Morphology Evolution of Stimuli-Responsive Triblock Copolymer

Jul 9, 2018 - The length of rod micelles exponentially decreased with solvent ... The hybrid assemblies showed acid–base responsibility due to the ...
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Cite This: Langmuir 2018, 34, 8975−8982

Morphology Evolution of Stimuli-Responsive Triblock Copolymer Modulated by Polyoxometalates Junyan Tan, Dandan Chong, Yue Zhou, Rong Wang, Xinhua Wan,* and Jie Zhang* Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China

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S Supporting Information *

ABSTRACT: Polyoxometalate (POM) H3PMo12O40 was coassembled with stimuli-responsive triblock copolymer poly(ethylene oxide)-block-polystyrene-block-poly(2(dimethylamino)ethyl methacrylate) (PEO-b-PS-b-PDMAEMA) by electrostatic interactions. Depending on the POM contents, the hybrid complexes can self-assemble into a series of morphologies: micelles, rods, toroids, and vesicles. Unlike traditional morphology transition of amphiphilic block copolymer derived from a broad range of hydrophobic volume fractions, POM-induced morphology transitions just occurred in a narrow range of volume fractions. The length of rod micelles exponentially decreased with solvent compositions (tetrahydrofuran/H2O). The hybrid assemblies showed acid−base responsibility due to the PDMAEMA block. Rod micelles could further assemble and disassemble reversibly upon adding acid/base. Fluorescent polyoxometalate Na9EuW10O36 was also complexed with PEO-b-PS-b-PDMAEMA to prepare fluorescent vesicles. The vesicles showed off−on switchable fluorescence behavior accompanied with reversible vesicle-to-micelle transformation in response to pH stimuli.

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INTRODUCTION Self-assembly of block copolymers (BCPs) for construction of nanomaterials with diverse nanostructures1−3 and functionalities has had incredible development in the past decades.4−9 Artificial nanoassemblies are promising biomaterials due to their similarity to cells and viral capsids.10,11 However, even though we can control the molecular weight of BCPs via controlled/living polymerization to get desired morphologies,12 the synthesis process is always complicated and timeconsuming. Alternatively, morphologies of BCPs can also easily be changed by tuning pH, solvent composition, and temperature.13−15 Besides, introducing additives, such as ions,16 small organic molecules,17 CO2,18 and homopolymers,19 is also a facile method. Inorganic nanoparticles (NPs), such as Au NPs20 and quantum dots,21 can also modulate morphologies of BCPs. For example, hydrophobic Au NPs can co-assemble with the triblock copolymer poly(γ-benzyl-L-glutamate)-blockpoly(ethylene glycol)-block-poly(γ-benzyl-L-glutamate) and the morphology of the triblock copolymer transfers from ellipse micelles to hybrid vesicles.20 Inorganic nanoparticles can also endow the hybrid assemblies with comprehensive functionalities.22 Polyoxometalates (POMs) are a kind of inorganic nanoclusters with well-defined structure.23 Compared to organic polymers, POMs possess prominent properties in various applications, such as catalysis, fluorescence, photoelectricity, etc.24−26 POMs are usually complexed with BCPs to fabricate desired hybrid nanomaterials with synergistic functionalities, such as stimuli responsibility.27−30 Compatibility is one of the major problems in POM/BCP hybridization because of their © 2018 American Chemical Society

remarkable difference in polarity and solubility. Electrostatic binding is an efficient way to get out of this dilemma.31,32 Various morphologies can be obtained by direct combination of anionic polyoxometalate with cationic polyelectrolytes. Mizuno et al. incorporated PW12O403− into poly(styrene-b-4vinyl-N-methylpyridinium iodide) and obtained hybrid micelles as well as vesicles. 33 Wu’s group premodified [CoW12O40]6− with cationic polystyrene (PS) to get supramolecular star polymer (SSP) first, and this POM-containing SSP could be complexed with polystyrene-block-poly(ethylene oxide) (PS-b-PEO) to get micelles, toroids, or bicontinuous structures by changing its content.34 However, to the best of our knowledge, achieving a series of morphologies in one system by simply changing POM content is still rare. Herein, we prepared the triblock copolymer poly(ethylene oxide)-block-poly(styrene)-block-poly(2-(dimethylamino)ethyl methacrylate) (PEO-b-PS-b-PDMAEMA), which could combine with POM anions through electrostatic interactions. Compared to diblock copolymers, triblock copolymers exhibit more diverse, complex nanostructures and a broader range of applications, due to their asymmetry arising from the additional block.35 The introduction of the middle polystyrene block is expected to adjust the hydrophobic/hydrophilic ratio and to balance the electrostatic interaction to avoid excessive aggregation. The PEO block is the hydrophilic segment to ensure the stability of assemblies in water. After complexation, Received: June 7, 2018 Revised: July 2, 2018 Published: July 9, 2018 8975

DOI: 10.1021/acs.langmuir.8b01908 Langmuir 2018, 34, 8975−8982

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Langmuir

Figure 1. TEM images of O114S146A66/PMo assemblies: (a) r = 10%, micelles; (b) r = 25%, rods; (c) r = 25%, toroids; (d) r = 35%, vesicles; (e) Rh distributions of O114S146A66/PMo complex; (f) ζ-potential of O114S146A66/PMo complexes; (g) illustration of PMo-induced morphology transitions; and (h) the inner structure of O114S146A66/PMo complex. alumina column to remove the inhibitor, dried with anhydrous calcium hydride, and then vacuum-distilled. Methacryloyl chloride, 4dimethylaminopyridine (DMAP), and N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) were purchased from Tokyo Chemical Industry Co. Methacryloyl chloride was redistilled, and DMAP and PMDETA were used without further purification. Tetrahydrofuran (THF) was distilled over sodium. Chromatographic grade dimethylformamide (DMF) was used directly without further purification. Sample Preparation. Synthesis of the Macroinitiator PEO114-Br. PEO114-OH (2.0 g, 0.40 mmol) was purified in 100 mL of toluene, by distillation to remove the water, and 50 mL of anhydrous dichloromethane was added to dissolve poly(ethylene glycol). Then, excess triethylamine (0.44 g, 4.3 mmol) and DMAP (0.036 g, 0.3 mmol) were added, and the mixture was cooled in an ice bath for 15 min. 2-Bromo-2-methylpropionyl bromide (0.68 g, 3 mmol) was slowly injected into the mixture and allowed to react for 24 h, and then the reaction mixture was filtrated. The solution was precipitated in Et2O twice to get a white powder. Yield 99%. Gel permeation chromatography (GPC) shows polydispersity index (PDI) = 1.06, Mn

the Keggin-type H3PMo12O40 could induce PEO114-b-PS146-bPDMAEMA66 (OnSmAx, where O, S, and A represent PEO, PS, and PDMAEMA, respectively, and n, m, and x denote their repeating units, respectively) to form a series of morphologies, including micelles, rods, toroids, and vesicles, by simply tuning its molar ratio. Furthermore, this preparation method was proved to be appropriate for other kinds of POMs. Na9EuW10O36 could be complexed with PEO114-b-PS146-bPDMAEMA66 to form responsive vesicles, which showed off− on switchable fluorescence behavior and morphology transition simultaneously upon acid−base stimulation.

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EXPERIMENTAL SECTION

Materials. H3PMo12O40 was purchased from Sinopharm Chemical Reagent Co. Ltd. Na9EuW10O36·9H2O was synthesized as described by Yamase.36 2-(Dimethylamino)ethyl methacrylate (DMAEMA) and styrene were purchased from J&K Chemical. DMAEMA was distilled under reduced pressure. Styrene was purified through a neutral 8976

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Langmuir = 7200 g/mol. 1H NMR (CDCl3, δ/ppm): 1.94 (s, 6 H; C(CH3)2Br), 3.38 (s, 3 H; OCH3), 3.65 (bs, 456 H; OCH2CH2O of PEO). Synthesis of the Diblock Copolymer PEO114-b-PS146-Br. Styrene (1 g, 9.6 mmol), PEO114-Br (120 mg, 0.024 mmol), PMDETA (7 μL, 0.036 mmol), CuBr (3.5 mg, 0.029 mmol), and anisole (1.5 mL) were added into a glass tube. The solution was degassed by three freeze− thaw cycles, and the glass tube was sealed. Polymerization took place at 90 °C for 27 h, and the reaction was then quenched by immersing in liquid nitrogen and exposed to air. The mixture was passed through a neutral alumina column to remove the copper salt. The solution was precipitated in cold ether three times. White powders were gained after vacuum drying. 1H NMR (CDCl3, δ/ppm): 0.8−2.0 (−CH− CH(Ph)− (main chain)), 3.65 (OCH2CH2O of PEO), 6.18−7.70 (CH of aromatic ring). Synthesis of the Triblock Copolymer PEO114-b-PS146-b-PDMAEMA66. DMAEMA was dried with anhydrous CaH2 and vacuumdistilled. DMAEMA (234 mg, 1.49 mmol), PEO114-b-PS146-Br (150 mg, 0.0074 mmol), PMDETA (2.6 μL, 0.013 mmol), CuBr (1.5 mg, 0.012 mmol), and THF (0.5 mL) were added into a glass tube and degassed by three freeze−thaw cycles, and the glass tube was sealed. The mixture was reacted at 50 °C for 40 min and then quenched by liquid nitrogen and exposed to air. The reaction liquid was washed with ethylenediaminetetraacetic acid water solution to remove the copper catalyst. The organic phase was dried by anhydrous Na2SO4 and precipitated in cold ether three times. A faint yellow flake solid was gained after vacuum drying. GPC shows PDI = 1.10, Mn = 25k. 1 H NMR (CDCl3, δ/ppm): 0.8−2.0 (−CH2−CH− (main chain)), 2.2−2.6 (N−CH3 of DMAEMA), 2.6−3.0 (N−CH2 of DMAEMA), 3.65 (OCH2CH2O of PEO), 4.0−4.4 (OCH2 of DMAEMA), 6.18− 7.70 (CH of aromatic ring). Preparation of Hybrid Assemblies. Triblock copolymer (5 mg) was dissolved in 0.5 mL of THF to get 1 wt % solution and PMo12O403− solution (1 wt % in THF) was added to the polymer solution, which turns the solution yellow. Water was dropwise added into the mixture at a speed of about 0.5 mL/h until the corresponding volume ratio was reached, and 4 mL of water was added suddenly to freeze the aggregates. THF could be entirely removed by volatilizing in air overnight. The preparation of the assemblies of O114S146A66/ EuW10 in DMF/H2O was similar to this protocol. Characterization. Transmission electron microscopy (TEM) observation was conducted on a JEM-2100 transmission electron microscope (JEOL, Japan) operated at 200 kV. A drop of the solutions of assemblies was placed on a copped grid coated with carbon film for 30 s. For dynamic light scattering (DLS) experiment, a vertically polarized, 100 mW solid-state laser (GXC-III, CNI, Changchun, China) operating at 633 nm was used as the light source, and a BI-TurboCo digital correlator (Brookhaven Instruments Corporation) was used to collect and process data. 1H NMR spectrum (400 MHz) was recorded on a Bruker Avance III spectrometer at room temperature in CDCl3. GPC was performed on an apparatus equipped with a Waters 2414 refractive index detector, a Waters 1525 binary HPLC pump, and three Waters Styragel columns with DMF as the eluent (1.0 mL/min). Photoluminescence experiments were performed on an FLS920 steady state and time-resolved fluorescence spectrometer (Edinburgh Instruments Ltd.). ζ-Potential was measured on NanoBrook 90Plus PALS (Brookhaven Instruments Corporation).

micelles were formed (Figure 1a), the average diameter of which was about 20 nm. As r increased from 10 to 25%, the dominating assemblies were rodlike micelles coexisting with a small amount of toroids under TEM observation (Figure 1b). The width of the rods (∼35 nm) was relatively uniform, and the length showed a relatively large distribution ranging from 100 to 600 nm. The average hydrated radius Rh of those assemblies was found to be ∼85 nm by dynamic light scattering (Figure 1e). The toroids have a similar average width of ∼35 nm compared to that of rods (Figure 1c). Toroids were always considered as the kinetic result during the fusion of cylindrical micelles,37 and therefore, the inner structure of toroids and rods may be the same. The intermediate state of formation of toroids was observed by TEM (Figure S1). A knot in the ring suggests that a bended long rod may connect its own head and tail to form a toroid because the closing fusion can reduce the terminal capping interfacial energy in the rod. In the case of r = 35%, the O114S146A66/PMo complex formed vesicles with average diameter of ∼65 nm and wall thickness of ∼20 nm (Figure 1d). Rh of the assemblies was about 120 nm, which was larger than the result of TEM observed at the dry state of the assemblies. In the control experiments, the solution of the triblock copolymer O114S146A66 was still transparent after adding equivalent HCl aqueous solution instead of H3PMo12O40, and there were no apparent assemblies observed in TEM or DLS. Therefore, we can deduce that the existence of PMo anions contributed to the formation of assemblies and the morphology transitions. To demonstrate this speculation, energy dispersive spectroscopy (EDS) was conducted and the results showed that P and Mo existed in the assemblies (Figure S2). In addition, we measured the ζ-potentials of the assemblies and the control group (Figure 1f). The ζ-potential of rods or toroids at r ∼ 25% was +9.95 mV, which is much less than that of the control solution (+27.05 mV). This difference may be helpful to explain the structure of the assemblies of hybrid complexes. First, the cationic protonated PDMAEMA chain was mainly located in the hydrophobic core by electrostatic binding with PMo (Figure 1h), leading to less positive charge distribution in the outer periphery, which caused lower ζ-potential. In addition, the PS block formed the hydrophobic shell and the PEO block formed the corona. This structure model can account for the characteristic size of these kinds of assemblies. When more PMo anions were involved in the core, the PDMAEMA chains also became more protonated, and the PDMAEMA chain become stretched to interact with more PMo anions, so the width of rods increased to 35 nm, which is larger than 20 nm of spherical micelles. As for r ∼ 35%, the PDMAEMA chain may take the interdigitated packing mode in the membrane of the vesicles, instead of the possible bilayer structure in rod micelles, so the wall thickness of vesicles is just 20 nm, which is less than the width of rods. Consequently, it was convinced that PMo anion played a crucial role in the formation of the assemblies. The morphology depends on the PMo contents greatly. As the hydrophobic volume of the PDMAEMA/PMo complex increased with the PMo content, the hydrophilic/hydrophobic volume ratio decreased. The volume of a single H3PMo12O40 molecule is 0.596 nm3,31 which is larger than that of the common organic molecules. The hydrophilic volume fraction Φ at different PMo contents was quantitatively calculated and

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RESULTS AND DISCUSSION PMo-Induced Morphology Transitions of O114S146A66/ PMo Complexes. O114S146A66/PMo complex (PMo represents PMo12O403−) was prepared by the co-solvent-induced method. O114S146A66 and PMo in THF were mixed to form a yellow solution. After water was slowly injected into the mixture, the solution became turbid, which means that coassemblies were formed. TEM was used to observe assemblies of O114S146A66/PMo complexes. When the molar ratio r (H+ of H3PW12O40 to DMAEMA units) was 10%, uniform spherical 8977

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Langmuir are shown in Table 1. The Φ decreased from 14.6 to 12.0% as r increased from 10 to 35%. Such a slight variation of Φ can

volume change was needed for the transition of a similar triblock copolymer PEO-b-PS-b-PDEAEMA.18 Therefore, the self-assembly behavior of POMs/BCP complexes based on electrostatic interactions was dissimilar to that of amphiphilic BCPs. We ascribe this difference to the change of aggregation number Nagg because the increase of Nagg was deemed to induce morphological transition of block copolymers according to the theory of Eisenberg.38 When the amount of PMo increased, more cationic polymer chains were necessary to neutralize the negative charge of PMo; thus, the average number of polymer chains in an aggregate increased dramatically, which caused the morphology transition. Influence of Solvent Composition on Morphologies of Assemblies. During the process of water addition, the change of solvent composition shows different influences on

Table 1. Volume Fractions and Morphologies of Different r’s r (%)

Φ (philic) (%)a

morphology

10 25 35

14.6 12.9 12.0

spheres rods and toroids vesicles

a

Calculated volume fraction of hydrophilic parts (please see the calculation in the Supporting Information).

hardly induce morphology transition of conventional amphiphilic block copolymers empirically. For example, about 10%

Figure 2. TEM images of O114S146A66/PMo (r = 25%) in THF/H2O of (a) 5:2, (b) 5:3, (c) 5:4, (d) 5:5, and (e) 0; (f) TEM image of the intermediate state of fracture of rods; (g) correlation curve of the average length with solvent composition (THF/H2O); and (h) schematic diagram of the morphological response of rod micelles to addition of water (pink: PDMAEMA/PMo; cyan: PS; green: PEO; red sphere: THF; blue sphere: water). 8978

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Figure 3. TEM images of O114S146A66/PMo (r = 25%) in H2O (a) after adding equivalent NaOH: (b) head-to-head connection and (c) side-toside connection of rods; (d) after adding equivalent HCl subsequently; DLS data of O114S146A66/PMo (r = 25%) in H2O; (e) after adding NaOH gradually; and (f) after adding HCl gradually. (g) Schematic diagram of the formation of hierarchical assemblies upon adding base (red: PDMAEMA; blue: PS; green: PEO).

the three blocks (PEO, PS, and PDMAEMA/PMo). First, water is a poor solvent for PS, whereas THF is a good solvent for PS, PEO block, and PMo. On the other hand, the dielectric constant of water is higher than that of THF. As water contents increased, the electrostatic interaction between PDMAEMA and PMo became looser, whereas the hydrophobic interaction between PS blocks increased and thus the two contradictory interplays may tune up the solvent-induced morphological transition. Assemblies in various THF/water volume ratios were observed by TEM. Spherical micelles as well as vesicles were found to be not obviously related to VTHF/Vwater (Figure S3). However, the length of rods exhibited a strong dependence on the solvent composition (Figure 2g), while the width kept

unchanged. When the water content was relatively low, in pure THF or THF/H2O = 5:1, excessively strong electrostatic interaction leads to irregular assemblies because the dielectric constant of the solvent was low (Figure S4). When the water content was increased to THF/H2O = 5:2, microscaled long rod micelles emerged from the huge assemblies (Figure 2a). The length of rod micelles takes on a Gauss distribution with the average length of ∼1200 nm, as estimated by ImageJ software based on 100 micelles (Figure S5a). Rod micelles in other solvent ratios, THF/H2O = 5:3, 5:4, 5:5, and 0 (Figure 2b−e), were also observed with average lengths of ∼600, 490, 390, and 270 nm (Figure S5b−e), respectively. The rod micelles became shorter as the water content increases, and they transformed to straight rods as the water content reached 8979

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Figure 4. TEM images of O114S146A66/Na9EuW10O36 complex in DMF/water solution (a) initially, (b) after adding equivalent amount of NaOH, and (c) after adding equivalent amount of HCl again. Fluorescence spectra of O114S146A66/Na9EuW10O36 complex in DMF/water solution (d) after gradually adding NaOH, (e) after adding HCl subsequently, and (f) after altering NaOH/HCl stimulation successively.

equiv NaOH was added (Figure 3e), and the first peak also slightly increased to 135 nm. Under TEM observation, the initial rods interconnected each other in either head-to-head (Figure 3b) or side-by-side (Figure 3c) ways to form secondary assemblies (Figure 3a). During the addition of NaOH, the inner PDMAEMA/PMo core will be solvated and swollen and some little cracks on the PS shell of rods would be generated. Similar cracks were also observed in PS shell in cryo-TEM previously reported by Eisenberg.40 Therefore, the hydrophobic PS was exposed to the solvent, which is not energy-favorable. To diminish the interfacial energy, they would connect with hydrophobic block chains of other rods in either head-to-head or side-by-side way. The secondary assemblies could also reversibly disassemble into individual rods after adding equivalent HCl, while the primary structure of rod was preserved (Figure 3d,f). The fluorescent polyoxometalate Na9EuW10O36 (EuW10) can be used as a probe molecule to confirm the above assembling mechanism. The photoluminescence of EuW10 was sensitive to its microenvironment, and hydrophobic environment would greatly enhance the fluorescent emission.41 EuW10 was mixed with O114S146A66 in DMF as f = 100% (f denotes the ratio of negative charge of EuW109− to the positive charge of O114S146A66). After adding water to ∼1:4 DMF/H2O, vesicles with diameter of ∼500 nm were formed (Figure 4a). As the strong electrostatic binding of PDMAEMA/EuW10 forms the hydrophobic and dense core, the fluorescence intensity of EuW10 greatly increased. Similarly, these hybrid vesicles were acid−base-responsive. When an equivalent amount of NaOH was added, the vesicles transformed into micelles (Figure 4b), accompanied with fluorescence quenching (Figure 4d). Adding equivalent HCl subsequently induced the micelles to form

100% finally. However, the average width showed no dependence on the solvent composition and kept constant at 35 nm during the whole water titration process. The correlation curve of the average length of rods with the solvent composition THF/H2O was plotted, and it was found that the average length decreased exponentially with the THF/ H2O ratios (Figure 2g). As normal TEM is just able to observe dry state of assemblies, the observed width mainly depends on the length of polymer chain, so the width keeps constant, whereas the changed length was related to the varied intermolecular force between the polymer chains highly depending on solvent compositions. The change of rod length could be explained as follows (Figure 2h): in rod micelles, PDMAEMA/PMo and PEO formed the inner core and corona, respectively, and PS formed the sandwiched shell. Even at the low water content (THF/H2O = 5:2), the mobility of hydrophobic PS chains had been frozen according to Eisenberg.39 As the water content increased, the electrostatic attraction between PDMAEMA and PMo was weakened and thus the PDMAEMA/PMo core was swollen and decreased. The interfacial energy decreased simultaneously, and consequently, the long rod micelles broke into short rods. The intermediate state of fracture (Figure 2f) was captured by TEM, which can support this explanation. Acid−Base Responsiveness of Rods. As the PDMAEMA block is pH-sensitive, we investigated the morphological transformation of hierarchical assemblies in response to pH stimuli. This process was also monitored by DLS. There was only a single peak at about 85 nm initially; after adding 0.5 equiv NaOH, a new and broad peak appeared at nearly 860 nm, indicating the formation of secondary assemblies. Rh of secondary assemblies increased continually to 1300 nm after 1 8980

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Langmuir vesicles again (Figure 4c), and the fluorescence intensity recovered to the previous extent (Figure 4e). Meanwhile, this behavior showed good reversibility (Figure 4f). Therefore, it is convinced that polyoxometalates dissociated with PDMAEMA in basic environment and recovered electrostatic combination in acid environment.

Crystalline-b-Coil Diblock Copolymer. Angew. Chem., Int. Ed. 2016, 55, 10102−10107. (3) Wong, C. K.; Mason, A. F.; Stenzel, M. H.; Thordarson, P. Formation of Non-Spherical Polymersomes Driven by Hydrophobic Directional Aromatic Perylene Interactions. Nat. Commun. 2017, 8, No. 1240. (4) Liu, S.; Zhang, J.; Dong, R.; Gordiichuk, P.; Zhang, T.; Zhuang, X.; Mai, Y.; Liu, F.; Herrmann, A.; Feng, X. Two-Dimensional Mesoscale-Ordered Conducting Polymers. Angew. Chem., Int. Ed. 2016, 55, 12516−12521. (5) Schöbel, J.; Burgard, M.; Hils, C.; Dersch, R.; Dulle, M.; Volk, K.; Karg, M.; Greiner, A.; Schmalz, H. Bottom-Up Meets Top-Down: Patchy Hybrid Nonwovens as an Efficient Catalysis Platform. Angew. Chem., Int. Ed. 2017, 56, 405−408. (6) Mei, S.; Qi, H.; Zhou, T.; Li, C. Y. Precisely Assembled Cyclic Gold Nanoparticle Frames by 2D Polymer Single-Crystal Templating. Angew. Chem., Int. Ed. 2017, 56, 13645−13649. (7) Yang, K.; Liu, Y.; Liu, Y.; Zhang, Q.; Kong, C.; Yi, C.; Zhou, Z.; Wang, Z.; Zhang, G.; Zhang, Y.; et al. Cooperative Assembly of Magneto-Nanovesicles with Tunable Wall Thickness and Permeability for MRI-Guided Drug Delivery. J. Am. Chem. Soc. 2018, 140, 4666− 4677. (8) Deng, Z.; Qian, Y.; Yu, Y.; Liu, G.; Hu, J.; Zhang, G.; Liu, S. Engineering Intracellular Delivery Nanocarriers and Nanoreactors from Oxidation-Responsive Polymersomes via Synchronized Bilayer Cross-Linking and Permeabilizing Inside Live Cells. J. Am. Chem. Soc. 2016, 138, 10452−10466. (9) Gaitzsch, J.; Huang, X.; Voit, B. Engineering Functional Polymer Capsules toward Smart Nanoreactors. Chem. Rev. 2016, 116, 1053− 1093. (10) Antonietti, M.; Förster, S. Vesicles and Liposomes: A SelfAssembly Principle Beyond Lipids. Adv. Mater. 2003, 15, 1323−1333. (11) Discher, D. E.; Ahmed, F. Polymersomes. Annu. Rev. Biomed. Eng. 2006, 8, 323−341. (12) Zhang, L.; Eisenberg, A. Multiple Morphologies and Characteristics of “Crew-Cut” Micelle-like Aggregates of Polystyrene-bPoly(Acrylic Acid) Diblock Copolymers in Aqueous Solutions. J. Am. Chem. Soc. 1996, 118, 3168−3181. (13) Xiao, X.; He, S.; Dan, M.; Huo, F.; Zhang, W. Nanoparticle-toVesicle and Nanoparticle-to-Toroid Transitions of PH-Sensitive ABC Triblock Copolymers by in-to-out Switch. Chem. Commun. 2014, 50, 3969. (14) Bhargava, P.; Zheng, J. X.; Li, P.; Quirk, R. P.; Harris, F. W.; Cheng, S. Z. D. Self-Assembled Polystyrene-Block-Poly(Ethylene Oxide) Micelle Morphologies in Solution. Macromolecules 2006, 39, 4880−4888. (15) Bhargava, P.; Tu, Y.; Zheng, J. X.; Xiong, H.; Quirk, R. P.; Cheng, S. Z. D. Temperature-Induced Reversible Morphological Changes of Polystyrene-Block-Poly(Ethylene Oxide) Micelles in Solution. J. Am. Chem. Soc. 2007, 129, 1113−1121. (16) Knight, A. S.; Larsson, J.; Ren, J. M.; Bou Zerdan, R.; Seguin, S.; Vrahas, R.; Liu, J.; Ren, G.; Hawker, C. J. Control of Amphiphile Self-Assembly via Bioinspired Metal Ion Coordination. J. Am. Chem. Soc. 2018, 140, 1409−1414. (17) Deng, R.; Derry, M. J.; Mable, C. J.; Ning, Y.; Armes, S. P. Using Dynamic Covalent Chemistry To Drive Morphological Transitions: Controlled Release of Encapsulated Nanoparticles from Block Copolymer Vesicles. J. Am. Chem. Soc. 2017, 139, 7616−7623. (18) Yan, Q.; Zhao, Y. CO2 -Stimulated Diversiform Deformations of Polymer Assemblies. J. Am. Chem. Soc. 2013, 135, 16300−16303. (19) Cai, C.; Li, Y.; Lin, J.; Wang, L.; Lin, S.; Wang, X.-S.; Jiang, T. Simulation-Assisted Self-Assembly of Multicomponent Polymers into Hierarchical Assemblies with Varied Morphologies. Angew. Chem., Int. Ed. 2013, 52, 7732−7736. (20) Yang, C.; Li, Q.; Cai, C.; Lin, J. Nanoparticle-Induced Ellipseto-Vesicle Morphology Transition of Rod−Coil−Rod Triblock Copolymer Aggregates. Langmuir 2016, 32, 6917−6927. (21) Liu, W.; Mao, J.; Xue, Y.; Zhao, Z.; Zhang, H.; Ji, X. Nanoparticle Loading Induced Morphological Transitions and Size

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CONCLUSIONS The triblock copolymer PEO-b-PS-b-PDMAEMA was complexed with Keggin polyoxometalate H3PMo12O40 by electrostatic interactions. By changing the contents of POMs, various morphologies, such as micelles, rods, toroids, and vesicles, could be obtained. The transition between those morphologies could take place in a narrow range of hydrophobic volume fractions, which is much more sensitive to that of conventional amphiphilic block copolymers. The length of rod micelles was related to the solvent composition (THF/H2O). In addition, rod micelles could hierarchically assemble and disassemble in response to pH stimuli. Responsive vesicles were also formed by PEO-b-PS-b-PDMAEMA with fluorescent polyoxometalate Na9EuW10O36. Fluorescent vesicles would transfer to micelles and recover to vesicles concomitantly with off−on switchable fluorescence behavior upon pH stimuli. This work provided a new sight into preparing organic/inorganic complexes with controlled morphologies and stimuli responsibility by adjusting electrostatic and hydrophobic interactions synergistically.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b01908. NMR spectra and GPC results of PEO-b-PS-bPDMAEMA; EDS images of the hybrid assemblies; TEM images of the assemblies at low water content and the intermediate state of the formation of toroids; distribution of the length of rods; and calculation methods for hydrophilic volume fraction of the assemblies (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.W.). *E-mail: [email protected] (J.Z.). ORCID

Xinhua Wan: 0000-0003-2851-6650 Jie Zhang: 0000-0002-6509-8614 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (Grant Nos. 51373001 and 51673002). REFERENCES

(1) Lin, Z.; Liu, S.; Mao, W.; Tian, H.; Wang, N.; Zhang, N.; Tian, F.; Han, L.; Feng, X.; Mai, Y. Tunable Self-Assembly of Diblock Copolymers into Colloidal Particles with Triply Periodic Minimal Surfaces. Angew. Chem., Int. Ed. 2017, 56, 7135−7140. (2) Presa-Soto, D.; Carriedo, G. A.; de la Campa, R.; Presa Soto, A. Formation and Reversible Morphological Transition of Bicontinuous Nanospheres and Toroidal Micelles by the Self-Assembly of a 8981

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DOI: 10.1021/acs.langmuir.8b01908 Langmuir 2018, 34, 8975−8982