New Morphologies and Phase Transitions of Rod–Coil Dendritic

Article pubs.acs.org/Macromolecules

New Morphologies and Phase Transitions of Rod−Coil Dendritic− Linear Block Copolymers Depending on Dendron Generation and Preparation Procedure Huanhuan Cai,† Guoliang Jiang,† Chongyi Chen,‡ Zhibo Li,‡ Zhihao Shen,*,† and Xinghe Fan*,† †

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 ‡ Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: Amphiphilic rod−coil dendritic−linear block copolymers PEG(Gm)-b-PMPCS (where m is the number of dendron generation, and m = 1, 2, 3) composed of a semirigid Percec-type dendron with hydrophilic poly(ethylene glycol) (PEG) tails and a rodlike mesogen-jacketed liquid crystalline polymer, poly{2,5-bis[(4′methoxy-phenyl)oxycarbonyl]styrene} (PMPCS), were successfully prepared. The self-assembled structures undergo a transition from vesicles through large compound vesicles (LCVs) to short cylindrical micelles with increasing dendron generation. PEG(G2)-b-PMPCS forms stable LCVs with porous surfaces of a narrow size distribution in a mixed solvent of tetrahydrofuran and water. The formation mechanism of the supramolecular structure with nano- and microsized scales is studied through changing the rate of water addition. It is composed of two steps: morphological transformation and vesicles fusion or differentiation. Vesicles are precursors for LCVs regardless of what the initial morphology is. However, the final LCV structures are different. Slow addition of water produces spherical LCVs, while those formed during fast water addition are irregular (like deformed spherical) LCVs.

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control36 or fixed in the later stage.37 Yan and co-workers reported LCVs from an amphiphilic hyperbranched multiarm copolymer by the “solid hydration” method.38 Lee et al. prepared a similar nanostructure, supramolecular hollow capsules with gated pores from dumbbell-shaped rod amphiphiles.39 No other papers have reported stable LCV aggregates under thermodynamic conditions in aqueous solutions. Herein, we prepare rod−coil PEG(Gm)-b-PMPCS (where m is the number of dendron generation, and m = 1, 2, 3) DLBCPs (Chart 1) that contain a semirigid Percec-type dendron with hydrophilic poly(ethylene glycol) (PEG) tails and poly{2,5bis[(4′-methoxyphenyl)oxycarbonyl]styrene} (PMPCS), which is a rod-like mesogen-jacketed liquid crystalline polymer (MJLCP), and study the effects of the dendron generation and the characteristics of the linear rod block on the selfassembled structures in aqueous solutions. For PEG(G2)-bPMPCS, the final morphologies formed in aqueous solutions and details of the formation process as well as mechanism from intermediate morphologies were explored using two different

INTRODUCTION Morphologies formed from the self-assembly of amphiphilic block copolymers (ABCs) in selective solvents are rich and colorful.1 The conformational asymmetry between rod and coil blocks in rod−coil block copolymers (BCPs) endows rod−coil BCPs with specific self-assembled morphologies in solution.2−4 The introduction of the rigid block5−10 plays an important role during the self-assembling process compared to that of coil− coil BCPs.11,12 In hybrid dendritic−linear block copolymers (DLBCPs),13−21 the dendron generation is also a factor influencing the self-assembly in solution.22−25 Despite tremendous development in recent years, reports on self-assembly of DLBCPs containing rigid linear chains in solution are limited, and uncommon morphologies like microspheres 26 and toroids27 are observed occasionally. The preparation procedure also greatly affects morphologies of the aggregates28,29 as BCPs are composed of repeated solvophobic units in each chain and exchange between these chains in aggregates is quite slow30,31 compared with small surfactants and phospholipids. In some ways, the morphologies are labile.32,33 However, the mechanism of their formations and transformations between each other may be obtained because of the slow chain exchange.34,35 As far as we know, large compound vesicles (LCVs) are mostly obtained under kinetic © 2014 American Chemical Society

Received: October 13, 2013 Revised: December 19, 2013 Published: January 2, 2014 146

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scattering cell; then pure water was added to the cell until the water content reached 50%.

Chart 1. Chemical Structure of PEG(Gm)-b-PMPCS Rod− Coil DLBCPs

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RESULTS AND DISCUSSION All the molecular characteristics of the DLBCPs are included in Table 1. The three DLBCPs were synthesized by the macroinitiator technique and vary in the number of dendron generation. In addition, two reference samples of a similar dendritic−linear architecture with a coil−linear block were prepared specifically for comparison. The molecular weights, polydispersity indexes (PDIs), and compositions of the DLBCPs were determined by gel permeation chromatography (GPC), matrix-assisted laser desorption ionization time-offlight mass spectrometry (MALDI-TOF MS), and 1H NMR. Critical water content (CWC) was obtained from turbidity curves. Weight fraction of PEG (wPEG) increases as the dendron generation increases for DLBCPs, while CWC decreases with increasing dendron generation. Clearly, the typical nonlinear geometry of the DLBCPs plays an important role in the selfassembly behavior. Effect of Dendron Generation on the Self-Assembled Structure. The aggregates solution was prepared by cosolvent dissolution42 and controlled precipitation method. TEM and SEM samples were prepared by drying solutions at ambient temperature, and it was expected that the aggregate structure would be retained in the solid state after solvent evaporation. PEG(G1)-b-PMPCS forms large polydispersed vesicles with diameters of 1−3 μm (Figure 1a,b). PEG(G2)-b-PMPCS forms polydispersed LCVs of as large as 3 μm (Figure 1c and Figure S6). SEM experiments further reveal that porous microstructures with pore sizes of about 20−30 nm distribute regularly on surfaces of the aggregates (Figure 1d). The supramolecular structure combining nano- and microsized scales is similar to but not totally the same as the gated capsules found by Lee et al.39 For PEG(G3)-b-PMPCS, a few loops mixed with a large amount of short cylindrical micelles are observed (Figure 1e,f). As the samples for TEM are all prepared by drying solutions at ambient temperature, whether the self-assembled structures will change when the THF cosolvent is allowed to evaporate is unknown. Cryo-TEM experiments were performed to solve this problem. Figure 2 reveals that PEG(G2)-b-PMPCS forms LCVs of different sizes in the aqueous solution, which proves that evaporation of THF has no effect on the final self-assembled structures and these LCVs do not reorganize or deform in air. Such stability suggests that the LCVs are thermodynamically stable in aqueous solutions. Effect of Rigidity of the Linear Block on the SelfAssembled Structure. LCVs with diameters of 1−3 μm were observed for PEG(G2)-b-PMPCS with a rod-like linear block in aqueous solutions (Figure 3a). However, for the dendritic−coil PEG(G2)-b-PS reference sample (see Table S1 for molecular

preparation procedures. To the best of our knowledge, stable LCVs with porous surfaces in thermodynamic equilibrium assembled in aqueous solutions have never been reported before. They may serve as possible templates to prepare mesoporous materials.

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EXPERIMENTAL SECTION

Materials and Synthesis of Polymers. PEG (molecular weight of 750 g/mol) was dried under vacuum overnight after it was received from commercial sources. The preparation of the dendritic macroinitiator was reported previously,40 and the PMPCS rod block was synthesized according to an established procedure.41 The amphiphilic PEG(Gm)-b-PMPCS DLBCPs used in this work were prepared by nitroxide mediated living radical polymerization (NMRP) in solution at 125 °C for 24 h. Characterization. The turbidity measurements were performed on a PerkinElmer Lambda 35 spectrophotometer at 650 nm where there was no absorption for the DLBCP/terahydrofuran (THF) solutions. Transmission electron microscopy (TEM) micrographs were obtained on a JEM-2100 electron microscope at an acceleration voltage of 200 kV and scanning electron microscopy (SEM) micrographs on a Hitachi S4800 electron microscope at an acceleration voltage of 1 kV. Cryogenic transmission electron microscopy (Cryo-TEM) samples were prepared in a controlled environment vitrification system (CEVS) at room temperature. Solution samples were pipetted onto a lacey support TEM grid, which was held by tweezers. The excess solution was blotted with a piece of filter paper, resulting in the formation of thin films that suspended the mesh holes, and the samples were quickly plunged into a reservoir of liquid ethane (cooled by liquid nitrogen) at its melting temperature. The vitrified samples were then stored in liquid nitrogen until they were transferred to a cryogenic sample holder (Gatan 626) and examined with a JEM2200FS TEM (200 keV) at about −174 °C. Dynamic light scattering (DLS) experiments were performed using a commercialized spectrometer (ALV/DLS/SLS-5022F). A solid-state laser polarized at the vertical direction operating at 632 nm was used as the light source. The viscosity of the THF/water mixture is 1.703, and the refractive index is 1.368. The amphiphilic PEG(G2)-b-PMPCS was dissolved in THF overnight with a concentration of 0.1 wt % to ensure complete dissolution of the polymer. The solution was filtered through nylon filters (Millipore, 0.45 μm) into a dust-free cylindrical light-

Table 1. Molecular Characteristics and CWC Values of All the Polymers sample

Mna (×104 g/mol)

PDI

Mwb (dendritic macroinitiator)

DPPMPCSc

wPEGd (%)

CWCe (%)

PEG(G1)-b-PMPCS PEG(G2)-b-PMPCS PEG(G3)-b-PMPCS

0.68 1.12 1.61

1.21 1.37 1.26

1933 4133 7297

88 108 143

4.04 6.46 9.43

31.7 24.6 22.5

a Number-average molecular weights measured by GPC calibrated with polystyrene standards. bAbsolute molecular weights of the dendritic macroinitiators determined by MALDI-TOF MS. cDegrees of polymerization of the PMPCS block estimated from 1H NMR results and Mw’s of the dendritic macroinitiator. dWeight fraction of PEG. eObtained with an initial concentration of 0.1 wt % from turbidity curves.

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nm and wall thicknesses of about 25 nm were observed (Figure 3b). The molecular architectures are almost the same except that the chemical characteristics of the linear blocks are different. Therefore, the relatively stiff PMPCS block plays an important role in stabilizing the aggregate structure at the high water content even in the aqueous solution. Previously, our group has proven the stretched conformation of MJLCPs in dilute solutions.12 Tang et al. also reported stable microspherical compound micelles self-assembled by a DLBCP of the similar rod−coil molecular characteristics in p-xylene.26 Therefore, the dendron with a semirigid core and the rod-like linear block are responsible for the formation of thermodynamically stable LCVs. When the volume fraction of the PEG block was increased (sample PEG(G2)-b-PS74, Table S1) to ensure that the CWC value was the same with that of PEG(G2)-b-PMPCS, vesicles were observed as well (Figure S5). Effect of Preparation Procedure on the SelfAssembled Structure. Aware of the special surface morphology of the stable LCVs, we studied the formation and evolution process in THF/water mixed solvents via two different preparation procedures, namely, cosolvent dissolutions followed by controlled precipitation or rapid precipitation. In the former procedure, the DLBCP was dissolved in a common solvent for the two blocks followed by addition of water, 1 wt % at every interval, until reaching the final water content, whereas in the latter protocol the required water content was added all at once into the system. The controlled precipitation process was probed by UV−vis and TEM. Figure 4 shows the turbidity curve and aggregated morphologies at the corresponding water contents indicated by the arrows. Cylindrical micelles were observed for the water content in the range of 24.8−27.2% (Figure 4b). When the water content was further increased, small vesicles along with elongated vesicles were detected (Figure 4c). Then they grew into LCVs (Figure 4d) eventually, and structures of the LCVs were retained at high water contents (Figure 4e). In this case, the formation process of LCVs is composed of two steps with the addition of water. First, cylindrical micelles transform into vesicles to decrease the free energy of the system in THF/water mixed solvents. Then, fusion between vesicles occurs to form LCVs. A few loose LCV particles (Figure S6) are observed in some areas, which also proves that LCVs are formed from vesicles and elongated vesicles. Moreover, the inner part is interconnected. DLS experiments were performed to study the final morphologies at high water contents. Two types of species were coexistent in the supernatant (Figure 5), indicated by two peaks: one centered around 79 nm and the other at 650 nm. After the solution was hand-shaken for a minute, the two aggregates disappeared, and instead even smaller aggregates appeared. These smaller aggregates grew quickly into large aggregates with time, and at one moment the smaller aggregated species appeared again. Finally, after 30 min the system returned to the original state before shaking, while the ratio of intensities of peaks for the two aggregates was different due to precipitation. In combination with TEM results, we deduce that the first peak at a small size may be assigned to the vesicular precursors, and the second peak may correspond to LCVs. Although LCVs dissociate after shaking, they quickly form again by adhesion and fusion of vesicular precursors. In addition, the LCV structures retain after dialysis, indicating that they are in thermodynamic equilibrium state in the mixed solvents.

Figure 1. TEM and SEM micrographs of aggregates self-assembled by PEG(G1)-b-PMPCS (a: TEM; b: SEM), PEG(G2)-b-PMPCS (c: TEM; d: SEM), and PEG(G3)-b-PMPCS (e: TEM; f: SEM) formed in aqueous solutions.

Figure 2. Cryo-TEM micrographs of aggregates self-assembled by PEG(G2)-b-PMPCS in aqueous solutions. (a: LCVs of smaller sizes; b: LCVs with larger sizes).

Figure 3. TEM micrographs of aggregates self-assembled by PEG(G2)b-PMPCS (a) and PEG(G2)-b-PS-ref (b) in aqueous solutions.

characteristics and the CWC value) with the identical PEG weight fraction, polydisperse vesicles with diameters of 80−150 148

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mechanism of LCVs. PEG(G2)-b-PMPCS with an initial concentration of 0.1 wt % form spherical micelles with a narrow size distribution when the water content reaches 50% immediately. We expected that the spherical micelles would eventually evolve into LCVs upon annealing for a period of time. The development process was monitored by TEM, with the results shown in Figure 6. As expected, morphological

Figure 6. TEM micrographs of PEG(G2)-b-PMPCS in THF with a water content of 50% at different aging times: 0 days, spherical micelles (a); 18 days, spherical micelles coexistent with differentiated vesicles (b); 77 days, spherical micelles coexistent with LCVs (c, d). The initial concentration is 0.1 wt %.

changes took place. Spherical micelles transformed into small vesicles, which then stuck to each other and fused together. Finally, LCVs were detected. The process is also composed of two steps: morphological transformation and vesicles differentiation. The process continued for several months, and not all of the spherical micelles reached the stable LCVs eventually. Besides, sizes and shapes are strongly dependent on the preparation methods. These LCVs with smaller sizes are deformed and not as regular as those obtained in the controlled precipitation method (Figure 6c,d). The results can be attributed to different mobility of the rod-like PMPCS chains in mixed solvents with different water contents. In the process of slow water addition, PMPCS chains move faster, and they quickly alter the chain conformation in mixed solvents with low water contents to change the self-assembled morphologies, while in the second method, the chains move slower. Nevertheless, it indicates that the mobility of the rod-like PMPCS chains is not truly “frozen” at the water content of 50%, and the chains are still able to stick to each other further. Moreover, the chain dynamics is deeply dependent on the initial concentration. When the initial concentration was decreased to 0.05 wt %, spherical micelles as well as a few small vesicles were observed for PEG(G2)-b-PMPCS at a water content of 50%. After 18 days, almost all the spherical micelles transformed into vesicles (Figure 7), while a few spherical micelles changed to vesicles for the sample with an initial concentration of 0.1 wt %. The chain dynamics is faster in comparison with that of the sample having a higher initial concentration. The evolution of the aggregates in solution was studied further by DLS, with the results shown in Figure 8. At

Figure 4. Turbidity curve (a) monitoring morphological changes that occur during the addition of water and TEM micrographs of selfassembled morphologies (b: short cylindrical micelles; c: vesicles coexistent with elongated vesicles; d, e: LCVs) from PEG(G2)-bPMPCS at the corresponding water contents.

Figure 5. Normalized size distribution of PEG(G2)-b-PMPCS in THF with a water content of 50% at different aging times after shaking. The initial concentration was 0.1 wt %.

Because LCVs are under thermodynamic control at high water contents, the questions are what kind of aggregates will be formed if the solvent quality is immediately changed in the cosolvent dissolution and rapid precipitation method and whether they may provide additional proof for the formation 149

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transition from vesicles through LCVs with porous surfaces to short cylindrical micelles mixed with loops as the dendron generation increases. As far as we know, the effect of dendron generation on the self-assembled structures of DLBCPs with a hydrophilic dendron block in this system is different from previous interpretations. It has been demonstrated for the first time that the thermodynamic stable supramolecular structures of PEG(G2)-b-PMPCS on both nano- and microsized scales form in aqueous solutions. The formation and evolution mechanism of LCVs was explored with different preparation methods. It involves morphological transformation and vesicles fusion or differentiation. Vesicles are precursors of LCVs no matter what the initial morphologies and the preparation pathway are. Such an architecture of DLBCPs can provide access to obtain stable LCVs on both nano- and microsized scales which may serve as templates to prepare potential mesoporous materials.

Figure 7. Morphological changes of 0.05 wt % PEG(G2)-b-PMPCS in a THF/water mixture with a water content of 50% observed by TEM after 0 (a) and 18 days (b).

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ASSOCIATED CONTENT

S Supporting Information *

Molecular characteristics and CWC values of the reference sample PEG(G2)-b-PS and PEG(G2)-b-PMPCS, GPC profiles of the target DLBCPs, 1H NMR of PEG(G2)-TEMPO and PEG(G2)-b-PMPCS, turbidity curves of PEG(G2)-b-PS and PEG(Gm)-b-PMPCS, TEM micrographs of aggregates selfassembled by PEG(G2)-b-PS74 in the aqueous solution, TEM micrographs of PEG(G2)-b-PMPCS aggregates formed in solution at the water content of 50%, and SEM micrograph of PEG(G2)-b-PMPCS aggregates formed in the aqueous solution. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (Z.S.). *E-mail [email protected] (X.F.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (Grants 21174006 and 21134001).

Figure 8. DLS profiles of 0.05 wt % PEG(G2)-b-PMPCS in a THF/ water mixture with a water content of 50% at different aging times (a: 0 days; b: 2 months).

REFERENCES

(1) Mai, Y.; Eisenberg, A. Chem. Soc. Rev. 2012, 41, 5969−5985. (2) Jenekhe, S. A.; Chen, X. L. Science 1998, 279, 1903−1907. (3) Jenekhe, S. A.; Chen, X. L. Science 1999, 283, 372−375. (4) Koh, H.-D.; Changez, M.; Rahman, M. S.; Lee, J.-S. Langmuir 2009, 25, 7188−7192. (5) Pochan, D. J.; Pakstis, L.; Ozbas, B.; Nowak, A. P.; Deming, T. J. Macromolecules 2002, 35, 5358−5360. (6) Halperin, A. Macromolecules 1990, 23, 2724−2731. (7) Wang, X.; Guerin, G.; Wang, H.; Wang, Y.; Manners, I.; Winnik, M. A. Science 2007, 317, 644−647. (8) Palmer, L. C.; Stupp, S. I. Acc. Chem. Res. 2008, 41, 1674−1684. (9) Mabrouk, E.; Cuvelier, D.; Brochard-Wyart, F.; Nassoy, P.; Li, M.-H. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 7294−7298. (10) Jia, L.; Li, M.-H. Liq. Cryst. 2013, 1−17. (11) Lin, W.; Zhang, J.; Wan, X.; Liang, D.; Zhou, Q. Macromolecules 2009, 42, 4090−4098. (12) Tu, Y.; Wan, X.; Zhang, D.; Zhou, Q.; Wu, C. J. Am. Chem. Soc. 2000, 122, 10201−10205. (13) Ge, Z.; Luo, S.; Liu, S. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 1357−1371.

the beginning, the DLS profile shows two peaks at 38 and 179 nm corresponding to the spherical micelles and a few small vesicles, respectively. After annealing for two months, the smaller peak disappears, leaving vesicles alone. The DLS experiments are in agreement with the TEM results. On the basis of the results from the above two methods, the formation mechanism of LCVs can be proposed. It is composed of two steps: morphological transformation and vesicles fusion or differentiation. No matter what the initial morphology and the preparation procedure are, it must go through the vesicular precursor before the formation of LCVs.

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CONCLUSIONS In summary, self-assembly of a series of amphiphilic DLBCPs with a semirigid linear block in solution was investigated. TEM results show that the self-assembled structures undergo a 150

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(14) Guida, V. Adv. Colloid Interface Sci. 2010, 161, 77−88. (15) Gillies, E. R.; Jonsson, T. B.; Fréchet, J. M. J. J. Am. Chem. Soc. 2004, 126, 11936−11943. (16) Chapman, T. M.; Hillyer, G. L.; Mahan, E. J.; Shaffer, K. A. J. Am. Chem. Soc. 1994, 116, 11195−11196. (17) Chang, Y.; Park, C.; Kim, K. T.; Kim, C. Langmuir 2005, 21, 4334−4339. (18) Peng, S.-M.; Chen, Y.; Hua, C.; Dong, C.-M. Macromolecules 2008, 42, 104−113. (19) van Hest, J. C. M.; Delnoye, D. A. P.; Baars, M. W. P. L.; van Genderen, M. H. P.; Meijer, E. W. Science 1995, 268, 1592−1595. (20) Iyer, J.; Fleming, K.; Hammond, P. T. Macromolecules 1998, 31, 8757−8765. (21) Tian, L.; Nguyen, P.; Hammond, P. T. Chem. Commun. 2006, 3489−3491. (22) van Hest, J. C. M.; Delnoye, D. A. P.; Baars, M. W. P. L.; Elissen-Román, C.; van Genderen, M. H. P.; Meijer, E. W. Chem. Eur. J. 1996, 2, 1616−1626. (23) del Barrio, J.; Oriol, L.; Sánchez, C.; Serrano, J. L.; Di Cicco, A.; Keller, P.; Li, M.-H. J. Am. Chem. Soc. 2010, 132, 3762−3769. (24) Shi, Z.; Lu, H.; Chen, Z.; Cheng, R.; Chen, D. Polymer 2012, 53, 359−369. (25) Wurm, F.; Frey, H. Prog. Polym. Sci. 2011, 36, 1−52. (26) Wu, J.; Tang, H.; Wu, P. Soft Matter 2011, 7, 4166−4169. (27) Jeong, M. G.; van Hest, J. C. M.; Kim, K. T. Chem. Commun. 2012, 48, 3590−3592. (28) Hayward, R. C.; Pochan, D. J. Macromolecules 2010, 43, 3577− 3584. (29) Wang, C.-W.; Sinton, D.; Moffitt, M. G. J. Am. Chem. Soc. 2011, 133, 18853−18864. (30) Dormidontova, E. E. Macromolecules 1999, 32, 7630−7644. (31) Lund, R.; Willner, L.; Richter, D.; Dormidontova, E. E. Macromolecules 2006, 39, 4566−4575. (32) Li, Y.; Armes, S. P. Angew. Chem., Int. Ed. 2010, 49, 4042−4046. (33) Sugihara, S.; Blanazs, A.; Armes, S. P.; Ryan, A. J.; Lewis, A. L. J. Am. Chem. Soc. 2011, 133, 15707−15713. (34) Blanazs, A.; Madsen, J.; Battaglia, G.; Ryan, A. J.; Armes, S. P. J. Am. Chem. Soc. 2011, 133, 16581−16587. (35) Lin, W. R.; Zheng, C.; Wan, X. H.; Liang, D. H.; Zhou, Q. F. Macromolecules 2010, 43, 5405−5410. (36) Lin, C.-H.; Chen, W.-C.; Chen, H.-L. Macromol. Chem. Phys. 2008, 209, 2349−2358. (37) Du, J.; Chen, Y. Angew. Chem., Int. Ed. 2004, 43, 5084−5087. (38) Mai, Y. Y.; Zhou, Y. F.; Yan, D. Y. Small 2007, 3, 1170−1173. (39) Kim, J.-K.; Lee, E.; Lim, Y.-b.; Lee, M. Angew. Chem., Int. Ed. 2008, 47, 4662−4666. (40) Cai, H.; Jiang, G.; Shen, Z.; Fan, X. Macromolecules 2012, 45, 6176−6184. (41) Ye, C.; Zhang, H.-L.; Huang, Y.; Chen, E.-Q.; Lu, Y.; Shen, D.; Wan, X.-H.; Shen, Z.; Cheng, S. Z. D.; Zhou, Q.-F. Macromolecules 2004, 37, 7188−7196. (42) Cai, H.; Jiang, G.; Shen, Z.; Fan, X. Soft Matter 2013, 9, 11398− 11404.

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