Amphiphilic Dual Brush Block Copolymers as “Giant Surfactants

pubs.acs.org/Langmuir © 2009 American Chemical Society

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Amphiphilic Dual Brush Block Copolymers as “Giant Surfactants” and Their Aqueous Self-Assembly )

Daniel Zehm,† Andr
e Laschewsky,*,†,‡ Michael Gradzielski,*,§ Sylvain Pr
evost,§ Hua Liang, J€urgen P. Rabe, Ralf Schweins,^ and J
er
emie Gummel# †

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Institut f€ ur Chemie, Universit€ at Potsdam, Karl-Liebknecht-Str. 24-25, 14476 Potsdam-Golm, Germany, ‡ Fraunhofer Institut f€ ur Angewandte Polymerforschung IAP, Geiselbergstr. 69, 14476 Potsdam-Golm, Germany, §Stranski-Laboratorium f€ ur Physikalische und Theoretische Chemie, Institut f€ ur Chemie, Technische Universit€ at Berlin, Strasse des 17. Juni 124, Sekr. TC7, 10623 Berlin, Germany, Department of Physics, Humboldt-Universit€ at zu Berlin, Newtonstrasse 15, 12489 Berlin, Germany, ^Institut Max von Laue-Paul Langevin (ILL), F-38042 Grenoble Cedex 9, France, and #European Synchrotron Radiation Facility, BP 220, F-38042 Grenoble, France Received August 18, 2009. Revised Manuscript Received October 6, 2009 Amphiphilic dual brush diblock as well as symmetrical triblock polymers were synthesized by the overlay of the reversible addition-fragmentation chain transfer (RAFT) and the nitroxide mediated polymerization (NMP) techniques. While poly(ethylene glycol) brushes served as hydrophilic block, the hydrophobic block was made of polystyrene brushes. The resulting “giant surfactants” correspond structurally to the established amphiphilic diblock and triblock copolymer known as macrosurfactants. The aggregation behavior of the novel “giant surfactants” in aqueous solution was studied by dynamic light scattering, small-angle neutron scattering (SANS), and small-angle X-ray scattering (SAXS) over a large range in reciprocal space. Further, the self-assembled aggregates were investigated by scanning force microscopy (SFM) after deposition on differently functionalized ultraflat solid substrates. Despite the high fraction of hydrophobic segments, the polymers form stable mesoscopic, spherical aggregates with hydrodynamic diameters in the range of 150-350 nm. Though prepared from well-defined individual polymers, the aggregates show several similarities to hard core latexes. They are stable enough to be deposited without much changes onto surfaces, where they cluster and show spontaneous sorting according to their size within the clusters, with the larger aggregates being in the center.

Introduction Compared to low molar mass surfactants, polymeric amphiphiles are characterized by a much larger number of molecular variables. Thus, they offer an enormous wealth of possible architectures and resulting self-organized structures in selective solvents, in particular in aqueous media.1-4 Polymeric amphiphiles are conveniently subdivided in two classes. The first class comprises all architectures, in which the monomer units (or at least short monomer sequences) are inherently amphiphilic, with the most notable case being the so-called polysoaps.5 In the second class, the amphiphilic nature of the polymers originates from the combination of individual hydrophilic and hydrophobic blocks; that is, amphiphilicity is the result of the overall macromolecular architecture. This second class comprises, for instance, graft and block copolymers as well as many dendrimers, when made of a hydrophobic core and a hydrophilic shell, or vice versa. In particular, amphiphilic block copolymers (often referred to as “macrosurfactants”) excel by their rich variability and have *To whom correspondence should be addressed. E-mail: laschews@rz. uni-potsdam.de (A.L.); [email protected] (M.G.). (1) Piirma, I. Polymeric surfactants. In Surfactant Science Series; Marcel Dekker: New York, 1992; Vol. 42. (2) Garnier, S.; Laschewsky, A.; Storsberg, J. Tenside, Surfactants, Deterg. 2006, 43, 88–102. (3) F€orster, S.; Plantenberg, T. Angew. Chem., Int. Ed. 2002, 41, 688–714. (4) Beginn, U. Colloid Polym. Sci. 2008, 286, 1465–1474. (5) Laschewsky, A. Adv. Polym. Sci. 1995, 124, 1–86. (6) Riess, G. Prog. Polym. Sci. 2003, 28, 1107–1170. (7) Liu, F.; Eisenberg, A. J. Am. Chem. Soc. 2003, 125, 15059–15064. (8) Lei, L.; Gohy, J.-F.; Willet, N.; Zhang, J.-X.; Varshney, S.; J
er^ome, R. Macromolecules 2004, 37, 1089–1094.

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therefore attracted much interest in recent years.6-13 Moreover, the rise of new polymerization methods, notably of controlled free radical polymerizations, has facilitated the synthesis of various block copolymers and thus stimulated the development of new structures.14-19 Hence, the transition from low molar mass surfactants (typical molar mass 200-600 g/mol) to block copolymer “macrosurfactants” (typical molar mass 2000-50 000 g/ mol) may be extended to the amphiphilic structuring of block copolymer building blocks by chemical bonds into even larger constructs (molar mass above 200 000 g/mol) (Scheme 1). In comparison to the self-assembly of such building blocks by purely physical interactions, prestructuring by chemical bonding allows one to create equilibrium structures and colloidally stable nonequilibrium structures or to forge otherwise incompatible building blocks into novel combinations. Furthermore, this chemical (9) Rodrı´ guez-Hern
andez, J.; Ch
ecot, F.; Gnanou, Y.; Lecommandoux, S. Prog. Polym. Sci. 2005, 30, 691–724. (10) Dimitrov, I.; Trzebicka, B.; M€uller, A. H. E.; Dworak, A.; Tsvetanov, C. B. Prog. Polym. Sci. 2007, 32, 1275–1343. (11) Blanazs, A.; Armes, S. P.; Ryan, A. J. Macromol. Rapid Commun. 2009, 30, 267–277. (12) Zhou, Y.; Yan, D. Chem. Commun. 2009, 1172–1188. (13) Schmitz, C.; Mourran, A.; Keul, H.; M€oller, M.; Keerl, M.; Richtering, W. Colloid Polym. Sci. 2009, 287, 1183–1193. (14) Hawker, C. J. Nitroxide Mediated Living Radical Polymerization. In Handbook of Radical Polymerization; Matyjaszewski, K., Davis, T. P., Eds.; John Wiley and Sons, Inc.: Hoboken, NJ, 2002; pp 463-521. (15) Kamigaito, M.; Ando, T.; Sawamoto, M. Chem. Rev. 2001, 101, 3689–3745. (16) Mori, H.; M€uller, A. H. E. Prog. Polym. Sci. 2003, 28, 1403–1439. (17) Lutz, J.-F. Polym. Int. 2006, 55, 979–993. (18) Braunecker, W. A.; Matyjaszewski, K. Prog. Polym. Sci. 2007, 32, 93–146. (19) Moad, G.; Rizzardo, E.; Thang, S. H. Acc. Chem. Res. 2008, 41, 1133–1142.

Published on Web 10/30/2009

DOI: 10.1021/la903087p

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Zehm et al. Scheme 1. Examples for Amphiphiles at Different Size Levelsa

a

From left to right: nonionic low molar mass surfactant, amphiphilic diblock copolymer, amphiphilic dual brush. Typically, a, b < 10; m, n g 10; x, y g 10.

Scheme 2. Possible Topologies of Amphiphilic Polymeric Brushesa

a (A) Core-shell, (B) inverse core-shell, (C) Janus-like, and (D) dual brush. Light gray color indicates hydrophilic elements, and dark gray color indicates hydrophobic elements.

prestructuring introduces a higher hierarchical level in these amphiphilic systems, thereby giving them additional functional potential. One direction of this effort aims, for instance, at the combination of synthetic block copolymers with proteins20 or with DNA,21,22 another one at the preparation of amphiphilic polymer brushes.23-25 In fact, four amphiphilic types of polymer brushes can be distinguished according to the positioning of the hydrophilic and hydrophobic building blocks (Scheme 2).23 Inherently, such structures are not accessible by a one-shot preparation but require hierarchical synthetic strategies. For instance, core-shell brushes (Scheme 2A and B) may be constructed either by homopolymerization of amphiphilic block copolymer macromonomers or by two final successive “grafting-from polymerization” steps from a precursor polymer, which is made of constitutional repeat units with initiator character. The corresponding monomers are often referred to as “inimers”. While amphiphilic core-shell brushes have been reported only occasionally,24-33 reports on (20) Reynhout, I. C.; Cornelissen, J. J. L. M.; Nolte, R. J. M. Acc. Chem. Res. 2009, 42, 681–692. (21) Watson, K. J.; Park, S.-J.; Im, J.-H.; Nguyen, S. T.; Mirkin, C. A. J. Am. Chem. Soc. 2001, 123, 5592–5593. (22) B€orner, H. G.; Schlaad, H. Soft Matter 2007, 3, 394–408. (23) Ishizu, K.; Yamada, H. Macromolecules 2007, 40, 3056–3061. (24) Sheiko, S. S.; Sumerlin, B. S.; Matyjaszewski, K. Prog. Polym. Sci. 2008, 33, 759–785. (25) Zhang, M.; M€uller, A. H. E. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 3461–3481. (26) Djalali, R.; Hugenberg, N.; Fischer, K.; Schmidt, M. Macromol. Rapid Commun. 1999, 20, 444–449. (27) Djalali, R.; Li, S.-Y.; Schmidt, M. Macromolecules 2002, 35, 4282–4288. (28) Cheng, G.; B€oker, A.; Zhang, M.; Krausch, G.; M€uller, A. H. E. Macromolecules 2001, 34, 6883–6888. (29) Zhang, M.; Breiner, T.; Mori, H.; M€uller, A. H. E. Polymer 2003, 44, 1449– 1458. (30) Cheng, C.; Qi, K.; Khoshdel, E.; Wooley, K. L. J. Am. Chem. Soc. 2006, 128, 6808–6809. (31) Liu, Q.; Chen, Y. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 6103–6113. (32) Tang, C.; Dufour, B.; Kowalewski, T.; Matyjaszewski, K. Macromolecules 2007, 40, 6199–6205. (33) Yuan, W.; Yuan, J.; Zhang, F.; Xie, X.; Pan, C. Macromolecules 2007, 40, 9094–9102. (34) Venkatesh, R.; Yajjou, L.; Koning, C. E.; Klumperman, B. Macromol. Chem. Phys. 2004, 205, 2161–2168.

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“Janus”-like brushes (Scheme 2C),34-36 preferentially made by alternating copolymerization of hydrophilic and hydrophobic macromonomers,23,37-39 are even scarcer, and examples of amphiphilic “dual brushes” (Scheme 2D) are hardly known.40-42 When considering amphiphilic block copolymers as “macrosurfactants”, amphiphilic dual brush polymers may be envisaged as “giant surfactants” as inherent to their molecular architecture; their molar mass is 1-2 orders of magnitude larger. Simplified miniaturized analogues were obtained by block copolymerizing poly(ethylene oxide) (PEO) macromonomers with long chain alkyl methacrylates43,44 or by complexing an ionic-nonionic block copolymer with oppositely charged hydrocarbon and fluorocarbon surfactants.45 True amphiphilic dual brush polymers however require more complex synthetic strategies with at least three independent steps. Here, we present the overlay of two methods of controlled free radical polymerization, namely, of the reversible addition-fragmentation chain transfer (RAFT)19 and the nitroxide mediated polymerization (NMP)14 techniques, in combination with the use of a hydrophilic poly(ethylene glycol) (PEG)-based macromonomer to prepare dual brush polymers. This leads to novel “giant surfactants” (Figure 1) that correspond structurally to the smaller amphiphilic diblock and triblock copolymer macrosurfactants (Scheme 1) but are more complex with respect to their molecular buildup. The aggregation behavior of the novel “giant surfactants” in aqueous solution was studied by means of combining small-angle neutron and Xray scattering (SANS and SAXS) over a large range in reciprocal space. This is needed because the formed aggregates are hierarchically structured over a very large overall size range, starting from the large overall extension down to increasingly smaller units, and further to the individual chains. Further, the self-assembled aggregates were investigated by scanning force microscopy (SFM) after deposition on differently functionalized ultraflat solid substrates. (35) Neugebauer, D. Polymer 2007, 48, 4966–4973. (36) Xie, M.; Dang, J.; Han, H.; Wang, W.; Liu, J.; He, X.; Zhang, Y. Macromolecules 2008, 41, 9004–9010. (37) Gu, L.; Shen, Z.; Feng, C.; Li, Y.; Lu, G.; Huang, X. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 4056–4069. (38) Zhu, H.; Deng, G.; Chen, Y. Polymer 2008, 49, 405–411. (39) Yin, J.; Ge, Z.; Liu, H.; Liu, S. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 2608–2619. (40) Cheng, Z.; Zhu, X.; Fu, G. D.; Kang, E. T.; Neoh, K. G. Macromolecules 2005, 38, 7187–7192. (41) Lanson, D.; Schappacher, M.; Borsali, R.; Deffieux, A. Macromolecules 2007, 40, 9503–9509. (42) Huang, K.; Rzayev, J. J. Am. Chem. Soc. 2009, 131, 6880–6885. (43) Street, G.; Illsley, D.; Holder, S. J. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 1129–1143. (44) Yi, Z.; Liu, X.; Jiao, Q.; Chen, E.; Chen, Y.; Xi, F. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 4205–4217. (45) Laschewsky, A.; Mertoglu, M.; Kubowicz, S.; Th€unemann, A. F. Macromolecules 2006, 39, 9337–9345.

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Figure 1. Molecular structure of the amphiphilic block-graft-copolymers. (left) Diblock dual brush and (right) symmetrical triblock dual brushes: (a) polymers prepared and studied; schematic architecture as “giant surfactants” (b) in nonselective solvents and (c) in selective solvents for the hydrophilic blocks.

Experimental Section Materials. Styrene (Aldrich, 99%) and poly(ethylene glycol monomethyl ether acrylate) (methoxy poly(ethylene glycol) acrylate, PEGA, Aldrich [32171-39-4], number average degree of ethoxylation = 9-10 according to 1H NMR) were passed through a column filled with basic Al2O3 (Merck, activity I, 0.0630-0.200 mm) to remove the inhibitor prior to polymerization. 2,20 -Azobis(isobutyronitrile) (AIBN, Acros-Organics, 98%) was recrystallized from methanol and dried in vacuo. Tetrahydrofuran (THF, Merck, extra pure) was distilled over K-Na. Other solvents used for polymerization and purification were analytical grade and used as received. Zellu-Trans dialysis tubes (normal molar mass cutoff of 4000-6000 D) were from Roth. Benzyl butyl trithiocarbonate (CTA1) was prepared by a modified literature procedure,46,47 while 2,6-bis-butylsulfanylthiocarbonylsulfanyl-heptanedioic acid dimethyl ester (CTA2) was made as reported before.47 Inimer phenyl-2-(2,2,6,6-tetramethyl-piperidin-1-oxyl)-ethylacrylate (ΦTEA) was synthesized by combining separate procedures for 2-phenyl-2-(2,2,6,6-tetramethyl-piperidine-1-oxyl)-ethanol48 and for coupling this alcohol with acryloyl chloride,49 thus improving considerably the overall yield and simplifying the necessary purification steps compared to the previously reported pathways.49-53 Synthesis of poly(ΦTEA). In a typical procedure, a mixture of phenyl-2-(2,2,6,6-tetramethyl-piperidin-1-oxyl)-ethylacrylate (2.12 g, 6.4 mmol), CTA2 (16.5 mg, 3.2  10-5 mol), and AIBN (1 mg, 6.4  10-6 mol) in dry THF (10 mL) is degassed by three freeze-pump-thaw cycles, sealed, and placed in an oil bath at 65 C. After 14.5 h, the reaction is stopped by cooling. The (46) Bowes, A.; McLeary, J. B.; Sanderson, R. D. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 588–604. (47) Bivigou-Koumba, A. M.; Kristen, J.; Laschewsky, A.; M€uller-Buschbaum, P.; Papadakis, C. M. Macromol. Chem. Phys. 2009, 210, 565–578. (48) Jahn, U. J. Org. Chem. 1998, 63, 7130–7131. (49) Voccia, S.; J
er^ome, C.; Detrembleur, C.; Leclere, P.; Gouttebaron, R.; Hecq, M.; Gilbert, B.; Lazzaroni, R.; J
er^ome, R. Chem. Mater. 2003, 15, 923–927. (50) Hawker, C. J.; Hedrick, J. L. Macromolecules 1995, 28, 2993–2995. (51) Gravert, D. J.; Janda, K. D. Tetrahedron Lett. 1998, 39, 1513–1516. (52) Morgan, A. M.; Pollack, S. K.; Beshah, K. Macromolecules 2002, 35, 4238– 4246. (53) Beil, J. B.; Zimmermann, S. C. Macromolecules 2004, 37, 778–787.

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solution is twice precipitated into ice-cold methanol, and the recovered polymer is dried under high vacuum. Yield = 0.71 g. GPC (RI detector): Mn = 7 kg/mol, Mw/Mn = 1.33. The homopolymers with CTA1 are synthesized and purified analogously. Synthesis of poly(PEGA-b-ΦTEA-b-PEGA). In a typical procedure, a mixture of poly(ΦTEA) (0.1 g, Mn = 28 300), PEGA (0.464 g, 1.02 mmol), and AIBN (0.06 mg, 3.4  10-6 mol) in dry THF (2.5 mL) is degassed by three freeze-pump-thaw cycles and placed in an oil bath at 65 C. After 2 h, the reaction is stopped by cooling. The solution is dialyzed against deionized water (dialysis membranes ZelluTrans from Roth (Germany), nominal cutoff 4000-6000). The aqueous polymer solution is lyophilized and dried under high vacuum. Yield = 0.23 g. GPC (RI detector): Mn = 4.3 kg/mol, Mw/Mn = 1.5. The diblock copolymer is synthesized and purified analogously. Grafting of Polystyrene (PS). In a typical procedure, a mixture of poly(PEGA-b-ΦTEA-b-PEGA) (0.130 g, Mn = 58 300, 0.196 mmol based on TEMPO groups) and styrene (2.05 g, 19.6 mmol) in toluene (2 mL) is degassed by three freeze-pump-thaw cycles and placed in an oil bath at 135 C for 19.5 h. Then the reaction is quenched and precipitated into icecold methanol (twice). The polymer is filtered and dried under high vacuum. Yield = 0.950 g. GPC (RI detector): Mn = 76 kg/mol, Mw/Mn = 1.15. The diblock copolymer brush is synthesized and purified analogously. Cleavage of the PS Branches. In a typical procedure, 10 drops of 5 M KOH in methanol are added to 20 mg of amphiphilic brush copolymer in 1 mL of THF. The mixture is sealed and heated at 60 C for 2 days. After cooling, the mixture is concentrated and precipitated in methanol, and then the precipitated polystyrene is recovered by centrifugation and dried. Preparation of Aqueous Solutions. Aqueous solutions of amphiphilic brush block copolymers were prepared by two protocols. In protocol A, amphiphilic brush block copolymer was dissolved in THF, water was added dropwise under stirring, and subsequently the THF was slowly evaporated under ambient conditions. Finally, the concentration of the polymer solution was adjusted to 1 wt % by adding pure water. Protocol B is analogous, except for adding a mixture DOI: 10.1021/la903087p

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of water/dimethylformamide (DMF) (10/1 v/v) to the solution in THF. Deposition onto Solid Substrates. A droplet of an aqueous solution of dual brush polymer (40 μL, prepared according to protocol B described above, at a concentration of 0.1 mg/mL) was deposited onto the three different surfaces (mica, poly-L-ornithine precoated mica, and octadecylamine precoated highly oriented pyrolytic graphite (HOPG)) and then spun off after 5 s on a spin coater. The preparations of the precoated mica54 and the precoated HOPG55 were described before. Methods. 1H NMR and 13C NMR spectra were taken with a Bruker Avance 300 apparatus. All spectra are referenced to the solvent residual peak (CHCl3 at 7.26 ppm). Details on the apparatus and methods used for size exclusion chromatography (SEC) were described before.56 Monomer conversions were measured before workup by comparing the intensity of the vinyl proton signals of the monomers PEGA or ΦTEA with the intensity of characteristic signals of the polymers. For the dual brush polymers, monomer conversions were approximated via the polymer yields as determined gravimetrically, and the theoretically expected molar mass values were estimated according to eq 1: M n, theor ¼ ðconversion  M r, monomer  ½M =½alkoxaminesÞ þ M r, precursor

ð1Þ

Thermal properties were measured with a DSC 822 differential scanning calorimeter (Mettler Toledo) under a nitrogen atmosphere, heating rate 10 K min-1, and cooling rate 5 K min-1. Dynamic light scattering (DLS) data were accumulated at a scattering angle of θ = 173 (backscattering detection) with a high-performance particle sizer (HPPS-ET, Malvern Instruments, U.K.) equipped with a He-Ne laser (λ = 633 nm) and a thermoelectric Peltier element for temperature control. Autocorrelation functions were analyzed with the CONTIN method. Apparent hydrodynamic diameters Dh were calculated according to the Stokes-Einstein equation, Dh = kT/3πηDapp, with Dapp being the apparent diffusion coefficient and η being the viscosity of the solution. Prior to measurements, the polymer solutions were filtered into a quartz glass cuvette (Suprasil from Hellma, Germany) with an optical path length of 1.9 cm using a WICOM OPTI-Flow 0.45 μm disposable filter. Samples employed for the SANS and SAXS measurements were additionally characterized by angle dependent dynamic and static light scattering (SLS) at 25 C using a setup consisting of an ALV 7004 correlator, an ALV CGS-3 goniometer, and a He-Ne Laser with a wavelength of 632.8 nm. Cylindrical sample cells were placed in an index matching vat filled with toluene. Autocorrelation functions as well as the mean intensity were recorded under different angles between 30 and 150. From the static intensity, the radius of gyration was determined according to eq 3 in the Supporting Information. SANS experiments were performed on the instrument D11 of the Institut Laue-Langevin (ILL, Grenoble, France), with scattered neutrons recorded on a 128  128 He3 detector of 96  96 cm2. A wavelength of 0.6 nm (fwhm 10%) and sample-todetector distances of 1.2, 8, and 34 m were employed, with collimation at 4, 8, and 34 m, respectively, thereby covering a q-range of 0.02-5 nm-1, where q is the magnitude of the scattering vector defined as q ¼

  4π θ sin λ 2

ð2Þ

(54) G€ossl, I.; Shu, L.; Schl€uter, A. D.; Rabe, J. P. J. Am. Chem. Soc. 2002, 124, 6860–6865. (55) Severin, N.; Barner, J.; Kalachev, A. A.; Rabe, J. P. Nano Lett. 2004, 4, 577– 579. (56) Skrabania, K.; Laschewsky, A.; Berlepsch, H. v.; B€ottcher, C. Langmuir 2009, 25, 7594–7601.

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where θ is the scattering angle and λ is the wavelength. The sensitivity of the detector was accounted for by comparison with the scattering of a 1 mm sample of water, the level of which being known from calibration with polymer standards was used for absolute scaling. The samples’ thicknesses (2 mm), transmissions, dead time, and electronic background were accounted for, and the background due to the scattering of the beam with an empty cell was subtracted. Hence, the scattering intensities given still contain the scattering contribution of the solvent and the incoherent scattering.57 The obtained data were finally radially averaged and merged. The SAXS experiments were done on the instrument ID02 of the European Synchrotron Radiation Facility (ESRF), with a wavelength of 0.1 nm and sample-to-detector distances of 0.75 and 8 m, thereby covering a q-range of 0.015-6.7 nm-1. This large q-range is required in order to characterize the large aggregates present comprehensively. The same quartz capillary was used for all measurements (samples, empty container, water). Data reduction was processed similarly as for SANS except that the scattering by a mixture of water/DMF 10/1 v/v was subtracted rather than only the empty capillary. Absolute scale was achieved by calibrating with a scattering intensity of water of 1.6  10-2 cm-1.58 Multiple short (10 ms) scattering patterns on the same sample volume were recorded to make sure that no radiation damage occurred during the exposure. Multiples of such 10 ms shots on different sample volumes were then averaged to obtain the final spectra. Scanning force microscopy (SFM) was performed in tapping mode under ambient conditions with a Nanoscope 3a (Veeco) instrument, using silicon cantilevers (Olympus, Japan) with a typical resonant frequency of 300 kHz and a spring constant of about 42 N/m. Both height and phase images are recorded.

Results and Discussion Amphiphilic dual brush copolymers were synthesized by combining reversible addition-fragmentation chain transfer (RAFT) polymerization with nitroxide mediated polymerization (NMP). The general synthetic strategy is outlined in Scheme 3 by the example of the triblock systems. The diblock systems are made analogously (Supporting Information). The polymers carried polystyrene as a hydrophobic brush block and poly(ethylene oxide) (PEO) as a nonionic hydrophilic brush block. Frequently, polymer brushes have been synthesized by polymerizing (protected) 2-hydroxyethyl methacrylate, which eventually after deprotection is converted into the 2-bromoisobutyrate moiety and subsequently used as an initiating site for ATRP.28,29,32,59-61 In contrast, our approach implied the polymerization of inimer 2-phenyl-2-(2,2,6,6-tetramethyl-piperidine-1-oxyl)-ethylacrylate (ΦTEA) to give directly the macroinitiators, without the need for postpolymerization modifications. Depending on the RAFT agent employed (monofunctional or bifunctional trithiocarbonate), amphiphilic reactive diblock as well as triblock copolymers were accessible by the consecutive RAFT copolymerization of the hydrophilic macromonomer PEGA. Subsequently, the pendant alkoxyamine moieties served as initiating sites for the final polymerization of styrene via a “grafting-from” approach, yielding the dual brush polymers (Figure 1a). Synthesis of the Linear Macrosurfactant Precursor Block Copolymers. RAFT polymerizations were performed with (57) Chen, S. H. Annu. Rev. Phys. Chem. 1986, 37, 351–399. (58) Orthaber, D.; Bergmann, A.; Glatter, O. J. Appl. Crystallogr. 2000, 33, 218– 225. (59) Zhang, M.; Drechsler, M.; M€uller, A. H. E. Chem. Mater. 2004, 16, 537– 543. (60) Khelfallah, N.; Gunari, N.; Fischer, K.; Gkogkas, G.; Hadjichristidis, N.; Schmidt, M. Macromol. Rapid Commun. 2005, 26, 1693–1697. (61) Wan, L.-S.; Lei, H.; Ding, Y.; Fu, L.; Li, J.; Xu, Z.-K. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 92–102.

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Article Scheme 3. Synthesis of Poly(PEGA-b-(ΦTEA-g-PS)-b-PEGA) Dual Brush Triblock Copolymer

CTA1 and CTA2 as monofunctional and bifunctional RAFT agents, respectively. Both have been reported to produce welldefined polymers.47,49,62 The polymerization of ΦTEA mediated (62) Troll, K.; Kulkarni, A.; Wang, W.; Darko, C.; Bivigou-Koumba, A. M.; Laschewsky, A.; M€uller-Buschbaum, P.; Papadakis, C. M. Colloid Polym. Sci. 2008, 286, 1079–1092.

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by both CTA1 and CTA2 provided poly(ΦTEA) with monomodal molar mass distributions (Table 1). The relatively narrow polydispersities of 1.3-1.5 indicate good control over the polymerization process. As the molar masses measured by GPC analysis are only approximate because of the calibration by PS standards, molar masses were calculated by end group analysis via UV DOI: 10.1021/la903087p

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Table 1. Synthesis and Characterization of Poly(ΦTEA-b-PEGA) and Poly(PEGA-b-ΦTEA-b-PEGA) Reactive Macrosurfactants by RAFT Polymerization at 65 C, Initiated by AIBN number average molar mass Mn entry

polymer

RAFT agent used

[M]/[CTA]/ [AIBN]

time [h]

conversion [%]a

theoretically expected, end group analysis, [103 g mol-1]b Mtheo Mn [103 g mol-1] n

by SEC, Mapp n [103 g mol-1]c

Mw/Mn (SEC)c

(ΦTEA)77 CTA1 200:1:0.2 16 31 21 25d 2.7 1.56 CTA2 200:1:0.2 14.5 46 30 29d 7.0 1.33 (ΦTEA)88 d CTA2 200:1:0.2 15 43 27 36 2.5 1.40 (ΦTEA)108 (ΦTEA)77 300:1:0.1 3 45 75 74e 2.4 1.44 (ΦTEA)77b-(PEGA)108 e 5 (ΦTEA)77(ΦTEA)77 400:1:0.1 4 65 143 112 2.2 1.40 b-(PEGA)191 (ΦTEA)88 300:1:0.1 2 28 51 58e 4.3 1.51 6 (PEGA)32b-(ΦTEA)88b-(PEGA)32 7 (PEGA)160(ΦTEA)108 600:1:0.1 4 45 158 181e 4.7 1.36 b-(ΦTEA)108b-(PEGA)160 a Determined by 1H NMR analysis of the crude product. b Calculated from conversion  [M]/[CTA]. c Eluent THF, RI detection, calibration by polystyrene standards. d Calculated by end group analysis via UV band (λ = 309 nm, εCTA1 = 14 700 L mol-1 cm-1, and εCTA2 = 30 930 L mol-1 cm-1 in CH2Cl2. e Calculated from the compositional data according to 1H NMR, assuming that Mn of the first block is preserved in the block copolymer. 1 2 3 4

Figure 2. (A) 1H NMR spectra of (i) poly(ΦTEA)88, (ii) poly(PEGA)32-b-(ΦTEA)88-b-(PEGA)32, and (iii) TriB-3 in CDCl3 . (B) SEC traces of (i, continuous line) (ΦTEA)88, (ii, dashed line) (PEGA)32-b-(ΦTEA)88-b-(PEGA)32, and (iii, continuous line) TriB-3. (C) SEC traces of (i, dashed line) TriB-3 and (ii, continuous line) corresponding PS graft chains after cleavage.

spectroscopic determination of the trithiocarbonate content, under the assumption that the polymer bears exactly one trithiocarbonate end group, when monofunctional RAFT agent CTA1 was used, or two end groups, when the bifunctional CTA2 was used. The obtained values agree well with the theoretically predicted ones, indicating the successful synthesis of defined poly(ΦTEA) macroinitiators. Note that, under RAFT polymerization conditions, the pendant alkoxyamine moiety is thermally stable according to thermogravimetric and 1H NMR analysis. Thus, every repeat unit of poly(ΦTEA) bears one pendant alkoxyamine, that is, functionalization is quantitative. 3150 DOI: 10.1021/la903087p

Reactive diblock and triblock copolymers were prepared by a “grafting-through” approach using macromonomer PEG acrylate (PEGA, Mn = 475 g/mol), starting from monofunctional macro-RAFT agent poly(ΦTEA)77 and bifunctional macroRAFT agents poly(ΦTEA)88 and poly(ΦTEA)108, respectively. The polymerization conditions and the results of 1H NMR and SEC characterization are summarized in Table 1. The SEC elugrams in Figure 2B illustrate the shift of the distributions after the block copolymerization step. The shifted, monomodal, and relatively narrow distributions demonstrate the successful copolymerization, as corroborated by NMR analysis (Figure 2A). Langmuir 2010, 26(5), 3145–3155

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Table 2. Synthesis and Characterization of the Amphiphilic Poly((ΦTEA-g-PS)-b-PEGA) and Poly(PEGA-b-(ΦTEA-g-PS)-b-PEGA) Dual Brush Block Copolymera brush copolymer theoryc polymer

macroinitiator

time (h)

SECd

NMRe

cleaved PS graftf

convn Mn,app Mn of Mn Mn (%)b [10-3 g mol-1] [10-3 g mol-1]g Mw/Mng [10-3 g mol-1] PS graftsh Mn Mw/Mn f [%]i

DiB-1 DiB-2 TriB-3

(ΦTEA)77-b-(PEGA)108 18 32 335 29 1.23 290 2800 4900 1.32 57 40 432 43 1.27 352 3120 3500 1.29 89 (ΦTEA)77-b-(PEGA)191 22 43 452 76 1.15 431 4160 8500 1.24 49 (PEGA)32-b-(ΦTEA)88- 19.5 b-(PEGA)32 17 20 408 22 1.07 381 1760 2000 1.32 88 TriB-4 (PEGA)160b-(ΦTEA)108b-(PEGA)160 a Conditions for NMP graft polymerization: 135C, [monomer]/[macroinitiator] = 100/1. b Conversion determined gravimetrically. c Calculated according to eq 1. d Eluent THF, RI detection, calibration by polystyrene standards. e Calculated by 1H NMR knowing the molar mass of the precursor and the weight fraction of the brushes. f After cleavage, measured by SEC. g Apparent values of brush copolymer fraction. h Number average molar mass of PS grafts assuming 100% grafting efficiency. i Grafting density, f = DPsc,NMR/(Msc,SEC/104).

However, despite their higher molar masses, both poly(ΦTEA-bPEGA) and poly(PEGA-b-ΦTEA-b-PEGA) block copolymers eluted slower than their poly(ΦTEA) precursors. This behavior is attributed to interactions of the copolymers with the column material, resulting in an interference of adsorption and size exclusion mechanisms.34,42,47,63 Figure 2A depicts exemplarily the 1H NMR spectrum of (PEGA)32-b-(ΦTEA)88-b-(PEGA)32, demonstrating the presence of both poly(PEGA) and poly(ΦTEA) in the products. Accordingly, from their overall composition determined from the integrated 1H NMR spectra, the overall molar masses of reactive diblock and triblock copolymers were calculated, assuming that the molar mass values of the poly(ΦTEA) blocks remained unchanged in the copolymerization steps. The calculated values agreed well with the theoretical molar masses derived from the conversion and the engaged molar amounts of monomer and RAFT agent. Therefore, the combined SEC and 1H NMR analyses show the successful chain extension yielding amphiphilic reactive diblock as well as symmetrical triblock copolymers. Polystyrene Grafting. The pendant alkoxyamine groups of the poly(ΦTEA) block were used to initiate the graft polymerization of styrene by NMP at 130 C, in analogy to the synthesis of linear PS by similar alkoxyamines.14 The monomer-to-initiator ratio was chosen to be about 100, and toluene was added as solvent to mitigate possible problems associated with high viscosity. The polymerization was stopped at monomer conversions below 50%. The polymerization conditions and the results of 1H NMR and SEC are summarized in Table 2. Accurate estimation of molar mass distributions of the amphiphilic copolymer dual brushes was complicated by the occurrence of non-size-exclusion effects during SEC analysis, as found for the precursor block copolymers. Nevertheless, one peak at considerably shorter elution times, corresponding to higher molar mass, was observed in the elugrams, indicating successful grafting of styrene to the backbone (Figure 2B). The polydispersity indices of the high molar mass peaks were in the range of 1.15-1.25, indicating controlled grafting of styrene. In addition, a smaller second peak at longer elution times was also observed in the SEC elugrams. This peak was attributed to a minority population of linear styrene homopolymers formed due to the self-initiation of styrene, as discussed below. Attempts to separate the styrene homopolymer by precipitation from the amphiphilic styrene brush did not succeed. Quantification of the polystyrene fraction from UV

detection, together with overall compositional analysis by 1H NMR analysis, enables calculation of the molar masses of the brush copolymers. Cleavage of the PS Branches. To ensure the successful synthesis of novel amphiphilic dual brush copolymers, the ester bonds anchoring the PS grafts to the backbone were cleaved with KOH/methanol at 60 C. Analysis of the cleaved PS chains by SEC shows monomodal molar mass distributions with polydispersities in the range of 1.24-1.32. Importantly, the molar masses of the PS graft chains correspond exactly to the minority peak at lower molar masses seen in the SEC elugrams of the brush copolymers (Figure 2C). This finding is explained by the thermal self-initiation of styrene at 130 C, which plays an important role in the controlled radical polymerization.64 Consequently, in parallel to the main process of styrene graft polymerization by the dissociated macroinitiator radicals, some polymer chains are newly initiated in solution. Due to the reversible termination by dissociated persistent radicals (TEMPO) originating from the graft process, this leads to NMP of styrene in solution yielding the minority fraction of linear PS and limits the grafting efficiencies (see Table 2). Thermal Properties. Differential scanning calorimetry (DSC) revealed that all the dual brush block copolymers exhibit an intense melting peak Tm at about -10 C and a glass transition Tg at about -65 C, which is characteristic for the poly(PEGA) blocks,65 as well as a second glass transition in the range from 79 to 92 C, which is attributed to the PS grafts, with Tg increasing according to their molar mass.66 Hence, the hydrophilic poly(PEGA) brushes and the hydrophobic PS brushes are thermodynamically incompatible with each other in DiB-1, DiB-2, TriB-3, and TriB-4. This and the resulting tendency for microphase separation in bulk should favor the tendency of the dual brush copolymers to self-organize in selective solvents, for instance, in aqueous solutions, in addition to their amphiphilic nature. Self-Assembly in Solution. Aqueous dispersions of the copolymers were prepared starting from a common solvent that is miscible with water to accelerate their dispersion and the formation of aggregates.6 THF was used as common solvent, which can easily be removed by evaporation by virtue of its low boiling point. The aqueous dispersions were prepared under ambient conditions by two protocols: in pure water (protocol A) and in the presence of 9.1 wt % DMF as cosolvent (protocol B). The latter protocol was found helpful to ensure long time stability

(63) Rinaldi, D.; Hamaide, T.; Graillat, C.; D’Agosto, F.; Spitz, R.; Georges, S.; Mosquet, M.; Maitrasse, P. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 3045– 3055.

(64) Goto, A.; Fukuda, T. Prog. Polym. Sci. 2004, 29, 329–385. (65) Garnier, S.; Laschewsky, A. Macromolecules 2005, 38, 7580–7592. (66) Blanchard, L.-P.; Hesse, J.; Malhotra, S. L. Can. J. Chem. 1974, 52, 3170– 3175.

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Table 3. Characteristics of the Aggregates of Poly((ΦTEA-g-PS)-b-PEGA) and Poly(PEGA-b-(ΦTEA-g-PS)-b-PEGA) Dual Brush Block Copolymers as Studied by DLS aggregate size by DLSc entry

polymer

contour length [nm]

hydrophobe content [wt %]

preparation protocol

d

Dh [nm]

PDI

Dh [nm]e

PDI

f 450 0.13 DiB-1 46 83 Aa DiB-1 Bb 285 0.15 f 410 0.15 DiB-2 67 75 Aa DiB-2 Bb 220 0.02 f 380 0.02 TriB-3 38 92 Aa TriB-3 Bb 175 0.12 185 0.04 B 170 0.04 TriB-3g a f 210 0.08 TriB-4 108 59 A TriB-4 Bb 125 0.14 115 0.07 B 130 0.08 TriB-4g a Preparation of 0.1 wt % aqueous solutions by solvent exchange at 25 C. bAs protocol A, but replacing pure water by D2O/DMF (10:1 w/w) mixture. c Mean hydrodynamic diameter Dh according to the volume distribution. dMeasured 1 day after preparation. e Measured 6 months after preparation. f Visible phase separation after a few days of storage. g Samples employed for the SAXS and SANS measurements, diluted by 1:50 with D2O/DMF (10:1 w/w) mixture.

1 2 3 4 5 6 6a 7 8 8a

and prevent phase separation with storage. The high turbidity indicates already visually the presence of rather large aggregates. First, aqueous dispersions of the amphiphilic dual brush copolymers were studied by DLS (Table 3, protocol A). The diblock as well as the triblock copolymers produced relatively large aggregates with hydrodynamic diameters (Dh) in the range of 125-450 nm, where always somewhat smaller aggregates are observed in the D2O/DMF mixtures. The hydrodynamic diameters of the aggregates formed by the diblock copolymers DiB-1 and DiB-2 are 445 and 413 nm, respectively, larger than the diameters of the aggregates formed by the triblock analogues. The aggregate size distributions tended to be broader in the case of the diblock copolymer dual brushes, too. The comparison within the pairs of diblock and triblock copolymers shows that the aggregate size decreases with DiB-1 > DiB-2, and TriB-3 >TriB-4, that is, with increasing length of the hydrophilic brush, while keeping the lengths of the hydrophobic blocks about constant. Except for triblock copolymer TriB-4, the Dh values are considerably larger than twice the contour lengths of the polymers and therefore too big for simple spherical micelles. For those, the core diameter for the diblock copolymer aggregates should be in the order of twice the contour length of the hydrophobic block but similar to the length of a single hydrophobic block for the case of the triblock copolymers. Only in the case of triblock copolymer TriB-4, the much smaller aggregates with a hydrodynamic diameter of 200 nm would be compatible with the formation of spherical micelles. Hence, both the diblock and triblock copolymer dual brushes seem to yield well-defined amphiphilic aggregates but of more complex structure. In order to learn more about these aggregates, stable colloidal dispersions are mandatory for additional investigation. However, the purely aqueous dispersions become unstable after several days, and phase separation occurs. Therefore, aqueous dispersions of the dual brush polymers were prepared by adding a small amount of DMF (Table 3, protocol B). Strikingly, protocol B favors the formation of considerably smaller aggregates of 150-200 nm hydrodynamic diameters. Hence, the added DMF helps not only to stabilize the colloids but also to disperse the polymers more efficiently. The diameters measured seem still too high for the presence of classical spherical micelles in the cases of at least DiB1, DiB-2, and TriB-3. Still, the aggregates are colloidally stable and do not phase separate even after 6 months of storage. On the contrary, the size distribution gets narrower upon storage while the hydrodynamic diameters shrink slightly. It may seem surprising that stable colloids are obtained, considering the large fraction of polystyrene in the brush polymers and the geometrical constraints for packing such molecules. Still, stable crew-cut 3152 DOI: 10.1021/la903087p

micelles from block copolymers with very small hydrophilic blocks exemplify that under such conditions stable colloids can be obtained, if the latter are strongly hydrophilic.7,67 This is the case for the poly(PEGA) brushes. Moreover, the true packing situation of the dual brushes in solution is not obvious from the relative molecular dimensions of the hydrophilic and hydrophobic brushes. While the hydrophobic polystyrene blocks, being the major segment in all the dual brushes DiB-1, DiB-2, TriB-3, and TriB-4, should give rise to thicker brushes in nonselective solvents (see Figure 1b), their collapse in a selective solvent as water may invert the relations (see Figure 1c). The colloidal stability of samples prepared by protocol B allowed for further studies by SANS and SAXS experiments68 to obtain a more detailed structural picture of the aggregates formed, focusing on the novel dual brush triblock copolymers TriB-3 and TriB-4. The turbid solutions remained dispersed homogeneously and unaltered for at least 6 months. A plot of the SANS data is given in Figure 3A. The data prove that for both polymers rather large structures must be present as evidenced by the high scattering intensity and the large intensity increase at low q-values. At the same time, both samples show the scattering of rather well-defined particles with intensity oscillations at 0.064 and 0.125 nm-1 for TriB-3 and TriB-4, respectively. For TriB-3, even a second order oscillation around 0.105 nm-1 is observed. Accordingly, the scattering arises from the hydrophobic core of aggregates with particle radii of about 70 and 35 nm for TriB-3 and TriB-4, respectively, proving the formation of larger aggregates for the more hydrophobic TriB-3. With the much more pronounced oscillations, TriB-3 apparently forms better defined and homogeneous aggregates, in agreement with the DLS data (Table 3). In order to extend this structural picture, SAXS measurements (Figure 3B) were done on the same samples 12 days later. Note that the SANS patterns did not show any noticeable change when repeated after 2 months. They show basically the same features as the SANS experiments, with scattering curves indicative of globular objects and, as for the case of SANS, with a much more well-defined scattering pattern for TriB-3. As a first step, SAXS and SANS data were analyzed with respect to their structural dimensions, by means of various modelfree analysis (for details, see the Supporting Information). The radius from the Guinier approximation R(Rg) is representative for the hydrated aggregate, while the radius from the Porod (67) Cameron, N. S.; Corbierre, M. K.; Eisenberg, A. Can. J. Chem. 1999, 77, 1311–1326. (68) Bou
e, F.; Cousin, F.; Gummel, J.; Oberdisse, J.; Carrot, G.; Harrak, A. E. C. R. Phys. 2007, 8, 821–844.

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Figure 3. (A) SANS intensity I and (B) SAXS intensity I as a function of the magnitude of the scattering vector q for 1 wt % samples of TriB-3 (, lower curve) and TriB-4 (0, upper curve) in D2O. Table 4. Parameters of the Micellar Aggregates as Deduced from a Model-Free Analysis of the SANS and SAXS Data, in Comparison to Hydrodynamic Radii Rh and Rg Obtained by DLS and SLS, Respectivelya TriB-3

TriB-4

-1

431 000 381 000 Mw (polymer)/g mol contour length/nm 38 108 95.2 65.0 Rh (DLS)/nmb b 100.1 70.6 Rg (SLS)/nm 71.5 45.1 Rg (SANS)/nm 92.3 58.2 R(Rg)/nm 76.5 44.5 RP (SANS)/nm 24 100 5675 I(0) (Guinier) (SANS)/cm-1 8 8.30  10 1.91  108 Mw (I(0) (SANS)/g/mol Rcore/nm 66.9 37.4 1530 440 Nagg (SANS) 77.1 43.6 Rg (SAXS)/nm -1 10.14 3.89 I(0) (SAXS)/cm 6.05  108 8.06  107 Magg (I(0) (SAXS)/g/mol Rcore/nm 60.0 28.0 1100 190 Nagg (SAXS) a Given are the molecular weight of the respective polymer molecules Mw, the radius of gyration Rg, the radius of a homogeneous sphere corresponding to Rg, R(Rg), the extrapolated intensity at zero-scattering angle I(0), the deduced molar mass Magg of the aggregates, the radius of the PS core (Rcore) corresponding to Magg, and the aggregation numbers Nagg of the copolymer calculated from Magg. b Samples for SLS/DLS were diluted by 1:50 with D2O/DMF (10:1 w/w) mixture.

volume RP is not really sensitive to the actual solvation. The comparison of both gives some hint about the solvation of the aggregate. A decisive comparison for hydration is made between the volume of the sphere of radius R(Rg) and the mass obtained from I(0). These model-free structural parameters were determined from the SANS scattering curves, and the corresponding values are given in Table 4. Assuming a density of the polymer of 1.056 and 1.085 g/mL for TriB-3 and TriB-4, respectively (based on the relative content of EO, AA, and PS, and using density data for these homopolymers), yields scattering length densities (SLDs) of 13.5 and 12.1  109 cm-2 for TriB-3 and TriB-4, respectively, and a scattering length density of the solvent of 57.9  109 cm-1. The SAXS data were analyzed in an identical fashion (Table 4). In our analysis, we also took into consideration the amount of PS homopolymer contained in the samples which was 23 and 10 wt % for TriB-3 and TriB-4, respectively. In general, we find very good agreement between the Rg values deduced from the shape of the SANS and SAXS curves. Also, the values deduced from the absolute intensities are in good agreement. This proves the presence of micellar aggregates of the amphiphilic polymers, which contain a large number of individual Langmuir 2010, 26(5), 3145–3155

polymer molecules and have a PS core and a PEO corona. The smaller aggregation number deduced by SAXS for TriB-4 might not be significant given the errors introduced by the uncertainty for the SLD. Note that, for the SAXS data, the differences of the scattering length densities (contrast) of polymer and DMF þ water are extremely small, so that errors in the density assumption for the polymers become very significant. For instance, an error in the density assumption of 1% for TriB-4 translates into an error of 35% with respect to the deduced aggregation number. Assuming that SAXS sees only the PS-based core, the aggregation numbers would jump to 2000 and 1000 for TriB-3 and TriB-4, respectively, instead of 1160 and 150, which also means that the error essentially affects TriB-4, which has by far the largest hydrophilic part (the fraction of the EO block being 41 wt %, as opposed to 8 wt % of the EO block for TriB-3). All such uncertainties do not arise for the SANS contrast. In contrast, the interpretation of the SANS intensity is very reliable, revealing aggregation numbers of about 1530 for TriB-3 and of 440 for TriB-4. For both samples, the radius of gyration, Rg, is larger than the calculated radius of a compact PS core, by 7% for TriB-3 and by 12% for TriB-4, in agreement with the more extended PEO corona expected for TriB-4. This is further corroborated by the fact that the radius derived from the Porod analysis, RP, is smaller than Rg only for TriB-4. The relatively large values for Rg suggest that the aggregates are not compact in structure but have a diffuse outer shell composed of the PEO brushes that extend deep into the surrounding water phase. For such a structure, one only expects the hydrophobic PS core to be rather compact, while the PEO corona would be strongly solvated with water molecules where one might expect to have up to 100 water molecules associated loosely per EO unit69,70 and about 3 water molecules per EO unit to be bound rather strongly.71 The sizes determined by SANS and SAXS are in good agreement not only with each other but also with respect to the values determined by DLS and SLS. The radius of gyration obtained by SLS is always significantly larger than the one obtained from SANS or SAXS. This arises from the fact that light scattering is more sensitive for the larger sizes than in the more limited q-range accessible by SANS and SAXS. Accordingly, the sizes observed by SLS are more realistic and comprise the full PEO corona. For both cases, we observe that the ratio between hydrodynamic radius and radius of gyration, Rh/Rg, is 1.05-1.09. This value is (69) Kjellander, R.; Florin, E. J. Chem. Soc., Faraday Trans. 1 1981, 77, 2053– 2077. (70) Keys, K. B.; Andreopoulos, F. M.; Peppas, N. A. Macromolecules 1998, 31, 8149–8156. (71) Smart, T. P.; Mykhaylyk, O. O.; Ryan, A. J.; Battaglia, G. Soft Matter 2009, 5, 3607–3610.

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Figure 4. SFM height image of TriB-4 on poly-L-ornithine precoated mica in (A) top view and (B) 3D representation. (C) Height profile along the line shown in (A). (D) Histogram of 61 particle heights in (A). Average height amounts to 65 ( 20 nm.

much larger than for a hard sphere, and is close to that of a star-like polymer with many arms of uniform arm length,72 thereby indicating that the aggregate structure is that of a rather extended PEO corona. The difference is similar for both triblock copolymers, which indicates that a rather compact structure is present, too, for TriB-4 with its much longer PEO brush. Obviously, the densely grafted PEO brushes form rather compact PEO coronas, in comparison to much more open structures encountered for copolymer micelles with linear PEO chains.73,74 When comparing to the molecular geometry and the size of the building blocks, one finds for the case of the TriB-4 that the value obtained from SANS/SAXS corresponds very well to half the length of the stretched molecule (54 nm). In the case of TriB-3 (19 nm), a much larger experimental value is observed. In order to interpret this discrepancy, one should keep in mind that both aggregates contain in addition to the amphiphilic double brush copolymers also the hydrophobic homopolymer polystyrene that allows swelling the hydrophobic core of the aggregates beyond the value expected just from the hydrophobic domain of the copolymers. It is now interesting to calculate the area required per PEO brush at the interface between the hydrophobic PS core and the aqueous environment containing the PEO brush. Assuming a spherical shape of the aggregates, we obtain an area per PEO brush of 18.4 nm2 for TriB-3 and 20.0 nm2 for TriB-4. These values compare rather well with the theoretical upper value of the (72) Burchard, W. Adv. Polym. Sci. 1999, 143, 113–194. (73) Pedersen, J. S.; Gerstenberg, M. C. Macromolecules 1996, 29, 1363–1365. (74) Stellbrink, J.; Rother, G.; Laurati, M.; Lund, R.; Willner, L.; Richter, D. J. Phys.: Condens. Matter 2004, 16, S3821–S3834.

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area obtained by taking the stretched PEO graft chain of the polymer as an effective radius of the brush (lPEO), yielding an area of 48 nm2 (= πlPEO2). Furthermore, it shows very nicely the expected effect that the much longer PEO brush of TriB-4 requires a larger area at the interface of the aggregates. The values themselves further corroborate our classification of these molecules as “giant surfactants” because the head group areas are by about a factor of 40-50 larger than the values encountered for standard surfactants,75 in agreement with the much more bulky “head group” of our giant surfactants. Self-Assembly of the Aggregates at Surfaces. Aggregates of TriB-4 formed in solution were deposited exemplarily onto three ultraflat surfaces with different surface chemistries, namely, on mica, on poly-L-ornithine precoated mica, and on octadecylamine precoated HOPG, in order to investigate their shape and size distributions at surfaces. In all cases, 2D clusters of slightly hexagonally distorted spherical particles with similar sizes and size distributions were observed. However, the size of the clusters varies largely, indicating different mobilities on the three surfaces. Figure 4A, B displays SFM images of TriB-4 on poly-Lornithine precoated mica. It shows a compact 2D island with the largest particles in the center and the smallest ones at the periphery. Height profiles (Figure 4C) indicate that the spacings between the particles correspond to their respective thicknesses, indicating almost spherical shapes, which seem slightly hexagonally distorted parallel to the surface (Figure 4A). The histogram of the particle heights exhibits a maximum around 55 nm and an average of 65 ( 20 nm. On octadecylamine precoated HOPG, (75) Rosen, M. J. Surfactants and Interfacial Phenomena, 3rd ed.; Wiley: New York, 2004.

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Article Scheme 4. Schematic Illustrating the Self-Sorting of Spherical Colloidal Particles with Hydrophilic Surfaces According to Their Size during Drying from an Aqueous Solution

the present case to the drying process, during which the particles move toward the center of an evaporating droplet (Scheme 4).

Figure 5. SFM height image of TriB-4 on mica.

many smaller 2D clusters were observed (Supporting Information) with an average height of 73 ( 22 nm, similar to the case on precoated mica. On pure mica, even larger 2D clusters are observed (Figure 5), surrounded by large empty areas. Again, the largest particles are found in the center of the cluster, and the height distribution is similar to the cases described above. The similarity of the particle shapes and of the size distributions on the three different substrates indicates that the particles are preformed in solution and do not change much upon selfassembly on the surfaces. Furthermore, the sizes are in good agreement with the ones obtained from SANS/SAXS (Table 4), confirming the stability of these micellar aggregates. The slight hexagonal distortion of the particles seen in the inset of Figure 5 is a reminder of the deformation of latexes in the initial stage of film formation, for example, by capillary forces,76,77 and points to hard spheres with a soft shell. This picture would correspond to particles containing a PS core with a high Tg, surrounded by a collapsed soft shell of poly(PEGA), in agreement with the DSC results. The obtained numbers for the average particle diameters in the range of 70 nm may be compared to the particle sizes in solution. In fact, this value matches well the diameter of the hydrophobic core of 75 nm determined by SANS (Table 4) in solution, also when assuming that the collapsed hydrophilic brush should increase the diameter of the particles by 15-20%. The increasing 2D cluster size in the order precoated HOPG > precoated mica > mica indicates that the mobility on pure mica is the highest under ambient conditions. This may be due to different wettabilities and also mobilities of the dissolved adsorbates, which both decrease from mica over the precoated mica to the precoated HOPG. Particularly interesting is the ordering of the particles within a 2D cluster according to their sizes. It has been reported before that lyophilic colloidal particles can sort according to their sizes on surfaces of patterned wettability,78 and also that polyelectrolyte-amphiphile complexes can self-sort on HOPG according to their lengths.79 We attribute the ordering in (76) Dobler, F.; Holl, Y. Trends Polym. Sci. 1996, 4, 145–151. (77) Film Formation in Waterborne Coatings; Provder, T., Winnik, M. A., Urban, M. W., Eds.; ACS Symposium Series 648; American Chemical Society: Washington, DC, 1996. (78) Fan, F.; Stebe, K. J. Langmuir 2005, 21, 1149–1152. (79) Severin, N.; Sokolov, I. M.; Miyashita, N.; Kurth, D. G.; Rabe, J. P. Macromolecules 2007, 40, 5182–5186.

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Conclusions The orthogonal overlay of two methods of controlled free radical polymerization, here of RAFT and NMP, enables the synthesis of “giant surfactants” in the form of amphiphilic dual brushes, either as diblock or as symmetrical triblock copolymers. These novel amphiphilic polymers undergo microphase separation in bulk and can self-assemble into spherical micellar aggregates in aqueous media. The aggregates formed are in the size range of polymer latexes, as evidenced by DLS, SAXS, and in particular SANS experiments, and possess micellar molar masses of 108-109 g/mol, that is, about 10 000 times more than those of conventional surfactant micelles. These polymer micelles are long-time stable and consist of a PS core surrounded by a PEO corona of densely packed PEO brushes. The area required at the PS/PEO interface is given by the size of the PEO brush and controls by simple packing arguments the size of the micelles formed in a similar fashion as for normal surfactants. These micelles are sufficiently stable to allow their intact deposition onto flat surfaces, where they form clusters, with similarities to hard core latexes, and show spontaneous sorting according to their size within the clusters, most probably induced by the drying process. The results demonstrate that it is possible by the new polymerization tools to prepare defined macromolecules which self-assemble in a straightforward way to micellar aggregates in the upper mesoscopic size range, which is generally rather associated with top-down than with bottom-up approaches. Acknowledgment. We thank C. Wieland and J.-F. Lutz (Fraunhofer IAP, Potsdam-Golm) for help with SEC measurements, and M. Heydenreich and A. Krtitschka for NMR measurements (Universit€at Potsdam). Furthermore, we would like to thank P. Heunemann (ILL, Grenoble) and T. Narayanan (ESRF, Grenoble) for help with the SANS, SAXS, and corroborating SLS and DLS experiments, and N. Severin (Humboldt-Universit€at zu Berlin) for fruitful discussions. Financial support was given by Sfb 448 of Deutsche Forschungsgemeinschaft DFG and by Fonds der Chemischen Industrie. Supporting Information Available: Details on the synthesis of CTA1, inimer ΦTEA, and brush diblock copolymer poly((ΦTEA-g-PS)-b-PEGA) and details for analysis by SAXS and SANS. DSC traces of the amphiphilic dual brush copolymers. DLS size distribution curves of dual brush aggregates in aqueous dispersion. SFM height and phase image of aggregates of TriB-4 deposited on octadecylamine precoated HOPG. This material is available free of charge via the Internet at http://pubs.acs.org. DOI: 10.1021/la903087p

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