Novel Fluoroalkyl End-Capped Amphiphilic Diblock Copolymers with

4,4,5,5,6,6,7,7,7-Nonafluoro-1-heptanol (NFHOH, Aldrich) was used as received. ... NFHO-PDMA30-b-PDEA20, 0.1474/3.68, 0.2196/0.79, DMA, 3.7094/23.63, ...
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Langmuir 2008, 24, 4647-4654

4647

Novel Fluoroalkyl End-Capped Amphiphilic Diblock Copolymers with pH/Temperature Response and Self-Assembly Behavior Hu Zhang, Peihong Ni,* Jinlin He, and Cuicui Liu Key Laboratory of Organic Chemistry of Jiangsu ProVince, College of Chemistry and Chemical Engineering, Soochow UniVersity, Suzhou 215123, China ReceiVed December 26, 2007. In Final Form: February 3, 2008 A series of fluoroalkyl end-capped diblock copolymers of poly[2-(N,N-dimethylamino)ethyl methacrylate] (PDMAEMA or PDMA) and poly[2-(N,N-diethylamino)ethyl methacrylate] (PDEAEMA or PDEA) have been synthesized via oxyanion-initiated polymerization, in which a potassium alcoholate of 4,4,5,5,6,6,7,7,7-nonafluoro1-heptanol (NFHOK) was used as an initiator. The chemical structures of the NFHO-PDMA-b-PDEA and NFHOPDEA-b-PDMA depended on the addition sequence of the two monomers and the feeding molar ratios of [DMA] to [DEA] during the polymerization process. These copolymers have been characterized by 1H NMR and 19F NMR spectroscopy and gel permeation chromatography (GPC). The aggregation behavior of these copolymers in aqueous solutions at different pH media was studied using a combination of surface tension, fluorescence probe, and transmission electron microscopy (TEM). Both diblock copolymers exhibited distinct pH/temperature-responsive properties. The critical aggregation concentrations (cacs) of these copolymers have been investigated, and the results showed that these copolymers possess excellent surface activity. Besides, these fluoroalkyl end-capped diblock copolymers showed pH-induced lower critical solution temperatures (LCSTs) in water. TEM analysis indicated that the NFHO-PDMA30b-PDEA10 diblock copolymers can self-assemble into the multicompartment micelles in aqueous solutions under basic conditions, in which the pH value is higher than the pKa values of both PDMA and PDEA homopolymers, while the NFHO-PDEA10-b-PDMA30 diblock copolymers can form flowerlike micelles in basic aqueous solution.

1. Introduction Fluoroalkyl end-capped polymers have received great interest recently in both academic and technological fields because a small amount of fluorinated groups is able to improve surfaceactive properties, molecular aggregation, self-assembling morphology, and even nanobiological activities, which cannot be achieved by the corresponding nonfluorinated, randomly or blocktype fluoroalkylated polymers and low molecular weight fluorinated surfactants.1-4 The incorporation of terminal fluoroalkyl groups, which are more mobile than the internal ones, with amphiphilic copolymers affords copolymers characterized by good solubility, low toxicity, and more economy.5 Moreover, these F-groups can easily aggregate with rather than repel one another in aqueous solution to give rise to self-assembled molecular aggregates with controlled nanometer size.6 Various fluoroalkyl end-capped polymers have been synthesized by a number of polymerization techniques,7 including the use of fluoroalkanoyl peroxide initiators,5,8-11 esterification with per* To whom correspondence should be addressed. E-mail: phni@ suda.edu.cn. (1) Sawada, H. Prog. Polym. Sci. 2007, 32, 509-533, and references therein. (2) Sta¨hler, K.; Selb, J.; Candau, F. Langmuir 1999, 15, 7565-7576. (3) Koberstein, J. T. J. Polym. Sci., Part B: Polym. Phys. 2004, 42, 29422956. (4) Synytska, A.; Appelhans, D.; Wang, Z. G.; Simon, F.; Lehmann, F.; Stamm, M.; Grundke, K. Macromolecules 2007, 40, 297-305. (5) Sawada, H.; Ikeno, K.; Kawase, T. Macromolecules 2002, 35, 43064313. (6) Lodge, T. P.; Rasdal, A.; Li, Z. B.; Hillmyer, M. A. J. Am. Chem. Soc. 2005, 127, 17608-17609. (7) Hansen, N. M. L.; Jankova, K.; Hvilsted, S. Eur. Polym. J. 2007, 43, 255-293. (8) Sawada, H. Chem. ReV. 1996, 96, 1779-1808. (9) Sawada, H.; Shikauchi, Y.; Kakehi, H.; Katoh, Y.; Miura, M. Colloid Polym. Sci. 2007, 285, 499-506. (10) Mugisawa, M.; Ohnishi, K.; Sawada, H. Colloid Polym. Sci. 2007, 285, 737-744. (11) Sawada, H.; Narumi, T.; Kodama, S.; Kamijo, M.; Ebara, R.; Sugiya, M.; Iwasaki, Y. Colloid Polym. Sci. 2007, 285, 977-983.

fluoroacyl chloride,12,13 telomerizations with fluorinated mercaptan and fluorinated alky iodides,14 nitroxide-mediated polymerization (NMP),15-17 atom transfer radical polymerization (ATRP),18-20 and reversible addition fragmentation chain transfer (RAFT) polymerization.21,22 From the viewpoint of structures, several recent studies have been devoted mainly to the combination of end-capped fluoroalkyl groups with hydrophobic polymers15,19-22 or amphiphilic block copolymers.18,23,24 Furthermore, some block copolymers consisted of a poly(fluoroalkyl methacrylate) moiety (or F-alkyl in the side chain), and common hydrocarbon segments have also been reported.25-28 However, as far as we know, only a few studies (12) Su, Z. H.; Wu, D. C.; Hsu, S. L.; McCarthy, T. J. Macromolecules 1997, 30, 840-845. (13) Su, Z. H.; McCarthy, T. J.; Hsu, S. L.; Stidham, H. D; Fan, Z. Y.; Wu, D. C. Polymer 1998, 39, 4655-4664. (14) Boutevin, B.; Mouanda, J.; Pietrasanta, Y.; Taha, M. J. Polym. Sci. Part A Polym. Chem. 1986, 24, 2891-2903. (15) Yusa, S.; Yamamoto, T.; Hashidzume, A.; Morishima, Y. Polym. J. 2002, 34, 117-124. (16) Andruzzi, L.; Chiellini, E.; Galli, G.; Li, X. F.; Kang, S. H.; Ober, C. K. J. Mater. Chem. 2002, 12, 1684-1692. (17) Lacroix-Desmazes, P.; Delair, T.; Pichot, C.; Boutevin, B. J. Polym. Sci. Part A Polym. Chem. 2000, 38, 3845-3854. (18) Shi, Z. Q.; Holdcroft, S. Macromolecules 2005, 38, 4193-4201. (19) Feiring, A. E.; Wonchoba, E. R.; Davidson, F.; Percec, V.; Barboiu, B. J. Polym. Sci. Part A Polym. Chem. 2000, 38, 3313-3335. (20) Destarac, M.; Matyjaszewski, K.; Silverman, E.; Ameduri, B.; Boutevin, B. Macromolecules 2000, 33, 4613-4615. (21) Lebreton, P.; Ameduri, B.; Boutevin, B.; Corpart, J.-M. Macromol. Chem. Phys. 2002, 203, 522-537. (22) Monteiro, M. J.; Adamy, M. M.; Leeuwen, B. J.; Van Herk, A. M.; Destarac, M. Macromolecules 2005, 38, 1538-1541. (23) Sawada, H.; Takebayashi, A.; Uejima, M.; Murakami, T. Polym. AdV. Technol. 2006, 17, 479-483. (24) 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. (25) Nishino, T.; Urushihara, Y.; Meguro, M.; Nakamae, K. J. Colloid Interface Sci. 2004, 279, 364-369. (26) Zhou, X. D.; Ni, P. H.; Yu, Z. Q.; Zhang, F. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 471-484. (27) Busch, P.; Krishnan, S.; Paik, M.; Toombes, G. E. S.; Smilgies, D-M.; Gruner, S. M.; Ober, C. K. Macromolecules 2007, 40, 81-89.

10.1021/la704036a CCC: $40.75 © 2008 American Chemical Society Published on Web 04/01/2008

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Scheme 1. A Representative Reaction Route for the Synthesis of NFHO-PDMAm-b-PDEAn Diblock Copolymer via Oxyanion-Initiated Polymerization. The Reverse Feed Order Was Used To Prepare NFHO-PDEAn-b-PDMAm Diblock Copolymer

have been reported on the synthesis of water-soluble polymers with small amounts of perfluorocarbon side chains.29,30 We have a particular interest in the combination of end-capped fluoroalkyl segments with double hydrophilic block copolymers (DHBCs) or other amphiphilic copolymers.31-33 In most cases, DHBCs can self-assemble and form micelles in aqueous solution with tunable factors, such as temperature, pH, and ionic strength.29,34 Two representative monomers and their copolymers, poly[2-(N,N-dimethylamino)ethyl methacrylate] (PDMA) and poly[2-(N,N-diethylamino)ethyl methacrylate] (PDEA), are considered the potentially use for targeted drug delivery.35-38 Armes and Gast et al. have synthesized the PDMA-b-PDEA diblock copolymers and used fluorescence spectroscopy, dynamic light scattering (DLS), and small-angle neutron scattering (SANS) to characterize the micellar structures.35,39 Above a critical pH, the PDEA block becomes hydrophobic to yield the micelle core due to its deprotonation while the hydrophilic PDMA block extends out into the water phase to form the micelle corona. However, the morphologies of micelles observed by TEM were not provided. If the PDMA-b-PDEA diblock copolymer is end-capped with an F-alkyl group, and if the linking sequence of the F-alkyl, PDMA, and PDEA blocks is different, are there more fascinating micelle structures formed by their self-assembly? To the best of our knowledge, such a kind of fluoroalkyl end-capped diblock (28) Li, H.; Zhang, Z. B.; Hu, C. P.; Ying, S. K.; Wu, S. S.; Xu, X. D. React. Funct. Polym. 2003, 56, 189-197. (29) Co¨lfen, H. Macromol. Rapid Commun. 2001, 22, 219-252. (30) Sawada, H.; Sumino, E.; Oue, M.; Baba, M.; Kira, T.; Shigeta, S.; Mitani, M.; Nakajima, H.; Nishida, M.; Moriya, Y. J. Fluorine Chem. 1995, 74, 21-26. (31) Xu, J.; Ni, P. H.; Mao, J. e-Polym. 2006, 015, 1-14. (32) Mao, J.; Ni, P. H.; Mai, Y. Y.; Yan, D. Y. Langmuir 2007, 23, 51275134. (33) He, J. L.; Ni, P. H.; Liu, C. C. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 3029-3041. (34) Mun˜oz-Bonilla, A.; Ferna´ndez-Garcı´a, M.; Haddleton, D. M. Soft Matter 2007, 3, 725-731. (35) Lee, A. S.; Gast, A. P.; Bu¨tu¨n, V.; Armes, S. P. Macromolecules 1999, 32, 4302-4310. (36) Tang, Y. Q.; Liu, S. Y.; Armes, S. P.; Billingham, N. C. Biomacromolecules 2003, 4, 1636-1645. (37) Xu, P. S.; Van Krik, E. A.; Murdoch, W. J.; Zhan, Y. H.; Isaak, D. D.; Radosz, M.; Shen, Y. Q. Biomacromolecules 2006, 7, 829-835. (38) Kusumo, A.; Bombalski, L.; Lin, Q.; Matyjaszewski, K.; Schneider, J.; Titon, R. Langmuir 2007, 23, 4448-4454. (39) Liu, S. Y.; Weaver, J. V. M.; Tang, Y. Q.; Billingham, N. C.; Armes, S. P.; Tribe, K. Macromolecules 2002, 35, 6121-6131.

copolymer has not been reported in literature.40 It is well-known that the fluorinated carbon atoms should be six or eight in order to obtain self-assembled molecular aggregates in aqueous solutions. However, McLain et al. reported that a shorter fluoroalkyl group (e.g., C4F9) could be effective in increasing the surface activity of polymer when they were linked to the end of polymer chains.41 In the present study, we have prepared a series of PDMAb-PDEA diblock copolymers end-capped with the fluoroalkyl C4F9 group via oxyanion-initiated polymerization. The initiator was a potassium alcoholate, CF3(CF2)3(CH2)3O-K+ (designated as NFHO-K+), which was derived from the reaction of a fluorinated alcohol and potassium hydride. The final copolymer structures depended on the addition sequence of the two monomers. Our aim is to exploit the effects of the end-capped fluoroalkyl segment on the pH/temperature-responsive behavior of the copolymers and to further manipulate the different morphologies of the supramolecular structures formed by these diblock copolymers in various pH media. Scheme 1 shows the representative synthesis route and the typical structure of NFHOPDMAm-b-PDEAn diblock copolymer. The reverse feed sequence was used to prepare NFHO-PDEAn-b-PDMAm diblock copolymer. The detailed characterization of these copolymers and the various morphologies resulting from the linking sequence in different pH media have been discussed in this report. 2. Experimental Section 2.1. Materials. 2-(N,N-Dimethylamino)ethyl methacrylate (DMAEMA or DMA, Wuxi Xinyu Chemical Reagent Co., China) and 2-(N,Ndiethylamino)ethyl methacrylate (DEAEMA or DEA, Aldrich) were respectively passed through a basic alumina column, dried over calcium hydride (CaH2), and distilled in vacuum immediately before use. 4,4,5,5,6,6,7,7,7-Nonafluoro-1-heptanol (NFHOH, Aldrich) was used as received. Potassium hydride (KH, Aldrich, 35 wt % dispersion in mineral oil) was washed three times with anhydrous tetrahydrofuran (THF) in an inert atmosphere before use. THF was initially dried over potassium hydroxide for at least 2 days and then refluxed over sodium wire with benzophenone as indicator until the color turned (40) Imae, T. Curr. Opin. Colloid Interface Sci. 2003, 8, 307-314. (41) McLain, S. J.; Sauer, B. B.; Firment, L. E. Macromolecules 1996, 29, 8211-8219.

Fluoroalkyl End-Capped Amphiphilic Diblock Copolymers

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Table 1. Recipes of Syntheses of Homopolymer and Block Copolymers with Different Monomer Sequence polymer formula (theor)

KH (g)/ (mmol)

NFHOH (g)/ (mmol)

NFHO-PDMA30 NFHO-PDMA30-b-PDEA10 NFHO-PDMA30-b-PDEA20 NFHO-PDMA40-b-PDEA13 NFHO-PDEA10-b-PDMA30 NFHO-PDEA20-b-PDMA30 BzO-PDMA30-b-PDEA10a

0.1577/3.94 0.1461/3.65 0.1474/3.68 0.0944/2.36 0.0968/2.42 0.1195/2.99 0.0865/2.16

0.6548/2.35 0.3028/1.09 0.2196/0.79 0.2244/0.81 0.3708/1.33 0.2069/0.74 0.2102/1.95 b)

a

the first monomer (g)/ (mmol) DMA DMA DMA DMA DEA DEA DMA

7.9346/50.54 5.1280/32.66 3.7094/23.63 5.0670/32.27 2.6085/14.10 2.7525/14.88 10.2793/65.47

the second monomer (g)/ (mmol) DEA DEA DEA DMA DMA DEA

2.0142/10.09 2.9215/15.79 1.9405/10.49 6.2700/39.87 3.5039/22.32 4.1115/22.22

BzO represents benzyloxy group. b Millimoles of benzyl alcohol. Table 2. Compositions, Molecular Weights, and Molecular Weight Distribution of a Series of Polymers

sample ID

polymer chemical formula (theor)

M h n of PDMA block (g mol-1) theor actuala

M h n of PDEA block (g mol-1) theor actuala

1 2 3 4 5 6 7

NFHO-PDMA30 NFHO-PDMA30-b-PDEA10 NFHO-PDMA30-b-PDEA20 NFHO-PDMA40-b-PDEA13 NFHO-PDEA10-b-PDMA30 NFHO-PDEA20-b-PDMA30 BzO-PDMA30-b-PDEA10 c)

4700 4700 4700 6300 4700 4700 4700

1800 3600 2400 1800 3600 1800

a

4910 4820 4600 4880 4580 3480 6000

M h w/M h nb of final copolymers

1890 2450 1870 1800 2035 2300

1.28 1.36 1.35 1.40 1.35 1.38 1.45

Calculated by 1H NMR spectra, measured in CDCl3. b Measured by GPC in THF. c BzO represents benzyloxy group.

to purple. Other reagents were purchased from Shanghai Chemical Reagent Co. and used as received. All polymerizations were carried out under a dry argon atmosphere. 2.2. Synthesis of Diblock Copolymers with an F-Alkyl End Group. The PDMA-b-PDEA diblock copolymers with a fluoroalkyl (F-alkyl) end group were prepared by oxyanion-initiated polymerization. The detailed process of oxyanion-initiated polymerization has been described in previous literature.31,32 A representative synthesis procedure is described as follows: all glassware was heated at 120 °C for 12 h and cooled in vacuum to eliminate moisture before use. A suspension of KH in mineral oil was first introduced in a dry, preweighed, 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 flask was weighed to determine the amount of KH, and then 25 mL of THF was added into the flask. A certain amount of 4,4,5,5,6,6,7,7,7-nonafluoro-1-heptanol (NFHOH) (OH molar amount equivalent to that of potassium hydride) was injected into the flask containing KH and THF. The mixed solution was stirred at 0 °C for 0.5 h to produce potassium F-alcoholate, i.e., CF3(CF2)3(CH2)3O-K+ (NFHO-K+). A required amount of DMA monomer was added into the reactor via a syringe, and the reaction was carried on at 30 °C for 40 min. Then the second monomer DEA was added into the flask, and the reaction was continued for another 40 min at 30 °C before being quenched with methanol. The solvent was removed by rotary vacuum, and the product NFHO-PDMA-b-PDEA diblock copolymer was then purified by precipitating in cold n-hexane three times to remove the residual monomers. Finally, the samples were dried in vacuum at 40 °C for 3 days. The overall conversion was more than 90%. The NFHO-PDEA-b-PDMA diblock copolymer was prepared by the same method with a reversed addition sequence of the two monomers. The accurate recipes are listed in Table 1. 2.3. Characterization. Nuclear Magnetic Resonance (NMR) Spectroscopy. The chemical structures of the diblock copolymers, NFHO-PDMA-b-PDEA and NFHO-PDEA-b-PDMA, were determined by 1H NMR and 19F NMR spectroscopy with deuterated water (D2O) or CDCl3 as solvent, in which tetramethylsilane (TMS) was used as an internal standard for 1H NMR measurement and CF3COOH as an external standard for 19F NMR analysis, respectively. All NMR spectra were recorded on a 400-MHz NMR instrument (INOVA-400). The 1H NMR spectra were used to calculate the composition of copolymers from the relative integrals at δ 2.2 ppm (N(CH3)2 of the PDMA block) and δ 0.8 ppm (N(CH2CH3)2 of the PDEA block).

Gel Permeation Chromatography (GPC). The molecular weight and polydispersity of the diblock copolymers were determined by a Waters 1515 gel permeation chromatograph (GPC) 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. THF was used as the eluent at 30 °C with a flow rate of 1.0 mL min-1 and a series of poly(methyl methacrylate) standards as the calibration. Surface Tension Measurements. Surface tension (γ) of dilute aqueous solutions of copolymers NFHO-PDMA-b-PDEA and NFHO-PDEA-b-PDMA were measured by Du Nou¨y method on a JK99C automatic surface tensiometer (Shanghai Zhongchen Co., China) equipped with a platinum ring. A stock polymer solution was prepared by dissolving the pure copolymer in deionized water and adjusting to the desired pH value using hydrochloric acid as a diluent. A series of polymer solutions at different concentrations were prepared by diluting the stock solution with an acidic aqueous solution which had the same pH as that of the stock solution. Each sample was measured three times at 25 °C, and then the average surface tension was calculated. Steady-State Fluorescence Measurements. Steady-state fluorescence spectra were recorded with a FLS920 spectrofluorometer (Edinburgh Co. UK) with a slit of 1 nm for both excitation and emission. The excitation wavelength was 335 nm, and pyrene was used as the probe. The intensity ratio of the third band to the first band of the pyrene emission spectrum, I3/I1, was used to indicate the polarity of the pyrene environment. Transmission Electron Microscopy (TEM). The micelle morphologies were observed on a TEM instrument (TECNAI G2 20, FEI Co.) at 200 kV. The micelle solutions were prepared by two methods: (1) the copolymers were directly dissolved in deionized water; (2) the copolymers were initially dissolved in acidic aqueous solution (pH 3.0), and then the pH value was adjusted by adding sodium hydroxide solution to 7.0 and finally to 9.0. The carboncoated copper grid (400 mesh) was immersed in the aqueous polymer solution, removed, and dried at room temperature for 1 day before measurement. Dynamic Light Scattering (DLS). The aqueous solutions were stirred in sealed vials at room temperature for at least 1 week and passed through 0.45 µm hydrophilic microfilters (Agilent Technologies) in a quartz sample cell prior to measurement. The dynamic light scattering measurements were performed with a high performance laser nanoparticle autosizer (NANOPHOX, Sympatec GmbH System-Partikel-Technik). The signals of scattering light were

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Figure 1. Typical GPC plots showing (a) the NFHO-PDMA30 homopolymer (sample 1 in Table 2) and (b) the NFHO-PDMA30b-PDEA10 diblock copolymer (sample 2 in Table 2).

Figure 3. 400 MHz 19F NMR spectra of (A) NFHO-PDEA10-bPDMA30, (B) NFHO-PDMA30-b-PDEA10 and (C) NFHOH in CDCl3. (samples 2 and 5 in Table 2).

Figure 2. 400 MHz 1H NMR spectra of (A) NFHO-PDMA30-bPDEA10 and (B) NFHO-PDEA10-b-PDMA30 in CDCl3 (samples 2 and 5 in Table 2). detected by two probes, and evaluation mode was 2ND_CUMULANT. All measurements were carried out at 30 °C. Lower Critical Solution Temperature (LCST). LCSTs of the copolymer solutions were characterized by cloud points. The latter was obtained by turbidimetric measurement using a Shimadzu 3150 UV-vis-NIR spectrophotometer. The quarts cell was thermostated with a circulating water jacket equipped with a temperature controller. Polymer solutions at different concentrations were heated slowly from a starting temperature of 10 °C with a heating rate of 0.5 °C min-1. The absorbance at the 289 nm wavelength were chosen because of the curves were smooth and clear. With increasing temperature, the break at this wavelength can be easily observed. It benefits to determine cloud points of the solutions. The cloud point was defined as the onset temperature at which the absorbance was abruptly augmented, which was considered as the LCST of the polymer solution.

3. Results and Discussion 3.1. Synthesis and Characterization of Fluoroalkyl-Capped Amphiphilic Diblock Copolymers. Several kinds of fluorinated alcohols have been used, such as 1,1,1,3,3,3-hexfluoropropan2-ol and 2,3,4,5,6-pentafluorobenzyl alcohol, but the initiations were not sufficient. The probable reason was that the electronegative fluorine atom was so close to the oxyanion that the activity of initiator was greatly restrained. So we chose 4,4,5,5,6,6,7,7,7nonafluoro-1-heptanol, possessing three methylene (C3H6) groups between the hydroxyl and fluoroalkyl group C4F9. After reaction with KH, the oxyanion in CF3(CF2)3(CH2)3O-K+ (NFHO-K+)

Figure 4. Surface tension curves as the function of polymer concentrations for NFHO-PDMA30-b-PDEA10 (sample 2 in Table 2) at (a) pH 7.0 and (b) pH 5.0. Plot c shows the surface tension vs the concentrations of BzO-PDMA30-b-PDEA10 copolymer (sample 7 in Table 2) at pH 7.0.

can successfully initiate DMA and DEA monomers. A series of fluoroalkyl end-capped, AB- or BA-typed PDMA-b-PDEA diblock copolymers were synthesized via oxyanion-initiated polymerization. Scheme 1 shows a representative synthesis procedure. The linking orders of PDMA and PDEA blocks with the fluoroalkyl group depend on the addition sequence of the two monomers. Figure 1 shows the GPC curves of the NFHO-PDMA30 homopolymer and NFHO-PDMA30-b-PDEA10 diblock copolymer. The curve b of the diblock copolymer moves toward higher molecular weight compared with the first block PDMA homopolymer (curve a). There is no detectable homopolymer that contaminates the block copolymer, indicating that the second monomer DEA has reacted with the living anions of first block PDMA. Table 2 summarizes the compositions, molecular weights, and molecular weight distribution of the homopolymer and a series of diblock copolymers. In order to compare the solution behaviors of fluorinated copolymers with the nonfluorinated copolymer, a benzyloxy-capped diblock copolymer, BzOPDMA30-b-PDEA10, was prepared via oxyanion-initiated polymerization using potassium benzyl alcoholate as the initiator.

Fluoroalkyl End-Capped Amphiphilic Diblock Copolymers

Figure 5. Surface tension curves as the function of polymer concentrations for NFHO-PDEA10-b-PDMA30 (sample 5 in Table 2) at (a) pH 7.0 and (b) pH 5.0.

Figure 6. Intensity ratio I3/I1 obtained from the fluorescence excitation spectra of pyrene plotted versus the copolymer concentrations of (a) NFHO-PDMA30-b-PDEA10 (sample 2 in Table 2) and (b) NFHO-PDEA10-b-PDMA30 (sample 5 in Table 2), measured at pH 7.0 and 25 °C.

Figure 2 depicts the 1H NMR spectra for (A) NFHO-PDMA30b-PDEA10 and (B) NFHO-PDEA10-b-PDMA30 copolymers in CDCl3. With the comparison of the two 1H NMR spectra, we have found that almost all chemical shift peaks, which are attributed to the protons of NFHO-PDMA30-b-PDEA10 in Figure 2A, are consistent with those of NFHO-PDEA10-b-PDMA30 in

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Figure 2B. We have used the integrals of these two peaks to calculate the molecular weights of the diblock copolymers. The overlapped signals at δ 2.62 ppm for the protons in N(CH2CH3)2 and all protons in >NCH2CH2 of PDMA and PDEA may be ignored. The molecular weights of the two block copolymers calculated by 1H NMR spectra are very close, which are in good agreement with the theoretical values obtained from the molar ratios of monomers to initiator, as shown in Table 2. This fact shows that the two diblock copolymers have the identical composition and target molecular weight corresponding to the initial molar ratios of [NFHOK]/[DMA]/[DEA], but they have different linking sequences, as expected. These results indicate that the feeding sequence of DMA and DEA monomers has influenced the chemical structure and composition of desired copolymers in the oxyanion-initiated polymerization. To further confirm the incorporation of PDMA or PDEA with the fluorinated alkyloxy group, CF3(CF2)3(CH2)3O, we used 19F NMR spectroscopy to determine the fluorocarbon moiety in the copolymers and made a comparison with the 19F NMR spectrum of the initiator precursor, 4,4,5,5,6,6,7,7,7-nonafluoro-1-heptanol (NFHOH). From Figure 3A,B, we can find that the chemical shifts of the four peaks (a, b, c, d) for NFHO-PDMA30-b-PDEA10 and NFHO-PDEA10-b-PDMA30 diblock copolymers are consistent with those of the NFHOH (Figure 3C). The signal at δ -3.5 ppm is ascribed to the fluorine atoms of CF3 group, and the chemical shift of CF2 close to that of CF3 group can be detected at δ -37.0 ppm. The chemical shifts of the other four fluorine atoms (CF2CF2) can be observed at δ -46.9 ppm and δ -48.5 ppm, respectively. These results further demonstrate the successful copolymerization of the two monomers with the fluorinated alkyl initiator. 3.2. Self-Assembly Behavior of Fluoroalkyl-Capped Amphiphilic Diblock Copolymers. It is well-known that the introduction of fluorine atoms into polymer structures can efficiently improve their stability and other properties.4,8,23,41 The dominant characteristics of fluorinated surfactants lie in their high surface activity, chemical and thermal stability, low surface energy, and strong tendency to self-aggregate into stable well-organized supramolecular assemblies such as vesicles and tubules. In this paper, we used several techniques to study the self-assembly behavior of the fluoroalkyl-capped amphiphilic diblock copolymers.

Figure 7. 400 MHz 1H NMR spectra of (A) NFHO-DEA10-b-DMA30 D2O (pH 9.0), (B) NFHO-DMA30-b-DEA10 in D2O (pH 9.0), and (C) NFHO-DMA30-b-DEA10 in CDCl3 (samples 5 and 2 in Table 2).

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Figure 8. TEM images of (a) multicompartment micelles obtained from NFHO-PDMA30-b-PDEA10 copolymer (sample 2 in Table 2) aqueous solution at pH 9.0 with a concentration of 15 g L-1, bar ) 0.5 µm and (b) the high-magnification image of a, bar ) 50 nm. The sample was prepared by method 1 described in section 2.3.

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Figure 10. TEM images of the self-assembled micelles by the NFHO-PDMA30-b-PDEA10 copolymer (sample 2 in Table 2) at (a) pH 7.0 and (b) 9.0. The samples were prepared by the method 2 described in section 2.3. The concentration of aqueous solution was 3 g L-1.

Figure 11. Schematic illustration of structures of (A) unimers, (B) micelles, and (C) multicompartment micelles obtained by the selfassembly of NFHO-PDMA30-b-PDEA10 copolymer. Figure 9. TEM images of (a) flowerlike multicompartment micelles obtained from NFHO-PDEA10-b-PDMA30 copolymer (sample 5 in Table 2) aqueous solution at pH 9.0 with a concentration of 15 g L-1, bar ) 0.2 µm, and (b) the high-magnification image of a, bar ) 100 nm. The sample was prepared by method 1 described in section 2.3.

3.2.1. Measurements of Surface Tension. For surfactants, surface tension is a key physical parameter and is commonly used to describe the surface activity. For NFHO-PDMA30-bPDEA10 and NFHO-PDEA10-b-PDMA30 copolymers, the variations of surface tension as the function of copolymer concentrations were depicted in Figure 4, in which Figure 4c was the comparison experiment of the nonfluorinated diblock copolymer, BzO-PDMA30-b-PDEA10. Comparing plots a and c in Figure 4, it is obvious that the lowest surface tension of NFHO-PDMA30b-PDEA10 is less than that without fluoroalkyl group at pH 7.0. This result implies that even a short fluorocarbon group can strongly influence the surface activity of the diblock copolymers. Furthermore, this copolymer exhibited different surface activities when the pH value of aqueous solution was changed. From Figure 4a and 4b we can find that for NFHO-PDMA30-b-PDEA10 diblock copolymer system, the final surface tension reaches around 43 mN m-1 at pH 5.0 and 36 mN m-1 at pH 7.0, respectively. The reason can be explained by the electrostatic interaction between the cationic polymer chains, which were produced by the partial protonation of the tertiary amine groups of both PDMA and PDEA segments at acidic media (pH 5.0), which would suppress the formation of the micelles and lower the surface activity of the copolymers. On the other hand, these cationic polyelectrolytes could be easily dispersed in acidic aqueous solution so that the adsorption of the polymer chains at the air-water interface was decreased. In contrast with the above-mentioned situation, the surface tension at pH 7.0 was always lower than that system at pH 5.0, indicating that the NFHO-PDMA30-b-PDEA10 diblock copolymer was preferential to form micelles in neutral media.

Figure 12. TEM images of (a) anomalous vesicles self-assembled by NFHO-PDEA10-b-PDMA30 copolymer (sample 5 in Table 2) in aqueous solution, adjusting pH from 3.0 to 9.0, bar ) 0.2 µm, (b) the high-magnification image of a, bar ) 200 nm. The sample was prepared by method 2 described in section 2.3. The concentration of aqueous solution was 3 g L-1.

The similar phenomenon can be also found in the aqueous solution of NFHO-PDEA10-b-PDMA30. Figure 5 shows the curves of surface tension vs copolymer concentrations at pH 5.0 and pH 7.0, respectively. The lowest surface tension declines to 38 mN m-1 at pH 5.0, whereas the value is only 33 mN m-1 at pH 7.0. Comparing Figure 4 with Figure 5, one can find that the linking sequence of PDMA and PDEA blocks has influenced the surface active properties in aqueous solutions. The linkage of CF3(CF2)3(CH2)3O group with the PDEA block first, and then the PDMA block, favors enhancement of the surface activity. 3.2.2. Measurements of Critical Aggregation Concentration. Fluorescence probe is a powerful method to study critical aggregation concentration (cac) values of amphiphilic copolymer solutions.42 A higher peak ratio of the intensity of the third band (42) Che´cot, F.; Lecommandoux, S.; Gnanou, Y.; Klok, H. Angew. Chem., Int. Ed. 2002, 41, 1339-1343.

Fluoroalkyl End-Capped Amphiphilic Diblock Copolymers

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Table 3. Effect of pH Values on the Aggregating Particle Size and Size Polydispersity particle size (nm) size PDI

pH 3.05

pH 5.97

pH 6.90

pH 7.55

pH 7.90

pH 9.00

58 0.82

98 0.84

86 0.54

97 0.84

183 0.98

245 0.73

(383 nm, I3) to the first band (372 nm, I1) of pyrene, I3/I1, obtained from the emission spectra of pyrene was observed when pyrene was located in a more hydrophobic environment. Figure 6 shows the intensity ratio I3/I1 versus the copolymer concentrations of the two kinds of diblock copolymers, from which we can obtain cac values of 0.6 g L-1 for NFHO-PDMA30-b-PDEA10 and 0.4 g L-1 for NFHO-PDEA10-b-PDMA30. In the experimental range, the I3/I1 values for NFHO-PDEA10-b-PDMA30 in aqueous solution are always higher than those of NFHO-PDMA30-b-PDEA10 under identical conditions, implying that NFHO-PDEA10-b-PDMA30 has a stronger aggregating tendency than NFHO-PDMA30-bPDEA10 in neutral aqueous solution. This result is in good agreement with that obtained by surface-tension measurements. 3.2.3. Self-Assembly Analysis by 1H NMR. NMR spectroscopy is indirect and effective means to study self-assembly behavior.34,35,39 1H NMR spectra of micelles formed respectively from NFHO-PDMA30-b-PDEA10 and NFHO-PDEA10-b-PDMA30 diblock copolymers were recorded in D2O, as shown in Figure 7A,B. Each testing sample was obtained by dissolving the corresponding copolymer of 5.0 mg into D2O of 0.5 mL, and the pH value was adjusted to 9.0 using a little deuterated hydrochloric acid (DCl). Figure 7C shows the 1H NMR spectrum of complete unimer of NFHO-PDMA30-b-PDEA10 in CDCl3, which is different from those in Figure 7A,B. In the case of CDCl3, the characteristic shift for N(CH3)2 at δ 2.36 ppm (peak a) and the chemical shift for N(CH2CH3)2 at δ 0.86 ppm (peak h) can be certainly ascribed to PDMA and PDEA blocks, respectively, indicating that the PDMA and PDEA blocks extended well in the organic solvent. However, in the D2O systems, it is clear that all of the peaks of PDEA become weak and the proton peaks of NFHO (OCH2) at δ 3.45 ppm disappear completely, as shown in Figure 7A,B. These phenomena further confirm that the micelles have formed because the nonsolvated PDEA blocks and insoluble fluoroalkyl groups are now in the micellar core, while the partial solvate PDMA blocks form the micellar shell. 3.2.4. Self-Assembly Analysis by TEM. Transmission electron microscopy (TEM) is one of the most powerful tools to investigate self-assembly behavior. We first used TEM to observe the micelle structures that resulted from the samples by directly dissolving the copolymers in deionized water at pH 9.0. We found that the NFHO-PDMA30-b-PDEA10 and NFHO-PDEA10-b-PDMA30 copolymers exhibit very different micelle morphologies in aqueous solutions. Figure 8a is the TEM image of micelles for the NFHOPDMA30-b-PDEA10 copolymer in aqueous solution, in which the size of the micelles is ca. 100-200 nm. Figure 8b is a magnified TEM image of Figure 8a, from which one can see the obvious multicompartment micelles. Several researchers have reported the compositions and structures of multicompartment micelles.6,24,43,44 At pH 9.0, the hydrophobic PDEA block and fluoroalkyl group are strongly incompatible, and thus they form segregated microphase in the micellar core, whereas the partially hydrophilic PDMA block as the corona. The dark domains distributed on the micellar surface in the TEM image can be attributed to the presence of electron-rich fluorine atoms because they minimize contact with the neighboring aqueous phase.24,43 (43) Li, Z. B.; Kesselman, E.; Talmon, Y.; Hillmyer, M. A.; Lodge, T. P. Science 2004, 306, 98-101. (44) Kubowicz, S.; Thu¨nemann, A. F.; Weberskirch, R.; Mo¨hwald. H. Langmuir 2005, 21, 7214-7219.

We then studied the micelle morphology of NFHO-PDEA10b-PDMA30 copolymer in aqueous solution using the same sample preparation method. Unexpectedly, flowerlike micelles were obtained from this kind of copolymer as shown in Figure 9, which are quite unlike those formed from NFHO-PDMA30-bPDEA10 in Figure 8. This phenomenon can also substantiate that the linking sequence of PDMA block and PDEA block with the fluoroalkyl group obviously influences the micelle structures. Some papers reported that hydrophilic/hydrophobic block ratios can influence aggregation morphologies of amphiphilic copolymers.45,46 As expounded by Eisenberg et al.,47 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. Therefore, it is easy to understand the different micelle morphologies between NFHO-PDMA30-b-PDEA10 and NFHOPDEA10-b-PDMA30 copolymers with two types of linking sequences under the same conditions. In the NFHO-PDMA30b-PDEA10 case, the fluorinated segment and hydrophobic PDEA block are separated by a partially hydrophilic PDMA block, and they form sphere-on-sphere multicompartment micelles, as shown in Figure 8b. In contrast, in the NFHO-PDEA10-b-PDMA30 system, the fluorinated segment directly links with the hydrophobic PDEA block to form the core, and the partially hydrophilic PDMA block can stabilize the micelles (Figure 9). Considering that both PDMA and PDEA blocks can be protonized in acidic media, we investigated the effect of pH value on the self-organization morphologies of the block copolymers in aqueous solutions. At low pH, for example pH 3.0, the tertiary amine groups of the PDMA and PDEA blocks could be protonized in the aqueous solution, which allowed the copolymer to exist in the aqueous solution in the form of unimer. No aggregation of the copolymers could be observed by TEM. With the increase of pH values, both PDEA and PDMA blocks was gradually deprotonized and became insoluble, which led to the aggregation. Figure 10 shows the TEM images of the selfassembled micelles for the NFHO-PDMA30-b-PDEA10 copolymer at pH 7.0 and 9.0. From Figure 10a, one can find that the obvious small dark particles with the size of ca. 3-5 nm, which can be ascribed to the existence of the fluoroalkyl group because of the strong electronegativity of fluorine atoms. The solvated PDMA and PDEA blocks can hardly be observed herein. With the gradual increase of pH value from 7.0 to 9.0, the immiscible segments of hydrophobic PDEA block and the fluoroalkyl group aggregated into the core, while the partially hydrophilic PDMA block formed the corona, as shown in Figure 10b. Diverse repulsion forces between the PDEA block and fluoroalkyl group dominated the self-assembly process and led to the formation of multicompartment micelles. We propose a schematic illustration for the self-assembly process of NFHO-PDMA30-b-PDEA10 copolymer in aqueous solution in different pH media, as displayed in Figure 11. For another copolymer, NFHO-PDEA10-b-PDMA30, the effect of pH value on the aggregating morphology was also investigated. We can see from Figure 12 that vesicles form with anomalous shape. At low pH media, the NFHO-PDEA10-b-PDMA30 (45) Li, Z. B.; Hillmyer, M. A.; Lodge, T. P. Langmuir 2006, 22, 9409-9417. (46) Guvendrien, M.; Shull, K. R. Soft Matter 2007, 3, 619-626. (47) Choucair, A.; Eisenberg, A. Eur. Phys. J. E: 2003, 10, 37-44.

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Figure 13. Schematic illustration of the change between the unimer at pH 3.0 and the anomalous vesicle at pH 9.0 for NFHO-PDEA10b-PDMA30 in aqueous solution.

copolymer can be well dispersed in water due to both the protonized PDMA and PDEA blocks. When the pH value was increased, PDEA block became hydrophobic and then aggregated into an interlayer, and the fluoroalkyl groups also cumulated to the interlayer to form a dark layer. We propose a schematic illustration about the vesicle structure as shown in Figure 13. As Eisenberg et al. indicated, long hydrophilic chains segregate to the outside of the vesicle while the short hydrophilic chains segregate to inside. So the repulsion among corona chains outside is stronger than that inside the vesicles, and the curvature is maintained in a thermodynamically stable manner.48,49 But in our case, the hydrophilic chains, regardless of being inside or outside the vesicles, were the identical length. Ill-defined vesicles, therefore, were obtained herein. 3.3. pH/Temperature-Responsive Properties of Fluoroalkyl-Capped Diblock Copolymers. It is well-known that both PDMA and PDEA possess pH- and temperature-responsive properties. For the NFHO-PDEA10-b-PDMA30 copolymer, the particle size of aggregation would be affected by the variation of pH values. In the range of lower pH, the NFHO-PDEA10b-PDMA30 copolymer could aggregate to form small micelles by means of the hydrophobic CF3(CF2)3(CH2)3O group as the core and the protonized PDEA10-b-PDMA30 moiety as the corona. However, large vesicles appear above the pKa values of PDMA and PDEA. We measured the particle size and size polydispersity (size PDI) in different pH media by DLS measurement. The particle size increased from 58 nm at pH 3.05 to 245 nm at pH 9.00, as shown in Table 3. The solution concentrations were kept at 3 g L-1, corresponding to the samples in the TEM measurement.. The temperature-responsive property of the fluoroalkyl-capped diblock copolymers can be investigated by the determination of cloud points, that is, lower critical solution temperature (LCST). We selected the NFHO-PDEA10-b-PDMA30 aqueous solution as a model. Figure 14a depicts the variation of cloud point with different pH values. With the increase of pH values, cloud points decline because of the decreasing solubility of PDMA and PDEA in aqueous solution. For the identical copolymer, cloud point also depends on the concentration of the copolymer aqueous solution. Figure 14b shows the variation of cloud point with the concentration of NFHO-PDEA10-b-PDMA30 at pH 9.0, and the LCSTs decrease with increasing polymer concentrations. The reason is that the appearance of LCST depends on the aggregation of the hydrophobic portions of the copolymers at a certain temperature. These results indicate that the fluoroalkyl-capped diblock copolymer has been successfully synthesized and possess pH/temperature-responsive behavior. (48) Luo, L.; Eisenberg, A. J. Am. Chem. Soc. 2001, 123, 1012-1013. (49) Luo, L.; Eisenberg, A. Langmuir 2001, 17, 6804-6811.

Figure 14. (A) Effect of pH values on cloud points, C ) 2.0 g L-1, and (B) effect of concentrations on cloud points at pH 9.0. The copolymer was NFHO-PDEA10-b-PDMA30 (sample 5 in Table 2).

4. Conclusions PDMA and PDEA are both outstanding pH/temperatureresponsive polymers. They can be protonized in acidic media and possess LCSTs in aqueous solution. In this paper, a series of fluoroalkyl end-capped, AB- or BA-typed PDMA-b-PDEA diblock copolymers have been successfully synthesized via oxyanion-initiated polymerization. The surface tension of endcapped fluoroalkyl copolymers in aqueous solution could decline to ca. 33 mN m-1 and exhibited a much better surface-active property than those without fluoroalkyl groups, implying that even a relatively short fluoroalkyl segment could greatly influence the solution behavior of the copolymers. The most interesting result is that various morphologies, such as sphere-on-sphere and flowerlike multicompartment micelles, as well as anomalous vesicles, have been obtained by changing the linking order of PDMA and PDEA blocks or adjusting the pH values of the aqueous solutions. LCST of the copolymer solutions declined with the increase of concentrations or pH values. DLS results indicated that the particle size increased with the increasing pH value of aqueous solution. These results have proved that the incorporation of a short fluoroalkyl group into amphiphilic copolymers conveys versatile properties of solution behavior and self-organized morphology. Acknowledgment. The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 20474041), the Key Laboratory of Molecular Engineering of Polymers, Ministry of Education of China, and the Natural Science Foundation of Educational Department of Jiangsu Province (03KJD150188). LA704036A