investigated the morphological transformation between pupa-like particle and reverse onion-like particle, which was also proved to be reversible (Figure S3, Supporting Information). As stated above, a series of annealing experiments via different pathways were carried out. We can conclude that the morphological transformation is reversible and pathway independent. The interfacial interaction is dominative to the shape and internal structure of the polymer particles under weak confinement. The morphology is the equilibrium state at the certain boundary selectivity. Once the surfactant is established, certain morphology appears and has nothing to do with the fabrication pathway. This can be attributed to the confining effect which can decrease the metastable states in the energy landscape and narrow down the polydispersity of the morphology.9 Though all the three blocks in the ABC triblock copolymers we used are hydrophobic, the surfactants act as a bridge to modulate the interfacial interaction. The shape and internal structure of the particles can be precisely tailored by

Figure 2. (a−d) TEM images for the reversible transformation between onion- and pupa-like ABC triblock copolymer particles upon solvent-absorption annealing, starting from the onion-like particles. (a) Initial onion-like particles. (b) Particles annealed for 4 h after replacement of PVA by surfactant mixture with R = 3:1. (c) Particles annealed for 24 h after the replacement of the surfactant mixture. (d) Particles annealed for 4 h after replacing the mixed surfactant by PVA. After being annealed for 24 h, the pupa-like particles in (c) will reverse to onion-like particles in (a).

with terrace tip and onion hemisphere can be observed (Figure 2b and Figure S2 in the Supporting Information). This structure is the intermediate structure between onion- and pupa-like particles. The bud-like particles will transform to pupa-like ones after another 20 h annealing (Figure 2c). No obvious difference can be found between such pupa-like particles and that shown in Figure 1c, indicating that pupa-like morphology is the equilibrium state at this interfacial condition (R = 3:1). If the mixed surfactant was then replaced by PVA solution, the pupa-like particles will reverse to onions upon annealing. During this process, the intermediate structure, i.e., bud-like particles, can also be observed (Figure 2d and Figure S2 in the Supporting Information). Therefore, both shape and internal structure transformation of the particle are reversible upon solvent vapor annealing when changing the boundary selectivity alternately. Moreover, we predict the pathway independence of the morphological transformation upon solvent annealing. To prove that, we conducted the annealing process where pupalike particles were applied as the initial state. The mixed D


Article

Macromolecules

Figure 4. TEM images demonstrating the effect of the confining size on the morphology of the ABC triblock copolymer particles when PVA is used as the surfactant. Insets in (a) and (b) are the cartoons showing the different phases in the particles: yellow, red, and green represent P2VP, PI, and PS, respectively.

Figure 5. TEM images of the ABC triblock copolymer particles obtained at different confining size when the mixed surfactant of PVA and CTAB (R = 3:1) is used as the surfactant. Insets in (a) and (b) are the cartoons showing the different phases in the particles: yellow, red, and green represent P2VP, PI, and PS, respectively.

regions in our case. In section 3.1, we discussed the particles under weak confinement, where the boundary selectivity dominates their morphologies. In this section, we will discuss the effect of confining size on the morphology of the BCP particles. To decrease size of the particles, initial polymer concentration was decreased to 3 mg/mL while surfactant concentration was increased to 5 mg/mL. Ultrasonication was employed to obtain much smaller droplets than that obtained through membrane extrusion route. Notably, particle size in this case is much smaller while size distribution is broader. We thus investigated many TEM images of these smaller particles and presented representative morphologies (Figure 4, where PVA is used as surfactant). The value of D/L0 is obtained by measuring more than 50 particles having the same structure.

modulating the property and concentration of polymeric, small molecular, and nanoparticle surfactants.12,17,18,48,50−52 The morphological control of the diblock copolymer particles in 3D confinement has been well studied in both simulation8 and experiment.7 Yet, it is difficult to control the internal structures of ABC triblock copolymers particles, as the interfacial interactions are more complicated. Using the strategy in this report, morphology of the particles can be readily tailored by simply changing the interfacial interaction through manipulating properties of the surfactants. 3.2. Effect of the Confining Size. Besides the boundary selectivity, the confining size plays another important role in determining the morphology of BCP particles. The strength of the 3D confinement can be divided into weak (D/L0 > 4), moderate (2 < D/L0 < 4), and strong (D/L0 < 2) confinement E


Article

Macromolecules

or various isolated nano-objects,13,53−55 depending on the volume fraction of the soluble block.11,39 Here we investigated the selective disassembly behavior of the ABC triblock copolymer particles in ethanol. The onion-like particles (Figure 1a) can be disassembled into microcapsules (Figure 6a). Since

Figure 4a shows the Janus particles (D/L0 = 0.65), in which the dark side is P2VP phase while the gray side is PS and PI phases. In this case, the spherical space is not large enough to hold a whole periodicity dimension of the ABC triblock copolymer (L0 ∼ 138 nm). As a result, asymmetric structure is observed. The polymer chains in such a tiny space are severely frustrated, leading to unusual phase behavior and unique structures. Simulation results from Li and co-worker demonstrated that symmetric BCPs formed onion-like particles with lessening layers as D/L0 decreased from 4.5 to 1.5. Yet, no asymmetric structures can be observed in their study, presumably because the confining size is not small enough.22 Recently, they found that ABC triblock copolymers could form Janus NPs when D/ L0 = 0.7.35 In our experiment, strong confinement forces the polymer chains to stretch at the cost of the increasing interfacial repulsion, resulting in asymmetric Janus structure at D/L0 = 0.65. Thus, our experimental results confirmed the simulative prediction. When D/L0 is increased to ∼1.0 (Figure 4b), the polymer chains are less frustrated and symmetric core−shell spheres (single-layer onions) are observed due to the commensurability between D and L0. When further increasing confinement size, the number of the layer in the onions increases (Figure 4c−f). Generally, increasing the D/L0 will weaken the confinement effect. At D/L0 = 2.31, the onions are almost the same as that shown in Figure 1a, even though D and L0 are incommensurable in this case. As a result, we conclude that under weak confinement the boundary selectivity plays a dominating role in determining the particle morphology; while under strong confinement, the confining size is more important. To further prove the above deduction, we investigated the size effect of the confinement where the binary mixture of PVA and CTAB (R = 3:1) was employed as the surfactant. As shown in Figure 5a, Janus ellipsoids were obtained at D/L0 = 0.56. Slightly increasing D/L0 to 0.95, hamburger-like particles are obtained (Figure 5b), which can be regarded as a shortest pupalike particle. When size of the particles is further increased, for example, at D/L0 = 1.26 (Figure 5c) and D/L0 = 1.69 (Figure 5d), spherical particle with multicompartments are observed. Interestingly, it is not a simple addition of the number of the layer to the shortest pupa-like particles, since D mismatches with L0 (D/L0 is not an integer). Instead, multicompartmental spherical particles separated by branched P2VP domains are obtained. When D/L0 is increased to 2.02, implying that it is commensurable between D and L0, pupa-like particles with more layers appear (Figure 5e). However, under weak confinement (e.g., D/L0 = 4.59, Figure 5f), the size effects of the confining space are less important. In this case, although D is incommensurable with L0, the pupa-like particles can still be generated. It can be found that length of the pupa-like particles depends on the BCP concentration and the emulsion droplet size. By controlling these two parameters, pupa-like particles with length of ∼10 μm can be generated (Figure S4, Supporting Information). On the basis of the results of the size effect on onion-like and pupa-like particles, we can conclude that under weak confinement (D/L0 > 4) the boundary selectivity is more important than the confining size to determine the morphology of BCP particles, while under strong confinement (D/L0 < 2) the confining size is dominating. 3.3. Unique Nano-Objects from Disassembly of the Structured Particles. Selective disassembly of the structured particles has been applied for preparing mesoporous materials

Figure 6. TEM images of the nano-objects obtained by disassembly of the (a) onion- and (b) bud-like particles. The samples are stained by I2 vapor before TEM test. Inset in (b) shows the side view of the nanobowls, of which the PI phase was stained by OsO4.

the volume fraction of P2VP phase is ∼52.5%, the stretching of the P2VP chains in ethanol will enlarge the volume of the particles and force the PS and PI layer to break up, leading to the release of the inner layer of the onion-like particles. Morphology of the nano-objects is dominated by the structure of the particles.11 For instance, when the bud-like particles (Figure 1b) were disassembled in ethanol, nanobowls can be obtained (Figure 6b). In our previous report,39 pupa-like particles of symmetric PSb-P4VP were disassembled into Janus nanodiscs. Yet, a threestep disassembly−cross-linking−disassembly process is essential in that case. Müller et al. introduced a template-assisted method to fabricate Janus discs from bulky phase or film of ABC BCPs with cross-linkable B block.42 Yet, the shape of the discs is difficult to be tailored since the breakup of the bulky phase is out of control. Here we combine the two methods described above, i.e., 3D confined assembly and selective crosslinking−disassembly, to form Janus nanodiscs. This approach can avoid the drawbacks mentioned above. We started from the ABC triblock copolymer pupa-like particles, in which the three blocks form lamella−lamella−lamella structure (Figure 7a,f). We attempted two different pathways to fabricate Janus nanodiscs. The first one is the disassembly−cross-linking−disassembly route (Figure 7a−e), and the second is the cross-linking−disassembly route (Figure 7f−j). In the first pathway, the pupa-like particles were first disassembled in ethanol into nanodiscs (Figure 7b), which are symmetrically constructed with six layers of polymer chains (P2VP−PI−PS−PS−PI−P2VP). Then we tried to cross-link PI by OsO4, followed by the separation of the two PS layers in chloroform to generate Janus nanodiscs with three layers of polymer chains (PS−PI−P2VP). However, we found that enclosing nanodiscs were obtained after cross-linking PI domains. As shown in Figure 7c, a dark circle obviously appears at the peripheral of the nanodiscs. The edge-on nanodiscs can be observed (inset of Figure 7c), indicating that they are surrounded by cross-linked PI domain. As a result, even though the nanodiscs are dispersed in chloroform, they cannot be separated to get Janus nanodiscs. Instead, only hollow discs can be obtained due to the swelling of PS phase by chloroform (Figure 7d,e). Generally, the soluble P2VP chains F


Article

Macromolecules

Figure 7. TEM images showing the disassembly of pupa-like particles. (a) Pupa-like particles stained by I2. Dashed lines indicate the boundaries of the domains, which are illustrated based on the result of OsO4 staining in (e). (b) Nanodiscs obtained by disassembly of the pupa-like particles in ethanol. (c) Enclosing nanodiscs after cross-linking PI phase of the nanodiscs in (b) by OsO4. (d) Hollow nanodiscs obtained by disassembling the nanodiscs in (c). (e) Cross-sectional TEM images of the hollow nanodiscs obtained by ultramicrotome. (g) Partially disassembled pupa-like particles with cross-linked PI phase. (h) Sandwiched nanodiscs obtained by thoroughly disassembling of pupa-like particles with cross-linked PI in ethanol. (i) The Janus nanodiscs obtained by cleaving the PS phase in (h). (j) Cross-sectional TEM images of the Janus nanodiscs obtained by ultramicrotome. Insets in (b−d) and (g, h) are the side view of the nano-objects. Insets in (e) and (j) are the cartoons showing the different phases in the nanodiscs.

Figure 8. Photographs of the emulsions (a) as prepared, (b) stacked for 2 h, and (c) after gently shaking the emulsion in (b), where panels indicate the emulsion (A) without any stabilizer, (B) stabilized by ABC triblock copolymer chains, and (C) stabilized by Janus nanodiscs. Arrows in (b) and (c) show the large emulsion droplets. (d) Optical microscopy image of droplets of B. Arrows indicates the fusion of the droplets. (e, f) Optical microscopy image of droplets (e) at the initial state and (f) after partially evaporating the oil phase of emulsion C, respectively.

OsO4 was used to cross-link PI, and then particles were selectively disassembled in ethanol. The PI domains still do not completely enclose the PS phase during this process (Figure 7g,h). As a consequence, the sandwiched nanodiscs with open edge were successfully obtained (inset of Figure 7h). Finally, these six-layer nanodiscs were cleft to Janus nanodiscs in chloroform (Figure 7i). The Janus structure is further proved by TEM investigation for the thin cross sections, and the asymmetric structure with dark PI phase at the middle layer can be clearly seen (Figure 7j). Thus, the Janus structure of the nanodiscs is successfully obtained. The stepwise disassembly process described above is just given out for better understanding how the symmetric lamella−lamella−lamella structure

in ethanol will migrate and cover on the hydrophobic inner core (PS and PI) of the nanodiscs to decrease the interfacial energy. The migration of the P2VP blocks will force the PI blocks to move since the glass transition temperature of PI is relatively low (Tg = −64 °C), resulting in the coverage of the PS core by PI and P2VP. As a consequence, no cleavage of the nanodiscs can be observed after cross-linking PI phase and disassembling in chloroform. Therefore, it is important to avoid the coverage of the PS core by the cross-linkable PI blocks for the synthesis of Janus nanodiscs. Thus, we tried the second cross-linking−disassembly pathway. In Figure 7f, it is obvious that the PI domains do not enclose the PS domains in the initial pupa-like particles. Then, G


Article

Macromolecules

4. CONCLUSION We have demonstrated a strategy to tailor the shape and internal structure of the ABC triblock copolymer particles under 3D confinement. Under weak confinement, the boundary selectivity dominates the morphology, while under strong confinement, the size of the limited space is much more important. Furthermore, these structured polymer particles can be disassembled into various isolated nano-objects by selectively swelling. More interestingly, the pupa-like particle consisted with alternating lamellas can be selectively crosslinked and disassembled into Janus nanodiscs. Such amphiphilic nanodiscs can be applied as surfactants to stabilize oil-in-water emulsion droplets, of which the stability is greatly enhanced due to the limited mobility of the disc-like surfactants on the interface. The 3D confined assembly of BCPs is a facile yet powerful approach to tailor the morphology of the polymer particles. The selective cross-linking can be applied for fixation of such structures. The selective disassembly process produces welldesigned isolated nano-objects, some of which can not be readily obtained by direct synthesis in selective solvent and bulk/film. The combination of these strategies offers us a versatile approach to design and fabricate nano-objects with various structures, which can be applied as drug carrier, catalyst supporter, template for hybrid materials, and other aspects.

cleaves to asymmetric Janus nanodiscs. Actually, the pupa-like particles with cross-linked PI phase can be directly disassembled in chloroform to simultaneously cleave P2VP and PS phases and get the Janus nanodiscs. Thus, the confined assembly−selective cross-linking−disassembly approach is proved to be a facile yet effective strategy to fabricate Janus nanodiscs. The Janus nanodiscs are amphiphilic due to hydrophobic PS and hydrophilic P2VP on the opposite side in acidic solution. Such amphiphilic nano-objects can act as surfactants to stabilize oil/water emulsion droplets, which are very stable because of the limited mobility of the surfactants on the interface. Here, the Janus nanodiscs synthesized above (∼2 mg/mL) are used to stabilize the chloroform-in-water emulsions. In order to highlight the role of the Janus nanodiscs, two control experiments were conducted. The first one is to emulsify chloroform in water without any surfactants (panel A in Figure 8a), and the second one is to emulsify triblock copolymers chloroform solution (10 mg mL−1, panel B in Figure 8a). To enhance the hydrophilicity of the P2VP block, acidic water (pH = 3) was applied in the emulsion. As shown in Figure 8a, all of the three kinds of oil phase can be emulsified. However, the two kinds of emulsion droplets in the control experiment are not stable. Large droplets can be found after being stored for 2 h (Figure 8b,d). When the Janus nanodiscs are used to stabilize the droplets (panel C of Figure 8a−c), the size of the droplets (∼45 μm) is smaller than that shown in Figure 8d (∼130 μm). After stacking for 2 h, most of the droplets precipitate to the bottom of the vial. However, no large droplet is observed, implying that the emulsion is very stable. After being gently shaken, the droplets will suspend again in water (panel C, Figure 8c). It is also found that the droplet is very stable when the oil phase evaporates. No fusion can be clearly seen during the shrinkage of the droplets (Figure 8f). To directly observe the Janus nanodiscs on the oil/water interface, homopolymer PS30K (5 mg/mL) is added in the oil phase. After evaporation of chloroform, solid spheres form to immobilize the nanodiscs. TEM image (Figure 9a) shows that

■

ASSOCIATED CONTENT

S Supporting Information *

Additional TEM images of the ABC triblock copolymer particles (Figures S1−S4). This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work is funded by National Basic Research Program of China (973 Program, 2012CB821500), National Natural Science Foundation of China (91127046), and China Postdoctoral Science Foundation (2014M552034). We thank Ying Wang at Soochow University and Yonggui Liao at HUST for the help of ultramicrotomy experiment. We also thank the HUST Analytical and Testing Center for allowing us to use its facilities.

■

Figure 9. Janus nanodiscs stabilized PS microparticles. (a) TEM image shows dark area at the periphery of the PS sphere. (b) SEM image shows a layer of nanodiscs on the surface of the PS sphere.

REFERENCES

(1) Otsuka, H.; Nagasaki, Y.; Kataoka, K. Adv. Drug Delivery Rev. 2003, 55, 403−419. (2) Pham, H. H.; Gourevich, I.; Oh, J. K.; Jonkman, J. E. N.; Kumacheva, E. Adv. Mater. 2004, 16, 516−520. (3) Connal, L. A.; Lynd, N. A.; Robb, M. J.; See, K. A.; Jang, S. G.; Spruell, J. M.; Hawker, C. J. Chem. Mater. 2012, 24, 4036−4042. (4) Cui, J.; Li, W.; Jiang, W. Chin. J. Polym. Sci. 2013, 31, 1225−1232. (5) Vaine, C. A.; Patel, M. K.; Zhu, J.; Lee, E.; Finberg, R. W.; Hayward, R. C.; Kurt-Jones, E. A. J. Immunol. 2013, 190, 3525−3532. (6) Yang, K.; Ma, Y. Q. Nat. Nanotechnol. 2010, 5, 579−583. (7) Yabu, H.; Higuchi, T.; Jinnai, H. Soft Matter 2014, 10, 2919− 2931. (8) Shi, A.-C.; Li, B. Soft Matter 2013, 9, 1398−1413.

the PS spheres are surrounded by a layer of Janus nanodiscs (dark area), while SEM image (Figure 9b) gives a direct view of the nanodiscs on the surface of the particles. Therefore, Janus nanodiscs can be applied as a solid surfactant to stabilize the oil/water emulsions. The stability of such emulsion is much better than that stabilized by amphiphilic block copolymers in similar experimental condition. This particular emulsifier can potentially find application in high internal phase emulsion,56 supracolloidal dispersants,57 and other fields. H


Article

Macromolecules (9) Arsenault, A. C.; Rider, D. A.; Tétreault, N.; Chen, J. I.-L.; Coombs, N.; Ozin, G. A.; Manners, I. J. Am. Chem. Soc. 2005, 127, 9954−9955. (10) Chi, P.; Wang, Z.; Li, B.; Shi, A.-C. Langmuir 2011, 27, 11683− 11689. (11) Deng, R.; Liang, F.; Li, W.; Liu, S.; Liang, R.; Cai, M.; Yang, Z.; Zhu, J. Small 2013, 9, 4099−4103. (12) Deng, R.; Liang, F.; Li, W.; Yang, Z.; Zhu, J. Macromolecules 2013, 46, 7012−7017. (13) Deng, R.; Liu, S.; Li, J.; Liao, Y.; Tao, J.; Zhu, J. Adv. Mater. 2012, 24, 1889−1893. (14) He, X. H.; Song, M.; Liang, H. J.; Pan, C. Y. J. Chem. Phys. 2001, 114, 10510−10513. (15) Higuchi, T.; Tajima, A.; Motoyoshi, K.; Yabu, H.; Shimomura, M. Angew. Chem., Int. Ed. 2008, 47, 8044−8046. (16) Higuchi, T.; Tajima, A.; Motoyoshi, K.; Yabu, H.; Shimomura, M. Angew. Chem., Int. Ed. 2009, 48, 5125−5128. (17) 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. J. Am. Chem. Soc. 2013, 135, 6649−6657. (18) Jeon, S. J.; Yi, G.-R.; Yang, S. M. Adv. Mater. 2008, 20, 4103− 4108. (19) Li, L.; Matsunaga, K.; Zhu, J.; Higuchi, T.; Yabu, H.; Shimomura, M.; Jinnai, H.; Hayward, R. C.; Russell, T. P. Macromolecules 2010, 43, 7807−7812. (20) Yu, B.; Li, B.; Jin, Q.; Ding, D.; Shi, A.-C. Soft Matter 2011, 7, 10227−10240. (21) Yu, B.; Sun, P. C.; Chen, T. H.; Jin, Q. H.; Ding, D. T.; Li, B. H.; Shi, A.-C. Phys. Rev. Lett. 2006, 96, 138306. (22) Yu, B.; Li, B.; Jin, Q.; Ding, D.; Shi, A.-C. Macromolecules 2007, 40, 9133−9142. (23) Yabu, H.; Motoyoshi, K.; Higuchi, T.; Shimomura, M. Phys. Chem. Chem. Phys. 2010, 12, 11944−11947. (24) Higuchi, T.; Motoyoshi, K.; Sugimori, H.; Jinnai, H.; Yabu, H.; Shimomura, M. Soft Matter 2012, 8, 3791−3797. (25) Higuchi, T.; Tajima, A.; Yabu, H.; Shimomura, M. Soft Matter 2008, 4, 1302−1305. (26) Jeon, S. J.; Yi, G.-R.; Koo, C. M.; Yang, S. M. Macromolecules 2007, 40, 8430−8439. (27) Deng, R.; Liu, S.; Liang, F.; Wang, K.; Zhu, J.; Yang, Z. Macromolecules 2014, 47, 3701−3707. (28) Yabu, H.; Higuchi, T.; Shimomura, M. Adv. Mater. 2005, 17, 2062−2065. (29) Cui, H.; Chen, Z.; Zhong, S.; Wooley, K. L.; Pochan, D. J. Science 2007, 317, 647−650. (30) Zhong, S.; Cui, H.; Chen, Z.; Wooley, K. L.; Pochan, D. J. Soft Matter 2008, 4, 90−93. (31) Zhu, J.; Jiang, W. Macromolecules 2005, 38, 9315−9323. (32) Pochan, D. J.; Chen, Z. Y.; Cui, H. G.; Hales, K.; Qi, K.; Wooley, K. L. Science 2004, 306, 94−97. (33) Kong, W.; Li, B.; Jin, Q.; Ding, D.; Shi, A.-C. J. Am. Chem. Soc. 2009, 131, 8503−8512. (34) Dupont, J.; Liu, G.; Niihara, K.; Kimoto, R.; Jinnai, H. Angew. Chem., Int. Ed. 2009, 48, 6144−6147. (35) Yu, B.; Deng, J.; Li, B.; Shi, A.-C. Soft Matter 2014, 10, 6831− 6843. (36) Li, S.; Jiang, Y.; Chen, J. Z. Y. Soft Matter 2013, 9, 4843−4854. (37) Qiu, W. J.; Li, S. B.; Ji, Y. Y.; Zhang, L. X. Chin. J. Polym. Sci. 2013, 31, 122−138. (38) Shim, J. W.; Kim, S. H.; Jeon, S. J.; Yang, S. M.; Yi, G.-R. Chem. Mater. 2010, 22, 5593−5600. (39) Deng, R.; Liang, F.; Zhou, P.; Zhang, C.; Qu, X.; Wang, Q.; Li, J.; Zhu, J.; Yang, Z. Adv. Mater. 2014, 26, 4469−4472. (40) Walther, A.; Drechsler, M.; Rosenfeldt, S.; Harnau, L.; Ballauff, M.; Abetz, V.; Müller, A. H. E. J. Am. Chem. Soc. 2009, 131, 4720− 4728. (41) Walther, A.; Hoffmann, M.; Müller, A. H. E. Angew. Chem., Int. Ed. 2008, 47, 711−714.

(42) Walther, A.; André, X.; Drechsler, M.; Abetz, V.; Müller, A. H. E. J. Am. Chem. Soc. 2007, 129, 6187−6198. (43) Walther, A.; Müller, A. H. E. Chem. Rev. 2013, 113, 5194−5261. (44) Walther, A.; Müller, A. H. E. Soft Matter 2008, 4, 663−668. (45) Gröschel, A. H.; Walther, A.; Loebling, T. I.; Schacher, F. H.; Schmalz, H.; Müller, A. H. E. Nature 2013, 503, 247−251. (46) Bahrami, R.; Loebling, T. I.; Gröschel, A. H.; Schmalz, H.; Müller, A. H. E.; Altstaedt, V. ACS Nano 2014, 8, 10048−10056. (47) Tangirala, R.; Revanur, R.; Russell, T. P.; Emrick, T. Langmuir 2007, 23, 965−969. (48) 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. Angew. Chem., Int. Ed. 2014, 53, 7018−7022. (49) Deng, R.; Liang, F.; Qu, X.; Wang, Q.; Zhu, J.; Yang, Z. Macromolecules 2015, 48, 750−755. (50) Ku, K. H.; Shin, J. M.; Kim, M. P.; Lee, C. H.; Seo, M.-K.; Yi, G.R.; Jang, S. G.; Kim, B. J. J. Am. Chem. Soc. 2014, 136, 9982−9989. (51) Ku, K. H.; Yang, H.; Shin, J. M.; Kim, B. J. J. Polym. Sci., Polym. Chem. 2015, 53, 188−192. (52) Yang, H.; Kang, D. J.; Ku, K. H.; Cho, H. H.; Park, C. H.; Lee, J.; Lee, D. C.; Ajayan, P. M.; Kim, B. J. ACS Macro Lett. 2014, 3, 985− 990. (53) Wang, Y.; Li, F. Adv. Mater. 2011, 23, 2134−2148. (54) Mei, S.; Jin, Z. Small 2013, 9, 322−329. (55) Fan, H.; Jin, Z. Macromolecules 2014, 47, 2674−2681. (56) Chevalier, Y.; Bolzinger, M.-A. Colloids Surf., A 2013, 439, 23− 34. (57) Gröschel, A. H.; Löbling, T. I.; Petrov, P. D.; Müllner, M.; Kuttner, C.; Wieberger, F.; Müller, A. H. E. Angew. Chem., Int. Ed. 2013, 52, 3602−3606.

I


ABC Triblock Copolymer Particles with Tunable Shape and Internal

Recommend Documents