Langmuir 2004, 20, 3809-3812
Ring-Shaped Morphology of “Crew-Cut” Aggregates from ABA Amphiphilic Triblock Copolymer in a Dilute Solution Jintao Zhu, Yonggui Liao, and Wei Jiang* State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China Received November 17, 2003. In Final Form: January 7, 2004
Introduction Great efforts have been devoted to the study of the selfassembly of amphiphilic block copolymer in a dilute solution during the past few years.1-6 “Crew-cut” micellelike aggregates represent a new type of aggregate. They are formed via the self-assembly of highly asymmetric amphiphilic block copolymer, in which the insoluble coreforming blocks are much longer than the soluble coronaforming blocks.1,7 One of the noteworthy phenomena associated with “crew-cut” aggregates is the accessibility of a wide range of morphologies.1,7-10 These include spheres, rods, vesicles, lamellae, large compound micelles, large compound vesicles, a hexagonally packed hollow hoop structure (the “HHH” structure),11 onions,12 a bowl-shaped structure,13 and several others.14,15 It has been found that the aggregate morphology was controlled mainly by a force balance involving three parameters,7 i.e., the stretching (deformation) of the core-forming blocks in the core, the repulsive interaction among the corona chains, and the interfacial tension at the core-corona interface. Many factors may thus affect the final morphologies of the aggregates,7,16-18 such as the block length of the copolymer, initial polymer concentration in solution, common solvent used, precipitant, temperature, adding ions, etc., because of their effects on the three parameters. Because of their very interesting properties, block copolymer micelle systems play an important role in many * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Zhang, L. F.; Eisenberg, A. Science 1995, 268, 1728. (2) Stewart, S.; Liu, G. J. Angew. Chem., Int. Ed. 2000, 39, 340. (3) Won, Y. Y.; Davis, H. T.; Bates, F. S. Science 1999, 283, 960. (4) 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. (5) Jungmann, N.; Schmidt, M.; Maskos, M. Macromolecules 2001, 34, 8347. (6) Massey, J. A.; Winnik, M. A.; Manners, I. J. Am. Chem. Soc. 2001, 123, 3147. (7) Shen, H. W.; Zhang, L. F.; Eisenberg, A. J. Am. Chem. Soc. 1999, 121, 2728. (8) Zhang, L. F.; Yu, K.; Eisenberg, A. Science 1996, 272, 1777. (9) Zhang, L. F.; Eisenberg, A. J. Am. Chem. Soc. 1996, 118, 3168. (10) Discher, D. E.; Eisenberg, A. Science 2002, 297, 967. (11) Zhang, L. F.; Bartels, C.; Yu, Y. S.; Shen, H. W.; Eisenberg, A. Phys. Rev. Lett. 1997, 79, 5034. (12) Talingting, M. R.; Munk, P.; Webber, S. E. Macromolecules 1999, 32, 1593. (13) Riegel, I. C.; Eisenberg, A. Langmuir 2002, 18, 3358. (14) Yu, K.; Zhang, L. F.; Eisenberg, A. Langmuir 1996, 12, 5980. (15) Cameron, N. S.; Corbierre, M. K.; Eisenberg, A. Can. J. Chem. 1999, 77, 1311. (16) Desbaumes, L.; Eisenberg, A. Langmuir 1999, 15, 36. (17) Yu, G.; Eisenberg, A. Macromolecules 1998, 31, 5546. (18) Yu, Y. S.; Zhang, L. F.; Eisenberg, A. Langmuir 1997, 13, 2578.
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areas,19-22 such as biology, colloid science, drug and gene delivery, advanced materials formation, etc. Previous studies have extensively explored the crewcut aggregates with various morphologies from the amphiphilic diblock copolymer.1,7-10,16-18 Only a few studies have been reported on the aggregates of triblock copolymer in a dilute solution.2,13,23,24 In fact, the self-assembly behavior of the block copolymer strongly depends on the chain architecture.25 Novel morphologies and properties of aggregates made from the triblock copolymer in a dilute solution can be expected because their architecture is more complex than that of the diblock copolymer. Ring formation in wormlike micellar systems has been studied by theory and simulation.26-28 When the endcapping energy of the rodlike micelles is more important than the curvature energy, the micelles tend to form hoopstructure aggregates.11,27,29 It has been theoretically confirmed that the ring-shaped-structure-dominated regime is very narrow for semiflexible chains and the presence of rings may be difficult to observe in many micellar systems.27 The formation of a significant amount of closed-loop micelles would require a surfactant with a very low critical micelle concentration and a high end-cap energy.27,30 As a consequence, the observations of rings have been rather scarce in the wormlike micelle system.31-34 The dominating closed-loop micelle is first formed from the cationic surfactant tetramer.30 In this paper, we have investigated the aggregate properties of the amphiphilic polystyrene (PS)/poly(4vinypyridine) (P4VP) triblock copolymer in a dioxane/ water mixture solution and found that the morphology depends on the annealing time. It changes from a rodshaped structure to a ring-shaped structure with the increasing annealing time. To our knowledge, the ringshaped dominating morphology has not been observed in an experiment with crew-cut aggregates of the block copolymer systems until now. Experimental Section The copolymer used in this paper is a triblock of P4VP(43)b-PS(260)-b-P4VP(43). The numbers in the parentheses indicate the block lengths (polydispersity index ) 1.09). The triblock (19) Massey, J.; Power, K. N.; Manners, I.; Winnik, M. A. J. Am. Chem. Soc. 1998, 120, 9533. (20) Savic, R.; Luo, L. B.; Eisenberg, A.; Maysinger, D. Science 2003, 300, 615. (21) Rosler, A.; Vandermeuler, G. W. M.; Klok, H. A. Adv. Drug Delivery Rev. 2001, 53, 95. (22) Jenekhe, S. A.; Chen, L. D. Science 1998, 279, 1903. (23) Procha´za, K.; Martin, T. J.; Webber, S. E.; Munk, P. Macromolecules 1996, 29, 6526. (24) Gohy, J. F.; Willet, N.; Varshney, S.; Zhang, J. X.; Je´roˆme, R. Angew. Chem., Int. Ed. 2001, 40, 3214. (25) Booth, C.; Attwood, D. Macromol. Rapid Commun. 2000, 21, 501. (26) Kindt, J. T. J. Phys. Chem. B 2002, 106, 8223. (27) van der Schoot, P.; Wittmer, J. P. Macromol. Theory Simul. 1999, 8, 428. (28) Wittmer, J. P.; van der Schoot, P.; Milchev, A.; Barrat, J. L. J. Chem. Phys. 2000, 113, 6992. (29) Zhang, L. F.; Eisenberg, A. Macromolecules 1999, 32, 2239. (30) In, M.; Aguerre-Chariol, O.; Zana, R. J. Phys. Chem. B 1999, 103, 7747. (31) Won, Y. Y.; Brannan, A. K.; Davis, H. T.; Bates, F. S. J. Phys. Chem. B 2002, 106, 3354. (32) Lin, Z.; Scriven, L. E.; Davis, H. T. Langmuir 1992, 8, 2200. (33) Bernheim-Groswasser, A.; Zana, R.; Talmon, Y. J. Phys. Chem. B 2000, 104, 4005. (34) Clausen, T. M.; Vinson, P. K.; Minter, J. R.; Davis, H. T.; Talmon, Y.; Miller, W. G. J. Phys. Chem. 1992, 96, 474.
10.1021/la0361565 CCC: $27.50 © 2004 American Chemical Society Published on Web 03/30/2004
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Notes
Figure 2. Ring size distribution (histogram) of aggregates formed by a 1 wt % solution of P4VP(43)-b-PS(260)-b-P4VP(43) in a dioxane/water (21 wt % water content) mixture after annealing for 48 h, collected from the AFM height images. were visualized by transmission electron microscopy (TEM) and atomic force microscopy (AFM) with the tapping mode at room temperature.
Results and Discussion
Figure 1. Aggregates of the ring-shaped structure made from a 1 wt % solution of P4VP(43)-b-PS(260)-b-P4VP(43) in a dioxane/water (21 wt % water content) mixture after annealing for 48 h: (a) TEM micrograph; (b) AFM height image (the dimension of the image is 2 × 2 µm). copolymer was first dissolved in dioxane, which is a good solvent for both the PS and P4VP blocks. Then, deionized water (which is a precipitant for the PS blocks) was added slowly to the copolymer solutions. When the water content reached 21 wt %, the solution, at that water content, was kept stirring at room temperature for 2 days (or for varying times for the kinetic studies). Finally, a large amount of deionized water was added to quench the resulting aggregates. The resulting solution was dialyzed against distilled water to remove dioxane from the solution. The length of time between the water content reaching 21 wt % and the subsequent large amount of water addition is named the annealing time. The morphologies of the aggregates
Figure 1 shows the morphologies of the aggregates formed by a 1 wt % solution of P4VP(43)-b-PS(260)-bP4VP(43) in a dioxane/water (21 wt % water content) mixture after 48 h of annealing. The TEM micrograph and AFM height image show that the dominating morphology is the ring shape. Moreover, the ring with a long tail, spheres, and rods can also be observed. The TEM micrograph shows that the rings are not hollow in the center of the ring rod. For this block copolymer system, the aggregates involved looping of the hydrophobic PS middle block into the core of the aggregates and tailing of the two hydrophilic P4VP end blocks to form the corona of the aggregates.35 Therefore, the aggregates are very stable after dialysis because of the strong interactions between the P4VP blocks or aqueous corona-forming P4VP and water (under pH 5). Also, at room temperature, the deswelled cores are probably well below the Tg of the bulk PS.1 The contour lengths (L) of 400 ringlike micelles have been determined from the AFM height images (Figure 2). The average L is 185.4 nm. An important feature is the broad range of sizes observed, from 67.1 to 1000.0 nm. The average diameter for 100 ring rods is 31.7 ( 2.0 nm, observed from the particle heights by the AFM height images.36 This value is nearly the same as the diameter (31.3 ( 3.0 nm) of the stretching rods. The ring-shaped structure resembles the “HHH” structure or the “hollow doughnut” structure reported by the Eisenberg group11,14,15 and the network phase fragments structure very recently reported by the Bates group.37 However, the difference from these structures is obvious. The “HHH” structure has mesosized crystallike aggregates with an internal structure of hexagonally packed hollow hoops. The “hollow doughnut” is hollow with one or multiple holes and not uniform in diameter. Also, the network fragments structure relates to the spherical caps closed to the hoops. The structures found in this paper are single rings with very uniform diameters (see Figure 1b). When the annealing (35) Yuan, J. J.; Ma, R.; Gao, Q.; Wang, Y. F.; Cheng, S. Y.; Feng, L. X.; Fan, Z. Q.; Jiang, L. J. Appl. Polym. Sci. 2003, 89, 1017. (36) Huang, H. Y.; Kowalewski, T.; Remsen, E. E.; Gertzmann, R.; Wooley, K. L. J. Am. Chem. Soc. 1997, 119, 11653. (37) Jain, S.; Bates, F. S. Science 2003, 300, 460.
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Figure 3. Morphologies of the aggregates at different stages of ring-shaped aggregates formation observed by AFM height images [the dimension of the images is 5 × 5 µm (A-D) and 3 × 3 µm (E and F)]: (A) 0 h, (B) 2 h, (C) 7 h, (D) 14 h, (E) 23 h, and (F) 48 h.
time was lengthened further (5 days), the dominating morphologies were still rings. The only difference is that the compound ring structure could be found, which may
be the results of the rings contacting and fusing with each other. Therefore, the ring structure had a relatively stable morphology under our experimental time scale. Direct
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evidence of the ring-shaped dominating morphology in this system will offer a good example to refine the input parameters to the theory and simulation and to test the predictions against the results at other conditions.26 It is generally believed that the mechanism of ring formation is end-end bonding of a single chain.26 The ring closure is probable for long flexible polymers obeying Gaussian statistics but not valid for short semiflexible ones.30 Because the bending energy of a closed micelle is very high,30 end-end bonding should not be the major way for small ring formation (as small as 67.1 nm) because the two ends of a rod, especially for shorter rods, are difficult to connect with each other. Another possible mechanism of small ring formation is given below based on the experimental results. In general, the architectural change occurs quite rapidly in a small-molecule surfactant system; thus, the kinetics have been difficult to follow.38 Because of the relatively low mobility (high viscosity) of the polymer chains in the self-assembled regions under the experimental conditions, it allows us to study the process of block copolymer micelle formation. Figure 3 shows the morphologies of the aggregates at different stages of structural rearrangement observed by AFM. Long rods are found at the initial stages (Figure 3A). With the increase of time, long rods become shorter and shorter. After 2 h, more spheres can be observed (Figure 3B,C). Parts D-F of Figure 3 show that the spheres became larger (the TEM observations indicate that these larger spheres are hollow in the center, i.e., small vesicles) and tend to form a ring structure with increasing time. These results suggest that the ring formation (or the rod-to-ring transition) undergoes two main stages. First, long rods may break and tend to form small vesicles, and then the vesicles transfer to the rings. In the second stage, the vesicles are the possible precursors of the rings. In the first stage, it resembles the rod-tovesicle transition of the diblock copolymer observed by Chen et al.39 The process of the rod-to-vesicle transition is difficult to trap. The process of the vesicle-to-ring transition is most similar to the one encountered in the HHH11 and bowl-shaped structures.13 On the basis of the previously found morphologies in similar copolymer systems, a possible mechanism can be proposed for the vesicle-to-ring transition. Ring formation may involve an adhesive contact and fusion of the vesicles, which is largely dependent on the local concentration of the aggregates and the fusion rate.40,41 With increased annealing time, (38) Burke, S. E.; Eisenberg, A. Langmuir 2001, 17, 6705. (39) Chen, L.; Shen, H. W.; Eisenberg, A. J. Phys. Chem. B 1999, 103, 9488.
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the small vesicles may become adhesive contacts and fuse, collapse, and then break in the center of the vesicles. Once the breakthrough occurred and the healing of the periphery of the breakthrough took place, a small ring may be formed. Finally, the small rings may rearrange and fuse to form larger rings. This viewpoint can be supported from the morphologies with the various sizes marked by the squares a-e in Figure 3F. For the purpose of comparison, the stirring stopped when the water content reached 21 wt %. The resulting morphologies roughly remained long rods along with the occasional ring with an increase of the annealing time to 5 days. Therefore, stirring or shearing is one of the important driving forces for ring formation. Conclusion In summary, the novel morphology of rings was formed from the triblock copolymer in a dilute dioxane/water mixture solution. It gives an additional member to the crew-cut morphologies family. We can see that it is possible to form some novel morphology from the triblock copolymer in a dilute solution. In terms of the potential applications, it is conceivable that the rings may provide a template to fabricate nanocomposites because the P4VP block can form a complex with a wide range of metal ions.42 The process of the morphological change from rods to vesicles and then to rings is interesting and attractive in itself. A better understanding of the process may shed light onto the structural changes and assembly in biological cells. Acknowledgment. This paper is supported by the National Natural Science Foundation of China (NSFC) for the General Program (50073023 and 20274047), the special fund (20023003), and the Major Program (20394001), the Chinese Academy of Sciences for Intellectual Innovations Project (KJCX2-SW-H07), Special Funds for Major State Basic Research Projects (2003CB615600), and the Fund for the Progress Projects in Science and Technology of Jilin Province, China. Supporting Information Available: Information on the triblock copolymer used, experimental details, and TEM and AFM figures (PDF). This material is available free of charge via the Internet at http://www.pubs.acs.org. LA0361565 (40) Zhang, L. F.; Eisenberg, A. Polym. Adv. Technol. 1998, 9, 677. (41) Choucair, A. A.; Kycia, A. H.; Eisenberg, A. Langmuir 2003, 19, 1001. (42) Fo¨rster, S.; Antonietti, M. Adv. Mater. 1998, 10, 195.
