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From “Sunflower-like” Assemblies toward Giant Wormlike Micelles Redouane Borsali,*,† Edson Minatti,†,‡ Jean-Luc Putaux,§ Michel Schappacher,† Alain Deffieux,† Pascal Viville,| Roberto Lazzaroni,| and Theyencheri Narayanan⊥ LCPO-CNRS-ENSCPB and Bordeaux University I, 16 Avenue Pey Berland, 33607 Pessac Cedex, France, Departamento de Quimica, Universidade Federal de Santa Catarina, Florianopolis, Brazil, CERMAV-CNRS affiliated with Joseph Fourier University, Grenoble, France, CRESMAP, Universite´ de Mons-Hainaut, Mons, Belgium, and ESRF, Grenoble, France Received June 25, 2002. In Final Form: October 22, 2002 One of the most interesting and fascinating solution properties of block copolymers is their ability to self-assemble into micelles, lamellar aggregates, and vesicles. Such organized structures have made diblock copolymers of great importance in nanotechnology applications.1-11 For instance, the spontaneously formed vesicles in water are potentially useful as vehicles for delivering drugs and as biomimetic models of biological cells. We show here for the first time that the cyclization of a linear copolymer chain induces a remarkable change in the micellar morphology. This result is highlighted in the case of linear and cyclic polystyrenepolyisoprene (PS-PI) diblock copolymers having exactly the same molar weight and dispersed in a selective solvent for PI. The micelles arising from linear diblock copolymers exhibit a monodisperse spherical shape (50 nm in diameter) whereas those formed from cyclic copolymers are long (>1 µm) cylindrical (wormlike) objects resulting from the unidirectional self-assembly of “sunflower-like” elementary micelles whose architecture strongly favors the core-core (PS-PS) attractions. We expect these new cyclization-based design copolymer architectures morphologies will be of particular interest in future nanotechnology applications.
Block copolymers spontaneously self-assemble into welldefined micelles in the presence of a selective solvent for one block. For instance, when linear block copolymers are placed in solution, they can behave like surfactants if both blocks of the polymer have different affinities for the solvent and thus form micelles (spheres, cylinders), lamellar aggregates, and vesicles.12-14 In most studied cases the micelles are soft colloidal spherical particles with aggregation numbers in the range of 30-100. For nonionic copolymers the range of the intermicellar interactions can be modulated through chain architecture.15 * To whom correspondence should be addressed. E-mail: borsali@ enscpb.fr. † LCPO-CNRS-ENSCPB and Bordeaux University I. ‡ Departamento de Quimica, Universidade Federal de Santa Catarina. § CERMAV-CNRS affiliated with Joseph Fourier University. | CRESMAP, Universite ´ de Mons-Hainaut. ⊥ ESRF. (1) Thurn-Albrecht, T.; Schotter, J.; Ka¨stle, G. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290, 2126. (2) Morkved, T. L.; Lu, M.; Urbas, A. M.; Ehrichs, E. E.; Jaeger, H. M.; Mansky, P.; Russell, T. P. Science 1996, 273, 931. (3) Amundson, K.; Helfand, E.; Quan, X.; Hudson, S. D.; Smith, S. D. Macromolecules 1994, 27, 6559. (4) Park, M.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Adamso, D. H. Science 1997, 276, 1401. (5) Huber, K. Macromolecules 1987, 21, 1305. (6) Marko, J. F. Macromolecules 1993, 26, 1442. (7) Benmouna, M.; Borsali, R.; Benoıˆt,H. J. Phys. II 1993, 3, 1041. (8) Borsali, R.; Benmouna, M. Europhys. Lett. 1993, 23 (4), 263. (9) Borsali, R. Macromol. Chem. Phys. 1996, 197, 3947. (10) Leibler, L. Macromolecules 1980, 13, 1602. (11) Borsali, R.; Lecommandoux, S.; Pecora, R.; Benoıˆt, H.; Macromolecules 2001, 34, 4229. (12) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: Oxford, 1998. (13) Tuzar, Z.; Kratochvil, P. Adv. Colloid Interface Sci. 1976, 6, 201. (14) Halperin, A.; Tirrell, M.; Lodge, T. P. Adv. Polym. Sci. 1991, 100, 33. (15) McConnell, G. A.; Gast, A. P. Phys. Rev. E 1996, 54, 5447.
For example, when the core of the micelle is formed by the smallest block (“starlike” micelle), a body centered cubic phase can be observed at high concentrations. On the other hand, when the core of the micelle is formed by the longest block (“crew-cut” micelle), face centered cubic arrays of hard spheres are dominant.16 Ionic block copolymers, however, are more interesting as they exhibit a variety of morphologies such as ellipsoidal or cylindrical (wormlike) micelles, vesicles, bilayers, and multilayers.17-20 Nonspherical assemblies obtained from nonionic block copolymers have begun to receive more attention. As an example, it has been demonstrated that very long wormlike micelles can be formed from linear poly(butadiene)poly(ethylene oxide) (PB-PEO) diblocks in water21 similar to those arising from cationic surfactant systems with interesting rheological behavior.22 The PB-PEO system can also be cross-linked in situ providing permanent structures21 that may lead to remarkable materials. ABA triblock copolymers in a B-selective solvent is widely studied either in organic media (e.g., styrene-isoprenestyrene) or in water (e.g., PPO-PEO-PPO). At low concentrations, these molecules may form discrete micelles, called “flower-like” because the central coronaforming block must loop back to insert both A blocks into the core. However, at higher concentrations the middle block can bridge between clusters, leading to gel formation by intermicellization interactions.12 The micellar forma(16) Dormidontova, E. E.; Lodge, T. P. Macromolecules 2001, 34 (26), 9143. (17) Zhang, L.; Eisenberg, A. Science 1996, 272, 1777. (18) Discher, B. M.; Won, Y.-Y.; Ege, D. S.; Lee, J. C.-M.; Bates, F. S.; Discher, D. E.; Hammer, D. A. Science 1999, 284, 1143. (19) Zhang, L.; Eisenberg, A. Macromolecules 1999, 32, 2239. (20) Zheng, Y.; Won, Y. Y.; Bates, F. S.; Davis, H. T.; Scriven, L. E.; Talmon, Y. Phys. Chem. B 1999, 103, 10331. (21) Won, Y. Y.; Davis, H. T.; Bates, F. S. Science 1999, 283, 960. (22) Cates, M. E.; Candau, S. J. J. Phys. Condens. Matter 1990, 2, 6869.
10.1021/la026125u CCC: $25.00 © 2003 American Chemical Society Published on Web 12/02/2002
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tion in aqueous solutions of cyclic block copolymer chains oxyethylene-oxypropylene and ethylene oxide-1,2-butylene oxide, made from their corresponding triblock copolymers, was also studied using light scattering and surface tension experiments.23,24 Under similar conditions, cyclic and triblock copolymers showed similar values of the critical micellar concentration (cmc) but much larger than that of linear diblocks. Regular as well as reverse micelles have been studied using different techniques and are called “starlike” for small core and long corona micelles and “crew-cut” for large core and short corona micelles. In this Letter, we discuss the morphologies of crew-cut micelles formed from linear diblocks and those formed from cyclic diblock copolymers. Because of the cyclic architecture of the diblock copolymer, these molecules may form discrete micelles, that we call here “sunflower-like” (the short corona loops back into the large core) in analogy with the “crew-cut” morphology of linear diblock copolymer chains. We report here experiments describing a new morphology arising from the cyclization of linear diblock copolymer in the micellar state. In our earlier work,25 mainly an atomic force microscopy technique was used to identify the morphology of these objects after solvent evaporation. In the present Letter, four techniques were used to identify the size and the morphologies of these micelles: dynamic light scattering (DLS), small-angle X-rays scattering (SAXS), and the direct imaging techniques in situ freezedrying transmission electron microscopy (TEM), and atomic force microscopy (AFM). DLS26 was used to measure the hydrodynamic radii of the micelles. As shown in Figure 1, the autocorrelation functions measured at the concentration of 0.01 wt % in both systems are described by a single relaxation time as evidenced by the narrow (linear) and broader (cyclic) size distributions deduced from CONTIN analysis27 and illustrated in the insert of Figure 1. For this volume fraction, and as reported in the literature,12 the micelles formed from linear PS290-PI110 in heptane are spherical. The DLS results show a narrow distribution in sizes50 nm in diametersindependent of the concentration. In contrast, those formed from the cyclic diblock copolymer were found to strongly depend on the concentration with a hydrodynamic radius varying from 10 nm to several hundred nanometers when the concentration was changed from 0.01 to 0.5 wt %. At very low concentrations, the distribution in size is rather large and suggests the coexistence of several morphologies of the micelles in solution. As the short corona must loop back into the large core, we may at this stage speculate that the smallest size (20 nm) (23) Yu, G. E.; Yang, Z.; Attwood, D.; Price, C.; Booth, C. Macromolecules 1996, 29, 8479. (24) Yu, G. E.; Garrett, C. A.; Mai, S. M.; Altinok, H.; Attwood, D.; Price, C.; Booth, C. Langmuir 1998, 14, 2278. (25) (a) M. Schappacher, A. Deffieux, A. Makromol. Chem. Phys., in press. (b) Minatti, E.; Viville, P.; Borsali, R.; Schappacher, M.; Deffieux, A.; Lazaronni, R. Submitted for publication in Macromolecules. (c) Minatti, E.; Borsali, R.; Schappacher, M.; Deffieux, A.; Soldi, V.; Theyencheri, N.; Putaux, J.-L. Submitted for publication in Macromol. Rapid. Commun. (26) Berne, B. J.; Pecora, R. Dynamic Light Scattering, 2nd ed.; Dover: Mineola, NY, 2000. (27) Provencher S. W. Makromol. Chem. 1979, 180, 201. (28) Jackson, C. L.; Chanzy, H.; Booy, F.; Drake, B. J.; Tomalia, D. A.; Bauer, B. J.; Amis, E. J. Macromolecules 1998, 31, 6259. (29) Putaux, J.-L.; Bule´on, A.; Borsali, R.; Chanzy, H. Int. J. Biol. Macromol. 1999, 26, 145. (30) Chalaye S.; Bourgeat-Lami, E.; Putaux, J.-L.; Lang, J. Macromol. Symp. 2001, 169, 89. (31) Oostergetel, G. T.; Esselink, F. J.; Hadziioannou, G. Langmuir 1995, 11, 3721. (32) Dubochet, J.; Lepault, J.; Adriam, J.; McDowall, A.; Homo, J. C. A. Q. Rev. Biophys. 1998, 128, 219.
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Figure 1. Dynamic Light Scattering Experiments. Correlation functions C(q,t) measured using DLS at the scattering angle of 90° for both micellar systems at the concentration C ) 0.01 wt % in heptane. The insert illustrates the distribution of the relaxation time A(t) as revealed by CONTIN analysis deduced from the autocorrelation functions. The narrow peak corresponds to micelles formed from linear PS290-PI110 with a diameter ) 50 nm.The micelles formed from cyclic PS290-PI110 corresponds to a broader distribution peak whose average size is about 120 nm.
corresponds to a discrete “sunflower-like” architecture micelle (PS290-PI110, core-corona). At higher concentrations, these elementary “sunflower-like” micelles may interconnect to form a nonspherical but rather elongated morphology that is strongly favored by the core-core (PSPS) interactions. The nonspherical morphology of the micelles obtained from cyclic copolymer chains was confirmed using SAXS experiments under shear, performed at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). In the radial geometry (the incident beam is perpendicular to the shear flow direction) the results revealed the existence of scattering structure in both systems as expected for micellar particles. The effect of shear, however, made very clear the distinction between micellar systems made from linear and cyclic copolymer chains. At zero shear rate, both systems show an isotropic pattern in the radial geometry (Figure 2A,B). As the shear rate was increased as low as 10 s-1, the results revealed a clear and strong anisotropic pattern (Figure 2D) for the micelles made from cyclic diblock copolymer chains while the isointensity pattern corresponding to micelles obtained from the linear diblock copolymer remained isotropic (Figure 2C) (the slight observed anisotropy is due to the deformation of the soft spherical micelles). This result strongly supports the idea that the micelles formed from cyclic copolymer chains are not spherical but rather have an elongated shape (cylindrical or wormlike micelles). To observe the morphology of the micelles, we used an original method derived from the well-known cryo-TEM technique (see details the Methods section). Thin liquid films of 0.2 wt % suspensions of linear and cyclic copolymer micelles were fast-frozen and transferred at low temperature in the microscope. Then, the embedding heptane was freeze-dried in situ at -115 °C. In Figure 3A, micelles from linear copolymer chains clearly appear as individual spherical objects, with a diameter of about 35 nm, independent of the initial copolymer concentration. The micelles formed from in the cyclic copolymer solution are, however, long (>1 µm) and entangled cylinders with a regular diameter of about 20 nm (Figure 3B). The
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Figure 4. Schematic model of wormlike micelle. The model shows the relative positions of the poly(styrene) (gray) and poly(isoprene) (orange) in the aggregate.
Figure 2. Small-Angle X-ray scattering Experiments. 2d-SAXS pattern obtained on micelles formed from linear and cyclic copolymer PS290-PI110/Heptane at C ) 1.5 wt % in radial geometry (the incident beam is perpendicular to the shear flow direction): (A) linear at rest; (B) cyclic at rest, (C) linear at shear rate γ ) 10 s-1 and (D) cyclic at shear rate γ ) 10 s-1.
Figure 3. Direct Imaging Techniques. Cryo-TEM micrographs of (A) linear and (B) cyclic copolymer PS290-PI110 micelles from 0.2 wt % solutions. The wormlike micelles in (B) are entangled across a hole of the carbon membrane.T MAFM topographic images of the morphology of deposits formed from 0.5 wt % solutions of PS290-PI110 in heptane: micelles arising from (C) linear diblock copolymer and from (D) cyclic diblock copolymer chains.
difference in the diameter of the spherical micelles measured from TEM (35 nm) and DLS (50 nm) can be accounted for considering the conditions of observations of the samples. In DLS suspensions, the corona of the micelles is fully solvated. For TEM observation, during the in situ freeze-drying process, the temperature did not exceed -115 °C (much lower than the glass transition temperature of PS and PI). However, as no visual
distinction could be made between the corona and the core parts of the micelles from the TEM images, we can assume that a shrinkage of the diffuse PI corona occurred during the sublimation of heptane. This remark also applies for the wormlike micelles from cyclic copolymers. These morphologies were further investigated by AFM as it can provide high-resolution three-dimensional imaging of the surface organization of the copolymer molecules. The effect on the micellar morphology of the cyclization of the PS290-PI110 copolymer molecules is clearly illustrated in parts C and D of Figure 3. For linear copolymers, the micelles have a very narrow lateral size distribution and their average diameter, 40 ( 0.6 nm, is in agreement with the DLS and in situ freeze-drying TEM data. When deposited on mica from a 0.5 wt % solution in heptane (Figure 3C), the micelles are found to be densely packed within a homogeneous monolayer and are 40 nm high. We observe that the micelles can locally pack into different patterns. As an illustration, areas marked 1 and 2 highlight two locations where the micelles adopt either hexagonal or cubic packing, respectively. For micelles arising from cyclic copolymers, Figure 3D shows a deposit obtained from a solution of 0.5 wt % in heptane. Their morphology is made of entangled wormlike micelles whose extremities are hardly discernible. Their average width and height are 30 ( 1 and 20 ( 1 nm, respectively, in good agreement with in situ freeze-drying TEM. Although not shown here, in situ freeze-drying TEM and AFM imaging of micelles made from the cyclic copolymer obtained from dilute solutions (0.01 wt %) reveals the existence of very small and discrete “sunflower-like” micelles whose size is smaller than that of the micelles obtained from the linear copolymer (30 vs 40 nm, respectively). All the results and observations using DLS, SAXS, in situ freeze-drying TEM, and TM-AFM presented in this Letter demonstrate that, contrarily to the micelles obtained from linear PS290-PI110 copolymers that maintain the same size and the spherical shape under different concentration conditions, the morphology of the micelles made from PS290-PI110 cyclic copolymer having exactly the same molecular weight and volume fraction adopts an energetically more favorable long wormlike (>1 µm) morphology. Figure 4 illustrates a schematic model of wormlike aggregate made from cyclic copolymer chains assuming a “sunflower-like” elementary micelle. As usually observed for micelles arising from linear diblock
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Langmuir, Vol. 19, No. 1, 2003 9
Table 1. Characteristics of the Linear and Cyclic Copolymer PS-PI copolymer
DPPS
DPPI
Mw/(103 g/mol)a
linear PS-PI cyclic PS-PI
290 290
110 110
37 37
a The molar mass was determined by size exclusion chromatography/gel permeation chromatography analysis. The polydispersity index is less than 1.1 for both copolymer chains.
copolymer, the morphology could be manipulated by changing the volume fraction, the concentration, the temperature, and the solvent quality and we have shown in this report for the first time that the cyclization of the same diblock copolymer leads to a different morphology. As opposed to the spherical micelles formed from linear copolymers, the long (>1 µm) wormlike morphology adopted by cyclic copolymers results from the unidirectional self-assembly of “sunflower-like” elementary micelles whose architecture strongly favors the core-core attraction. Methods Synthesis. The cyclic diblocks were obtained by connecting intramolecularly, under high dilution, both extremities of R-isopropylidene-1,1-dihydroxymethyl-ω-diethylacetal heterodifunctional linear poly(styrene-b-isoprene) precursors prepared by living anionic polymerization. By this successful method,25a we were able to obtain linear and cyclic diblock copolymer chains with identical molar mass and chemical composition (see Table 1). Dynamic Light Scatering. Light scattering has provided an important method for characterizing macromolecules. Now through the use of intense, coherent laser light and autocorrelators, experiments in the time domains can be used to study molecular motions, diffusion, and dynamic process (hydrodynamic radii). Small-Angle X-ray Scattering under Shear. The effects of external fields such as that produced by shear flow are of great relevance when applied to spherical or nonspherical (elongated) particles and can result in fundamental differences. Consequently, subjecting elongated particles to shear flow induces an orientation in the direction of the shear field and enhances the intensity in the vorticity direction (anisotropy in the isointensity pattern).
In Situ Freeze-Drying Transmission Electron Microscopy. The cryo-TEM technique has proven useful to observe various polymer nanoparticles dispersed in water28-30 and, in very few cases, in organic solvents.31 As most organic solvents are known to dissolve in liquid ethane, thin films of copolymer solutions formed onto holey carbon films were fast frozen in liquid nitrogen, using a guillotine-type quenching device. However, as the freezing speed was not sufficiently high in this cryogen, it was not possible to properly vitrify the embedding heptane, as required for “regular” cryo-TEM.32 Frozen heptane appeared as crystalline lamellae with diffraction effects strongly affecting the contrast of the objects. Therefore, the crystalline solvent was freeze-dried in the microscope at -115 °C. Then the sample was allowed to cool back to -180 °C prior to image recording. After sublimation of the embedding heptane, many objects could be clearly identified, the smaller ones clinging to the supporting “lacey” carbon membrane while the larger ones entangled across the holes in the film. All samples were observed under low dose conditions, using a Philips CM200 “Cryo” microscope operated at 80 kV. Micrographs were recorded at a magnification of 11500×. Atomic Force Microscopy. Thin films for AFM analysis were prepared by solvent casting at ambient conditions starting from solutions in heptane (from 0.01 to 0.5 wt %). Typically, 20 µL of the solution was cast on a 1 × 1 cm2 piece of freshly cleaved mica. Samples were analyzed after complete evaporation of heptane at room temperature. The AFM microscope has been operated in Tapping Mode. All TM-AFM images were recorded in ambient atmosphere at room temperature with a Nanoscope IIIa (Veeco, Santa Barbara, CA). The probes are commercially available silicon tips with a spring constant of 24-52 N/m, a resonance frequency lying in the 264339 kHz range, and a typical radius of curvature in the 10-15 nm range. Acknowledgment. We gratefully acknowledge useful discussions with Y. Gnanou (LCPO), M. Fontanille (LCPO), and R. Pecora (Stanford University). R.B. acknowledges financial support from CNRS, Re´gion Aquitaine, and FEDER. LA026125U
