Multicompartment Micelles from Hyperbranched Star-Block

Mar 24, 2007 - These PDMAEMA arms were further extended by a sequential addition of second .... of fluorinated segments, they are apt to aggregate int...
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Langmuir 2007, 23, 5127-5134

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Multicompartment Micelles from Hyperbranched Star-Block Copolymers Containing Polycations and Fluoropolymer Segment Jiang Mao,† Peihong Ni,*,† Yiyong Mai,‡ and Deyue Yan‡ The Key Laboratory of Organic Chemistry of Jiangsu ProVince, College of Chemistry and Chemical Engineering, Soochow UniVersity, Suzhou 215123, China, and College of Chemistry and Chemical Engineering, Shanghai Jiao Tong UniVersity, 800 Dongchuan Road, Shanghai 200240, China ReceiVed December 11, 2006. In Final Form: January 30, 2007 In this Article, we have investigated the self-assembly of a series of amphiphilic hyperbranched star-block copolymers to form multicompartment micelles in acidic aqueous solution (pH 3.0) or in a dimethylformamide/water (pH 3.0) mixture. These hyperbranched star-block copolymers were prepared via oxyanion-initiated polymerization process, using hydroxyl-terminated hyperbranched poly[3-ethyl-3-(hydroxymethyl)oxetane] (HP) as a macroinitiator precursor with multi-reactive sites. It was turned into oxyanion end-capped macroinitiator through the reaction with potassium hydride, and followed by a sequential addition of 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA) and 2,2,3,3,4,4,5,5-octafluoropentyl methacrylate (OFPMA). The resultant HP-star-PDMAEMA-b-POFPMA copolymers were characterized via 1H NMR, 19F NMR, and gel permeation chromatography (GPC). The analyses of transmission electron microscopy (TEM), dynamic light scattering (DLS), and microelectrophoresis confirmed that these copolymers could directly self-organize into supramolecular multicompartment micelles with different diameters, depending on the length of the PDMAEMA segment, which can be protonated in acidic aqueous medium. The measurement of the zeta potential gave further evidence of the aggregating structures for the multicompartment micelles.

* To whom correspondence should be addressed. E-mail: phni@ suda.edu.cn. † Soochow University. ‡ Shanghai Jiao Tong University.

ABCBA pentablock,11 as well as miktoarm star µ-ABC copolymers.4 Various ideal models of multicompartment micelles were proposed by Kubowicz et al.6 and Weberskirch et al.,12 who studied a kind of well-defined polymer, in which a fluorocarbon end group was linked with a hydrophilic poly(Nacrylethyleneimine). Boschet et al.13 also suggested some associative possibility of perfluorooctyl end-functionalized PSb-PEO diblock copolymers. With respect to the visible morphology of multicompartment micelles, Lodge and co-workers4 first observed the phenomenon with cryo-TEM and demonstrated the segregated microdomains of a well-defined miktoarm star µ-ABC copolymer.14 Subsequently, Laschewsky’s group5 proposed a precise morphology of inner core for an ABC triblock copolymer containing a watersoluble segment and two insoluble (hydrocarbon and fluorocarbon) moieties. They concluded that the core was segregated into nanometer-sized compartments in which many small, fluorocarbon-rich domains coexisted with a continuous hydrocarbon-rich region. Most recently, Li et al.15-17 contributed a series of multicompartment micelle morphology diagrams regarding µ-ABC miktoarm star terpolymer systems. We have noted that only the linear block copolymers or miktoarm star terpolymers are available for multicompartment micelles in the literature to date. In fact, many other complex polymers, such as graft or brush,18-23 star copolymers,24-27 dendrimers,28,29 and hyperbranched copolymers,30-36 can form

(1) Sta¨hler, K.; Selb, J.; Candau, F. Langmuir 1999, 15, 7565-7576. (2) Laschewsky, A. Curr. Opin. Colloid Interface Sci. 2003, 8, 274-281. (3) Lutz, J. F.; Laschewsky, A. Macromol. Chem. Phys. 2005, 206, 813-817. (4) Li, Z. B.; Kesselman, E.; Talmon, Y.; Hillmyer, M. A.; Lodge, T. P. Science 2004, 306, 98-101. (5) Kubowicz, S.; Baussard, J. F.; Lutz, J. F.; Thu¨nemann, A. F.; von Berlepsch, H.; Laschewsky, A. Angew. Chem., Int. Ed. 2005, 44, 5262-5265. (6) Kubowicz, S.; Thu¨nemann, A. F.; Weberskirch, R.; Mo¨hwald. H. Langmuir 2005, 21, 7214-7219. (7) Lodge, T. P.; Hillmyer, M. A.; Zhou, Z. L. Macromolecules 2004, 37, 6680-6682. (8) Zhou, Z. L.; Li, Z. B.; Ren, Y.; Hillmyer, M. A.; Lodge, T. P. J. Am. Chem. Soc. 2003, 125, 10182-10183. (9) Choucair, A.; Eisenberg, A. Eur. Phys. J. E 2003, 10, 37-44. (10) Lutz, J. F. Polym. Int. 2006, 55, 979-993.

(11) Thu¨nemann, A. F.; Kubowicz, S.; von Berlepsch, H.; Mo¨hwald, H. Langmuir 2006, 22, 2506-2510. (12) Weberskirch, R.; Preuschen, J.; Spiess, H. W.; Nuyken, O. Macromol. Chem. Phys. 2000, 201, 995-1007. (13) Boschet, F.; Branger, C.; Margaillan, A.; Hogen-Esch, T. E. Polym. Int. 2005, 54, 90-95. (14) Li, Z. B.; Hillmyer, M. A.; Lodge, T. P. Macromolecules 2004, 37, 89338940. (15) Li, Z. B.; Hillmyer, M. A.; Lodge, T. P. Langmuir 2006, 22, 9409-9417. (16) Li, Z. B.; Hillmyer, M. A.; Lodge, T. P. Nano Lett. 2006, 6, 1245-1249. (17) Li, Z. B.; Hillmyer, M. A.; Lodge, T. P. Macromolecules 2006, 39, 765771. (18) Zhang, M. F.; Breiner, T.; Mori, H.; Mu¨ller, A. H. E. Polymer 2003, 44, 1449-1458.

Introduction With the increasing research of self-assembly of block copolymers, the multicompartment micelle is now becoming one of the most attractive subjects in polymer chemistry because this kind of nanoparticle has potential applications in biomedicine, pharmacy, and biotechnology.1-3 From the point of view of the structure, multicompartment micelles have water-soluble shells for stabilizing nanoparticles, and multicompartment hydrophobic cores for accommodating two or more incompatible drugs simultaneously.4-8 It has been realized that the aggregated morphology of the multicompartment micelles may depend on many factors, including the chemical structure of copolymers, the block sequence, the relative lengths of the hydrophobic and hydrophilic blocks, and even the nature of solvents.8-10 Therefore, the combination of one hydrophilic segment and two incompatible hydrophobic segments, for example, distinct hydrocarbon and fluorocarbon blocks, has been considered as a preferable strategy. To design the architecture of polymers, several pioneering works have focused on the fluorocarbon/hydrocarbon surfactants1 and the other block copolymers containing a fluorinated segment and an amphiphilic block copolymer, such as ABC triblock,5,7,8

10.1021/la063576w CCC: $37.00 © 2007 American Chemical Society Published on Web 03/24/2007

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Scheme 1. Representative Reaction Route for the Preparation of the HP-star-PDMAEMA-b-POFPMA Copolymers via Oxyanion-Initiated Polymerization with a Multi-Reactive-Site Macroinitiatora

a

The dimethylamino groups of PDMAEMA can be protonated in acidic medium.

more fascinating structures by their self-assembly. If these species are incorporated with the fluorinated segments, the tailored molecular architectures for multicompartment micelles will be extended to a wide scope. As a kind of complex copolymer, ill-defined hyperbranched copolymers instead of well-defined dendrimers have been studied on the self-assembly in recent years. For example, Yan et al. prepared a series of novel amphiphilic hyperbranched multiarm copolymers, which can self-organize into tubules,30 vesicles,31 or large micelles.32 Similarly, Shi et al. prepared an amphiphilic hyperbranched copolymer, which can also self-assemble into vesicles after cross-linking.33 Except for the morphologies aforementioned, the formation of supramolecular fibers was demonstrated by the self-assembly of hyperbranched copolymers.34-36 In contrast, some amphiphilic hyperbranched copolymers are unimolecular micelles themselves,37-40 which are different from those big micelles self-assembled by a number of copolymers. In the present work, we report the first example of the hyperbranched star-block, fluorine-containing copolymers and (19) Ishizu, K.; Satoh, J.; Sogabe, A. J. Colloid Interface Sci. 2004, 274, 472-479. (20) Hu. D. J.; Cheng, Z. P.; Zhu, J.; Zhu, X. L. Polymer 2005, 46, 75637571. (21) Cheng, Z. P.; Zhu, X. L.; Kang, E. T.; Neoh, K. G. Langmuir 2005, 21, 7180-7185. (22) Cheng, Z. P.; Zhu, X. L.; Fu, G. D.; Kang, E. T.; Neoh, K. G. Macromolecules 2005, 38, 71877192. (23) Ruotsalainen, T.; Turku, J.; Heikkila¨, P.; Ruokolainen, J.; Nyka¨nen, A.; Laitinen, T.; Torkkeli, M.; Serimaa, R.; Brinke, G.; Harlin, A.; Ikkala, O. AdV. Mater. 2005, 17, 1048-1052. (24) Jin, R. H. Macromol. Chem. Phys. 2003, 204, 403-409. (25) Jin, R. H. AdV. Mater. 2002, 14, 889-892. (26) Teng, J.; Zubarev, E. R. J. Am. Chem. Soc. 2003, 125, 11840-11841. (27) Wu, J. H.; Watson, M. D.; Mu¨llen, K. Angew. Chem., Int. Ed. 2003, 42, 5329-5333. (28) Percec, V.; Glodde, M.; Bera, T. K.; Miura, Y.; Shiyanovskaya, I.; Singer, K. D.; Balagurusamy, V. S. K.; Heiney, P. A.; Schnell, I.; Rapp, A.; Spiess, H.-W.; Hudson, S. D.; Duan, H. Nature 2002, 419, 384-387. (29) Wang, B. B.; Zhang, X.; Jia, X. R.; Li, Z. C.; Ji, Y.; Yang, L.; Wei, Y. J. Am. Chem. Soc. 2004, 126, 15180-15194. (30) Yan, D. Y.; Zhou, Y. F.; Hou, J. Science 2004, 303, 65-67. (31) Zhou, Y. F.; Yan, D. Y. Angew. Chem., Int. Ed. 2004, 43, 4896-4899. (32) Mai, Y. Y.; Zhou, Y. F.; Yan, D. Y. Macromolecules 2005, 38, 86798686. (33) Zou, J. H.; Ye, X. D.; Shi, W. F. Macromol. Rapid Commun. 2005, 26, 1741-1745. (34) Ornatska, M.; Peleshanko, S.; Genson, K. L.; Rybak, B.; Bergman, K. N.; Tsukruk, V. V. J. Am. Chem. Soc. 2004, 126, 9675-9684. (35) Ornatska, M.; Bergman, K. N.; Rybak, B.; Peleshanko, S.; Tsukruk, V. V. Angew. Chem., Int. Ed. 2004, 43, 5246-5249. (36) Ornatska, M.; Peleshanko, S.; Rybak, B.; Holzmueller, J.; Tsukruk, V. V. AdV. Mater. 2004, 16, 2206-2212.

their self-assembly for multicompartment micelles in acidic aqueous solution or in a DMF/water (pH 3.0) mixture. These copolymers were prepared by oxyanion-initiated polymerization,41,42 using the potassium alcoholate of hyperbranched poly(3-ethyl-3-(hydroxymethyl)oxatane) (HP) as a macroinitiator with multi-reactive sites. A hydrophilic monomer and an F-monomer, 2-(dimethylamino)ethyl methacrylate (DMAEMA) and 2,2,3,3,4,4,5,5-octafluoropentyl methacrylate (OFPMA), were sequentially copolymerized. Scheme 1 outlines the representative synthesis route and the typical structure of HP-starPDMAEMA-b-POFPMA. Experimental Section Materials. Hyperbranched poly[3-ethyl-3-(hydroxymethyl)oxetane] (HP, M h HP,GPC ) 7300 g mol-1, M h w/M h n ) 1.76) was prepared from 3-ethyl-3-(hydroxymethyl)oxetane, as reported in the literature.43 The average hydroxyl value of the polymer was 57 units mol-1, which was measured by titration method. 2-(N,N-Dimethylamino)ethyl methacrylate (DMAEMA, Wuxi Xinyu Chemical Reagent Co., China) was passed through a basic alumina column to remove the inhibitor and dried over calcium hydride (CaH2), then distilled in vacuum immediately before use. 2,2,3,3,4,4,5,5-Octafluoropentyl methacrylate (OFPMA, TCI) was used as received. Potassium hydride (KH, Aldrich, a 35 wt % dispersion in mineral oil) was washed with tetrahydrofuran (THF) in an inert atmosphere when used. THF was initially dried over potassium hydroxide at least overnight and then refluxed over sodium wire for 3 days before use. Other reagents were purchased from Shanghai Chemical Reagent Co. and used as received. All polymerizations were carried out under a dry argon atmosphere. Synthesis of Hyperbranched Star-Block Copolymers. HP-starPDMAEMA-b-POFPMA copolymers (denoted HDF) were prepared by oxyanion-initiated polymerization. All glassware was dried at 120 °C for 12 h and flamed in a vacuum to eliminate moisture before use. A suspension of KH in mineral oil was first introduced in a dry preweighted 100 mL round-bottom flask with a rubber septum and a magnetic bar. The purified KH powder was obtained by washing three times with anhydrous THF to remove the mineral oil. The (37) Newkome, G. R.; Moorefield, C. N.; Baker, G. R.; Saunders, M. J.; Grossman, S. H. Angew. Chem., Int. Ed. Engl. 1991, 30, 1178-1180. (38) Kim, Y. H.; Webster, O. W. J. Am. Chem. Soc. 1990, 112, 4592-4593. (39) Mekelburger, H.-B.; Jaworek, W.; Vo¨gtle, F. Angew. Chem., Int. Ed. Engl. 1992, 31, 15711576. (40) Xu, J.; Luo, S. Z.; Shi, W. F.; Liu, S. Y. Langmuir 2006, 22, 989-997. (41) Nagasaki, Y.; Sato, Y.; Kato, M. Macromol. Rapid Commun. 1997, 18, 827-835. (42) Zhao, Q.; Ni, P. H. Polymer 2005, 46, 3141-3148. (43) Mai, Y. Y.; Zhou, Y. F.; Yan, D. Y.; Lu, H. W. Macromolecules 2003, 36, 9667-9669.

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Figure 1. 1H NMR spectra of (A) the HP homopolymer, (B) the HP-star-PDMAEMA15 copolymer, and (C) the HP-star-PDMAEMA15b-POFPMA10 copolymer. flask was weighted to determine the amount of KH, and then 25 mL of THF was added into the flask. A definite amount of hyperbranched poly[3-ethyl-3-(hydroxymethyl)oxetane] (-OH molar amount equivalent to that of KH) was dissolved in dried 20 mL of THF in another flask and injected into the flask containing KH. The mixed solution was stirred at 0 °C for 1 h and continued at 30 °C in a water bath for next 0.5 h. A required amount of DMAEMA monomer was added into the reactor. The reaction was conducted at 30 °C for 1.5 h. Next, the second monomer OFPMA was added into the same flask and the reaction was continued for 1.5 h at 30 °C, before being quenched with methanol. The solvent was removed by rotary vacuum. The product was purified by three times precipitation in cold n-hexane for removal of unreacted monomers. Finally, the samples were dried in a vacuum oven at 40 °C for 3 days. The overall conversions of monomers were more than 90%. HP-star-PDMAEMA (denoted HD) was prepared by the same method without OFPMA before being quenched by methanol. The accurate recipes are listed in Table 1. Characterization. Nuclear Magnetic Resonance (1H NMR and 19F NMR) Spectroscopy. All 1H NMR and 19F NMR spectra were recorded using a 400-MHz NMR instrument (INOVA-400) with CDCl3 as a solvent. The number-average molecular weights (M h n) of HP-star-PDMAEMA and HP-star-PDMAEMA-b-POFPMA copolymers were calculated with the M h HP,GPC value of the HP precursor and the molar fractions of PDMAEMA and POFPMA in the copolymers, by using 1H NMR spectroscopic integration of the

Table 1. Recipes for the Synthesis of HP-star-PDMAEMA and HP-star-PDMAEMA-b-POFPMA Copolymers via Oxyanion-Initiated Polymerization sample ID HD15 HD30 HD15F10 HD30F10 HD60F10

KH HP (mmol) (g)/(-OH mmol) 1.87 2.16 1.92 1.12 0.71

0.2389/1.87 0.2776/2.17 0.2449/1.91 0.1441/1.12 0.0926/0.71

DMAEMA (g)/(mmol)

OFPMA (g)/(mmol)

4.4715/28.48 10.1958/ 64.94 4.5044/28.69 5.7362/19.11 5.2499/33.44 3.3192/11.06 6.6165/42.14 2.0440/6.81

-N(CH3)2 protons (δ ) 2.3 ppm) for PDMAEMA and -CHF2 proton (δ ) 5.9-6.2 ppm) for POFPMA. Gel Permeation Chromatography (GPC). The molecular weight distributions (denoted PDIs) of the dried copolymers were recorded on a Waters 1515 gel permeation chromatography instrument using a PLgel 5.0 µm bead-size guard column (50 × 7.5 mm), followed by two linear PLgel columns (500 Å and Mixed-C) and a differential refractive index detector. The eluent was THF at 30 °C with a flow rate of 1.0 mL min-1. Dynamic Light Scattering (DLS). All micelle solutions were prepared by dispersing the copolymers into diluted hydrochloric acid aqueous solution with pH 3.0. All of the solutions were stirred in sealed vials at room temperature for at least 1 week before measurement. These solutions were passed through 0.45 µm

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Figure 2.

19F

Mao et al.

NMR spectra of (A) the OFPMA monomer and (B) the HP-star-PDMAEMA15-b-POFPMA10 copolymer.

hydrophilic microfilters (Agilent Technologies) in a quartz sample cell. The dynamic light scattering measurements were performed using a high performance particle size HPPS 5001 autosizer (Malvern Instrument, U.K.). The Z-average particle sizes (D h z) and size polydispersity indexes (denoted size PDIs) of micelles were recorded. Transmission Electron Microscopy (TEM). The morphologies of micelles were observed on a TEM instrument (TECNAI G2 20, FEI Co.) at 200 kV. A micelle solution was placed onto 400-mesh carboncoated copper grids, followed by air-drying at room temperature for 1 day before measurement. Microelectrophoresis and Zeta Potential Measurements. The determination of aqueous microelectrophoresis of micelles was carried out in a JS94J microelectrophoresis instrument (Shanghai Zhongchen Co., China). The device used a CCD camera, frame grabber, and software to capture the image of the moving particles. The zeta potential data were directly calculated by the instrument.

Results and Discussion Synthesis of HP-star-PDMAEMA and HP-star-PDMAEMAb-POFPMA Copolymers. An HP macroinitiator was prepared by the process as shown in Scheme 1, in which all hydroxyl groups in hyperbranched poly[3-ethyl-3-(hydroxymethyl)oxetane] reacted completely with potassium hydride and yielded oxyanion-terminated hyperbranched polyether with multi-reactive sites. These oxyanions could polymerize DMAEMA (15 times the hydroxyl values in HP) to form HP-star-PDMAEMA15 copolymer (denoted HD15). If it was not quenched with methanol, the living chain of PDMAEMA could be subsequently used to yield block copolymers by adding the second feed of OFPMA monomer to the living system.44 Controlling the theoretical lengths of PDMAEMA and POFPMA, three samples of HP-starPDMAEAM-b-POFPMA copolymers with different chain lengths of hydrophilic segments (DMAEMA units as 15, 30, and 60) and an identical fluorinated block (10 units of OFPMA in theory) were synthesized and denoted HD15F10, HD30F10, and HD60F10, respectively. (44) Xu, J.; Ni, P. H.; Mao, J. e-Polym. 2006, No. 015.

To confirm the incorporation of hyperbranched polyether with the two kinds of monomers and their chemical structures, we used 1H NMR spectroscopy to characterize the three samples: (A) the original HP, (B) the HP-star-PDMAEMA copolymer, and (C) the HP-star-PDMAEMA-b-POFPMA copolymer. The results are shown in Figure 1. From Figure 1A, the apparent signal at δ ) 1.65 ppm (peak a) can be attributed to the protons of -OH groups at HP periphery. The chemical shifts in the region of 3.4-3.6 ppm are assigned to the methylene protons (-CH2OH) adjacent to peripheral hydroxyl groups in the HP homopolymer. For the HP-star-PDMAEMA copolymer, the characteristic peak corresponding to the protons of original -OH groups disappears completely, as shown in Figure 1B, indicating that the efficient initiation of the oxyanion groups has taken place. In this 1H NMR spectrum, several new peaks for HPstar-PDMAEMA are ascribed to the protons of the PDMAEMA segment, such as the six aliphatic protons of the dimethylamino group -N(CH3)2 at δ ) 2.3 ppm (peak a), the protons of methylene -CH2Nd linking to nitrogen atom at δ ) 2.6 ppm (peak e), and the protons of methylene (-OCH2-) in the ester group at δ ) 4.1 ppm (peak f). These PDMAEMA arms were further extended by a sequential addition of second monomer (OFPMA) and yielded an HP-star-PDMAEMA-b-POFPMA copolymer. A signal of the characteristic proton of -CHF2 for the POFPMA part is visible in the region of 5.9-6.2 ppm, as shown in Figure 1C. The signals of methylene protons (-OCH2CF2-) at δ ) 4.4 ppm (peak k) are due to the influence of ester group and difluoromethylene (-CF2-). To better characterize the structure of the HP-star-PDMAEMAb-POFPMA copolymer, 19F NMR was used to determine the fluorocarbon moiety in the copolymer, as shown in Figure 2. Comparing the 19F NMR spectrum of the HP-star-PDMAEMA15b-POFPMA10 copolymer with that of the OFPMA monomer, we can find that the chemical shifts of the three peaks (b, c, d) for the HD15F10 copolymer are in good agreement with those for the OFPMA monomer, except for peak a, which shifted from the original δ ) -41.8 ppm of the monomer to the current δ )

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Table 2. Molecular Parameters of the HP-star-PDMAEMA and HP-star-PDMAEMA-b-POFPMA Copolymers Used in This Study M h n of HP-star-PDMAEMA theor. actuala

sample ID

chemical structure (theor.)

HD15 HD30 HD15F10 HD30F10 HD60F10

HP-star-PDMAEMA15 HP-star-PDMAEMA30 HP-star-PDMAEMA15-b-POFPMA10 HP-star-PDMAEMA30-b-POFPMA10 HP-star-PDMAEMA60-b-POFPMA10

a

140 260 274 500 140 260 274 500 542 970

165 910 262 900 209 310 292 760 394 200

M h n of HP-star-PDMAEMA-b-POFPMA theor. actuala

311 260 445 500 713 970

348 360 441 040 536 260

M h w/M h nb 1.60 1.65 1.65 1.62 1.68

Calculated by 1H NMR spectra with CDCl3 as the solvent. b Measured by GPC with THF as the eluent.

-44.6 ppm of the HD15F10 copolymer, as shown in Figure 2B. This is due to the disappearance of the CdC double bonds in the OFPMA monomer and the formation of the new copolymer. These 1H NMR and 19F NMR results confirm that the PDMAEMA arms have been further extended by a sequential addition of the second monomer OFPMA, and the successful preparation of the HP-star-PDMAEMA-b-POFPMA copolymers has been achieved. The molecular weights of the HP-star-PDMAEMA and HPstar-PDMAEMA-b-POFPMA copolymers can be calculated by 1H NMR analyses. Assuming that each oxyanion in the hyperbranched polyether polymerizes DMAEMA monomer and the resultant chains are extended by the polymerization of OFPMA, the theoretical molecular weight (M h n,th) of the HPstar-PDMAEMA-b-POFPMA copolymer can be calculated by the initial molar ratios of the macroinitiator (based on the average hydroxyl value 57 units mol-1) to the two kinds of monomers as follows:

M h n,th ) M h HP,GPC +

[M1]0 [OH value]

× MDMAEMA + [M2]0 [OH value]

× MOFPMA (1)

where M h HP,GPC is the GPC-determined HP molecular mass (7300 g mol-1); [M1]0 and [M2]0 represent the initial molar concentrations of DMAEMA and OFPMA monomers in the feed; and MDMAEMA and MOFPMA represent the two monomer molecular weights of DMAEMA and ODPMA, respectively. The actual number average-molecular weights (M h n,NMR) of the copolymers can be calculated on the basis of the 1H NMR integration ratios of PDMAEMA to HP and POFPMA to HP as follows:

h HP,GPC + M h n,NMR ) M

AN(CH3)2 AOCH2

× DPHP × MDMAEMA + ACHF2 AOCH2

× DPHP × MOFPMA (2)

where DPHP is the average degree of polymerization of the hyperbranched poly[3-ethyl-3-(hydroxymethyl)oxetane] (HP). According to Figure 1C, the corresponding integration is denoted by AOCH2 for HP at δ ) 3.4-3.6 ppm, AN(CH3)2 for PDMAEMA at δ ) 2.3 ppm, and ACHF2 for POFPMA at δ ) 5.9-6.2 ppm, respectively. Table 2 summarizes the compositions, molecular weights, and molecular weight distributions of the HP-star-PDMAEMA and HP-star-PDMAEMA-b-POFPMA copolymers. From the GPC curves, as shown in Figure 3, one can observe the obvious evolution from the hyperbranched core to the second block PDMAEMA and finally third POFPMA block. Considering the facts that GPC analysis uses linear polystyrene as standard and the HP-star-PDMAEMA-b-POFPMA copolymers have hyperbranched star structures, the M h n measured by GPC for hyper-

Figure 3. GPC curves of the hyperbrached homopolymer (HP), the HP-star-PDMAEMA copolymer, and the HP-star-PDMAEMA-bPOFPMA copolymer.

branched star-block copolymers would underestimate the real molecular weight. However, the GPC-determined molecular weight distributions of the copolymers can be used as the reference. Self-Assembly of HP-star-PDMAEMA and HP-star-PDMAEMA-b-POFPMA in Acidic Aqueous Solution. We first investigated the self-assembly of the HD15 multiarm star copolymer and the HD15F10 star-block copolymer in diluted hydrochloric acid aqueous solution (pH 3.0). Figure 4 shows the TEM images of the micelles obtained by the self-assembly of the HD15 and HD15F10 copolymers. The protonated PDMAEMA block allows the amphiphilic copolymer to self-organize into micelles. From Figure 4a and b, one can find that the HD15 multiarm star copolymer has a good self-assembled morphology with core-shell structure, while the TEM image of HD15F10 micelles in Figure 4c reveals an obvious difference in structure, that is, multicompartment micelles. This can be ascribed to the existence of the fluorinated chains in the arms. Because of the lipophobic character of fluorinated segments, they are apt to aggregate into the core from the periphery and form a subdivided solvophobic core with the polyether segment. As a result, some small dark domains distribute inside the micelles or on the surface. The contrast of the objects is attributed to the presence of electronrich fluorine atoms in the core of the micelles. When the PDMAEMA block length was increased to 30 units, we found the different morphologies between fluorinated and non-fluorinated star copolymers. For HD30, the micelle surface looked not round due to the hydrophilic property of PDMAEMA, as shown in Figure 5a and b. Yet after being linked with POFPMA block, the micelles of HD30F10 changed into the obvious multicompartment micelles with small dark spots on the surface of the micelles, as shown in Figure 5c. Some peripheric fluorinated chains may still locate in the surface of micelles and associate weakly with the other chain-ends containing fluorinated segments, as reported by Weberskirch et al.12 Figure 6 presents a typical 1H NMR spectrum of partially associated micelles, which selfassembled by HP-star-PDMAEMA30-b-POFPMA10 copolymer. The sample was prepared by dissolving HD30F10 in deuteron water (D2O) and adjusting the pH value with deuteron hydrochloric acid (DCl). As compared to the original position in Figure

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Figure 4. TEM images of (a) micelles of HD15, bar ) 0.5 µm, (b) the high-magnification image of (a), and (c) multicompartment micelles by the self-assembly of HD15F10 hyperbranched star-block copolymer. The concentrations of samples were 0.5 wt % aqueous solutions (pH 3.0).

Figure 5. TEM images of micelles of (a) the HD30 copolymer, (b) the high-magnification image of (a), and (c) the HD30F10 hyperbranched star-block copolymer. The concentrations of samples were 0.5 wt % aqueous solutions (pH 3.0).

Figure 6. 1H NMR spectrum of the partially associated micelles self-assembled by the HP-star-PDMAEMA30-b-POFPMA10 copolymer in D2O (pH 3.0).

1C (peaks b, k, and m), the disappearance of the corresponding proton signals of the HP and POFPMA confirms that the hydrophobic HP and POFPMA block have aggregated into the cores, while the hydrophilic PDMAEMA block acts as the coronas in acidic aqueous solution. In our case, the fluorinated block and the hydrophobic polyether segment are separated by the protonated PDMAEMA block. This linking sequence of the hyperbranched polyether with the PDMAEMA-b-POFPMA diblock copolymer here is unlike that of the ABC triblock copolymers used by Laschewsky et al.5 In their work, the hydrophobicity increased along the direction of the linking sequence of hydrophilic-block-hydrophobic-blockfluorophilic parts. It is noted that the micelle particle sizes obtained from HD15F10 and HD30F10 were around 200-100 nm. Moreover, the micelles sizes decreased with the increasing chain length of the PDMAEMA block. This result deviated from our initial assumption: that is, such amphiphilic hyperbranched copolymers would behave as the unimolecular micelles in acidic medium because the electrostatic stability of protonated PDMAEMA chains might

result in the repulsion of macromolecules. However, unimolecular micelles have only a size scale in the range 1-10 nm according to the literature reported by Newkome et al.37 So, an interesting issue is what factor leads to such large micelles? We propose a schematic illustration to describe the selfassembly processes for the formation of supramolecular multicompartment micelles, as shown in Scheme 2. Although there was electrostatic stability due to the shorter hydrophilic protonated PDMAEMA chain, the electrostatic repulsion between macromolecules was not strong enough to overcome the affinity of the polar polyether segment. Therefore, the hydrophobic segments were prone to aggregate together to reduce the interface with water, which minimized the surface tension and interaction with environment. As a result, the unimolecular micelles would selforganize into the larger multicompartment micelles like those micelle particles shown in Figure 4c. In this case, POFPMA blocks would partially fold back to the hyperbranched polyether cores and form more separating compartments in the cores of the micelles. The aggregation of the peripheral fluorocarbon chain of the HP-star-PDMAEMA-b-POFPMA copolymer would

Multicompartment Micelles from Star-Block Copolymers

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Scheme 2. Schematic Representative Structures of the Unimolecular Micelles and the Multicompartment Micelles Obtained by the Self-Assembly of Star-Block Copolymers with Different PDMAEMA Chain Lengths in Acidic Aqueous Solution (pH 3.0)

result in the slight association between the different micelles, just as the model in Scheme 2a. To confirm the aforementioned hypothesis, we extended the PDMAEMA chain length to 60 units in arms and synthesized another copolymer HD60F10. The TEM images are shown in Figure 7. The micelle size did not grow bigger with the increasing arm lengths, but reduced to about 50-60 nm. This result is consistent with our assumption. The longer the protonated PDMAEMA chains are charged, the stronger electrostatic stability occurs. Because the HD60F10 copolymer had the longer hydrophilic segments in the arms, the stronger electrostatic stability made the hydrophobic segments hard to pack together. Consequently, only small amounts of unimolecular micelles aggregated into the smaller micelles. Self-Assembly of the HP-star-PDMAEMA-b-POFPMA Copolymers in a DMF/Water (pH 3.0) Mixture. It is well known that many factors affect the morphology of micelles from self-assembly of amphiphilic copolymers.9,10 We have investigated the morphology of HD30F10 in a DMF/water (pH 3.0) mixture (v/v ) 1:2). The mixed solvent is not a good solvent for both the HP segment and the POFPMA block. As shown in Figure 8, core-corona spherical or elliptic micelles with 100200 nm size are observed for the HD30F10 copolymer. Some dark small domains distribute inside the micelles or on their surface.

Figure 7. TEM images of (a) the micelles of the HD60F10 copolymer and (b) the high-magnification image of (a), bar ) 20 nm. The concentration of sample was 0.5 wt % aqueous solution (pH 3.0).

Figure 8. TEM images of multicompartment micelles of (a) HD30F10 in a DMF/water (pH 3.0) mixture (v/v ) 1:2) and (b) highmagnification image of the micelles of (a).

Scheme 3. Representative Model of the Threadlike Nanofibers Self-Assembled from HD60F10 in a DMF/Water (pH 3.0) Mixture (v/v ) 1:2)

They can be considered as the fluorocarbon segments. As indicated by Hillmyer and Lodge et al.,4 the strong contrast in TEM images could be attributed to the presence of electron-rich fluorine atoms. In our case, these fluoropolymers were incompatible with both hyperbranched polyether and hydrophilic PDMAEMA. This result gives further evidence that the multicompartment micelles consist of the HP-fluoropolymer cores and the protonated PDMAEMA coronas. The model of the micelles for this case is illustrated in Scheme 2b. A very interesting phenomenon is that the HD60F10 copolymer formed the long threadlike nanofiber morphology in a DMF/ water (pH 3.0) mixture (v/v ) 1:2), as shown in Figure 9. This result demonstrates that the hyperbranched star-block copolymer with longer hydrophilic block in the arm can self-organize into various morphologies, depending on their chain lengths and the nature of solution, as expounded by Tsukruk and co-workers.34-36 As compared to the morphology self-assembled by HD60F10 in water (pH 3.0), the morphology in mixed solvent is very attractive. Obviously, such change is induced by the nature of the solvent. As Eisenberg and co-workers pointed out, the aggregation morphology is determined primarily by a force balance among three contributions: the core-chain stretching, corona-chain repulsion, and interfacial tension between the core and the outside solution.9 Consequently, the hyperbranched starblock copolymer in the mixed solvent mentioned here will selforganize into the threadlike nanofiber by phase inversion to reduce the contact area with the mixed solvent, which is assumed to result in the aggregation formation with the lowest Gibbs free energy. The plausible process is presented in Scheme 3.

Figure 9. TEM image of the threadlike nanofibers of HD60F10 in a DMF/water (pH 3.0) mixture (v/v ) 1:2). Bar ) 100 nm.

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Figure 10. Optical micrographs of the micelle motion in acidic media (pH 3.0) under the drive of the alternating voltage. The sample was HD15F10. Table 3. Colloidal Characteristics of the Multicompartment Micelles in Acidic Aqueous Solutions (pH 3.0) sample

D h z (nm)

size PDI

zeta potential ξ (mV)

HD15 HD15F10 HD30F10 HD60F10

269 207 115 59

0.435 0.247 0.643 0.736

+42.7 +47.8 +51.5 +44.1

Colloidal Characteristics of the Multicompartment Micelles. To further confirm the various morphologies obtained by the self-assembly of hyperbranched star-block copolymers with different PDMAEMA lengths, we measured the Z-average diameters (D h z) and particle size distributions (size PDIs) of the micelles with a high performance particle size HPPS 5001 (Malvern) autosizer. The concentrations of the micelle solutions were kept at 0.5 wt % aqueous solutions at pH 3.0, corresponding to the samples used for TEM measurements. The zeta potentials (ξ) of these micelles were also determined with a microelectrophoresis instrument. These results are listed in Table 3. For the three samples of the HP-star-PDMAEMA-b-POFPMA copolymers, the micellar diameters decrease gradually with the increase of the PDMAEMA chain lengths, which is in good agreement with the results obtained by TEM measurements. Under the drive of the alternating voltage, the micelle particles bearing positive charges move forth and back, as shown in Figure 10, indicating that these micellar structures possess hydrophobic cores and hydrophilic/charged coronas. In principle, the micelles from HD15 and HD15F10 should have identical ξ values because they have the same PDMAEMA length. However, the experimental data for these two samples were different. The average zeta potential of HD15F10 (ξ ) +47.8 mV) was higher than that of HD15 (ξ ) +42.7 mV). This phenomenon can be explained

by the postulate that the fluorinated segments have a tendency to escape from the water phase to close each other due to their unique hydrophobic and lipophobic characters. This trend would result in the short PDMAEMA chain to spread well into the water phase and allow the tertiary amine groups to be sufficiently protonated. With the increase of the PDMAEMA length, the zeta potential of HD30F10 increased up to +51.5 mV, which was attributed to the higher cationic charge density on the micelle surface than that of HD15F10. Very interestingly, the zeta potential did not continue to augment with the increase of PDMAEMA chains. On the contrary, the zeta potential of HD60F10 (ξ ) +44.1 mV) was lower than those of HD15F10 and HD30F10. The reason might be that the relatively small micelle morphology of HD60F10 contained less aggregating numbers of unimolecular micelles, which would lead to the lower charge density on the micelle surface. These results are also in good agreement with those in the TEM measurements.

Conclusions We have successfully prepared a kind of novel hyperbranched star-block copolymers via oxyanion-initiated polymerization, which is composed of hyperbranched poly[3-ethyl-3-(hydroxymethyl)oxetane] as a core, and 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA) and 2,2,3,3,4,4,5,5-octafluoropentyl methacrylate (OFPMA) as the block arms. These copolymers can directly self-organize into supramolecular multicompartment micelles in acidic aqueous solution. When the DMF/water (pH 3.0) mixture (1:2 v/v) was used, the multicompartment micelles revealed the well-dispersed micelles for the HD30F10 system, while the tenuous nanofibers showed threadlike morphology for the HD60F10 system. The TEM and DLS analyses showed that the micellar sizes decreased gradually with the increase of the PDMAEMA chain lengths. The measurement of zeta potentials confirmed that the charge density of multicompartment micelles decreased with increasing PDMAEMA chain lengths. Acknowledgment. We gratefully acknowledge the financial support from the National Natural Science Foundation of China (20474041 and 50633010), the Key Laboratory of Molecular Engineering of Polymers, Ministry of Education of China, Fudan University, and the Natural Science Foundation of Educational Department of Jiangsu Province (03KJD150188) and the National Basic Research Program (2007CB808000). LA063576W