Morphologies of Multicompartment Micelles Formed by ABC Miktoarm

Sep 27, 2006 - Several new multicompartment micellar structures have been identified by cryogenic transmission electron microscopy (cryoTEM) from the ...
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Langmuir 2006, 22, 9409-9417

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Morphologies of Multicompartment Micelles Formed by ABC Miktoarm Star Terpolymers Zhibo Li,† Marc A. Hillmyer,*,† and Timothy P. Lodge*,†,‡ Departments of Chemistry and of Chemical Engineering and Materials Science, UniVersity of Minnesota, Minneapolis, Minnesota 55455 ReceiVed July 11, 2006. In Final Form: August 18, 2006 Several new multicompartment micellar structures have been identified by cryogenic transmission electron microscopy (cryoTEM) from the aqueous self-assembly of µ-[poly(ethylethylene)][poly(ethylene oxide)][poly(perfluoropropylene oxide)] (µ-EOF) miktoarm star terpolymers. This work extends our previous studies, in which it was found that, upon decreasing the length of the hydrophilic block (O), the resulting micelles evolved from “hamburger” micelles to segmented worms and ultimately to nanostructured bilayers and vesicles. In the terpolymers examined here segmented ribbons and bilayers were found at an intermediate composition between segmented worms and nanostructured bilayers, provided that the fluoropolymer (F) was the minority component in the micelle core. On the other hand, when the F block exceeded the chain length of the hydrocarbon block (E), the superhydrophobic F block imposed a “double frustration” on the self-assembly of the µ-EOF(2-9-5) terpolymer; while F prefers to minimize its interfacial contact with the O corona, it must occupy the majority of the micellar core. Therefore, a richer variety of multicompartment micelles, including well-defined segmented worms, raspberry-like micelles, and multicompartmentalized worms, were formed from one terpolymer, as revealed by cryoTEM. Despite the complexity and variety of the observed aggregate morphologies, a small number of common structural elements can be invoked to interpret the observed micelles and to relate a given structure to the terpolymer composition.

Introduction The self-assembly of multiblock copolymers into multicompartment micelles, which are nanoscopic aggregates with subdivided solvophobic cores, has received great interest recently.1-8 The concept of multicompartment micelles was originally inspired by biological systems in which a single eukaryotic cell, comprising many different subunits, can perform an array of distinct functions. However, the micellar structures available from AB two-component amphiphiles are limited to the partitioning of space into an “inside” and an “outside”. Synthetic ABC triblock terpolymers containing three mutually immiscible blocks can serve as a model system to create several compartments within one micelle core.6-11 For example, a hydrophilic block can stabilize the micelle in water while the other immiscible polymeric components form the segregated micellar core. In addition, the micelle corona could be composed of biocompatible materials labeled with selective recognition elements for targeting drug or gene delivery, and the segregated * To whom correspondence should be addressed. E-mail: hillmyer@ chem.umn.edu (M.A.H.), [email protected] (T.P.L). † Department of Chemistry. ‡ Department of Chemical Engineering and Materials Science. (1) Laschewsky, A. Curr. Opin. Colloid Interface Sci. 2003, 8, 274. (2) Thu¨nemann, A. F.; Kubowicz, S.; von Berlepsch, H.; Mo¨hwald, H. Langmuir 2006, 22, 2506. (3) Kubowicz, S.; Baussard, J.-F.; Lutz, J.-F.; Thu¨nemann, A. F.; von Berlepsch, H.; Laschewsky, A. Angew. Chem., Int. Ed. 2005, 44, 5262. (4) Kubowicz, S.; Thu¨nemann, A. F.; Weberskirch, R.; Mo¨hwald, H. Langmuir 2005, 21, 7214. (5) Lutz, J.-F.; Laschewsky, A. Macromol. Chem. Phys. 2005, 206, 813. (6) Li, Z.; Kesselman, E.; Talmon, Y.; Hillmyer, M. A.; Lodge, T. P. Science 2004, 306, 98. (7) Li, Z.; Hillmyer, M. A.; Lodge, T. P. Macromolecules 2006, 39, 765. (8) Li, Z.; Hillmyer, M. A.; Lodge, T. P. Nano Lett. 2006, 6, 1245. (9) Lodge, T. P.; Rasdal, A.; Li, Z.; Hillmyer, M. A. J. Am. Chem. Soc. 2005, 127, 17608. (10) Lodge, T. P.; Hillmyer, M. A.; Zhou, Z.; Talmon, Y. Macromolecules 2004, 37, 6680. (11) Zhou, Z.; Li, Z.; Ren, Y.; Hillmyer, M. A.; Lodge, T. P. J. Am. Chem. Soc. 2003, 125, 10182.

micelle core could in turn provide distinct chemical environments to store various drug molecules, gene therapy agents, or pesticides.9 Ideally, the discrete nanocompartments within one micelle core can accommodate two or more incompatible compounds, with the possibility of concurrent transport and subsequent “double delivery” of active agents in a prescribed manner. Therefore, understanding the underlying principles governing the self-assembly of the ABC terpolymer on the molecular level is of fundamental importance to the control and application of these hierarchical structures. Initial progress toward this goal has recently been made.2-7,9-19 For example, several years ago Gohy et al. reported the formation of core-shell-corona micelles containing a pH-responsive middle shell from a polystyrene-b-poly(2-vinylpyridine)-b-poly(ethylene oxide) (PS-P2VP-PEO) triblock terpolymer.12 Zhou et al. observed a disklike core-shell-corona micelle assembled from an ABC triblock composed of a fluorinated 1,2-polybutadiene (PBD), a PS, and a PEO block.10,11 Using cryogenic transmission electron microscopy (cryoTEM), Laschewsky and co-workers observed a raspberry-like micelle from the aqueous self-assembly of an ABC triblock containing a fluorocarbon, a hydrocarbon, and a charged block.3 They proposed that the fluoropolymer formed small spherical domains dispersed in a large hydrocarbon-rich domain. This structure is at least qualitatively consistent with the earlier prediction by Dormidontova (12) Gohy, J.-F.; Willet, N.; Varshney, S.; Zhang, J.-X.; Je´roˆme, R. Angew. Chem., Int. Ed. 2001, 40, 3214. (13) Sta¨hler, K.; Selb, J.; Candau, F. Langmuir 1999, 15, 7565. (14) Weberskirch, R.; Preuschen, J.; Spiess, H. W.; Nuyken, O. Macromol. Chem. Phys. 2000, 201, 995. (15) Kujawa, P.; Goh, C. C. E.; Calvet, D.; Winnik, F. M. Macromolecules 2001, 34, 6387. (16) Kotzev, A.; Laschewsky, A.; Rakotoaly, R. H. Macromol. Chem. Phys. 2001, 202, 3257. (17) Kotzev, A.; Laschewsky, A.; Adriaensens, P.; Gelan, J. Macromolecules 2002, 35, 1091. (18) Brannan, A. K.; Bates, F. S. Macromolecules 2004, 37, 8816. (19) Zhu, J.; Jiang, W. Macromolecules 2005, 38, 9315.

10.1021/la0620051 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/27/2006

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and Khokhlov.20 Additionally, some intricate micelle structures have also been formed from an ABC triblock containing a polyelectrolyte block, in which the micelle morphologies could be tuned by changing the solvent composition, solution pH, and salt concentration and the addition of multivalent ligands. For example, Yu and Eisenberg reported the formation of spherical, rodlike, and vesicular aggregates from a polystyrene-b-poly(methyl methacrylate)-b-poly(acrylic acid) (PS-PMMA-PAA) triblock by tuning the composition of the solvent mixtures.21 Pochan, Wooley, and co-workers systematically investigated the formation of toroids, disks, and cylinders formed by polystyreneb-poly(methyl acrylate)-b-poly(acrylic acid) (PS-PMA-PAA) triblock terpolymers by changing the THF/H2O mixture composition, selecting different divalent cation ligands, and modifying the copolymer architecture.22-24 Brannan and Bates documented the formation of monolayer vesicles from PEO-PS-PBDPEO linear tetrablock copolymers, in which the hydrophobic membrane could be internally segregated.18 Very recently, Thu¨nemann and co-workers explored a two-compartment cylindrical micelle formed from an ABCBA pentablock copolymer in dilute aqueous solution,2 and Balsara et al. created flat layered micelles from an olefinic ABCA system.25 In general, the above-mentioned micellar structures feature a concentric arrangement of core-shell-corona domains regardless of the overall micelle shape, due to the sequential linkage of the different blocks. For ABC miktoarm star terpolymers, in contrast, the mandatory convergence of the three immiscible blocks at one common junction point suppresses the formation of concentric structures and leads to an array of new morphologies with compartmentalized micellar cores. In the aqueous self-assembly of µ-[poly(ethylethylene)][(poly(ethylene oxide)][poly(perfluoropropylene oxide)] [µ-(PEE)(PEO)(PFPO)] star terpolymers, we have observed two distinct types of packing within the micellar cores, provided the PFPO (F) block is the smallest. At high PEO (O) volume fractions (fPEO), the F block forms a disklike layer that is sandwiched by two layers of hydrocarbon domains, as in the reported hamburger or segmented wormlike micelles.6 At low fPEO, the F block forms cylindrical domains immersed in a PEE (E) matrix, as in the observed nanostructured bilayers and vesicles.8 In both cases the hydrophilic O blocks emanate from the interfaces between hydrocarbon and fluorocarbon domains, thereby shielding the whole micellar core from water. In this paper we expand the range of terpolymer compositions, particularly in terms of the ratio of fPFPO to fPEE. Several new micelle structures were identified relative to the previous reports,6-8,10,11 and appropriate chain-packing models are proposed to understand the observed new micellar structures. Experimental Section Materials. The mid-hydroxyl-functionalized PEE-PEO diblock copolymers were synthesized by two successive anionic polymerizations, as described before.26 A heterobifunctional 1,2-polybutadiene precursor was first formed by end-capping living polybutadienyllithium chains with 2-[(methoxymethoxy)methyl]oxirane. The heterobifunctional PEE macroinitiator was obtained by catalytic hydrogenation of the 1,2-polybutadiene precursor, prior to initiation (20) Dormidontova, E. E.; Khokhlov, A. R. Macromolecules 1997, 30, 1980. (21) Yu, G.; Eisenberg, A. Macromolecules 1998, 31, 5546. (22) Pochan, D. J.; Chen, Z.; Cui, H.; Hales, K.; Qi, K.; Wooley, K. L. Science 2004, 306, 94. (23) Li, Z.; Chen, Z.; Cui, H.; Hales, K.; Qi, K.; Wooley, K. L.; Pochan, D. J. Langmuir 2005, 21, 7533. (24) Chen, Z.; Cui, H.; Hales, K.; Li, Z.; Qi, K.; Pochan, D. J.; Wooley, K. L. J. Am. Chem. Soc. 2005, 127, 8592. (25) Gomez, E. D.; Rappl, T. J.; Agarwal, V.; Bose, A.; Schmutz, M.; Marques, C. M.; Balsara, N. P. Macromolecules 2005, 38, 3567. (26) Li, Z.; Hillmyer, M. A.; Lodge, T. P. Macromolecules 2004, 37, 8933.

Li et al. Table 1. Molecular Parameters of µ-EOF Star Terpolymers sample ID

NPEO

NPFPO

fPEOa

fPFPOa

Mn

µ-EOF(2-4-2.5) µ-EOF(2-6-2.5) µ-EOF(2-7-2.5) µ-EOF(2-9-2.5) µ-EOF(2-13-2.5) µ-EOF(2-26-2.5) µ-EOF(2-9-3.5) µ-EOF(2-13-3.5) µ-EOF(2-6-5) µ-EOF(2-7-5) µ-EOF(2-9-5) µ-EOF(2-13-5) µ-EOF(1.4-1-2.5) µ-EOF(1.4-2-2.5) µ-EOF(1.4-3-2.5) µ-EOF(1.4-5-2.5)

82 128 148 197 285 601 197 285 128 148 197 285 24 46 74 115

14 14 14 14 14 14 20 20 31 31 31 31 14 14 14 14

0.48 0.59 0.62 0.69 0.76 0.87 0.66 0.74 0.50 0.54 0.61 0.69 0.24 0.38 0.50 0.61

0.20 0.15 0.14 0.12 0.09 0.05 0.15 0.12 0.28 0.26 0.22 0.17 0.34 0.28 0.22 0.18

8100 10100 10900 13100 17000 30900 14000 17900 12900 13700 15900 19800 5900 5900 7200 9000

a The volume fractions were calculated using the molecular weight from NMR spectroscopy and amorphous densities at room temperature: FPEE ) 0.866 g/cm3,39 FPEO ) 1.12 g/cm3 (amorphous),40 and FPFPO ) 1.9 g/cm3.41 b NPEE is 31 or 22, and the total molecular weight of the terpolymer was calculated from the 1H NMR data.

of the polymerization of ethylene oxide and subsequent end-capping of the PEO block by ethyl bromide. After deprotection of the methoxymethyl group, well-defined µ-[poly(ethylethylene)][poly(ethylene oxide)][poly(perfluoropropylene oxide] (µ-EOF) miktoarm star terpolymers were obtained via coupling reactions between the mid-hydroxyl-functionalized EO diblock and the acid chloride endcapped PFPO. Three monocarboxylic acid end-capped PFPO homopolymers with molecular weights of 2500, 3500, and 5100 (determined by 19F NMR spectroscopy) were provided by Dupont. The acid group was converted into the acid chloride by treatment with oxalyl chloride in methoxynonafluorobutane (3M) at 60 °C. The resulting diblock and triblock copolymers are designated EO(x-y) and µ-EOF(x-y-z), where x, y, and z denote the molecular weights divided by 1000 of the E, O, and F blocks, respectively. Six EO diblocks with a PEE number-average degree of polymerization (NPEE) ) 31 and 0.59 < fPEO < 0.91 and four with NPEE ) 22 and 0.37 < fPEO < 0.74 were prepared. Sixteen µ-EOF miktoarm star terpolymers with different compositions and overall molecular weights from the parent EO diblocks were synthesized; their characteristics are summarized in Table 1. Of particular interest in this paper, the series of µ-EOF(2-y-2.5) and µ-EOF(1.4-y-2.5) terpolymers maintain constant E and F block lengths with varying O block lengths, and the series of µ-EOF(2-9-z) and µ-EOF(2-13-z) terpolymers vary the molar mass of the F block while keeping the E and O blocks constant. All solutions were prepared gravimetrically by direct dispersion of the copolymers into deionized water, to make 1 wt % solutions. All the solutions were stirred in sealed vials at room temperature for at least 2 weeks before measurement. CryoTEM. CryoTEM samples were prepared in a controlled environment vitrification system (CEVS) at room temperature.27 A micropipet was used to load a drop of micelle solution (ca. 5 µL) onto a lacey Formvar-supported TEM grid, which was held by tweezers. The excess solution was blotted with a piece of filter paper, resulting in the formation of thin films with thicknesses of ca. 100-300 nm in the mesh holes. After at least 20 s was allowed for relaxation of any stresses induced during the blotting, the samples were quickly plunged into a reservoir of liquid nitrogen cooled liquid ethane at its melting temperature (-183 °C). The vitrified samples were then stored in liquid nitrogen until they were transferred to a cryogenic sample holder (Gatan 626) and examined with a JEOL 1210 TEM (120 keV) at about -178 °C. The phase contrast was enhanced by underfocus. The images were recorded on a Gatan 724 multiscan CCD and processed with DigitalMicrograph, version 3.3.1. The ramp-shaped optical density gradients in the background were digitally corrected. (27) Bellare, J. R.; Davis, H. T.; Scriven, L. E.; Talmon, Y. J. Electron Microsc. 1988, 10, 87.

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Dynamic Light Scattering (DLS). All the µ-EOF micelle solutions were passed through 0.45 µm hydrophilic microfilters (Millipore) into 0.25 in. diameter glass tubes. The samples were investigated using a home-built photometer equipped with an electrically heated silicon oil index-matching bath, a Lexel 75 Ar+ laser operating at 488 nm, a Brookhaven BI-DS photomultiplier, and a Brookhaven BI-9000 correlator. Intensity autocorrelation functions, g(2)(t), were recorded at room temperature, and the measurements were typically made at angles of 45° and 90°. When the measured and the calculated baselines were in good agreement (difference < 0.1%), the correlation function was accepted. For the decay rate distribution, an inverse Laplace transform of the field correlation functions was performed using CONTIN.28 The corresponding micelle size distributions were then obtained using the Stokes-Einstein equation Rh )

k BT 6πηD0

(1)

where kb, T, η, and D0 are the Boltzmann constant, absolute temperature, solvent viscosity, and diffusion coefficient, respectively.

Background The factors that govern the self-assembly of nonionic diblock copolymers in selective solvents are rather well understood.29,30 Three classical morphologies, i.e., spheres, cylinders, and bilayer vesicles, are generally observed, and the predominant structure can be tuned from spheres to cylinders to vesicles by decreasing the corona block volume fraction or by increasing the interfacial tension, as schematically illustrated in parts a-c, respectively, of Figure 1.31-34 The interfacial curvature and the magnitude of the core block stretching decrease on transforming from spheres to cylinders to vesicles, while there is an increase in corona crowding. For the assemblies of µ-EOF star terpolymers where the F block volume is less than or equal to that of the E block, these three classical structures have been observed with decreasing O block length; however, each micelle features subdivided cores composed of nanoscopic E and F domains. The corresponding chain-packing motif and representative cryoTEM images are exhibited in parts d-f and parts g-i, respectively, of Figure 1. In these cryoTEM images (Figure 1g-i), the dark and gray domains are attributed to the fluoropolymer (F) and hydrocarbon (E), respectively, due to electron density difference; the wellsolvated O blocks are generally not distinguished from the vitrified water. The corresponding cryoTEM images for these structures, as well as the new ones to be described in this paper, are summarized as a function of composition in the morphologycomposition diagram shown in Figure 2. The above-mentioned results demonstrated that µ-EOF terpolymers can self-assemble into a variety of fascinating multicompartment micelles in dilute aqueous solution and the observed micellar structures are generally correlated with the O corona size and the relative length of the E and F blocks. These novel multicompartment structures can be understood, at least qualitatively, through the interplay of several factors. First, the strong incompatibility among the three polymeric components drives the formation of segregated micelle cores even at modest (28) Provencher, S. W. Comput. Phys. Commun. 1982, 27, 213. (29) Halperin, A.; Tirrell, M.; Lodge, T. P. AdV. Polym. Sci. 1992, 100, 31. (30) Zhulina, E. B.; Adam, M.; LaRue, I.; Sheiko, S. S.; Rubinstein, M. Macromolecules 2005, 38, 5330. (31) Won, Y.-Y.; Brannan, A. K.; Davis, H. T.; Bates, F. S. J. Phys. Chem. B 2002, 106, 3354. (32) Bang, J.; Jain, S.; Li, Z.; Lodge, T. P.; Pedersen, J. S.; Kesselman, E.; Talmon, Y. Macromolecules 2006, 39, 1199. (33) LaRue, I.; Adam, M.; Pitsikalis, M.; Hadjichristidis, N.; Rubinstein, M.; Sheiko, S. S. Macromolecules 2006, 39, 309. (34) Ding, J.; Liu, G. Macromolecules 1997, 30, 655.

Figure 1. Schematic representation of micellar structures selfassembled from AB diblocks and ABC star terpolymers: (a) spherical micelle, (b) wormlike micelle, and (c) bilayer vesicle formed from AB diblock copolymers; (d) hamburger micelle, (e) segmented wormlike micelle, and (f) nanostructured bilayer vesicle formed from ABC miktoarm star terpolymers. Representative cryoTEM images of (g) hamburger micelles, (h) segmented wormlike micelles, and (i) nanostructured vesicles from µ-EOF terpolymers. (h) was modified from ref 6.

molecular weight. Second, the extreme hydrophobicity of the F block places the system in the superstrong segregation regime (SSSR).10,35,36 Within the SSSR, the interfacial tension is so large that the minority core-forming block is essentially fully extended and the interfacial area per chain is minimized. The domain size thus is limited by the contour length of the F block (LF). We have demonstrated that the radii of the F disklike layer in both hamburger micelles and segmented worms are close to LF, consistent with the SSSR.6 Third, although the extremely large interfacial tension between the F domains and the solvated O coronas favors small and flat interfaces, this has to coordinate with the copolymer composition, which determines the interfacial curvature; i.e., large coronas prefer large interfacial curvature and thereby push the morphology from vesicles to cylinders and then to spheres, as demonstrated in diblock systems.31,37 In µ-EOF star terpolymers, because the O/F interfacial tension is much greater than those of O/E and E/F, the optimal chain-packing motif would have minimum interfacial contact between the F blocks and solvated O blocks. However, by virtue of the covalent connectivity of the E, F, and O blocks at a common junction point, a certain amount of interfacial contact between the F and O blocks as well as water is inevitable. This, in turn, imposes an important constraint on the microstructure of the micelle cores that suppresses the formation of concentric structures. Finally, for each terpolymer a broad distribution of micellar aggregates were typically observed. This is symptomatic of a nonergodic system, which is unable to find the global free energy minimum due to the virtual absence of unimer exchange, as documented in PBD-PEO diblock micelle solutions.38 Thus, the aggregates of varying sizes cannot equilibrate to a single preferred size or (35) Edmonds, W. F.; Li, Z.; Hillmyer, M. A.; Lodge, T. P. Macromolecules 2006, 39, 4526. (36) Semenov, A. N.; Nyrkova, I. A.; Khokhlov, A. R. Macromolecules 1995, 28, 7491. (37) Jain, S.; Bates, F. S. Science 2003, 300, 460. (38) Won, Y.-Y.; Davis, H. T.; Bates, F. S. Macromolecules 2003, 36, 953. Jain, S.; Bates, F. S. Macromolecules 2004, 37, 1511.

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Figure 2. Multicompartment micelle morphology diagram for µ-EOF miktoarm star terpolymers in dilute aqueous solution as a function of composition. fPEE, fPEO, and fPFPO are the volume fractions of the PEE, PEO, and PFPO blocks, respectively. The inset in the upper left images represents the three- and four-lobe multicompartment micelles formed from µ-EOF(2-13-3.5). The scale bar indicates 50 nm.

shape. Nevertheless, these multiple overall structures share common packing features at the compartmental level. The discrete micelle depicted by the so-called “hamburger” model is illustrated in Figure 1d.6 In this model the F blocks form a middle disklike layer, which is sandwiched by two E domains on the top and bottom, while the O blocks emanate from the E/F interface to wrap around the hydrophobic core. In this way the F block, being much more hydrophobic than the E block, tends to minimize surface contact with the solvated O corona. In this hamburger model, because the O blocks are significantly stretched from their random conformation, a “polymerization” of the individual hamburger units could occur due to the attraction between the less shielded hydrophobic ends. When two micelles approach one another, a concentration fluctuation of the O blocks that exposes the hydrophobic ends can induce a stack of different hamburger elements. This scenario naturally explains why the discrete hamburger micelles are stable only when the O blocks are extremely long; otherwise, the exposed hydrophobic ends drive the formation of segmented wormlike micelles, as observed, for example, in µ-EOF(2-9-2.5) and µ-EOF(2-7-2.5). By forming a worm, different hamburger elements are able to share their O blocks, thus protecting them from the highly unfavorable exposure to water and decreasing the total hydrophobic end concentration (Figure 1e). Further decreasing the O block results in the formation of nanostructured bilayer sheets and vesicles, which have approximately hexagonally packed F channels dispersed in the homogeneous hydrocarbon membrane (Figure 1f).8 As an elaboration on the above-mentioned results, we report new micellar structuresssegmented ribbons and bilayers (illustrated schematically in Figure 3)soccurring at an intermediate

Figure 3. Schematic illustration of chain-packing motifs for segmented ribbon-like and bilayer-like micelles.

composition between segmented worms and nanostructured bilayers. Moreover, when the chain length of the F block exceeds the chain length of the E blocks, the superhydrophobic F block becomes the majority component in the micelle core, thereby imposing a “double frustration” on the self-assembly of the µ-EOF(2-9-5) terpolymer; not only would F prefer to escape O, but it also occupies too much volume to be encased by E. While a rich variety of micellar structures are identified by cryoTEM in this paper, a small number of common structural elements can still be adapted to interpret the observed morphologies, as described next.

Results Using a synthetic protocol which allows us to systematically tune one block length while keeping the other two constant, we synthesized 16 µ-EOF terpolymers from 10 EO parent diblock

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Figure 5. CryoTEM images of nanostructured bilayer sheets and vesicles obtained from 1 wt % µ-EOF(2-4-2.5) aqueous solutions. The scale bar indicates 100 nm.

Figure 4. CryoTEM images of segmented ribbons, Y-junctions, toroids, network micelles, and segmented bilayers with anisotropic orientations obtained from 1 wt % µ-EOF(2-6-2.5) aqueous solutions. The scale bar indicates 50 nm.

precursors. The corresponding molecular parameters and respective cryoTEM images of these terpolymers are summarized in Table 1 and Figure 2, respectively. The EO diblock precursors with fPEO > 0.60 form spherical micelles in water, whereas EO(2-4) with fPEO ) 0.59 forms a mixture of spheres and cylinders, as confirmed by cryoTEM and DLS (see the Supporting Information). We thus survey the dependence of micelle morphology on the terpolymer composition by systematically tuning the volume fraction of the O or F block while keeping the other two constant. µ-EOF(2-y-2.5) Series. Here the volume fraction ratio of E (fPEE) to F (fPFPO) blocks is about 1.7 and fPEO ranges from 0.48 to 0.87. The contour lengths of the E (LE) and F (LF) blocks are about 8.3 and 5.2 nm, respectively, assuming planar zigzag conformations. The self-assembly of µ-EOF terpolymers occurs spontaneously upon dispersion in water to form various micellar structures depending on the terpolymer composition. For µ-EOF(2-26-2.5) (fPEO ) 0.87) having the longest O block among these terpolymers, distinct hamburger micelles are the dominant species (Figure S1 in the Supporting Information). In contrast, µ-EOF(2-13-2.5) (fPEO ) 0.74) with a shorter O block forms a mixture of hamburger micelles and segmented wormlike micelles (Figure S2 in the Supporting Information). µ-EOF(2-9-2.5) (fPEO ) 0.69) forms a mixture of segmented wormlike micelles with lengths between 20 and 200 nm (Figure S3 in the Supporting Information). In contrast, µ-EOF(2-7-2.5) (fPEO ) 0.62), having a slightly decreased O block length, self-assembles into elongated wormlike micelles (Figure S4 in the Supporting Information) with worm lengths ranging from 100 to 600 nm. However, both sets of worms share the same micellar microstructures characterized by alternating F domains and E domains along the major axis of the cylinder. When the fPEO of µ-EOF(2-6-2.5) is decreased to 0.59, cryoTEM images shown in Figure 4 reveal assemblies having more complicated morphologies, e.g., segmented ribbons, Yjunctions, toroids, network micelles, and segmented bilayers with anisotropic orientations. It is noteworthy that these images were taken from different solutions of the same µ-EOF(2-6-2.5) terpolymer after comparable annealing histories. Although these

structures maintain the alternating stack of E and F domains, the dimensions of the worms are much larger than those formed from µ-EOF(2-9-2.5) and µ-EOF(2-7-2.5). For example, the worms have F domain widths (w) ranging from 22 to 100 nm, which far exceeds both 2LE and 2LF. Clearly, the chain-packing motif shown in Figure 1e is not sufficient to explain these structures. We thus propose that these assemblies have ribbonlike micelle cores, as illustrated in Figure 3. Within this model, the E and F blocks form alternating ellipsoidal disk domains along the major axis of a ribbon and the E and F chains are extended from the O/E and O/F interfaces into the corresponding domains. The O blocks still emanate from the E/F interface to shield the hydrophobic core from water. Within the ribbon, the bilayer thickness (H) could be approximated by 2LF, while the width (w in Figure 3) could be significantly larger than the 2LF. On the ribbon edges the E and F blocks form half-cylinders with a curvature close to the reciprocal of LF. The large bilayers shown in Figure 4d presumably share the same chain-packing motif proposed in Figure 3, the only difference being the width of the E and F layers. CryoTEM images of the assemblies formed from µ-EOF(24-2.5) (fPEO ) 0.48), which has an even lower fPEO, reveal the formation of large bilayer sheets and vesicles as in Figure 5. The coexistence of segmented worms, bilayer sheets with protruding worms, and completed vesicles suggests a slow micellar evolution to stable vesicles. As we have discussed elsewhere, the cores of the bilayer sheets and vesicles feature cylindrical F domains distributed approximately hexagonally in a continuous and twodimensional hydrocarbon (E) matrix.8 µ-EOF(2-9-z) and µ-EOF(2-13-z) Series. CryoTEM images shown in Figures 6, 7, and 9 display the effects of varying the F block length on the micelle morphologies for the µ-EOF(29-z) and µ-EOF(2-13-z) series. µ-EOF(2-9-2.5) forms segmented worms with a broad length distribution (Figure S3); Figure 6 shows that µ-EOF(2-9-3.5) (fPEO ) 0.66) forms similar segmented wormlike micelles. However, µ-EOF(2-9-3.5) also exhibits Y-junctions, which were not seen in the µ-EOF(29-2.5) solutions. In contrast, µ-EOF(2-9-5) (fPEO ) 0.61), which has a longer F block than the E block, self-assembles into an even richer variety of multicompartment micelles, including well-defined segmented worms, raspberry-like micelles, and multicompartmentalized worms, as shown in Figure 7. Figure 7a provides a larger view of the micelle variety. Parts b and c of Figure 7 clearly identify the raspberry-like and multicompartmentalized wormlike micelles. Both types of aggregates contain a continuous dark regime, attributed to the F domains, while the E blocks form isolated patches dispersed in the dark matrix. These novel structures are beyond the framework of the previous models;6,8 a new chain-packing model will be proposed in the Discussion.

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Figure 6. CryoTEM images of segmented worms and Y-junctions obtained from 1 wt % µ-EOF(2-9-3.5) aqueous solutions. The scale bar indicates 50 nm.

Figure 8. Apparent micelle size distributions of the series of µ-EOF(2-9-z) terpolymers from 1 wt % aqueous micelle solutions. The scattering angle is 90°.

Figure 7. CryoTEM images of segmented worms, raspberrylike micelles, and multicompartment worms obtained from 1 wt % µ-EOF(2-9-5) aqueous solutions. The scale bar indicates 50 nm.

The overall micelle size distributions for these three terpolymers as well as the EO(2-9) diblock precursor are characterized by DLS, as shown in Figure 8. All three µ-EOF(2-9-z) terpolymers formed large assemblies with much broader distributions than that of EO(2-9) diblock micelles. In particular, the average aggregate size increased slightly with an increase of the F block length, which is consistent with the cryoTEM results. We also investigated the µ-EOF(2-13-z) series, which have longer O blocks than the µ-EOF(2-9-z) series. As demonstrated previously,6 µ-EOF(2-13-2.5) forms predominantly discrete hamburger micelles with a small amount of short worms (Figure S2). Increasing the F block length induces substantial changes in the micelle morphologies, as shown in Figure 9. In addition to discrete hamburger micelles and segmented worms, µ-EOF(213-3.5) displays the ability to form some intricate multicompartment micelles with three-lobe or four-lobe facets (Figure 9a,b). For µ-EOF(2-13-5) segmented ribbons, bilayer-like structures, and raspberry-like micelles are observed by cryoTEM (Figure 9c,d).

Discussion Effect of the O Block Length on the Micelle Morphology. When the most hydrophobic block is the minority component

Figure 9. (a, b) Hamburger micelles and segmented worms formed from µ-EOF(2-13-3.5). (c, d) Segmented ribbons formed from µ-EOF(2-13-5). The cryoTEM images were obtained from 1 wt % aqueous solutions. The scale bar indicates 50 nm.

in the micelle core, the µ-EOF terpolymers exhibit the same morphology sequence as diblocks, i.e., spheres, cylinders, branched micelles, and bilayers, with decreasing O block length, as summarized in Figure 2. In particular, for fPEO > 0.6 the flat F disk elementary unit is shared by the hamburger and segmented wormlike micelles (Figure 1d,e); when fPEO < 0.48, nanostructured vesicles are the stable morphology (Figure 1f). In the intermediate regime with 0.48 < fPEO < 0.59, segmented ribbons, Y-junctions,

Micelles Formed by ABC Miktoarm Star Terpolymers Table 2. Interfacial Area Per Chain in Different Micelle Morphologies sample ID µ-EOF(2-26-2.5) µ-EOF(2-13-2.5) µ-EOF(2-9-2.5) µ-EOF(2-7-2.5) µ-EOF(2-6-2.5) EO(2-6)

ra (nm) 6.8 6.5 6.2 6.5 6.5 7d

Nb 400 300 310 360 360 513 390d

aEc aFc aOc (nm2) (nm2) (nm2) 1.93 2.15 0.82 0.95 0.96 0.64 NA

0.64 0.67 0.70 0.67 0.67 0.58 NA

2.57 2.82 1.52 1.62 1.64 1.22 1.6

morphology hamburger hamburger segmented worm segmented worm segmented worm segmented ribbon sphere

a Radius of the F disklike layer. b Aggregation number of each F disklike domain. c Interfacial area per E, F, and O block, respectively. d These numbers correspond to the radius of the spherical PEE micelle core (7 nm) and EO diblock micelle aggregation number (390).

networks, and bilayerlike structures are preferred by µ-EOF(26-2.5) (see Figure 4). The formation of Y-junctions and networks in cylinder-forming three-component (surfactant/oil/water) microemulsions was theoretically anticipated several years ago42,43 and experimentally demonstrated by cryoTEM.44,45 More recently, Y-junctions and networks have been observed in aqueous dispersions of PBD-PEO diblock copolymers under appropriate conditions.37 For cylinder-forming systems the penalty for forming “end caps” favors the formation of Y-junctions and networks. The opposing factor in the copolymer case is the steric repulsion arising from corona crowding, especially in the vicinity of a branch. As mentioned before, the micelle free energy within the SSSR is primarily determined by the interfacial tension and corona entropy. Because the free energy of micellar cores is already in some sense saturated, i.e., with fully stretched F blocks and minimized F/O and F/E interface areas, the interfacial curvature is primarily controlled by the O block length, which in turn becomes the critical factor in determining the optimal micelle morphologies. Therefore, we propose that µ-EOF(2-6-2.5) lies at a threshold O composition, above which the cylinder geometry is preferred, but below which bilayer vesicles are the stable structures. A compromise between the cylinder and bilayer vesicle results in the formation of segmented ribbon-like micelles, which will have an intermediate curvature between those of the cylinder and flat bilayer. This concept of a threshold composition has already been documented in a diblock copolymer system.37 Branched and network micelles were observed at a particular composition within the regime where cylinders and bilayers coexisted. Ribbon-like micelles have also been demonstrated to be the intermediate structures between the cylinders and bilayer sheets in hybrid gemini surfactants that consist of more than one hydrophobic tail. Using cryoTEM, Oda et al. observed a continuous transition from cylinders into bilayer sheets through ribbons with continuously increasing width.46 Estimation of the interfacial area per O block (aO) for the different morphologies also supports the idea that the ribbonlike structure is preferred by µ-EOF(2-6-2.5), as compared to cylinders. The data are compared in Table 2. The calculation of aOstar for hamburger micelles, segmented worms, and segmented ribbons is based on the models in parts d and e of Figure 1 and (39) Fetters, L. J.; Lohse, D. J.; Richter, D.; Witten, T. A.; Zirkel, A. Macromolecules 1994, 27, 4639. (40) Smith, G. D.; Yoon, D. Y.; Jaffe, R. L.; Colby, R. H.; Krishnamoorti, R.; Fetters, L. J. Macromolecules 1996, 29, 3462. (41) Provided by Dupont. (42) Tlusty, T.; Safran, S. A.; Strey, R. Phys. ReV. Lett. 2000, 84, 1244. (43) Tlusty, T.; Safran, S. A. Science 2000, 290, 1328. (44) Bernheim-Groswasser, A.; Wachtel, E.; Talmon, Y. Langmuir 2000, 16, 4131. (45) Bernheim-Groswasser, A.; Tlusty, T.; Safran, S. A.; Talmon, Y. Langmuir 1999, 15, 5448. (46) Oda, R.; Huc, I.; Danino, D.; Talmon, Y. Langmuir 2000, 16, 9759.

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in Figure 3, respectively. For the latter two models, end effects are neglected in the calculations. For the segmented ribbon-like micelle, the radius of a cylindrical edge is approximated by LF and H is approximated by 2LF (a sample calculation is given in the Supporting Information). In Table 2, the hamburger micelles have the largest aOstar compared to the others, as expected. The F block has almost constant interfacial area in the hamburger micelles and in segmented worms, indicative of the SSSR. The aOstar of the cylindrical morphology has a value comparable to that of the EO(2-6) diblock, which adopts a spherical shape with larger curvature. Notably, µ-EOF(2-6-2.5) has an unambiguously smaller aOstar than either µ-EOF(2-9-2.5) or µ-EOF(2-7-2.5), which further confirms the key role of the O coronas in determining the interfacial curvature as well as the micellar morphology. Another possible contribution to forming stable ribbon-like micelles is the polydispersity of the O blocks. Because both E and F blocks have low glass transition temperatures, i.e., -20 °C39 and -63 °C,47 respectively, terpolymers with relatively long O blocks can easily migrate to the curved half-cylinder regime while others with relatively short O blocks will occupy the bilayer part of the ribbon. However, neither F nor E blocks can easily cross the opposite domain due to the strong incompatibility. This factor also inhibits global equilibration, in contrast to low molecular weight gemini surfactants.48 The effects of the F block length on micellar structure were explored using the µ-EOF(2-9-z) and µ-EOF(2-13-z) series. µ-EOF(2-9-2.5) forms well-defined segmented wormlike micelles, broadly distributed in length, with an F domain radius of 6.5 nm, consistent with the SSSR. For µ-EOF(2-9-3.5) with LF ) 7.5 nm, segmented worms with several Y-junctions are observed (Figure 6). For these worms, the most prominent change is that the F domain radius is increased to 7.5 nm (averaged from the worm in the bottom of Figure 6a), again suggesting the SSSR limit. The model proposed in Figure 1e is presumably applicable to these worms. The Y-junctions indicate that the E and F blocks may form a bilayer-like structure at the joint, similar to the model proposed for µ-EOF(2-6-2.5) micelles (Figure 3). When LF > LE in the µ-EOF(2-9-5) terpolymer, more complicated morphologies are revealed by the cryoTEM images shown in Figure 7, in which a mixture of segmented worms, raspberry-like micelles, and multicompartmentalized worms are identified. To understand these observed structures, we may need to take into account the polydispersity of the E and F blocks to interpret the coexistence of multiple micellar structures formed from µ-EOF(2-9-5). From the MALDI-TOF mass spectrometry of the E (Figure S16 in the Supporting Information) and F (Figure S17 in the Supporting Information) blocks, we can obtain their corresponding contour length ranges, i.e., 5 nm < LE < 12 and 7 nm < LF < 19 nm, respectively, which could offer two possibilities for the self-assembly of µ-EOF(2-9-5). When LF ≈ LE, the segmented worms depicted in Figure 1e might be expected since the relatively long O block would oppose forming segmented ribbon-like or bilayer-like micelles (in contrast to µ-EOF(2-6-2.5)), due to coronal crowding. For example, the worms indicated in Figure 7d have radii for the F or E disklike domains of about 9 nm, which is comparable to both LE and LF. However, when LF becomes significantly longer than LE, two factors will compete to impose a double frustration on the selfassembly of µ-EOF(2-9-5). The superhydrophobic F block will try its best to minimize contact with water because of the (47) Zhu, S.; Edmonds, W. F.; Hillmyer, M. A.; Lodge, T. P. J. Polym. Sci., Part B: Polym. Phys. 2005, 43, 3685. (48) Kunitake, T.; Higashi, N. J. Am. Chem. Soc. 1985, 107, 692.

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Figure 10. Schematic chain-packing illustration of the micelles formed from a µ-EOF(2-9-5) terpolymer in which the F block is longer and more hydrophobic than the E block. Red, green, and blue represent the E, F, and O blocks, respectively.

extremely large interfacial tension, but the star architecture frustrates the desire to have an F core surrounded entirely by an E shell. Then, when the F block is also the majority component within the micelle core, it is difficult for the E block to screen most of the F block from the aqueous interface. We thus propose a chain-packing motif in which the F blocks form the matrix of the core but the E blocks form small, thin domains at the F/O interface, as in Figure 10. This picture can reconcile the microstructures of the raspberry-like micelles and multicompartmentalized worms. For the former (illustrated in Figure 10a,b), the E blocks form short, fat, and possibly cylindrical domains, whose length (dE) is comparable to LE, immersed in the outer shell of a spherical fluoropolymer matrix; the O blocks emanate from the E/F interface to wrap around the hydrophobic micellar core. Examination of the cryoTEM images provides direct comparison between the domain sizes and E and F chain lengths. Taking the raspberry-like micelle indicated in Figure 7b as a prototype, we determine the micelle core diameter (wF) and the E cylinder length (dE) and diameter (wE) to be about 32, 10, and 9 nm, respectively, which are fairly consistent with 2LF and LE. However, we cannot preclude the possibility that these raspberrylike micelles might be bilayer-like structures. Under this assumption, the aspect ratio of a raspberry-like bilayer would be less than 1.4 provided that the bilayer thickness and micelle diameter are about 23 nm (2LF) and 32 nm (determined from Figure 7b), respectively. However, in the absence of edge-on views of such bilayer structures, we must defer the unambiguous confirmation or rejection of this hypothesis to future work. Synthesis of a µ-EOF terpolymer having an F block much longer than the E block could help to elucidate this question. The chain-packing motif for the multicompartmentalized worms (Figure 7b,c) is illustrated in Figure 10c. Again, the F blocks form the matrix of the cylinder, while the E blocks presumably form wide, short cylinders to occupy the outer shell with a larger E/O interfacial area than the F/O interfacial area. Analyzing the cryoTEM images gives a diameter (wF) of worms and dE and wE of E domains of about 28, 8, and 7 nm, respectively,

Li et al.

Figure 11. (a-c) Chain-packing motif for a three-lobe multicompartment micelle formed from the self-assembly of µ-EOF(2-133.5) in dilute aqueous solution. (d) CryoTEM image obtained from a 1 wt % aqueous solution of µ-EOF(2-13-3.5).

which in turn compare favorably to 2LF and LE. By analogy to the growth of segmented worms from hamburger micelles, we speculate that these multicompartmentalized worms are “polymerized” from the discrete raspberry-like micelles on the basis of their common local chain-packing motifs. In addition, the cryoTEM images in Figure 7 also exhibit various raspberrylike micelles with distinct lengths. For the series of µ-EOF(2-13-x) terpolymers, µ-EOF(213-2.5) (Figure S2) and µ-EOF(2-13-3.5) (Figure 9) form predominantly hamburger micelles with a small fraction of segmented worms. Regarding the three- or four-lobe multicompartment micelles formed by µ-EOF(2-13-3.5), we suggest a bilayer-like structure as illustrated in Figure 11. The F blocks occupy the majority of the micelle core, however again having a small interfacial contact with surrounding O coronas. The E blocks fill the interstitial space between the F domains and have relatively large interfacial contact with water. A possible pathway to form such structures involves merging one edge of the F disklike layer from three hamburger micelles. Because of the steric repulsion arising from the large O corona, these micelles could not grow to form stable bilayer-like structures in contrast to µ-EOF(2-6-2.5). Compared to µ-EOF(2-9-5), µ-EOF(213-5) did not form extended wormlike micelles. Instead, it formed segmented ribbon-like and raspberry-like micelles with highly curved interfaces, presumably due to its longer O block than that of µ-EOF(2-9-5). Additionally, µ-EOF(2-7-5) (fPEO ) 0.54) (Figure 2), which has shorter O blocks than those of µ-EOF(2-9-5) and µ-EOF(2-13-5), forms segmented worms, raspberry-like micelles, and multicompartmentalized worms, similar to the assemblies of µ-EOF(2-9-5), consistent with the overall picture presented.

Summary Using cryoTEM, we have identified two new types of multicompartment micelles from the self-assembly of µ-EOF miktoarm star terpolymers in dilute aqueous solution. When the F block is the minority component in the segregated micelle core, segmented ribbons, Y-junctions, and networks are preferred

Micelles Formed by ABC Miktoarm Star Terpolymers

by µ-EOF(2-6-2.5), which has an intermediate O block volume fraction between those of a segmented worm-forming terpolymersµ-EOF(2-7-2.5)sand a nanostructured vesicle-forming terpolymersµ-EOF(2-4-2.5). When the F block becomes longer than the E block, a double frustration in the self-assembly of µ-EOF(2-9-5) results in another set of novel multicompartment micelles, e.g., raspberry-like micelles and multicompartmentalized worms, in which the F block forms the matrix of the segregated micelle core while still having the least interface contact with the O coronas. An extensive survey of the micellar morphology dependence on the terpolymer molecular weight and compositions from 16 µ-EOF terpolymers is summarized in a morphology-composition diagram shown in Figure 2. For the series of µ-EOF(2-y-2.5) terpolymers with fPEO/fPFPO ) 1.7, when fPEO > 0.76, µ-EOF(226-2.5) and µ-EOF(2-13-2.5) formed predominantly distinct hamburger micelles, when 0.60 < fPEO < 0.76, well-defined segmented worms with different length distributions were formed from µ-EOF(2-9-2.5) and µ-EOF(2-7-2.5), when fPEO < 0.48, the µ-EOF(2-4-2.5) terpolymer self-assembled into nanostructured bilayer sheets and vesicles, and for 0.48 < fPEO < 0.60, a µ-EOF(2-6-2.5) terpolymer with fPEO ) 0.59 formed various multicompartment micelles, e.g., segmented ribbons, Y-junctions, toroids, networks, and segmented bilayers with anisotropic orientations. The series of µ-EOF(1.4-y-2.5) terpolymers have lower overall molecular weights, smaller fE/fF values, and significantly lower fPEO values than the other series.6 In this case µ-EOF(1.4-5-2.5) (fPEO ) 0.61) formed well-defined segmented worms, as expected, while µ-EOF(1.4-3-2.5) (fPEO ) 0.50) formed laterally nanostructured bilayer sheets, which consist of hexagonally ordered F cylinders with radii of 4.1 nm immersed in a continuous E matrix. These polygons have been demonstrated to be metastable bilayer sheets, which evolve into

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closed vesicles upon thermal annealing.8 In addition, µ-EOF(1.42-2.5) (fPEO ) 0.38) formed thermodynamically stable vesicles with nanoscopic F channels dispersed in the bilayer membrane matrix composed of the E blocks. In contrast, µ-EOF(1.4-12.5) has too short an O block to spontaneously assemble into defined micellar objects in water. The effect of the fluoropolymer length on the micelle morphology was investigated by using the series of µ-EOF(29-z) and µ-EOF(2-13-z) terpolymers. Increasing the F block length drives the micellar structure from hamburger micelles or segmented worms to raspberry-like micelles or multicompartmentalized worms, due to the double frustration imposed by the longer F block compared to the E block. Quantitative analysis of these cryoTEM images suggests that the fluoropolymer domain lies in the superstrong segregation regime. Overall, these results suggest a promising strategy to tune the hierarchical assemblies with regularly segregated micelle cores for a range of advanced applications. Acknowledgment. This work was supported by the National Science Foundation through the University of Minnesota MRSEC, Award DMR-0212302. We thank Dr. Ellina Kesselman and Prof. Yeshayahu Talmon at TechnionsIsrael Institute of Technology (Israel) for initial help with the cryoTEM measurements. Supporting Information Available: Complementary cryoTEM images and micelle size distribution of µ-EOF terpolymers (Figures S1-S13), characterization of EO diblock micelle solutions by cryoTEM and DLS (Figures S14 and S15 and Table S1), MALDI-TOF MS of PEE and PFPO (Figures S16 and S17), and molecular parameters of the PFPO precursors (Table S2). This material is available free of charge via the Internet at http://pubs.acs.org. LA0620051