Article pubs.acs.org/Macromolecules
Tailoring the Multicompartment Nanostructures of FluoroContaining ABC Triblock Terpolymer Assemblies via PolymerizationInduced Self-Assembly Meng Huo,†,‡ Min Zeng,† Dan Li,† Lei Liu,† Yen Wei,*,‡ and Jinying Yuan*,† †
Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education and ‡Key Lab of Bioorganic Phosphorus Chemistry & Chemical Biology of Ministry of Education, Department of Chemistry, Tsinghua University, 100084 Beijing, China S Supporting Information *
ABSTRACT: Polymer self-assembly has been one of the most important strategies for preparation of multicompartment micelles (MCMs). However, the traditional self-assembly techniques are constrained by limited common solvent, complex kinetic factors, low solids content, etc. Polymerizationinduced self-assembly (PISA) is a novel technique for preparation of polymer assemblies at high solids content and has been exploited to produce MCMs. Nevertheless, the morphology evolution of the MCMs obtained through PISA has not yet been well understood. Herein, we study the compartmentalization behaviors of a series of MCMs constituted by poly(N,N-dimethylaminoethyl methacrylate)-b-poly(benzyl methacrylate)-b-poly(2-perfluorohexylethyl methacrylate) (PDMA-b-PBzMA-b-PFHEMA) triblock terpolymers, which were synthesized by seeded reversible addition− fragmentation chain transfer (RAFT) dispersion polymerization of FHEMA using PDMA-b-PBzMA micelles, wormlike micelles, or vesicles as the seeds. Because of the strong incompatibility between PBzMA and PFHEMA, MCMs with abundant compartmentalized nanostructures were produced. Phosphotungstic acid- and RuO4-stained TEM images of these MCMs indicate that their morphologies are controlled by both the DPs of PBzMA and PFHEMA. Our results suggest that PISA could serve as a reliable platform for revealing the compartmentalization behaviors of polymeric assemblies.
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INTRODUCTION Compartmentalization is ubiquitous in nature at different scales and plays important roles in realization of the function of proteins, the selective transport and communication among organelles and cells, etc.1−4 Inspired by these phenomena, Ringsdorf raised the concept of “multicompartment micelles” (MCMs) which refers to the colloids with multiple microphase separated nanodomains.5−7 Since the emergence of this concept, MCMs have drawn much attention and shown intriguing potential in a vast number of research areas, including drug delivery,8−10 cascade reactions,11,12 organic− inorganic hybrid materials,13 and hierarchical nanomaterials.14 To synthesize MCMs, polymer self-assembly in solution has been developed as one of the most significant avenues due to its robustness in functionalization and versatility and tailorability in molecular characteristics.15−17 In principle, the block− block and block−solvent interactions are the essential driving forces for the formation of MCMs.16 Accordingly, their nanostructures could be tuned by the adjustment of the polymer topology and the block−block and block−solvent incompatibility, which are defined by the Flory−Huggins parameter χ. As an example, Lodge and Hillmyer et al. designed a series of fluoro-containing miktoarm star terpolymers to prepare MCMs.18−21 Because of the strong incompatibility of the fluoro-containing block with the other core-forming block, a variety of MCMs were observed, including “hamburger” micelles, segmented worms, and © XXXX American Chemical Society
nanostructured vesicles. Besides miktoarm star terpolymers, fluoro-containing linear triblock terpolymers have also been exploited to fabricate MCMs. By delicately regulating the block−block incompatibility as well as the block sequences, different morphologies, such as raspberry-shaped and “football”-shaped micelles, etc., were obtained.22−30 In addition, the block−solvent incompatibility could also be tuned by variation of the quality of solvents or the solubility of the polymer blocks.31−37 Müller et al. have achieved controlled syntheses of MCMs by stepwise decreasing the degree of freedom of the block copolymers in solution.38 To realize this, a number of polystyrene-b-polybutadiene-b-poly(methyl methacrylate) (PSb-PB-b-PMMA) terpolymers with varying chain lengths for each block were first dissolved into a nonsolvent for the PB blocks, followed by dialysis in a nonsolvent for both PS and PB blocks. Monodisperse MCMs were prepared, whose morphologies were determined by the volume ratio of PS/PB, and the number of compartments was related to the corona volume. Despite these achievements, there are still restraints for the above strategies. For example, the preparation of MCMs usually entails a solvent for all of the constituent blocks, which is difficult, especially for the fluoro-containing terpolymers.39 Besides, the complex kinetic factors make it challenging to Received: July 30, 2017 Revised: September 24, 2017
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DOI: 10.1021/acs.macromol.7b01629 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules control the nanostructure of the MCMs. Furthermore, these traditional self-assembly strategies have been suffering from the low solids content, which largely limits their scale-up. As a result, development of new strategies for MCMs as well as the understanding of their morphology evolution is still needed. Polymerization-induced self-assembly (PISA) is a novel technique for preparation of polymer assemblies by chain extension of solvophilic macroinitiators with solvophobic blocks using “living”/controlled polymerization techniques, among which the most used is thermal-initiated reversible addition−fragmentation chain transfer (RAFT) polymerization.40−45 As the polymerization temperature of thermalinitiated RAFT polymerization is usually higher than the Tg of the solvated core-forming blocks, most PISA systems lead to ergodic polymer assemblies during the polymerization, which is quite different from the traditional self-assembly methods.46 Thus, polymer assemblies, such as spherical micelles, cylindrical micelles, and vesicles, could be facilely prepared in situ with high solids content.47−55 In view of these merits, PISA should be suitable to prepare MCMs.56−59 In fact, Armes et al. successfully prepared framboidal vesicles with multiple nanodomains within the vesicular wall by seeded emulsion polymerization of methyl methacrylate (MMA) or benzyl methacrylate (BzMA) in H2O using poly(glycerol methacrylate)-b-poly(2-hydroxypropyl methacrylate) (PGMA-bPHPMA) vesicles as the seeds.60 The incompatibility of PHPMA with PBzMA or PMMA results in the formation of the nanodomains, whose size increased with the length of the third block. Subsequently, they obtained poly(methacrylic acid)-b-poly(benzyl methacrylate)-b-poly(trifluoroethyl methacrylate) (PMAA-b-PBzMA-b-PTFEMA) triblock terpolymer vesicles by seeded dispersion polymerization of trifluoroethyl methacrylate (TFEMA) in ethanol with PMAA-b-PBzMA as the macro-chain-transfer agent (macro-CTA).61 Nevertheless, the incompatibility between PTFEMA and PBzMA blocks was not sufficient to prepare MCMs with enough extent of compartmentalization as the fluorine content of TFEMA is rather low. Semifluorinated methacrylates with long fluoro side chains would provide enough incompatibility, thus serving as the potential candidates for preparation of MCMs by PISA. Herein, we report the systematic study of the formation of MCMs by PISA of poly(N,N-dimethylaminoethyl methacrylate)-b-poly(benzyl methacrylate)-b-poly(2-perfluorohexylethyl methacrylate) (PDMA-b-PBzMA-b-PFHEMA) triblock terpolymers (Scheme 1). RAFT dispersion polymerization of 2perfluorohexyl ethyl methacrylate (FHEMA) is performed with PDMA-b-PBzMA assemblies as the seeds, producing MCMs with abundant internal compartments (Scheme S1). The nanostructures of these MCMs are controlled by both the chain lengths of PBzMA and PFHEMA, and the evolution of the internal compartments is exhaustively studied by transmission electron microscopy (TEM), which provides an instructive insights into the morphological regulation of MCMs by PISA.
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Scheme 1. Reaction Scheme for the Syntheses of PDMA-bPBzMA-b-PFHEMA Triblock Terpolymer Multicompartment Micelles by Seeded RAFT Dispersion Polymerization of FHEMA with PDMA-b-PBzMA Diblock Copolymer Assemblies as the Seedsa
a
Color scheme: D (red), B (gray), and H (black). D = PDMA, B = PBzMA, H = PFHEMA, FHEMA = 2-(perfluorohexyl)ethyl methacrylate, and the subscripts represent the DPs. Cyanopentanoic acid) dithiobenzoate (CPADB) was synthesized before and used directly.62 Poly(N,N-dimethylaminoethyl methacrylate) (PDMA33) (Mn,NMR = 5.5 kDa, Mn,SEC = 5.3 kDa, Đ = 1.23) macro-CTA was synthesized according to the procedure reported before.55 BzMA and FHEMA were passed through an alumina column to remove the inhibitors. AIBN was purified by recrystallization in ethanol. 1 H NMR spectra were recorded at ambient temperature on a 400 MHz JEOL JNM-EA 400 spectrometer using CDCl3 as the solvent. For the fluoro-containing polymers, CF2ClCFCl2 was added to improve the solubility. The 19F NMR spectrum was recorded on a 564 MHz JEOL JNM 600 spectrometer. THF/CDCl3 (10/1, v/v) was used as the solvent. Size-exclusion chromatography (SEC) of the polymers was performed at 35 °C on a Waters 1515 GPC system consisting of three styragel columns, a Waters 1515 isocratic HPLC pump, and a 2414 refractive index detector. Polystyrene standards with narrow distribution were used for calibration. THF containing 2% of triethylamine was used as the eluent at a flow rate of 1 mL min−1. TEM images were recorded on a 200 kV JEOL JEM-2010 highresolution transmission electron microscope and an 80 kV Hitachi H7650B transmission electron microscope. For TEM sample preparation, 10 μL of diluted dispersion was placed onto the carbon-coated copper grid for 1 min and was blotted up. This process was repeated for three times. To prepare PTA- or RuO4-stained samples, the above copper grids were further stained by PTA solution for 1 min or by RuO4 vapor for 20 min, respectively. All these samples were air-dried at room temperature before observation. SEM images were recorded on a Hitachi SU-8010 field emission scanning electron microscope. The accelerating voltage was set as 5 kV. The hydrodynamic sizes of the block copolymer assemblies were characterized by a Malvern zetasizer Nano ZS90. The samples were illuminated with a 633 nm He−Ne laser, and the scattering light at 90° angle was recorded by an avalanche photodiode detector. Differential scanning calorimetry (DSC) was performed on a TAQ2000 differential scanning calorimeter at a scanning rate of 10 K min−1 from −20 to 120 °C. PBzMA and PFHEMA homopolymers were synthesized for DSC characterization. The detailed synthetic procedures can be found in the Supporting Information. Preparation of D33-Bx (D = PDMA, B = PBzMA, Subscripts Represent the DPs) Assemblies by PISA. D33-Bx diblock copolymeric assemblies were prepared by RAFT alcoholic dispersion polymerization of BzMA with PDMA33 as the macro-chain-transfer agent (macro-CTA). The feed ratio of BzMA/PDMA33 was varied to target polymer assemblies with various morphologies. In a typical formulation targeting D33-B56 assemblies, AIBN (0.036 mmol, 6.0 mg), PDMA33 (1.5 mmol, 825 mg), and BzMA (10.5 mmol, 1.85 g, 1.77
EXPERIMENTAL SECTION
Materials and Instrumentation. 2-(Perfluorohexyl)ethyl methacrylate (FHEMA) (99%), 2,2′-azobis(2-methylpropionitrile) (AIBN) (99%), and ruthenium(VIII) oxide (RuO4) (0.5 wt % stabilized aqueous solution) were purchased from J&K. Benzyl methacrylate (BzMA) (98%) was a product of TCI (Shanghai). Phosphotungstic acid (PTA) (analytical grade) was purchased from Beijing Chemical Co. and was dissolved in water to form 0.5 wt % solution. 4-(4B
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Figure 1. (a−d) Unstained, (e−h) PTA-stained, and (i−l) RuO4-stained TEM images of D33-B56-Hy (y = 20, 42, 73, 161) MCMs. mL) were dissolved into 19.2 mL of ethanol in a 50 mL tube, which was sealed with rubber stopper, purged with Ar for 30 min, and then placed into an oil bath of 70 °C for 12 h. The target solids content after polymerization is 15 wt %. A small portion of the resulting dispersion was extracted by a syringe for monomer conversion measurement. The residual monomers were removed by dialysis against ethanol. After dialysis, a portion of the dispersion was evaporated and vacuum-dried for NMR, SEC, and DSC characterizations. The remaining part was stored for further use. Preparation of D 33 -B x -H y (H = PFHEMA, Subscripts Represent the DPs) Assemblies by PISA. D33-Bx-Hy triblock terpolymer assemblies were prepared by seeded RAFT alcoholic dispersion polymerization of FHEMA at 10 wt % solids using the D33Bx assemblies as the seeds. The ratio of FHEMA/D33-Bx was varied to prepare D33-Bx-Hy assemblies with tunable chain length of PFHEMA. Typically, AIBN (0.25 μmol, 0.4 mg), alcoholic dispersion of D33-B56 (1.0 mL, containing 7.5 μmol of D33-B56 macro-CTA), FHEMA (1.2 mmol, 520 mg, 344 μL), and 4.7 mL of ethanol were mixed in a 25 mL Schlenk tube. After Ar purging for 20 min, the Schlenk tube was immersed in a 70 °C oil bath. After 24 h, the reaction was quenched by liquid nitrogen, and the mixture was dialyzed against ethanol. An aliquot was further purified by evaporation and vacuum-drying for NMR, SEC, and DSC characterizations. The remaining dispersion was sealed in a vial for further characterization.
these diblock copolymeric assemblies were observed using TEM, which reveals spherical micelles for D33-B30 and D33-B56, wormlike micelles for D33-B92, and vesicles for D33-B167 (Figure S3). Preparation and Morphology Evolution of D33-B56-Hy MCMs. To study the morphology evolution of the MCMs, we first examined the seeded RAFT dispersion polymerization of FHEMA using D33-B56 micelles as the seeds. As a typical semifluorinated methacrylate, FHEMA is soluble in ethanol while the corresponding PFHEMA homopolymer is not, which meets the criterion of PISA. Moreover, because of the long fluorinated side chains, PFHEMA possesses both hydrophobic and lipophobic character and is incompatible with most of the lipophilic polymers, including PBzMA. As a result, PFHEMA would be a suitable block for preparation of the MCMs by PISA. We systematically varied the feed ratio of FHEMA/D33B56 to trace the evolution of the compartments within the micellar cores (Table S2). The successful chain extension of D33-B56 with FHEMA was confirmed by 1H and 19F NMR spectra (Figures S1 and S4). However, SEC analyses reveal no meaningful results because of the associative characters of fluoro-containing polymers in the SEC eluent as well as the triphilicity of the D-B-H terpolymers.27,30,39 As a result, the DPs of FHEMA for all these D-B-H terpolymers are calculated according to 1H NMR measurements. DLS demonstrates that the hydrodynamic size (Dh) of the micelles increases from 23 ± 5.3 to 33 ± 8.2 nm with the incorporation of the PFHEMA blocks, which also suggests the successful chain extension of the third block (Figure S5). The Dh further increases steadily as the DP of the PFHEMA block increases from 20 to 161. TEM was used to study the morphologies of the D33-B56-Hy (y = 20, 42, 73, 161) assemblies. As in Figure 1a−d, all of the assemblies are spherical micelles, whose sizes increase with the increasing DP of PFHEMA, which is in accordance with the DLS results. It should be noted that the spherical micelles in Figure 1a−d are
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RESULTS AND DISCUSSION Preparation and Characterization of D33-Bx Assemblies. D33-Bx assemblies with varying morphologies were prepared by RAFT dispersion polymerization of BzMA in ethanol with PDMA33 as the macro-CTA. The feed ratio of BzMA/PDMA33 was varied from 50 to 200, producing D33-Bx (x = 30, 56, 92, 167) diblock copolymers whose actual DPs of PBzMA block are 30, 56, 92, and 167, respectively (Table S1). Both NMR and SEC indicate that the molecular weights of these diblock copolymers increase with the feed ratio, and the dispersities of these D33-Bx (x = 30, 56, 92, 167) copolymers are all lower than 1.20, suggesting controllable RAFT dispersion polymerizations (Figures S1 and S2). The morphologies of C
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that PISA could be a robust and reliable approach to study the MCMs. RuO4 staining provides a direct access to study the microphase separation of these D33-B56-Hy (y = 20, 42, 73, 161) MCMs by selective staining the PBzMA blocks. Figure 1i−l shows the morphologies of the RuO4-stained D33-B56-Hy MCMs, in which the dark areas of the micelles represent the PBzMA domains, while the light areas and the notches represent the PFHEMA domains. PDMA corona is invisible in RuO4-stained TEM images. These TEM images manifest similar morphologies to that in Figure 1e−h. Especially, for the D33-B56-H73 and D33-B56-H161 MCMs in Figures 1k and 1l, the inner compartments of the ribbon-shell micelles are clearly presented. Thus, these RuO4-stained TEM images could provide direct evidence for the microphase segregation within the core of these MCMs (see Supporting Information for additional discussion about the RuO4-stained TEM images). According to the TEM analyses above, we propose the models to delineate these MCMs as shown in Figure 2. To convince that it is the incompatibility between PBzMA and PFHEMA blocks that leads to the formation of the MCMs, we assessed the compatibility of PDMA, PBzMA, and PFHEMA blocks by DSC. As shown in Figure 3, the Tg values
actually the core of the D33-B56-Hy (y = 20, 42, 73, 161) micelles, as the PDMA corona has low electron density and is invisible without staining agent in TEM observation.38 To observe the multicompartmental inner structure of these micelles, we further stained these TEM samples with PTA and RuO4, respectively. Because of the acid−base interaction between PTA and PDMA blocks, PTA solution with proper concentration can be used to selectively stain the PDMA corona, which is chemically bonded with the PBzMA blocks. As a result, TEM images of PTA-stained samples would help to determine the distribution of PBzMA domains on the surface of the micellar core. As shown in Figures 1e−h and 2, the original
Figure 2. High-resolution TEM images and cartoon models of PTAstained D33-B56-Hy (y = 20, 42, 73, 161) MCMs.
spherical micellar cores manifest a variety of surface outlines on their surfaces, confirming the effectiveness of discriminating the inner micellar structures of MCMs by PTA staining. For the PTA-stained MCMs, the dark parts represent the PDMA and PBzMA blocks, while the light parts and the notches compared with the regular sphere in Figure 1a−d represent the PFMA domains. For example, about 1−2 compartments composed of PFHEMA blocks appear in the core of D33-B56-H20 MCMs, and the size of the compartments increases as the chain length of PFHEMA increases to 42. With the further increment of the DP of PFHEMA to 73, the morphology of the MCMs transforms to micelles with spiral outlines on their core. Figure 2c shows the high-resolution TEM image of the D33-B56-H73 MCMs. By comparing the TEM images in Figures 2c and 1g, we speculate the morphology of these micelles to be ribbonshell micelles in which the PBzMA and PFHEMA helical cylinders constitute the micellar core. Eventually, the size of the micellar core enlarges and the number of these helical cylinders increases as the DP of PFHEMA increases to 161. Actually, the morphology evolution of these MCMs from patchy-like micelles to ribbon-shell micelles agrees with Li et al.’s simulation results, in which they simulated the self-assembly of ABC triblock terpolymer in C-selective solvent and found that patchy-like micelles, ribbon-shell micelles emerged in turn with increased volume fraction of A block.63 The coincidence of the computer simulation with our experimental results suggests
Figure 3. DSC characterizations of (a) PDMA33, (b) PBzMA88, (c) PFHEMA80, (d) D33-B56, and (e) D33-B56-H73.
of PDMA, PBzMA, and PFHEMA are 35.4, 56.8, and 36.5 °C, respectively, while only one Tg at 40.0 °C is detected for D33B56, indicating the compatibility of PDMA with PBzMA at bulk state. However, two Tg values are detected for D33-B56-H73 terpolymers, one of which located at 56.1 °C corresponds to the Tg of PBzMA block. The other Tg located at 37.1 °C corresponds to the Tg values of PDMA and PFHEMA blocks, which are too close to be distinguished. By comparing the DSC traces of D33-B56-H73 with its corresponding homopolymers, we confirm that PBzMA and PFHEMA are incompatible in the bulk state. As a result, MCMs with various compartmentalization behaviors in micellar cores could be controlled by the variation of the relative chain lengths of PFHEMA and PBzMA blocks. Preparation and Morphology Evolution of D33-B30-Hy MCMs. Having confirmed the phase-segregated structures of the D33-B56-Hy micelles, we then performed the seeded RAFT dispersion of FHEMA using the D33-B30 micelles as the seeds to ensure the generality of seeded RAFT dispersion polymerization as the approach to MCMs (Figures 4 and 5). By regulating the relative feed ratio of FHEMA/D33-B30, we were D
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Figure 4. (a−e) Unstained, (f−j) PTA-stained, and (k−o) RuO4-stained TEM images of D33-B30-Hy (y = 9, 19, 38, 75, 136) MCMs.
able to prepare D33-B30-Hy (y = 9, 19, 38, 75, 136) assemblies with tunable chain lengths of PFHEMA (Table S3). As is similar to the PISA of D 33 -B 56 -H y terpolymers, the morphologies of the D33-B30-Hy (y = 9, 19, 38, 75, 136) assemblies were confirmed to be spherical micelles, and the size of the micelles increases with the DP of the PFHEMA block (Figure 4a−e and Figure S6). However, TEM images for the PTA-stained D33-B30-Hy (y = 9, 19, 38, 75, 136) micelles manifest different compartmentalization behaviors in the cores with those of D33-B56-Hy MCMs (Figures 4f−j and 5). Most of the assemblies in Figure 4g remain intact spherical cores for the D33-B30-H9 micelles, which we speculate them to be core− shell−corona structure. With the increment of the PFHEMA chain length, the PFHEMA domain protrudes from the inner core and forms a small patch on the surface of the micellar core, followed by the growth of the patch size (Figure 4g,h). Regretfully, more detailed information about the compartmentalization of these MCMs could not be clearly provided because of the limited contrast of the samples. When the number of repeating units of PFHEMA block increased to 75, the morphology of the corresponding MCMs transforms to raspberry micelles (Figure 4i), with many small PFHEMA nanodomains dispersing on the surface of the micellar core (Figure 5d), which is further confirmed by the RuO4-stained TEM images in Figure 4n. More interestingly, as the DP of PFHEMA reaches to 136, the PTA-stained TEM image shows raspberry-shaped MCMs for the D33-B30-H136 micelles (Figure 5e), while the RuO4-stained TEM images in Figure 4o and Figure S7b display several small black spots in a gray spherical matrix, which is diametrically opposed compartmentalization
Figure 5. High-resolution TEM images and cartoon models of PTAstained D33-B30-Hy (y = 9, 19, 38, 75, 136) MCMs.
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Figure 6. (a−d) Unstained, (e−h) PTA-stained, and (i−l) RuO4-stained TEM images of D33-B92-Hy (y = 21, 40, 79, 164) MCMs.
behavior compared with that of D33-B30-H75 MCMs. Since the chain length of D33-B30-H136 is far longer than that of D33-B30H75, PISA of D33-B30-H136 terpolymer generates raspberryshaped MCMs with PBzMA nanodomains dispersing in the PFHEMA continuous phase. Whereas raspberry-like MCMs have been prepared by the nanoprecipitation method, long annealing time was usually needed to acquire the MCMs at equilibrium state.25,27,29 In our experiments, the polymerization temperature is higher than the Tg values of PBzMA and PFHEMA; rearrangement of the segments in the core is thus allowed to reach the energy minimum. Consequently, PISA possesses prominent advantages regarding the morphology regulation and the large-scale preparation. In addition, we notice that Lin et al. simulated the formation of raspberry-like micelles by self-assembly of ABC triblock terpolymers in A-selective solvents using dissipative particle dynamics simulation.64 They found a morphology transition between core−shell−corona micelles and raspberry-like micelles, which is controlled by the chain length of the B block. The decrease in the length of B block favored the transition from core−shell−corona micelles to B-bump-C micelles, which is in good agreement with our experiments. In our experiments, concentric micelles form when the DP of PFHEMA is 9, while raspberry-like micelles with PBzMA bumps on the PFHEMA core form when the DP of PFHEMA is 136. Besides, we successfully prepared raspberry-like micelles with PFHEMA bumps on the PBzMA core for D33-B30-H75, which was not observed in their simulations. Preparation of D33-B92-Hy MCMs Using D33-B92 Wormlike Micelles as the Seeds. D33-B92 wormlike micelles were also exploited as the seeds and macro-CTA for the seeded RAFT dispersion polymerization of FHEMA to explore the possibility of preparing MCMs from wormlike micelles (Figures 6 and 7 and Table S4). As shown in Figure S8, DLS characterization indicates that the spherical equivalent Dh of the original D33-B92 wormlike micelles is about 56 nm, which
Figure 7. High-resolution TEM images and cartoon models of PTAstained D33-B92-Hy (y = 21, 40, 79, 164) MCMs.
decreases to 41 nm for the D33-B92-H21. With the increment of the PFHEMA block, the Dh of the assemblies increases gradually from 41 to 164 nm. The trend for the size variation in DLS characterization could be well explained by TEM, which reveals that linearly connected spherical clusters constitute the major morphology of D33-B92-H21 assemblies (Figure 6a−d). These spherical clusters further crumble to form disperse spherical micelles as the DP of PFHEMA increases to 40 (Figure 6b). On the basis of the strong incompatibility of PFHEMA and PBzMA blocks, we speculate that the short F
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Figure 8. (a−d) Unstained, (e−h) PTA-stained, and (i−l) RuO4-stained TEM images of D33-B167-Hy (y = 18, 36, 67, 138) MCMs. (m−p) Cartoon models for D33-B167-Hy (y = 18, 36, 67, 138) MCMs.
Afterward, the microphase separation in the core of the segmented wormlike micelles strengthens as the DP of PFHEMA further increases, leading to formation of spherical micelles with core−shell−corona structure (Figure 8c,g,k,o). Accordingly, the Dh of the assemblies decreases from 208 to 109 nm as their morphology transforms from vesicles to micelles (Figure S11). Actually, this process is similar to the shape transformation of D33-B92-Hy assemblies upon increment of the chain length of PFHEMA block. For the D33-B167-H138 terpolymers, the corresponding assemblies appear to be vesicles with cavities of about 26 nm in diameter, which is confirmed by the comparison of Figure 8d,l. There are two mechanisms rationalizing the formation of polymer vesicles, one of which involves the growth of small micelles to large micelles and the subsequent chain rearrangement of the large micelles to form the cavity, while the other one includes the sphere-to-worm-tolamella-to-vesicle route.65 Although the most prevailing explanation for the vesicle formation during PISA is the lamella-to-vesicle transition mechanism,46 our finding indicates that the first mechanism above could predominate at some occasions.
PFHEMA blocks are phase-separated with the PBzMA blocks and form spherical core within the wormlike micelles, leading to the rupture of the wormlike micelles and the formation of spherical micelles. The compartmentalization within the micellar cores was clearly imaged by PTA- and RuO4-stained TEM (Figure 6e−l). As shown in Figure 6e, there are 1−2 breaches on each spherical micelles, suggesting the formation of compartments in the core. It is noteworthy that “clover”structured MCMs with three distinct compartments are produced by PISA of the D33-B92-H40 terpolymers. Besides the typical “clover” structure, MCMs with 1−3 compartments are also shown in Figures 6f and 6j due to the various deposition angles of these “clover” structures on the copper grid. One of the most intriguing application of MCMs is to construct hierarchical nanomaterials, which necessitates delicate control of the nanostructure of the constituting subunits.14,38 The successful preparation of “clover”-structured MCMs shows the potential of PISA to precisely control the nanostructure of MCMs in a facile way. On further increasing the DP of PFHEMA to 79, the “clover”-structured MCMs transform to MCMs with 4−7 compartments. When the DP of PFHEMA reaches to 164, the morphology of the MCMs remains almost unchanged, while the size of the micelles and the number of the compartments within the micelles enlarge accordingly. Preparation of D33-B167-Hy Assemblies Using D33-B167 Vesicles as the Seeds. Different from the above morphology evolutions of the MCMs, incorporation of PFHEMA block to the D33-B167 vesicles leads to the formation of partially coalesced wormlike micelles (Table S5 and Figure 8a,m). After staining, spherical protuberances at the end of the branches could be easily discriminated (Figure 8e,i), suggesting that the shape transformation from vesicles to branched worms resulted from the microphase separation between the PFHEMA and the PBzMA blocks. Further, these branched worms degenerate to segmented wormlike micelles, which are confirmed by the PTA- and RuO4-stained TEM images and SEM characterization (Figure 8b,f,j,n and Figures S9 and S10).
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CONCLUSION To conclude, PISA was exploited to study the self-assembly behaviors of fluoro-containing triblock terpolymer assemblies. PDMA-b-PBzMA-b-PFHEMA triblock terpolymers with variable chain lengths of PBzMA and PFHEMA were facilely prepared by seeded RAFT dispersion polymerization of FHEMA with PDMA-b-PBzMA micelles, wormlike micelles, or vesicles as the seeds. Because of the phase segregation between PBzMA and PFHEMA blocks, MCMs with abundant compartmentalized nanostructures were produced, including core−shell−corona micelles, patchy-like micelles, ribbon-shell micelles, raspberry-like micelles, core−shell−corona vesicles, etc. The morphology evolution of these MCMs was controlled by both the chain lengths of PBzMA and PFHEMA blocks, which was clearly presented via both PTA- and RuO4-stained G
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TEM techniques. Hence, PISA could be not only a robust approach to prepare MCMs in large scale but also a reliable platform for understanding their compartmentalization behaviors. Besides FHEMA, a series of semifluorinated (meth)acrylates66 could be used in this strategy, which would greatly promote the understanding of MCMs.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01629. Synthetic procedures for PBzMA and PFHEMA homopolymers, reaction scheme for PDMA-b-PBzMAb-PFHEMA, molecular characterizations for PDMA-bPBzMA and PDMA-b-PBzMA-b-PFHEMA, TEM images of PDMA-b-PBzMA, DLS of PDMA-b-PBzMA and PDMA-b-PBzMA-b-PFHEMA, high-resolution TEM images of D33-B30-H75 and D33-B30-H136 assemblies, SEM images of D33-B167 and D33-B167-Hy (y = 18, 36, 67, 138) assemblies (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (Y.W.). *E-mail:
[email protected] (J.Y.). ORCID
Jinying Yuan: 0000-0002-1667-9252 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The National Natural Science Foundation of China (Project Nos. 21374053 and 51573086) is acknowledged for financial support.
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