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Novel Morphologies of “Crew-Cut” Aggregates of Amphiphilic Diblock Copolymers in Dilute Solution Kui Yu, Lifeng Zhang, and Adi Eisenberg* Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2K6 Received September 16, 1996X Several novel morphologies of “crew-cut” aggregates have been prepared from highly asymmetric diblock copolymers of polystyrene-b-poly(ethylene oxide) (PS-b-PEO) and polystyrene-b-poly(acrylic acid) (PS-bPAA) in dilute solution. The morphologies include simple, circular, and interconnected tubules, large compound rods, a structure of interconnected bicontinuous rods, branched short rods, spheres with protruding rods, and porous spheres, as well as aggregates containing combined morphologies. Some of these structures are biomimetic. Branched short rods and spheres with protruding rods appear to be intermediate structures trapped during morphological transitions. This study illustrates the high degree of kinetic control which can be exercised over the morphologies of molecular self-assembled nanostructures. Some of the aggregates may have potential applications in areas such as separations and drug delivery systems.
Introduction Amphiphilic diblock copolymers, when dissolved in a solvent which is good for only one of the blocks, can form nanosized aggregates as a result of the self-assembly of the less soluble segment.1 Most of the aggregates are spherical, with a core-shell structure, and are frequently called “star” micelles if the corona block is much longer than that of the core. When the soluble block is much shorter than the insoluble block, the aggregates are called “crew cut”.2 Recently, we described several morphologies found in crew-cut aggregates made from two families of diblock copolymers, polystyrene-b-poly(acrylic acid) (PSb-PAA) and polystyrene-b-poly(ethylene oxide) (PS-bPEO), in dilute solution.3,4 As the soluble PAA or PEO blocks are made progressively shorter, the morphology of the aggregates changes from spherical to rodlike to lamellar or vesicular and finally to large compound vesicles (LCVs) and large compound micelles (LCMs). It is also possible to form aggregates of these morphologies from one single diblock copolymer, by the addition of ions in micromolar (CaCl2 or HCl) or millimolar (NaCl, etc.) concentrations.5 This Letter describes the preparation and observation of several additional morphologies of crew-cut aggregates. The morphologies include hollow tubes (linear, circular, and interconnected), large compound rods (LCRs), a bicontinuous structure of interconnected rods, branched * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, November 15, 1996. (1) (a) Price, C. In Developments in block copolymers; Goodman, I., Ed.; Applied Science Publishers: London, 1982; Vol. 1, p 39. (b) Selb, J.; Gallot, Y. In Developments in block copolymers; Goodman, I., Ed.; Applied Science Publishers: London, 1985; Vol. 2, p 27. (c) Whitmore, M. D.; Noolandi, J. Macromolecules 1985, 18, 657. (d) Halperin, A. Macromolecules 1987, 20, 2943. (e) Nagarajan, R.; Ganesh, K. J. Chem. Phys. 1989, 90, 5843. (f) Hilfiker, R.; Wu, D. Q.; Chu, B. J. Colloid Interface Sci. 1990, 135, 573. (g) Tuzar, Z.; Kratochvil, P. In Surface and Colloid Science; Matijevic, E., Ed.; Plenum Press: New York, 1993; Vol. 15, p 1. (h) Gast, A. P.; Vinson, P. K.; Cogan-Farinas, K. A. Macromolecules 1993, 26, 1774. (i) Astafieva, I.; Zhong, X. F.; Eisenberg, A. Macromolecules 1993, 26, 7339. (j) Qin, A.; Tian, M.; Ramireddy, C.; Webber, S. E.; Munk, P. Macromolecules 1994, 27, 3276. (k) Antonietti, M.; Heinz, S.; Schmidt, M.; Rosenauer, C. Macromolecules 1994, 27, 6046. (l) Glatter, O.; Scherf, G.; Schille´n, K.; Brown, W. Macromolecules 1994, 27, 6055. (2) (a) Halperin, A.; Tirrel, M.; Lodge, T. P. Adv. Polym. Sci. 1992, 100, 31. (b) Gao, Z.; Varshney, S. K.; Wong, S.; Eisenberg, A. Macromolecules 1994, 27, 7923. (3) (a) Zhang, L.; Eisenberg, A. Science 1995, 268, 1728. (b) Zhang, L.; Eisenberg, A. J. Am. Chem. Soc. 1996, 118, 3168. (4) Yu, K.; Eisenberg, A. Macromolecules, in press. (5) Zhang, L.; Yu, K.; Eisenberg, A. Science 1996, 272, 1777.
S0743-7463(96)00894-3 CCC: $12.00
short rods, spheres with protruding rods, porous spheres, and several combined morphologies. To our knowledge, these morphologies have not been seen before in diblock copolymer aggregates in dilute solution and are an addition to the growing field of molecular self-assembled nanostructures. Furthermore, because of their shapes and hydrophilic surfaces, some may have potential applications. Experimental Section PS-b-PEO and PS-b-PAA diblock copolymers used in this study were prepared by sequential anionic polymerization in tetrahydrofuran (THF) at -78 °C under an ultrapure nitrogen atmosphere, generally following published procedures.6-8 Two methods have been employed to prepare the aggregates. In the first method, PS-b-PEO or PS-b-PAA diblock copolymers were dissolved in pure N,N-dimethylformamide (DMF) to give stock solutions of various polymer concentrations (expressed in weight percent). In the second method, the diblocks were dissolved in water-DMF mixtures of varying water contents (also expressed in weight percent). In both methods, deionized water was then added to the solutions with stirring, until the water content reached ca. 25%. The change in the water content per step was ca. 0.25%, with 15 s between steps, which averaged to ca. 0.17 µL of water per milliliter of solution per second. On occasion, much more rapid water addition was employed, as will be pointed out. At a certain water content, the aggregates become kinetically frozen; finally the solutions were dialyzed against distilled water to remove the DMF. After dialysis, the solutions were further diluted by a factor of 20. A drop of the diluted solution was placed onto a copper EM grid which has been pre-coated with a thin film of Formvar (J. B. EM Services Inc.) and then coated with carbon. Excess solution was removed, and the sample grids were allowed to dry in air for a few hours. The sample grids were shadowed with a palladium/platinum alloy. A detailed description of the experimental conditions can be found elsewhere.3,4 The morphologies of the aggregates were observed by transmission electron microscopy (TEM) on a Phillips EM410 microscope.
Results and Discussion A typical example of tubules made from a 1.5% diblock copolymer solution in pure DMF is shown in Figure 1A. The diblock copolymer is PS(240)-b-PEO(15) (the numbers in parentheses indicate the block lengths). The tubules (6) O’Malley, J. J.; Marchessault, R. H. Macromol. Synth. 1972, 4, 35. (7) Hruska, Z.; Hurtrez, G.; Walter, S.; Riess, G. Polymer 1992, 33, 2447. (8) Zhong, X. F.; Varshney, S. K.; Eisenberg, A. Macromolecules 1992, 25, 7160.
© 1996 American Chemical Society
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Figure 1. Bilayer aggregates made from PS(240)-b-PEO(15): (A) tubules; (B) hollow “doughnut”; (C) interconnected tubular structure; (D) tubule appearing in the barolinic mode.
have a wall thickness of ca. 20 nm, have an outside diameter of ca. 150 nm, and are frequently many micrometers in length. Aggregates made from the same block copolymer at the same concentration but dissolved in a 4.5% water-DMF mixture are shown in Figure 1B-D. Under those conditions, the tubules can form circular hollow “doughnuts”, shown in Figure 1B, or complex interconnected structures (“plumber’s nightmare”), shown in Figure 1C. Some parts of the tubules, on occasion, appear in the baroclinic mode,9,10 shown in Figure 1D. Tubules have also been observed from PS(240)-b-PEO(45) and PS(240)-b-PEO(80) in water-DMF mixtures under generally similar conditions. It was found that water-DMF mixtures are better than pure DMF solutions for the preparation of tubules. However, the optimum range of copolymer concentrations is narrow, and the higher the water content in water-DMF mixture, the lower the copolymer concentration at which tubules can be prepared. It should be remembered that aggregates of several morphologies are frequently seen in the same micrograph,3 as has been discussed before. The tubules are biomimetic, in that they resemble, among others, the microtubules composed of tubulin subunits which exist in eukaryotic cells.11 The circular hollow “doughnuts” in Figure 1B look like some carnivorous fungi that trap nematodes.12 The complex interconnected structures (“plumber’s nightmare”) in Figure 1C show some resemblance to the endoplasmic reticulum, (9) Brochard, F.; de Gennes, P.-G. Pramana Suppl. 1975, 1, 1. (10) Nallet, F.; Roux, D.; Prost, J. J. Phys. (Paris) 1989, 50, 3147. (11) Lodish, H.; Baltimore, D.; Berk, A.; Zipursky, S. L.; Matsudaira, P.; Darnell, J. In Molecular Cell Biology, 3rd ed.; W. H. Freeman and Company: New York, 1995. (12) (a) Slack, A. In Carnivorous Plants; The MIT Press: Cambridge, Massachusetts, 1980. (b) Juniper, B. E.; Robins, R. J.; Joel, D. M. In The Carnivorous Plants; Academic Press, New York, 1989.
Golgi apparatus, and sponges.11,13 Many other examples of biological microtubules can be cited.11 The determination of the factors which regulate the self-assembly of tubules of diblock copolymers may improve our understanding of the self-assembly in biological systems. The preparation and characterization of nanosized tubules from small molecule amphiphiles have received considerable attention recently.14-16 These tubules tend to be structurally fragile, and efforts have been made to improve their stability by polymerization or templating.14-16 Tubules from diblock copolymers would obviously not require such efforts because of their greater inherent stability. The potential applications of tubules made from diblock copolymers may include chemical encapsulation and controlled release. The next morphologies to be discussed, which are shown in Figure 2, are rodlike. An example of a large compound rod (LCRs) with a diameter of ca. 550 nm made from a 3.0% PS(240)-b-PEO(45) solution in DMF is shown in Figure 2A. It is evident that the LCRs must consist of aggregates of primary structures, because even if the 240unit PS chains were fully stretched, they would not be able to form rods of that diameter. The formation mechanism of these rods, as well as their internal structure, may be similar to those of the spherical large compound micelles (LCMs) described previously,3 which consist of assemblies of reverse micelles with hydrophilic surfaces. A bicontinuous rod structure from a 4.3% PS(190)-bPAA(20) solution in a 8.5% water-DMF mixture is shown (13) Sheeler, P.; Bianchi, D. E. In Cell and Molecular biology, 3rd ed.; John Wiley & Sons, Inc.: New York, 1987. (14) Yager, P.; Schoen, P. E. Mol. Cryst. Liq. Cryst. 1984, 106, 371. (15) Schnur, J. M. Science 1993, 262, 1669. (16) Thomas, B. N.; Safinya, C. R.; Plano, R. J.; Clark, N. A. Science 1995, 267, 1635.
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Figure 2. Aggregates of rodlike morphologies: (A) large compound rod (LCR) made from PS(240)-b-PEO(45); (B) bicontinuous structure of interconnected rods made from PS(190)-b-PAA(20); (C) branched short rods made from PS(240)-b-PEO(15); (D) spheres with protruding rods (LCRMs or “pincushions”) made from PS(240)-b-PEO(80).
in Figure 2B. The structure is three dimensional and bicontinuous as judged from its appearance and that of the shadowed regions. The average distance between neighboring junctions is ca. 50 nm, with a rod diameter of ca. 25 nm. Bicontinuous structures have been observed in block copolymers in bulk or in copolymer/homopolymer blends, as well as in solutions of small molecule amphiphiles,17,18 but not in dilute solutions of block copolymers. This morphology in thin film form might yield a highly porous layer, with potential uses in separation technology. An example of branched short rods with diameters of ca. 75 nm made from a 1.5% PS(240)-b-PEO(15) solution in a 5.5% water-DMF mixture is shown in Figure 2C. From the shadowing it is evident that the branches are not coplanar. Branched short rods are also formed by the rapid addition of water to a PS(240)-b-PEO(80) solution in DMF. Filtration of these branched rods would also yield a highly porous layer which could be of some practical interest. A typical example of spheres with protruding rods (“pincushions”) made from a 1.0% PS(240)-b-PEO(80) solution in a 5.5% water-DMF mixture is shown in Figure 2D; the name “large compound rod micelles” (LCRMs) is suggested for these assemblies. Aggregates of this morphology can be prepared from solutions of all the PSb-PEO samples used in this work. The sizes of the LCRMs (17) (a) Aggarwal, S. L. Polymer 1976, 17, 938. (b) Alward, D. B.; Kinning, D. J.; Thomas, E. L.; Fetters, L. J. Macromolecules 1986, 19, 215. (18) (a) Forster, S.; Khandpur, A. K.; Zhao, J.; Bates, F. S.; Hamley, I. W.; Ryan, A. J.; Bras, W. Macromolecules 1994, 27, 6922. (b) Hajduk, D. A.; Harper, P. E.; Gruner, S. M.; Honeker, C. C.; Kim, G.; Thomas, E. L.; Fetters, L. J. Macromolecules 1994, 27, 4063.
are polydisperse, and they do not seem to be primary structures either. Frequently, when pincushions were made from water-DMF mixtures, small branched rods were also observed on the same EM grid. The pincushions are biomimetic, in that they bear some resemblance to amoebas with pseudopods. Very small pincushions with diameters of ca. 300 nm sometimes show a resemblance to erythrocytes in echinocytic shapes.19 An example of porous spheres made from a 1.5% PS(240)-b-PEO(45) solution in a 5.5% water-DMF mixture is shown in Figure 3. The mechanism of pore formation probably involves a classical phase inversion process, which is commonly used in the production of various microporous polymeric membranes.20,21 Porous solids of this type might be attractive as supports for immobilized enzymes or other catalysts or in other applications such as chromatography, where stability, a large hydrophilic surface area, and easy accessibility to liquids are required. Several examples of combined morphologies are shown in Figure 4. Lamellae with rods (“pancake with fingers”) and a vesicle with attached rods made from a 1.5% PS(240)-b-PEO(45) solution in DMF are shown in parts A and B of Figure 4, respectively. The resemblance of the lamellae or vesicles with protruding rods to neurons with dendrites11 or fruiting bodies is noteworthy.12 Vesicles with connected tubules made from a 1.5% PS(240)-b-PEO(15) solution in a 4.0% water-DMF mixture is shown in Figure 4C. All these combined morphologies illustrate (19) Elgsaeter, A.; Stokke, B. T.; Mikkelsen, A.; Branton, D. Science 1986, 234, 1217. (20) (a) Loeb, S. Desalination 1966a, 1, 35. (b) Sourirajan, S. In Reverse Osmosis; Academic Press: New York, 1970. (21) Mulder, M. In Basic Principles of Membrane Technology; Kluwer Academic Publishers: Dorodecht, 1991.
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Figure 3. Porous spheres made from PS(240)-b-PEO(45).
the relatively small differences in the free energy between the various primary morphologies under the preparation conditions or the small changes in external conditions which induce different morphologies under kinetic control. The structures in Figure 4C are of special interest. The appearance of pearling states, facilitated by laser tweezers in small molecule amphiphiles,22,23 is frequent in the present materials under a range of preparation conditions. Similarly, the dimeric and linear trimeric vesicular aggregates, which might be examples of budding transitions predicted by curvature models,24,25 are easy to detect. Also, starfish-like vesicular aggregates with three arms, which are rarely seen in small molecule amphiphiles, are common here and are expected to be of some theoretical importance.26 Finally, the ease of observation of interconnected structures (Figure 1C) is also noteworthy. All these examples illustrate the ease of preparation and observation in the present block copolymers of different bilayer topologies which, in some cases, are much less easy to produce in lipid bilayers, and which have generated intense interest for the last 20 years.27-30 We now turn our attention to thermodynamic versus kinetic aspects of morphological control. Since bicontinuous interconnected rods (along with spheres, simple rods, and vesicles) have been found in small molecule amphiphiles and in phase separated block copolymer systems, they resemble morphologies formed under thermodynamic control. However, most of the other morphologies, such as the spheres with protruding rods (LCRMs) or branched short rods, appear to result from kinetic control. The thermodynamics and kinetics of the self-assembly of the crew-cut aggregates are obviously complicated. In the early stages of aggregate formation, while the water content in the system is relatively low, (22) Bar-Ziv, R.; Moses, E. Phys. Rev. Lett. 1994, 73, 1392. (23) Bar-Ziv, R.; Moses, E. Phys. Rev. Lett. 1995, 75, 3481. (24) Miao, L.; Fourcade, B.; Rao, M.; Wortis, M; Zia, R. K. P. Phys. Rev. A 1991, 43, 6843. (25) Miao, L.; Seifert, U.; Wortis,M.; Dobereiner, HG. Phys. Rev. E 1994, 49, 5839. (26) Wintz, W.; Dobereiner, HG.; Seifert, U. Europhys. Lett. 1996, 33, 403. (27) Scriven, L. E. Nature 1976, 263, 123. (28) Longley, W.; Mclntosh, T. J. Nature 1983, 303, 612; (29) Kantor, Y.; Nelson, D. R. Phys. Rev. Lett. 1987, 58, 2774. (30) (a) Canham, P. B. J. Theor. Biol. 1970, 26, 61. (b) Helfrich, W. Z. Naturforsch.Teil C Biochem. Biophys. Biol. Virol. 1973, 28, 693. (c) Evans, E. A. Biophys. J. 1974, 14, 923. (d) Deuling, H. J.; Helfrich, W. J. Phys. (Paris) 1976, 37, 1335. (e) Mutz, M.; Bensimon, D. Phys. Rev. A 1991, 43, 4525. (f) Seifert, U.; Berndl, K.; Lipowsky, R. Phys. Rev. A 1991, 44, 1182. (g) Lipowsky, R. Nature 1991, 349, 475. (h) Gompper G.; Kroll, D. M. Phys. Rev. E 1995, 51, 514. (i) Michalet, X.; Bensimon, D. Science 1995, 269, 666. (j) Seifert, U. Curr. Opin. Colloid Interface Sci. 1996, 1, 350.
Figure 4. Aggregates of combined morphologies: (A) lamellae with rods (“pancake with fingers”) and (B) a vesicle with attached rods made from PS(240)-b-PEO(45); (C) vesicles with connected tubules made from PS(240)-b-PEO(15).
it is likely that the exchange of the copolymer single chains between unimers and aggregates proceeds at a significant rate. Therefore, the aggregation is controlled by thermodynamics. As the water content increases, the copolymer chains may reorganize to form aggregates of other morphologies if a morphological boundary is crossed; alternatively, the new morphology may form while leaving the first structure intact if the reorganization kinetics are slow. Which of the two possibilities is dominant depends on the DMF content in the core, the rate of change of the water concentration, and the polymer concentration, along with other parameters. This introduces the aspect of kinetic control. At relatively high water contents, the rate of single chain exchange decreases with increasing water concentration both because of the decrease in the solubility of single chains and because of the increase in the viscosity of the polystyrene cores, which may even become glassy as the DMF is removed. Therefore, an intermediate structure, produced by kinetic control, can
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be frozen in, provided that the morphological transition is slow on the time scale of the water addition. The branched short rods and the spheres with protruding rods (LCRMs) are typical examples. For these two morphologies, it was found that, if the addition of water is accelerated, most of the aggregates are large spheres without protruding rods; on the other hand, if water addition is slowed down, most of the aggregates are rodlike. This illustrates that the formation of branched short rods and LCRMs is kinetically controlled. Trapped intermediates such as branched short rods and LCRMs are of great interest. For aggregates of small molecule surfactants, fusion processes and rates of morphological transitions are very fast, usually of the order of 0.1-1 ms. Consequently, it is not easy to trap the intermediate structures of fusion, although a number of fusion mechanisms have been suggested.31 However, the fusion process between morphologically different copolymer aggregates is much slower because of the lower mobility of these high molecular weight molecules. Therefore, it becomes possible to trap intermediate structures, such as the branched short rods or spheres with protruding rods (LCRMs). The observation of these intermediate structures from diblock copolymers may provide a deeper insight into the process of amphiphile self-assembly. Conclusions In this Letter, we have described several novel morphologies of crew-cut aggregates prepared from PS-b-PEO and PS-b-PAA diblock copolymers in dilute solution. These include tubules, large compound rods, intercon(31) Israelachvili, J. N. In Intermolecular and Surface Forces, 2nd ed.; Academic Press, London, 1992.
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nected rods, branched short rods, spheres with protruding rods, and porous spheres, as well as aggregates of combined morphologies. The structures described in this paper join the morphologies prepared earlier, i.e., spheres, rods, lamellae, spherical vesicles, large compound micelles, macroscopic needles and reverse crew cuts,3,4 large compound vesicles (which also have biomimetic aspects), irregular bilayer surfaces and “pearl necklaces”.5 The morphologies described here bring the total number found in the crew-cut micelles to about 15. Some of the morphologies are biomimetic and some are trapped intermediates resulting from kinetically controlled copolymer self-assembly. The morphologies are stable and easy to observe. This morphological variety is unprecedented for any system of small or large molecule amphiphiles, and makes the range of morphologies observed in block copolymers especially in terms of the bilayer structures, comparable to that seen until now only in a wide range of small molecule amphiphile families. Because of their specific structures and hydrophilic surfaces, some, such as tubules, porous spheres, and interconnected rods, may have potential applications in areas such as drug delivery systems and separations. In addition, the preparation of stable nanotubules from block copolymers is novel and interesting in its own right. A detailed study of morphogenesis of the present system, especially for the biomimetic structures, may contribute to our understanding of corresponding biological processes. Acknowledgment. We thank The Natural Science and Engineering Research Council of Canada (NSERC) for financial support of this research. We are also indebted to L. Cuccia, C. J. Clarke, and M. Moffitt for useful discussions. LA960894U
