Additives Induced Structural Transformation of ABC Triblock

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Additives Induced Structural Transformation of ABC Triblock Copolymer Particles Jiangping Xu,†,‡ Yi Yang,† Ke Wang,† Jingyi Li,† Huamin Zhou,‡ Xiaolin Xie,† and Jintao Zhu*,† †

State Key Laboratory of Materials Processing and Mold Technology, School of Chemistry and Chemical Engineering, and ‡School of Materials Science and Engineering, Huazhong University of Science and Technology (HUST), Wuhan 430074, China ABSTRACT: Here we report the structural control of polystyrene-b-polyisoprene-bpoly(2-vinylpyridine) (PS-b-PI-b-P2VP) asymmetric ABC triblock copolymer particles under 3D confinement by tuning the interactions among blocks. The additives, including 3n-pentadecylphenol, homopolystyrene, and solvents, which can modulate the interactions among polymer blocks, play significant roles in the particle morphology. Moreover, the structured particles can be disassembled into isolated micellar aggregates with novel morphologies or mesoporous particles with tunable pore shape. Interestingly, the formed pupa-like PS-b-PI-b-P2VP particles display interesting dynamic stretch−retraction behavior when the solvent property is changed after partial cross-linking of the P2VP block. We further prove that such dynamic behavior is closely related to the density of cross-linking. The strategies presented here are believed to be promising routes to rationally design and fabricate block copolymer particles with desirable shape and internal structure.

1. INTRODUCTION Three-dimensional (3D) confined assembly of block copolymers (BCPs) provides a powerful route to manipulate the shape and internal structure of BCP particles.1−3 Various morphologies, such as onion-like particles,4−7 pupa-like ellipsoids,8−10 Janus particles,11 and patchy particles,12,13 have been produced through this strategy. Due to the morphological diversity, these particles have broad applications in the fields of guiding deposition of inorganic particles,14,15 drug delivery,16 bioengineering,17 cosmetics,18 and advanced materials formation.19 Particle shape and internal structure play critical roles in particle functions, including targeting ability,20 cellular uptake,21 immune response,22 flow separation,23 rheological property,24 colloidal self-assembly,25 catalysis efficiency,26 etc. Therefore, how to systemically control the morphology of the polymer particles is of great significance. Three main factors, including (1) interaction among blocks, (2) interfacial interaction, and (3) strength of confinement, are proved to be crucial to the confining assembly behavior of BCPs based on the simulative and experimental investigations.27−30 Although these factors have been intensively studied for diblock copolymers (dBCP) under confinement,1,2 it is still not clear how these factors affect the confined assembly of ABC triblock copolymers (tBCPs) due to the much more complicated interactions among blocks and interfaces. Recently, Yu13 and Li31,32 studied the confining assembly of ABC linear and star tBCPs through computer simulation, respectively, revealing the complexity of phase diagram under 3D confinement by considering the above three factors. In our previous experimental study, ABC tBCP particles with tunable shape and internal structure were fabricated by tailoring the interfacial interaction and confining strength.28 Some theoretically predicted structures were confirmed by carefully tuning the © XXXX American Chemical Society

parameters, yet the effect of the block interaction on the 3D confined assembly of tBCPs was not demonstrated in the report of that experiment. The interaction among blocks, χN (where χ is the Flory− Huggins interaction parameter and N is the polymerization degree), is a function of the temperature, composition, and molecular weight of the polymer.33 Thus, BCPs with different block ratios and molecular weights can be applied to tailor the interactions among blocks. However, tedious steps are needed to synthesize tBCPs with precise and controllable structure. Alternatively, the introduction of additives into BCPs is a simple yet effective route to tailor the interactions and thus morphologies of the assemblies. For example, in dilute solution, the addition of ions can significantly influence the morphology of “crew-cut” BCP micelles.34,35 In polymer thin film/bulky phase, small molecule, polymer, or inorganic nanoparticle addition could induce the phase transformation or enhance the regularity of the BCP domains.36−43 Recently, similar methods have been employed to control the morphology of BCP particles under 3D confinement. For example, addition of homopolymers or BCPs to dBCP particles can regulate the selfassembled morphology.27,30,44−48 Moreover, addition of small molecules that can form hydrogen bonding (HB) with one block of the BCP will form comb-like supramolecules.36,37 Such HB agents can affect the relative volume fraction of the host block, the hydrophilicity of the block, and the Flory−Huggins interaction parameter (χ) between the blocks.49−51 As a consequence, the morphology of the particles can be easily tailored in the same BCP system by varying the content of HB Received: July 31, 2015 Revised: September 17, 2015

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Langmuir agent.11,50,52,53 Furthermore, solvent can be regarded as a small molecular additive and plays a critical role in determining the phase structure of the BCP particles.54−56 On the basis of the solubility parameters (δ) of solvents and polymers, different solvents affect the affinity and interactions among blocks.11,12 Additionally, solvent vapor can be applied in postprocessing of the BCP particles. For instance, a solvent absorption annealing strategy was developed to induce the morphology evolution of PS-b-polyisoprene (PS-b-PI) particles, and transition of PI phase from lamellae to cylinder was observed.56 Therefore, the additives will significantly affect the Flory−Huggins interaction parameter among blocks and show great promise in regulating the morphology of polymer particles. Most of the above studies focused on the case of dBCPs, and limited structures can be obtained. Due to their large parameter spaces, the reconstruction of ABC tBCPs in confinement geometry is interesting to investigate. By using such asymmetric tBCPs, we are able to readily fabricate multicompartmental nano-objects, which can be applied to load several cargoes simultaneously.57 Herein, we demonstrate the additives-induced morphology evolution of PS-b-PI-b-P2VP tBCP particles under 3D confinement via introducing HB agent 3-n-pentadecylphenol (PDP) and homopolystyrene (hPS). Also, common and selective solvents are used to reshape the polymer particles. Moreover, these particles can be disassembled in ethanol, and isolated micellar aggregates or mesoporous materials can thus be obtained. Furthermore, selective cross-linking and disassembly strategy are combined to fabricate pupa-like particles, which can change shape reversibly upon variation of the solvent property alternatively. These results are believed to be helpful for understanding the phase behavior of ABC tBCPs in confined space and offer us sophisticated approaches to fabricate BCP particles with designable structures.

was diluted by the same surfactant solution to 0.5 mL. For common solvent annealing, 0.05 mL of the particle suspension was first washed at least three times to remove the surfactant and then redispersed in 0.5 mL of a different surfactant solution. Afterward, the suspensions with particle content of ∼0.1 mg/mL in a small vial (5 mL) were placed inside a large vial (25 mL) with 1.0 mL of volatile solvents (chloroform, toluene, or ethanol) at 30 °C for 24 h. Subsequently, the inner small vial was taken out to release the absorbed organic solvent for 12 h at 30 °C. Then, the structure of the particles was kinetically frozen after removal of the organic solvent, and the particles were collected by centrifugation (14 000 rpm, 6 min). 2.4. Cross-Linking and Disassembly of the Polymer Particles. The non-cross-linked SIV(PDP)x particles and SIV particles can be disassembled into various isolated nano-objects or mesoporous particles in selective solvent (ethanol) of P2VP.14 In a typical disassembly process, 0.1 mL of the particle aqueous suspension was centrifuged to remove the surfactant. Then, 0.5 mL of ethanol was added to redisperse the particles under gentle ultrasonication. The suspension was then stirred for 24 h to complete the disassembly process. On the other hand, the dynamic stretch−retraction behavior was investigated on the basis of the pupa-like particles obtained from emulsions using mixed surfactant of PVA and CTAB (weight ratio 3:1).28 After cross-linking P2VP phase by DIB at 40 °C for 3 days, the P2VP phase was selectively swollen in ethanol or acidic water (pH 3), whereas PS and PI phases remained unchanged. For the retraction process, the swollen pupa-like particles were collected by centrifugation and redispersed in neutral or basic water. 2.5. Characterization. The structures of the particles were investigated using FEI Tecnai G2 20 TEM operated at an accelerated voltage of 200 kV. Before TEM characterization, the samples were stained with iodine vapor at 30 °C for 2 h (for the P2VP block) or with OsO4 solution overnight (for PI block).

3. RESULTS AND DISCUSSION 3.1. HB Agent Induced Structural Transition of PS-bPI-b-P2VP Particles. PDP will form HB with a 2VP unit of PS-b-PI-b-P2VP to produce comb-like supramolecules. In our previous studies, we have proved that PDP significantly influenced the structure of the PS-b-P4VP dBCP particles.11,50 Here we show that this facile strategy can also be applied to the ABC tBCPs system. The morphological evolution of the particle as the increase of PDP content (x) is shown in Figure 1. When x = 0, onion-like particles with P2VP at the outermost layer can be obtained. All of the three layers, PS, PI, and P2VP, are proved to form lamella structure by staining P2VP with iodine and PI with OsO4.28 The PDP molecules located in P2VP phase will enlarge its volume fraction, change the hydrophilicity of the P2VP block,49 and decrease the Flory− Huggins parameter (χ) among the blocks.51 All of the changes will induce transformation of the particle morphology. Indeed, when a trace amount of PDP is added (x = 0.05), the interconnected onion-like particles can be observed (Figure 1b). Spherical particles with a perforated shell and twist cylindrical cores can be obtained at x = 0.2 (Figure 1c). When x = 0.3, ellipsoids consisting of stacked rings are observed (Figure 1d). With further increase of x to 0.6, particles consisting of severely distorted cylinders are obtained (Figure 1e). Finally, the internal structure of the particles transforms to a sphere at x = 1.0 (Figure 1f). The internal structure of the particles will be further confirmed under section 3.4, where the particles are disassembled into small fractions. One of the advantages of our strategy is that the shape and internal structure of the particles can be readily tailored by small molecules without bothering the synthesis of tBCPs with various block ratios and molecular weights. The other advantage is that the HB agent can be easily

2. EXPERIMENTAL SECTION 2.1. Materials. The ABC tBCP PS40K-b-PI33K-b-P2VP87K (SIV; the subscripts are the Mn of the blocks, Mw/Mn = 1.15), homopolymer hPS2.8K (Mw/Mn = 1.09), and hPS876 K (Mw/Mn = 1.19) were purchased from Polymer Source, Inc., Canada. Poly(vinyl alcohol) (PVA; average Mw = 13−23K g mol−1, 87−89% hydrolyzed), cetyltrimethylammonium bromide (CTAB; purity ≥ 99%), and 3-npentadecylphenol (PDP; purity ≥ 90%, recrystallized twice from hexane before use) were purchased from Aldrich. 1,4-Diiodobutane (DIB; purity = 99%) was supplied by Alfa Aesar. 2.2. Preparation of tBCP Particles. The emulsion−solvent evaporation method was applied to prepare the polymer particles.58 The copolymer SIV, hPS, and PDP were first dissolved in chloroform at a concentration of 10 mg/mL, respectively. For HB-assisted assembly, the copolymer solution was mixed with the desired amount of PDP solution, followed by stirring for 24 h to form SIV(PDP)x comb-like supramolecules, where x is the molar ratio of PDP to 2VP unit. For hPS modulated assembly, the copolymer solution was mixed with hPS solution with different weight fractions (whPS = mhPS/ (mhPS+mSIV)). For solvent-tailored assembly, the copolymer SIV was dissolved in the mixed solvent of chloroform and toluene at a concentration of 10 mg/mL. Subsequently, the above solutions (total volume = 0.1 mL) were emulsified with 1.0 mL of aqueous solution containing 3 mg/mL surfactant (PVA, CTAB, or their binary mixture,) through a hand-driven membrane extrusion emulsification route.28,50,59 The resulting emulsions were diluted by 0.5 mL of the same surfactant solution. The organic solvent was then allowed to slowly evaporate for 3 days at 30 °C. Subsequently, the formed polymer particles were separated by centrifugation (14 000 rpm, 6 min) to remove the surfactant. 2.3. Solvent Adsorption Annealing. For selective solvent (toluene or ethanol) annealing, 0.05 mL of the particle suspension B

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obtained (Figure 2b). With further increase of whPS to 50 wt %, onion-like particles with two domes can be observed (Figure 2c). A similar structure has been observed in a previous study by Yi et al.,44 who added hPS to PS-b-poly(butadiene) (PS-bPB) in the dry-brush regime. The dome-like PB phase was obtained when whPS > 50 wt % due to the random segregation triggered by macrophase separation. In the present case, the closed onion-like lamellae is crushed to two pieces by the enlarged PS phase. The dome-like P2VP domains located at the opposite sides of the particle, reshaping the spherical particles to ellipsoids. Such a unique structure has not been reported in ABC tBCP/homopolymer blend under confinement. When whPS is further increased, twisted cylindrical (whPS = 70 wt %, Figure 2d) and spherical (whPS = 80 wt %, Figure 2e) P2VP phases are observed in the particles. Thus, by tuning the fraction of homopolymer in the wet-brush regime, the morphology of the ABC tBCP particles can be well tailored. In the dry-brush regime, hPS876 K is added to the SIV particles to tailor the structures. The hPS876 K chains will trigger a macrophase separation inside the particles. Thus, particles with both microphase and macrophase separation structures were obtained. When SIV is minority in the blend (Figure 3a,b), Janus particles with an hPS876 K “head” and an SIV “cap”

Figure 1. TEM images of PS-b-PI-b-P2VP(PDP)x particles with various PDP contents: (a) x = 0, (b) x = 0.05, (c) x = 0.20, (d) x = 0.30, (e) x = 0.60, and (f) x = 1.0. All of the samples are stained by I2 vapor before TEM investigation. (Insets) Cartoons showing the particle structures: yellow, red, and green represent P2VP, PI, and PS, respectively.

removed in selective solvent, leading to the disintegration of the particles, which will be displayed in more detail in section 3.4. 3.2. hPS Induced Particle Structural Transformation. The effect of hPS on the 3D confined assembly behavior of the ABC tBCPs has also been investigated. The structure of the blend system dramatically depends on the mixing ratio, molecular weight, and the interaction parameter among different components.45−48,60 Here we focus on the effect of the molecular weight of homopolymers. Generally, the homopolymer chains are immiscible with the BCP chains when the molecular weight is similar to or higher than that of the matrix polymer. This is the so-called dry-brush regime, where the mixture will segregate to form macrophase separation structure. When the molecular weight of the homopolymer is smaller than that of the matrix, the blended polymers are miscible and a wet-brush regime is formed.44 Such a relationship between homopolymer and BCP can be applied to control the internal structure of the BCP particles. In the wet-brush regime, hPS2.8K is used to tailor the structure of the SIV particles. The molecular weight of hPS2.8K is much smaller than that of the PS block (Mn = 40K) in SIV copolymer. Thus, hPS2.8K can penetrate into the PS microdomains formed by SIV, which will increase the volume fraction of PS phase and induce phase transition. When the weight fraction of hPS2.8K (whPS) is below 20 wt %, the onion-like structure will remain nearly unchanged (Figure 2a). When whPS reaches 25 wt %, particles with severely curved lamellas are

Figure 3. TEM images of the SIV/hPS876 K particles with different weight ratios of SIV (wSIV): (a) 25 wt %, (b) 33 wt %, (c) 50 wt %, (d) 67 wt %, (e) 75 wt %, and (f) 90 wt %. Dashed lines indicate the interfaces between hPS876 K and SIV. (Insets) Cartoons showing the particle structures: yellow, red, and green represent P2VP, PI, and PS, respectively.

can be observed. Because the hPS876 K cannot insert into the PS phase of SIV, the intrinsic structure (lamellae) of SIV is not affected by the addition of hPS876 K. When wSIV reaches 50 wt %, core−shell particles with hPS876 K core and SIV shell are

Figure 2. TEM images of the SIV/hPS2.8K particles with different weight fractions of hPS2.8K (whPS): (a) 5 wt %, (b) 25 wt %, (c) 50 wt %, (d) 70 wt %, and (e) 80 wt %. (Insets (c−e)) Cartoons showing the particle structures: yellow, red, and green represent P2VP, PI, and PS, respectively. The outermost layers of the particles are omitted for clarity. C

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Langmuir obtained (Figure 3c). With further increase of wSIV to 67 wt %, Janus particles with a multilayered “cap” can be observed. The surface of the hPS core is completely covered by SIV (Figure 3d), yet the thickness of the SIV shell is not uniform. As shown in Figure 3d, on one side of the hPS core, there is only one layer of SIV, whereas on the opposite side, two layers of SIV can be found. Interestingly, at wSIV = 75 wt %, core−shell particles with two layers of SIV in the shell appear (Figure 3e). When wSIV is increased to 90 wt %, Janus particles are obtained again (Figure 3f). Clearly, there are two layers of SIV on one side of the particles, whereas, on the other side, there are four to six layers of SIV. Therefore, core−shell structures (including the Janus structures shown in Figure 3d,f) are favorable in SIV/ hPS876 K blends when wSIV reaches a critical value (∼50 wt %). Presumably, the core−shell structure can decrease the interfacial energy between hydrophobic hPS876 K and water. In addition, the surfactant PVA has a preference for the P2VP block of the copolymer. Such selective attractive interaction will pull P2VP out to the surface of the particles. 3.3. Effect of Solvent Property on Particle Morphology. Solvent property affects the affinity of solvent to the blocks of the copolymers, which determines the configuration of the chains and the phase separation behavior. In this section, we will discuss the influence of solvent property on the internal structures of the particles. The solubility parameters of the solvents and polymers are shown in Table 1.

Figure 4. TEM images of SIV particles obtained from the emulsion droplets containing 1 wt % SIV and a mixture of chloroform and toluene with various volume fractions of toluene (φtoluene): (a) 10 vol %, (b) 50 vol %, (c) 100 vol %, and (d) 100 vol %. (d) TEM image showing the core−shell particles with smaller diameter (∼180 nm). (Insets) Cartoons showing the particle structures: yellow, red, and green represent P2VP, PI, and PS, respectively. The outermost layers of the insets in panels a−c are omitted for clarity.

Table 1. Solubility Parameters of the Polymers and Solvents33 polymer

solubility parameter δ (MPa1/2)

solvent

solubility parameter δ (MPa1/2)

PS PI P2VP

17.6 16.5 20.6

chloroform toluene ethanol

19.0 18.2 26.0

After evaporation of the mixed solvents, the particle structure is frozen. By using solvent absorption annealing strategy, the polymer chains can be endowed with mobility. Thus, there is a good opportunity to reshape the particles through this route. We have proved that, during the reshaping process, the surfactants played an important role in determining the morphology.28 Here we focus on the effect of the solvent property on particle morphology through solvent adsorption annealing strategy. Chloroform, a common solvent for the three blocks of SIV, is used for annealing the particle suspension. The particles with spherical P2VP phase (Figure 4c) were applied as the initial state. After being annealed in chloroform for 24 h, the particles dispersed in PVA (preferential to P2VP) solution will transform to onions (Figure 5a). If the particles are annealed in mixed surfactants of PVA:CTAB = 3:1 (no preference for the blocks),28 pupa-like particles can be obtained (Figure 5b). Therefore, we can conclude that the particles obtained in a selective solvent can be reconstructed by annealing them in a nonselective solvent. The surfactants with different preferences

Clearly, toluene is a selective solvent for PS and PI but a poor solvent for P2VP. Thus, addition of toluene to a chloroform solution containing SIV copolymer will induce the deswelling of the P2VP segments and then decrease the effective volume fraction of P2VP. Figure 4 shows the morphologies of the particles obtained from emulsion droplets containing a mixture of chloroform/toluene with a different volume fraction of toluene (φtoluene). At φtoluene = 10 vol %, P2VP phase changes from lamella to entangled long cylinder (Figure 4a). Then, long cylinders transform to short cylinders at φtoluene = 50 vol % (Figure 4b). When neat toluene is applied to dissolve the copolymer, a turbid suspension rather than a clear solution is obtained, due to the formation of spherical micelles with P2VP as the core and PS/PI as the corona. After evaporation, the micelles aggregate to form solid particles, in which P2VP blocks segregate to spheres while PS and PI blocks form a continuous phase (Figure 4c). The internal structure of the particles can also be tuned by varying the size of the particles. As shown in Figure 4d, when the size of particles (Figure 4c) is decreased to ∼180 nm, core−shell−corona particles can be obtained. The size-dependent structural evolution can be ascribed to the 3D confining effect within the confined space. Thus, by adding selective solvent to the solution of ABC block copolymer, the internal structure of the particles can be readily tailored. This represents a convenient and versatile methodology for manipulation of the particle morphology.

Figure 5. TEM images of the particles obtained by chloroform annealing of particles in Figure 4c in different surfactants, (a) PVA and (b) PVA/CTAB = 3:1, for 24 h. D

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Langmuir to blocks will dominate the final shape and internal structure of the particles. Thus, by the combination of solvent annealing and surfactant selection, the shape and internal structure of the ABC tBCP particles can be readily tailored. The absorption of nonselective solvent will endow all three blocks with mobility, yet when selective solvent is used for annealing, it will selectively swell part of the BCPs while leaving other parts glassy. Here, toluene, a selective solvent for PS and PI, is employed to anneal pupa- and onion-like particles without changing surfactants. As shown in Figure 6a,b, the PS

simply disassembled into isolated nano-objects by rupturing the HB in ethanol.50 More interestingly, from the fragments obtained by disintegrating the particles, we can deduce their initial internal structure by reconstructing the fragments. Figure 7 shows the TEM images of the isolated nano-objects obtained by disassembling the particles in Figure 1. The onion-

Figure 7. TEM images of isolated nano-objects obtained by disassembling the SIV(PDP)x particles in Figure 1: (a) x = 0, (b) x = 0.05, (c) x = 0.20, (d) x = 0.30, (e) x = 0.60, and (f) x = 1.0.

like particles disassemble into hollow capsules (Figure 7a).28 The interconnected onions (Figure 1b) disintegrate to perforated vesicles (Figure 7b), which have potential applications in drug delivery and controlled release. The particles shown in Figure 1d−f can be disassembled into ringlike, cylindrical, and spherical micelles, respectively (Figure 7d− f). In these cases, the perforated vesicles (Figure 7b) and nanorings (Figure 7d) are difficult to synthesize through direct selfassembly of the ABC triblock copolymers in dilute solution.64,65 Clearly, through combining 3D confined assembly and selective disassembly, several novel micellar aggregates can be readily obtained. Moreover, the disassembly strategy is helpful for analyzing the initial structures of the particles, especially those with complicated structures. For instance, Figure 1c does not clearly demonstrate the internal structure of the particles due to the overlapped projection. Although 3D microtomography technique can analyze the internal structure, it critically depends on the special TEM facility and is time-consuming. Here we first disassemble the complicated particles into small fractions and then attempt to deduce the original structure of the particles. As shown in Figure 7c, two types of micellar aggregates, perforated vesicles and isolated cylinders, can be observed. Thus, by combining the TEM images before (Figure 1c) and after (Figure 7c) disassembly, we deduce that the particle consists of a perforated shell and a core of twisted cylinders. The proposed internal structure of the particles is given in the inset of Figure 1c. Then, we focus on the selective swelling of SIV particles with a discrete P2VP domain. The initial particles we used are shown in Figure 4a,c, and ethanol is applied to swell the P2VP chains. After removal of ethanol during sample preparation for TEM investigation, the P2VP chains shrink quickly, leading to the appearance of mesopores.61,62 The shape of the mesopores is dependent on the initial morphology of the P2VP domain. As a consequence, mesoporous particles with cylindrical or spherical pores are thus obtained (Figure 8). In previous studies, the

Figure 6. TEM images of (a) pupa- and (b) onion-like particles annealing in toluene and (c) pupa- and (d) onion-like particles annealing in ethanol for 24 h.

and PI domains are swollen to increase their thicknesses, whereas the P2VP domain remains nearly unchanged. When ethanol, a selective solvent for P2VP, is applied to anneal the particles in aqueous suspension, no obvious structure change can be observed (Figure 6c,d). Presumably, partition of organic solvent between particle and surrounding water is an important parameter to trigger the morphological transition. Toluene is immiscible with water, but can slightly dissolve in water (0.52 g/L)33 and fully dissolve in PS/PI blocks, indicating that the chemical potential of toluene in water is much higher than that in PS/PI domains. Thus, the dissolved toluene will be absorbed and enriched in the PS and PI domains until the chemical potential of toluene is equal in each domain and in the aqueous phase.56 Therefore, the structural transformation is initiated. Because ethanol is soluble in water, the low chemical potential of ethanol in water prevents the enrichment of ethanol inside the particles. Thus, the P2VP phase cannot be effectively swollen. As a result, no obvious change can be observed even when the particles are annealed in ethanol for 48 h. Generally, a high concentration of ethanol is essential to swell the P2VP phase and initiate the structure change of the particles. Actually, neat ethanol was needed to swell the particles into mesoporous materials or isolated nano-objects.14,61−63 The swelling behavior of the particles in ethanol will be discussed in the next section. 3.4. Disassembly of the Particles in Selective Solvent. In the case of HB-assisted confined assembly, P2VP(PDP) forms the continuous phase, whereas the PS and PI phases are discrete in the particles. As a result, these particles can be E

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More interestingly, we find that the dynamic shape changing upon the alteration of solution property depends on the crosslinking degree of the P2VP phase. When excess DIB (10-fold of 2VP unit) is added to cross-link the P2VP phase, no obvious shape changing can be observed when the pH value is lowered or water is replaced by ethanol (Figure 9d−f). Only slight stretching of P2VP domains can be observed. The thickness of P2VP domains enlarges from 76 to 82 nm when being dispersed in acidic water, whereas an 11 nm enlargement of P2VP domains can be observed when ethanol is added to swell the particles. In this case, the dense cross-linkage severely frustrates the swelling of P2VP chains, resulting in their subtle stretching. Therefore, we are able to control the dynamic shape changing by tailoring the cross-linking degree of the particles.

Figure 8. Mesoporous particles with (a) cylindrical and (b) spherical pores, obtained by swelling P2VP phase in ethanol. The original particles of (a) and (b) are those shown in Figure 4, panels a and c, respectively.

pore shape was dominated by the block ratio of BCPs61 or the content of PDP.50 Here we demonstrate that the mesopores can be tailored by adding selective solvent, which can induce the shrinkage of P2VP domain. We believe it is a much more convenient strategy to prepare mesoporous polymeric particles. 3.5. Dynamic Shape Changing of Cross-Linked Particles. In section 3.4, we have shown that the disassembly of the particles relied on the swelling of P2VP phase in ethanol. However, if P2VP chains were partially cross-linked, the disassembly would be frustrated, resulting in a dynamic shape variation. Hawker et al. have studied the shape changing of PSb-P2VP pupa-like particles triggered by pH alteration.10 In our case, the P2VP phase was slightly cross-linked by DIB (5 mol % of 2VP unit) and then dispersed in HCl aqueous solution at pH 3. As shown in Figure 9a,b, the pupa-like particles stretch along the major axis. The non-cross-linked P2VP segments will be ionized by HCl and the repulsion among the lamellas will be increased, resulting in the stretching of the pupa-like particles. When NaOH was added to increase the pH to 10, the elongated particles will contract and the stacked structure can be observed again. Moreover, the P2VP phase can also be swollen by ethanol, resulting in the stretching of the particles (Figure 9c). We further prove that the shape changing by ethanol swelling is reversible when ethanol is replaced by water.

4. CONCLUSION The phase behavior of ABC triblock copolymer is quite complicated due to the complex interactions among blocks and solvent.66 In a confining space, the situation is much more intricate because of the additional effects of confinement and the interaction between polymer and interface. However, we simplified this problem and focused on the phase behavior of ABC tBCPs under soft confinement. In this case, the phase separation is mainly affected by the composition of copolymer itself and the interface interaction. We found that the composition of ABC triblock copolymer can be readily tailored by adding HB agent PDP and hPS. Moreover, the particle structure can be well-designed by varying the properties of solvents and surfactants during the formation of particles or the solvent annealing processing. In addition, these particles can be swollen to isolated micellar aggregates or mesoporous materials in selective solvent, depending on the volume fraction of P2VP phase. Interestingly, the combination of selective cross-linking and disassembly strategies is used to fabricate deformable pupalike particles upon solvent changing. It is proved that the dynamic shape-changing process relies on the cross-linking density. These findings will enrich the phase diagram of ABC triblock copolymer, and the strategies introduced here will offer

Figure 9. TEM images showing (a−c) the reversible shape change of the pupa-like particles with slightly cross-linked P2VP phase and (d−f) the insignificant shape change of particles with densely cross-linked P2VP phase. (Insets (a, b)) Cartoons showing the original particle and the elongated particle, respectively. Yellow, red, and green represent P2VP, PI, and PS, respectively. F

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(15) Cui, J.; Li, W.; Jiang, W. Simulation Study of Co-assembly of ABC Triblock Copolymer/Nanoparticle into Multicompartment Hybrids in Selective Solvent. Chin. J. Polym. Sci. 2013, 31, 1225−1232. (16) Otsuka, H.; Nagasaki, Y.; Kataoka, K. PEGylated nanoparticles for biological and pharmaceutical applications. Adv. Drug Delivery Rev. 2003, 55, 403−419. (17) Yang, K.; Ma, Y.-Q. Computer Simulation of the Translocation of Nanoparticles with Different Shapes Across a Lipid Bilayer. Nat. Nanotechnol. 2010, 5, 579−583. (18) Souto, E. B.; Müller, R. H. Cosmetic Features and Applications of Lipid Nanoparticles (SLN®, NLC®). Int. J. Cosmet. Sci. 2008, 30, 157−165. (19) Im, S. H.; Jeong, U.; Xia, Y. Polymer Hollow Particles with Controllable Holes in Their Surfaces. Nat. Mater. 2005, 4, 671−675. (20) Mitragotri, S.; Lahann, J. Physical Approaches to Biomaterial Design. Nat. Mater. 2009, 8, 15−23. (21) Agarwal, R.; Singh, V.; Jurney, P.; Shi, L.; Sreenivasan, S. V.; Roy, K. Mammalian Cells Preferentially Internalize Hydrogel Nanodiscs over Nanorods and Use Shape-Specific Uptake Mechanisms. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 17247−17252. (22) Vaine, C. A.; Patel, M. K.; Zhu, J. T.; Lee, E.; Finberg, R. W.; Hayward, R. C.; Kurt-Jones, E. A. Tuning Innate Immune Activation by Surface Texturing of Polymer Microparticles: the Role of Shape in Inflammasome Activation. J. Immunol. 2013, 190, 3525−3532. (23) Masaeli, M.; Sollier, E.; Amini, H.; Mao, W.; Camacho, K.; Doshi, N.; Mitragotri, S.; Alexeev, A.; Di Carlo, D. Continuous Inertial Focusing and Separation of Particles by Shape. Phys. Rev. X 2012, 2, 031017. (24) Yamamoto, T.; Suga, T.; Mori, N. Brownian Dynamics Simulation of Orientational Behavior, Flow-Induced Structure, and Rheological Properties of a Suspension of Oblate Spheroid Particles under Simple Shear. Phys. Rev. E 2005, 72, 021509. (25) Zhang, Z.; Pfleiderer, P.; Schofield, A. B.; Clasen, C.; Vermant, J. Synthesis and Directed Self-Assembly of Patterned Anisometric Polymeric Particles. J. Am. Chem. Soc. 2011, 133, 392−395. (26) Lu, Z.; Liu, G.; Phillips, H.; Hill, J. M.; Chang, J.; Kydd, R. A. Palladium Nanoparticle Catalyst Prepared in Poly(Acrylic Acid)-lined Channels of Diblock Copolymer Microspheres. Nano Lett. 2001, 1, 683−687. (27) Jeon, S.-J.; Yi, G.-R.; Yang, S.-M. Cooperative Assembly of Block Copolymers with Deformable Interfaces: Toward Nanostructured Particles. Adv. Mater. 2008, 20, 4103−4108. (28) Xu, J.; Wang, K.; Li, J.; Zhou, H.; Xie, X.; Zhu, J. ABC Triblock Copolymer Particles with Tunable Shape and Internal Structure through 3D Confined Assembly. Macromolecules 2015, 48, 2628− 2636. (29) Yu, B.; Li, B.; Jin, Q.; Ding, D.; Shi, A.-C. Confined SelfAssembly of Cylinder-Forming Diblock Copolymers: Effects of Confining Geometries. Soft Matter 2011, 7, 10227−10240. (30) Yang, R.; Li, B.; Shi, A.-C. Phase Behavior of Binary Blends of Diblock Copolymer/Homopolymer Confined in Spherical Nanopores. Langmuir 2012, 28, 1569−1578. (31) Li, S.; Jiang, Y.; Chen, J. Z. Morphologies and Phase Diagrams of ABC Star Triblock Copolymers Confined in a Spherical Cavity. Soft Matter 2013, 9, 4843−4854. (32) Jiang, W.; Lang, W.; Li, S.; Wang, X. Morphologies of CoreShell-Cylinder-Forming ABC Star Triblock Copolymers in Nanopores. Chin. J. Chem. Phys. 2014, 27, 337−342. (33) Brandrup, J., Immergut, E. H., Grulke, E. A., Eds. Polymer Handbook; Wiley: New York, 1999; Section VII, pp 675−714. (34) Zhang, L.; Yu, K.; Eisenberg, A. Ion-Induced Morphological Changes in “Crew-Cut” Aggregates of Amphiphilic Block Copolymers. Science 1996, 272, 1777−1779. (35) Moffitt, M.; Eisenberg, A. Scaling Relations and Size Control of Block Ionomer Microreactors Containing Different Metal Ions. Macromolecules 1997, 30, 4363−4373. (36) Ikkala, O.; ten Brinke, G. Functional Materials Based on SelfAssembly of Polymeric Supramolecules. Science 2002, 295, 2407− 2409.

a new opportunity to control the shape and internal structure of complicated polymer particles.

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AUTHOR INFORMATION

Corresponding Author

*(J.Z.) E-mail: [email protected]. Phone: 86-2787793240. Fax: 86-27-87543632. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work is funded by the National Basic Research Program of China (973) (2012CB821500), the National Natural Science Foundation of China (91127046), and the China Postdoctoral Science Foundation (2014M552034). We thank the HUST Analytical and Testing Center for allowing us to use its facilities.

(1) Yabu, H.; Higuchi, T.; Jinnai, H. Frustrated Phases: Polymeric Self-Assemblies in a 3D Confinement. Soft Matter 2014, 10, 2919− 2931. (2) Shi, A.-C.; Li, B. Self-assembly of Diblock Copolymers under Confinement. Soft Matter 2013, 9, 1398−1413. (3) Jin, Z.; Fan, H. Self-Assembly of Nanostructured Block Copolymer Nanoparticles. Soft Matter 2014, 10, 9212−9219. (4) Arsenault, A. C.; Rider, D. A.; Tétreault, N.; Chen, J. I.-L.; Coombs, N.; Ozin, G. A.; Manners, I. Block Copolymers under Periodic, Strong Three-Dimensional Confinement. J. Am. Chem. Soc. 2005, 127, 9954−9955. (5) Deng, R.; Liang, F.; Li, W.; Yang, Z.; Zhu, J. Reversible Transformation of Nanostructured Polymer Particles. Macromolecules 2013, 46, 7012−7017. (6) He, X. H.; Song, M.; Liang, H. J.; Pan, C. Y. Self-Assembly of the Symmetric Diblock Copolymer in a Confined State: Monte Carlo Simulation. J. Chem. Phys. 2001, 114, 10510−10513. (7) Higuchi, T.; Motoyoshi, K.; Sugimori, H.; Jinnai, H.; Yabu, H.; Shimomura, M. Phase Transition and Phase Transformation in Block Copolymer Nanoparticles. Macromol. Rapid Commun. 2010, 31, 1773−1778. (8) Chi, P.; Wang, Z.; Li, B.; Shi, A.-C. Soft Confinement-Induced Morphologies of Diblock Copolymers. Langmuir 2011, 27, 11683− 11689. (9) Jang, S. G.; Audus, D. J.; Klinger, D.; Krogstad, D. V.; Kim, B. J.; Cameron, A.; Kim, S.-W.; Delaney, K. T.; Hur, S.-M.; Killops, K. L.; Fredrickson, G. H.; Kramer, E. J.; Hawker, C. J. Striped, Ellipsoidal Particles by Controlled Assembly of Diblock Copolymers. J. Am. Chem. Soc. 2013, 135, 6649−6657. (10) Klinger, D.; Wang, C. X.; Connal, L. A.; Audus, D. J.; Jang, S. G.; Kraemer, S.; Killops, K. L.; Fredrickson, G. H.; Kramer, E. J.; Hawker, C. J. A Facile Synthesis of Dynamic, Shape-Changing Polymer Particles. Angew. Chem., Int. Ed. 2014, 53, 7018−7022. (11) Deng, R.; Liu, S.; Liang, F.; Wang, K.; Zhu, J.; Yang, Z. Polymeric Janus Particles with Hierarchical Structures. Macromolecules 2014, 47, 3701−3707. (12) Deng, R.; Liang, F.; Qu, X.; Wang, Q.; Zhu, J.; Yang, Z. Diblock Copolymer Based Janus Nanoparticles. Macromolecules 2015, 48, 750− 755. (13) Yu, B.; Deng, J.; Li, B.; Shi, A.-C. Patchy Nanoparticles SelfAssembled from Linear Triblock Copolymers under Spherical Confinement: a Simulated Annealing Study. Soft Matter 2014, 10, 6831−6843. (14) Deng, R.; Liang, F.; Li, W.; Liu, S.; Liang, R.; Cai, M.; Yang, Z.; Zhu, J. Shaping Functional Nano-Objects by 3D Confined Supramolecular Assembly. Small 2013, 9, 4099−4103. G

DOI: 10.1021/acs.langmuir.5b02843 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir (37) Ruokolainen, J.; Makinen, R.; Torkkeli, M.; Makela, T.; Serimaa, R.; ten Brinke, G.; Ikkala, O. Switching Supramolecular Polymeric Materials with Multiple Length Scales. Science 1998, 280, 557−560. (38) Lin, Y.; Daga, V. K.; Anderson, E. R.; Gido, S. P.; Watkins, J. J. Nanoparticle-Driven Assembly of Block Copolymers: a Simple Route to Ordered Hybrid Materials. J. Am. Chem. Soc. 2011, 133, 6513− 6516. (39) Stoykovich, M. P.; Muller, M.; Kim, S. O.; Solak, H. H.; Edwards, E. W.; de Pablo, J. J.; Nealey, P. F. Directed Assembly of Block Copolymer Blends into Nonregular Device-Oriented Structures. Science 2005, 308, 1442−1446. (40) Tanaka, H.; Hasegawa, H.; Hashimoto, T. Ordered Structure in Mixtures of a Block Copolymer and Homopolymers. 1. Solubilization of Low Molecular Weight Homopolymers. Macromolecules 1991, 24, 240−251. (41) Hashimoto, T.; Tanaka, H.; Hasegawa, H. Ordered Structure in Mixtures of a Block Copolymer and Homopolymers. 2. Effects of Molecular Weights of Homopolymers. Macromolecules 1990, 23, 4378−4386. (42) Matsen, M. W. Phase Behavior of Block Copolymer/ Homopolymer Blends. Macromolecules 1995, 28, 5765−5773. (43) Wang, J.; Li, W.; Zhu, J. Encapsulation of Inorganic Nanoparticles into Block Copolymer Micellar Aggregates: Strategies and Precise Localization of Nanoparticles. Polymer 2014, 55, 1079− 1096. (44) Jeon, S.-J.; Yi, G.-R.; Koo, C. M.; Yang, S.-M. Nanostructures Inside Colloidal Particles of Block Copolymer/Homopolymer Blends. Macromolecules 2007, 40, 8430−8439. (45) Yabu, H.; Sato, S.; Higuchi, T.; Jinnai, H.; Shimomura, M. Creating Suprapolymer Assemblies: Nanowires, Nanorings, and Nanospheres Prepared from Symmetric Block-Copolymers Confined in Spherical Particles. J. Mater. Chem. 2012, 22, 7672−7675. (46) Tanaka, T.; Saito, N.; Okubo, M. Control of Layer Thickness of Onionlike Multilayered Composite Polymer Particles Prepared by the Solvent Evaporation Method. Macromolecules 2009, 42, 7423−7429. (47) Okubo, M.; Saito, N.; Takekoh, R.; Kobayashi, H. Morphology of Polystyrene/Polystyrene-block-Poly(Methyl Methacrylate)/Poly(Methyl Methacrylate) Composite Particles. Polymer 2005, 46, 1151−1156. (48) Yabu, H.; Motoyoshi, K.; Higuchi, T.; Shimomura, M. Hierarchical Structures in AB/AC Type Diblock-Copolymer Blend Particles. Phys. Chem. Chem. Phys. 2010, 12, 11944−11947. (49) Li, W.; Liu, S.; Deng, R.; Zhu, J. Encapsulation of Nanoparticles in Block Copolymer Micellar Aggregates by Directed Supramolecular Assembly. Angew. Chem., Int. Ed. 2011, 50, 5865−5868. (50) Deng, R.; Liu, S.; Li, J.; Liao, Y.; Tao, J.; Zhu, J. Mesoporous Block Copolymer Nanoparticles with Tailored Structures by Hydrogen-Bonding-Assisted Self-Assembly. Adv. Mater. 2012, 24, 1889− 1893. (51) van Zoelen, W.; Asumaa, T.; Ruokolainen, J.; Ikkala, O.; ten Brinke, G. Phase Behavior of Solvent Vapor Annealed Thin Films of PS-b-P4VP(PDP) Supramolecules. Macromolecules 2008, 41, 3199− 3208. (52) Klinger, D.; Robb, M. J.; Spruell, J. M.; Lynd, N. A.; Hawker, C. J.; Connal, L. A. Supramolecular Guests in Solvent Driven Block Copolymer Assembly: from Internally Structured Nanoparticles to Micelles. Polym. Chem. 2013, 4, 5038−5042. (53) Ku, K. H.; Shin, J. M.; Kim, M. P.; Lee, C.-H.; Seo, M.-K.; Yi, G.R.; Jang, S. G.; Kim, B. J. Size-Controlled Nanoparticle-Guided Assembly of Block Copolymers for Convex Lens-Shaped Particles. J. Am. Chem. Soc. 2014, 136, 9982−9989. (54) Mei, S.; Wang, L.; Feng, X.; Jin, Z. Swelling of Block Copolymer Nanoparticles: a Process Combining Deformation and Phase Separation. Langmuir 2013, 29, 4640−4646. (55) Fan, H.; Jin, Z. Selective Swelling of Block Copolymer Nanoparticles: Size, Nanostructure, and Composition. Macromolecules 2014, 47, 2674−2681. (56) Li, L.; Matsunaga, K.; Zhu, J.; Higuchi, T.; Yabu, H.; Shimomura, M.; Jinnai, H.; Hayward, R. C.; Russell, T. P. Solvent-

Driven Evolution of Block Copolymer Morphology under 3D Confinement. Macromolecules 2010, 43, 7807−7812. (57) Moughton, A. O.; Hillmyer, M. A.; Lodge, T. P. Multicompartment Block Polymer Micelles. Macromolecules 2012, 45, 2−19. (58) Wyman, I.; Njikang, G.; Liu, G. When Emulsification Meets Self-Assembly: the Role of Emulsification in Directing Block Copolymer Assembly. Prog. Polym. Sci. 2011, 36, 1152−1183. (59) Tangirala, R.; Revanur, R.; Russell, T. P.; Emrick, T. Sizing Nanoparticle-Covered Droplets by Extrusion through Track-Etch Membranes. Langmuir 2007, 23, 965−969. (60) Li, W.; Jiang, W. Self-Consistent Filed Theory Study on SelfAssembly of Linear ABC Triblock Copolymer in Homopolymer C. Chem. J. Chin. Univ. 2010, 31, 1878−1883. (61) Wang, Y.; Li, F. B. An Emerging Pore-Making Strategy: Confined Swelling-Induced Pore Generation in Block Copolymer Materials. Adv. Mater. 2011, 23, 2134−2148. (62) Wang, Y.; Goesele, U.; Steinhart, M. Mesoporous Block Copolymer Nanorods by Swelling-induced Morphology Reconstruction. Nano Lett. 2008, 8, 3548−3553. (63) Wang, Y.; Goesele, U.; Steinhart, M. Mesoporous Polymer Nanofibers by Infiltration of Block Copolymers with Sacrificial Domains into Porous Alumina. Chem. Mater. 2008, 20, 379−381. (64) Pochan, D. J.; Chen, Z. Y.; Cui, H. G.; Hales, K.; Qi, K.; Wooley, K. L. Toroidal Triblock Copolymer Assemblies. Science 2004, 306, 94−97. (65) Zhu, J. T.; Liao, Y. G.; Jiang, W. Ring-Shaped Morphology of ″Crew-Cut″ Aggregates from ABA Amphiphilic Triblock Copolymer in a Dilute Solution. Langmuir 2004, 20, 3809−3812. (66) Bates, F. S.; Fredrickson, G. H. Block Copolymers − Designer Soft Materials. Phys. Today 1999, 52, 32−38.

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DOI: 10.1021/acs.langmuir.5b02843 Langmuir XXXX, XXX, XXX−XXX