Water Interface - Langmuir (ACS Publications)

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Langmuir 2001, 17, 6787-6793

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Janus Micelles at the Air/Water Interface Hui Xu,† Rainer Erhardt,‡ Volker Abetz,‡ Axel H. E. Mu¨ller,‡,§ and Werner A. Goedel*,† Organische Chemie III/Makromolekulare Chemie, Universita¨ t Ulm, D-89081 Ulm, Germany, Makromolekulare Chemie II Universita¨ t Bayreuth, D-95440 Bayreuth, Germany, and Bayreuther Zentrum fu¨ r Kolloide und Grenzfla¨ chen, Universita¨ t Bayreuth, D-95440 Bayreuth, Germany Received January 16, 2001. In Final Form: June 18, 2001 Janus micellessasymmetric star block copolymers resulting from cross-linking of the polybutadiene middle blocks of polystyrene-block-polybutadiene-block-poly(methyl methacrylate) triblock copolymerss and the corresponding non-cross-linked ABC triblock copolymer precursors form monolayers on a water surface. Analysis of the lateral pressure/area isotherms and scanning force microscopy images of monolayers spread from dilute chloroform solutions reveal a lateral structure: In monolayers of both polymers, one observes elevated nanometer-sized domains separated by flat regions. However, the monolayers made from the two copolymers differ in morphology and size of the observed domains. Janus micelle monolayers are predominantly composed of arrays of circular domains with height of 16-18 nm and diameter of 70-80 nm, while the monolayers of the precursor triblock copolymer are mainly composed of significantly smaller elongated domains of approximately 3 nm height, 40 nm width, and nonuniform length. The formation of domains and the shape of the lateral pressure/area isotherms are in accordance with the assumption that the poly(methyl methacrylate) chains spread out on the water surface, while the hydrophobic polystyrene and polybutadiene chains dewet from the water surface. Differences in domain morphology are probably due to the different chain architectures of the two polymers but may as well be influenced by the preaggregation in the spreading solvent.

Introduction Block copolymers composed of incompatible chains form fascinating, nanometer-sized structures in solution,1-9 in bulk,10-15 and at interfaces.16-21 The nanometer scale †

Universita¨t Ulm. Makromolekulare Chemie II Universita¨t Bayreuth. § Bayreuther Zentrum fu ¨ r Kolloide und Grenzfla¨chen, Universita¨t Bayreuth. ‡

(1) Bayer, U.; Stadler, R. Macromol. Chem. Phys. 1994, 195, 2709. (2) Alexandridis, P.; Olsson, U.; Lindman, B. Langmuir 1997, 13, 23. (3) Fo¨rster, S.; Antonietti, M. Adv. Mater. 1998, 10, 195. (4) Yu, K.; Eisenberg, A. Macromolecules 1998, 31, 3509. (5) Voulgaris, D.; Tsitsilianis, C.; Grayer, V.; Esselink, F. J.; Hadziioannou, G. Polymer 1999, 40, 5879. (6) Giebeler, E.; Stadler, R. Macromol. Chem. Phys. 1997, 198, 1385. (7) Yu, G.; Eisenberg, A. Macromolecules 1998, 31, 5546. (8) Stewart, S.; Liu, G. Chem. Mater. 1999, 11, 1048. (9) Bieringer, R.; Abetz, V.; Mu¨ller, A. H. E. Eur. Phys. J. E 2001, 5, 5. (10) Fredrickson, G. H.; Bates, F. S. Annu. Rev. Mater. Sci. 1996, 26, 501. (11) Bates, F. S.; Fredrickson, G. H. Phys. Today 1999, 52, 33. (12) Chen, Z.-R.; Issaian, A. M.; Kornfield, J. A.; Smith, S. D.; Grothaus, J. T.; Satkowski, M. M. Macromolecules 1997, 30, 7096. (13) Matsushita, Y.; Yamada, K.; Hattori, T.; Fujimoto, T.; Sawada, Y.; Nasagawa, M.; Matsui, C. Macromolecules 1983, 16, 10. (14) Stadler, R.; Auschra, C.; Beckmann, J.; Krappe, U.; Voigt-Martin, I.; Leibler, L. Macromolecules 1995, 28, 3080. (15) Abetz, V. In Supramolecular Polymers; Ciferri, A., Ed.; Marcel Dekker, Inc.: New York, 2000; p 215. (16) Zhu, J.; Eisenberg, A.; Lennox, R. B. J. Am. Chem. Soc. 1991, 113, 5583. (17) Cox, J. K.; Eisenberg, A.; Lennox, R. B. Curr. Opin. Colloid Interface Sci. 1999, 4, 52-59. (18) Park, M.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Adamson, D. H. Science 1997, 276, 1401. (19) Spatz, J. P.; Eibeck, P.; Mo¨ssmer, S.; Mo¨ller, M.; Kramarenko, E. Y.; Khalatur, P. G.; Potemkin, I. I.; Khokhlov, A. R.; Winkler, R. G.; Reineker, P. Macromolecules 2000, 33, 150. (20) Potemkin, I. I.; Kramarenko, E. Yu.; Khokhlov, A. R.; Winkler, R. G.; Reineker, P.; Eibeck, P.; Spatz, J. P.; Moeller, M. Langmuir 1999, 15, 7290. (21) Elbs, H.; Fukunaga, K.; Stadler, R.; Sauer, G.; Magerle, R.; Krausch, G. Macromolecules 1999, 32, 1204-1211.

structuring and compartmentalization of matter associated with these structures lead to a variety of new applications in fields as various as drug delivery, catalysis, materials science, and microelectronics. Investigations of block copolymers significantly contributed to our understanding on how these structures are determined by the interplay of covalent bonding, attractive/repulsive forces, and the conformational entropy of the polymer chains.22-24 Currently, investigations of bulk phase morphologies of block copolymers have been expanded to include more complex polymer architectures such as linear and branched copolymers composed of more than two incompatible chains and have led to the discovery of a large number of new three-dimensional structures.25-27 Investigations concerning the solution properties and surface activity of these polymer architectures, however, are still at the very beginning.6-8 Especially of interest in the context of this paper is the fact that in bulk of suitable ABC-triblock copolymers such as polystyrene-b-polybutadiene-b-poly(methyl methacrylate) (SBM), the middle blocks form spheres located at the interfaces of lamellae formed by the outer blocks. These spheres were selectively cross-linked to give rise to a soluble star-shaped polymer, subsequently termed a “Janus micelle” (Figure 1). This star polymer consists of a cross-linked core and a corona of two different types of (22) Encyclopedia of Polymer Science and Engineering; Kroschwitz, J. I., Ed.; Wiley-Interscience: New York, 1985; Vol. 2. (23) Polmer bends; Paul, D. R., Newman, S., Eds.; Academic Press: New York, 1978. (24) Thomas, E. I.; Anderson, D. M.; Henkee, C. S.; Hoffman, D. Nature 1988, 334, 598. (25) Okamoto, S.; Hasegawa, H.; Hashimoto, T.; Fujimoto, T.; Zhang, H.; Kazyma, T.; Takano, A.; Isono, Y. Polymer 1997, 38, 5275. (26) Hadjichristidis, N.; Pispas, S.; Pitsikalis, M. Prog. Polym. Sci. 1999, 24, 875. (27) Hu¨cksta¨dt, H.; Go¨pfert, A.; Abetz, V. Macromol. Chem. Phys. 2000, 201, 296.

10.1021/la010091t CCC: $20.00 © 2001 American Chemical Society Published on Web 10/06/2001

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Figure 1. Schematic representation of the bulk morphology of the polystyrene-b-polybutadiene-b-poly(methyl methacrylate) triblock copolymer precursor (SBM) and the architecture of the “Janus micelle” generated by cross-linking the spherical domains of the polybutadiene middle block followed by dissolution.

chains.28 Due to the method of preparation, one can expect the following: (i) the number of chains per “Janus micelle” is determined by the aggregation number of the middle blocks in bulk before cross linking; (ii) the corona of the “Janus micelles” is divided into a “northern” hemisphere composed of polystyrene and a “southern” hemisphere composed of poly(methyl methacrylate) (Figure 1). This unusual structure of bundled block copolymers combines the multiarm structure of simple star polymers29-33 with the amphiphilicity of simple block copolymers. One can expect that this combination affects the aggregation in solution as well as the interaction with interfaces. While aggregation of Janus micelles in solution has been the subject of a separate investigation,28 we report here on their interfacial properties. Janus micelles are spread on a water surface: Lateral pressure/area isotherms of the resulting monolayers as well as scanning force microscopy images of transferred monolayers are investigated. To highlight the special properties of the Janus micelles, these results are compared to the corresponding data of the non-cross-linked triblock copolymer SBM precursor. Experimental and Data Analysis Experimental Section. The SBM precursor triblock copolymer (51 wt % polystyrene (PS, 932 repeat units), 6 wt % polybutadiene (PB, 211 repeat units), and 43 wt % poly(methyl methacrylate) (PMMA, 817 repeat units)) with a number-average molecular mass Mn ) 190 kg/mol and a polydispersity index Mw/Mn ) 1.05 was prepared by living anionic polymerization.28,34

The polybutadiene spheres at the interface between the PS and PMMA lamellae formed by the aggregation of 13 ( 5 middle (28) Erhardt, R.; Bo¨ker, A.; Zettl, H.; Kaya, H.; Pyckhout-Hintzen, W.; Krausch, G.; Abetz, V.; Mu¨ller, A. H. E. Macromolecules 2001, 34, 1069. (29) Saville, P. M.; Reynolds, P. A.; White, J. W.; Hawker, C. J.; Frechet, J. M. J.; Wooley, K. L.; Penfold, J.; Webster, J. R. P. J. Phys. Chem. 1995, 99, 8283. (30) Schenning, A. P. H. J.; Elissen-Roman, C.; Weener, J. W.; Baars, M. W. P. L.; van der Gaast, S. J.; Meijer, E. W. J. Am. Chem. Soc. 1998, 120, 8199. (31) Iyer, J.; Hammond, P. T. Langmuir 1999, 15, 1299. (32) Ariga, K.; Urakawa, T.; Michiue, A.; Sasaki, Y.; Kikuchi, J. Langmuir 2000, 16, 9147. (33) Pao, W. J.; Stetzer, M. R.; Heiney, P. A.; Cho, W. D.; Percec, V. J. Phys. Chem. B 2001, 105, 2170. (34) Auschra, C.; Stadler, R. Polym. Bull. (Berlin) 1993, 30, 257.

Xu et al. blocks have been selectively cross-linked in bulk to give rise to a soluble star-shaped polymer, i.e., a Janus micelle.28 The bulk cross-linking was performed in the following way: A film of SBM cast from chloroform (CHCl3) solution was swollen in acetonitrile for 48 h (without destroying the morphology) and afterward crosslinked by adding 5% (v/v) disulfur dichloride (S2Cl2) and waiting for another 48 h.35-37 The product was purified as follows: The film was rinsed three times with acetonitrile and dried in a vacuum. Afterward it was redissolved in THF and reprecipitated in methanol twice and dried under vacuum at room temperature. The Janus micelles and the precursor triblock copolymer were spread onto the water surface from chloroform or tetrahydrofuran solutions at a concentration of approximately 0.015 g/L. Chloroform (Aldrich, 99.99% pure) and tetrahydrofuran (THF) (Aldrich, 99.99+% pure) were used as received. Water (resistivity 18.2 × 106 Ω‚cm-1, total dissolved organic carbon