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Langmuir 1998, 14, 4222-4226
Compression-Induced Formation of Surface Micelles in Monolayers of a Poly(2-oxazoline) Diblock Copolymer at the Air-Water Interface: A Combined Film Balance and Electron Microscopy Study Thomas R. Baekmark,† Irene Sprenger,† Matthias Ruile,‡ Oskar Nuyken,‡ and Rudolf Merkel*,† Fakulta¨ t fu¨ r Physik, Lehrstuhl fu¨ r Biophysik, E22, and Fakulta¨ t fu¨ r Chemie, Lehrstuhl fu¨ r Makromolekulare Stoffe, Technische Universita¨ t Mu¨ nchen, D-85748 Garching, Germany Received June 9, 1997. In Final Form: April 8, 1998 We investigated monolayers of the diblock copolymer poly(2-ethyl-2-oxazoline)-poly(2-nonyl-2-oxazoline) (E60N60) at the air-water interface using classical Langmuir film balance techniques and transmission electron microscopy on transferred Langmuir-Blodgett films. The isotherms exhibit three distinct regions. At low pressures the monolayer is relatively incompressible and the moderately water-soluble ethyloxazoline block is adsorbed to the interface. At intermediate pressures of approximately 10 mN/m the compressibility increases temporarily and decreases again at higher lateral pressures. Desorption of the ethyloxazoline block takes place in the intermediate high-compressibility region. This desorption process cannot be described as a phase transition. In the water-insoluble nonyloxazoline block the polymer backbone remains confined to the interface at all pressures. Electron micrographs of monolayers transferred onto mica at lateral pressures below approximately 10 mN/m showed no structures. In the high-compressibility region structures appeared that persisted at higher pressures. These structures can be described as twodimensional micelles. The driving force for the micelle formation is the only moderate water solubility of the ethyloxazoline block. For this reason, desorption of the moderately water-soluble ethyloxazoline block and the formation of surface micelles are coupled.
Introduction Diblock copolymers have found widespread interest for both scientific and practical reasons.1-4 The complex phase behavior of such copolymers has attracted much attention: Segmental density profiles were investigated at polymer-polymer interfaces5-7 and at interfaces between selective solvents.8,9 In cases where one block is hydrophilic and the other strongly hydrophobic, insoluble monolayers at the air-water interface can be prepared. Ultrathin films of such materials have been proposed for biocompatibilization of solid substrates.10 Classical film balance techniques11,12 were combined with neutron reflectometry to obtain density profiles of block copolymer films at the air-water interface13 and at the air-ethyl benzoate interface.8,14 In these studies lateral homogene* Corresponding author. E-mail:
[email protected]. † Fakulta ¨ t fu¨r Physik, Lehrstuhl fu¨r Biophysik, E22. ‡ Fakulta ¨ t fu¨r Chemie, Lehrstuhl fu¨r Makromolekulare Stoffe. (1) Argon, A. S.; Cohen, R. E. Adv. Polym. Sci. 1990, 91/92, 301. (2) Bates, F. S.; Fredrickson, G. H. Annu. Rev. Phys. Chem. 1990, 41, 525. (3) Halperin, A.; Tirrell, M.; Lodge, T. P. Adv. Polym. Sci. 1992, 100, 31. (4) Russell, T. P. Curr. Opin. Colloid Interface Sci. 1996, 1, 107. (5) Leibler, L. Physica A 1991, 172, 258. (6) Russell, T. P.; Anastasiadis, S. H.; Menelle, A.; Felcher, G. P.; Satija, S. K. Macromolecules 1991, 24, 1575. (7) Shull, K. R.; Kramer, E. J.; Hadziioannou, G.; Tang, W. Macromolecules 1990, 23, 4780. (8) Kent, M. S.; Lee, L. T.; Farnoux, B.; Rondelez, F. Macromolecules 1992, 25, 6240. (9) Johner, A.; Joanny, J. F.; Marques, C. Physica A 1991, 172, 285. (10) Sackmann, E. Science 1996, 271, 43. (11) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; John Wiley: New York, 1990. (12) Gaines, G. L. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience Publishers: New York, 1966. (13) Bijsterbosch, H. D.; de Haan, V. O.; de Graaf, A. W.; Mellema, M.; Leermakers, F. A. M.; Cohen Stuart, M. A.; van Well, A. A. Langmuir 1995, 11, 4467.
ity of the layers was assumed. Given the commonplace occurrence of lateral structuring in more conventional monolayers15,16 and the richness of the phase diagrams of block copolymers in three dimensions,17 lateral structuring should be expected for monolayers of block copolymers at the air-water interface as well. Using transmission electron microscopy, Zhu et al. demonstrated the formation of surface micelles for a certain class of diblock polyelectrolytes18-23 and later for nonionic diblock copolymers.24,25 Formation of surface micelles was also observed using evanescent light scattering.26 In all these cases, micelles were formed already at vanishing surface pressure. The same held for the polystyrene homopolymer27 which formed the hydrophobic block in the studies mentioned above. In this paper we present film balance studies of a poly(2-oxazoline) diblock copolymer and of the homopolymers that form the individual blocks. (14) Kent, M. S.; Lee, L. T.; Factor, B. J.; Rondelez, F.; Smith, G. S. J. Chem. Phys. 1995, 103, 2320. (15) Mo¨hwald, H. Annu. Rev. Phys. Chem. 1990, 41, 441. (16) McConnell, H. M. Annu. Rev. Phys. Chem. 1991, 42, 171. (17) Bates, F. S. Science 1991, 251, 898. (18) Zhu, J.; Eisenberg, A.; Lennox, R. B. J. Am. Chem. Soc. 1991, 113, 5583. (19) Zhu, J.; Lennox, R. B.; Eisenberg, A. Langmuir 1991, 7, 1579. (20) Zhu, J.; Lennox, R. B.; Eisenberg, A. J. Phys. Chem. 1992, 96, 4727. (21) Zhu, J.; Eisenberg, A.; Lennox, R. B. Macromolecules 1992, 25, 6547. (22) Zhu, J.; Eisenberg, A.; Lennox, R. B. Macromolecules 1992, 25, 6556. (23) Zhu, J.; Hanley, A.; Eisenberg, A.; Lennox, R. B. Makromol. Chem. 1992, 53, 211. (24) Li, S.; Hanley, S.; Varshney, S. K.; Eisenberg, A.; Lennox, R. B. Langmuir 1993, 9, 2243. (25) Meszaros, M.; Eisenberg, A.; Lennox, R. B. Faraday Discuss. 1994, 98, 283. (26) Lin, B.; Rice, S. A.; Weitz, D. A. J. Chem. Phys. 1993, 99, 8308. (27) Kumaki, J. Macromolecules 1988, 21, 749.
S0743-7463(97)00601-X CCC: $15.00 © 1998 American Chemical Society Published on Web 06/30/1998
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Figure 1. Molecular structure of E60N60.
Monolayers of the copolymer E60N60 and of the blockforming homopolymers were also examined using transmission electron microscopy. Poly(2-oxazoline) polymers have hydrophilic backbones containing an amide group to which a side chain is attached, typically an acyl group. These polymers are ideally suited for our study, as they can be produced with very narrow size distributions and a broad variety of side chains which allows simple tuning of the physical properties of the polymers. Poly(2-methyl2-oxazoline) and poly(2-ethyl-2-oxazoline) are watersoluble polymers, whereas longer side chains lead to polymers that cannot be dissolved in water. In the present work, the water solubility of the blocks was controlled via the length of the side chains. The insoluble block had nonyl moieties and the soluble block ethyl moieties. Using electron microscopy, we observed the formation of surface micelles in monolayers of this diblock copolymer. These micelles were only found at elevated surface pressures. In the present case, micelle formation appears to be driven by the properties of the water-soluble block. Materials and Methods We synthesized the diblock copolymer (E60N60) (Figure 1) using living, cationic polymerization. A detailed account of the synthesis and characterization will be presented elsewhere.28 The degree of polymerization was 60 for both blocks. It was determined by gel permeation chromatography (polystyrene standards) and 1H-NMR spectroscopy. No impurities could be detected within the resolution of these techniques. Homopolymers of poly(2-ethyl-2-oxazoline) (PEOX) and poly(2-nonyl-2oxazoline) (PNOX) were prepared and characterized by the same methods. The degree of polymerization was 50 for both homopolymers. The substances were dissolved in chloroform (Aldrich, Steinheim, Germany, HPLC grade) at a concentration of 0.1 nmol/ µL. A Langmuir film balance equipped with a paper Wilhelmy element was used. The Langmuir trough was cleaned until compression of the clean water surface yielded no detectable pressure increase and isotherms of arachidic acid (Avanti Polar Lipids, Birmingham, AL, purity 99.9%) could be reproduced. The subphase was ultrapure water produced by a Millipore apparatus (Millipore, Molsheim, France) and had a specific resistance of more than 18 MΩ cm. To cover the surface of the film balance (385 cm2), less than 1 nmol of E60N60 was needed. Monolayers were compressed at rates of 2 and 20 mm2/s. At pressures of 5, 12, and 25 mN/m monolayers of E60N60 were transferred using the vertical lifting technique29 at a constant speed of 0.05 mm/ s. The experimental error in the transfer pressures was 0.5 mN/ m. Substrates were freshly cleaved mica (TAAB Laboratories Equipment, Aldermaston, U.K.) and carbon films that were deposited onto freshly cleaned glass microslides (Schultheiss, Munich, Germany) by vacuum evaporation. Monolayers were transferred immediately after substrate preparation. (28) Ruile, M.; Nuyken, O. Designed Monomers and Polymers. Manuscript in preparation. (29) Blodgett, K. B.; Langmuir, I. Phys. Rev. 1937, 37, 3150.
Figure 2. Isotherms of E60N60 (full line), the homopolymers poly(2-ethyl-2-oxazoline) (dashed line) and poly(2-nonyl-2oxazoline) (dotted line), and an equimolar mixture of both homopolymers (dash-dotted line) at a temperature of 20 °C on pure water. (A) Areas are calculated as area per polymer. (B) Areas are calculated as area per nonyloxazoline residue. The only exemption is poly(2-ethyl-2-oxazoline). Here the area is calculated as area per ethyloxazoline group. The surface pressure was kept constant during transfer. The trough was thermostated to 20 °C. One sample of each substrate charge was prepared identically but without any polymer present. The coated substrates were transferred to an ultrahigh vacuum apparatus and shadowed with 2-3 nm of platinum/carbon (Balzers, Liechtenstein) at an angle of 8°. The average grain size of the coating was 3 nm. Both unidirectional and rotary shadowing were used. The samples were observed using a Philips 400T transmission electron microscope at an acceleration voltage of 100 kV. Pictures were taken on Agfa Scientia EMfilm. Micellar heights were estimated from the lengths of the “shadow” regions without metal coating in unidirectional shadowing. Monolayers of the respective homopolymers were also characterized by electron microscopy. For monolayers of PEOX transfer pressures of 5 and 7 mN/m were used. PNOX was transferred at pressures of 5, 12, and 25 mN/m. The substrate was freshly cleaved mica. All other procedures were as in the case of E60N60.
Results Isotherms. The isotherm of E60N60 is presented in Figure 2 together with the isotherms of poly(2-ethyl-2oxazoline) (PEOX), poly(2-nonyl-2-oxazoline) (PNOX), and an equimolar mixture of both homopolymers. The results for PNOX agreed in all aspects very well with published isotherms for poly(2-octyl-2-oxazoline) and poly(2-decyl2-oxazoline).30,31 For E60N60 the onset of the isotherm occurs at approximately 70 nm2/polymer. This corresponds to 0.58 nm2/oxazoline unit (total of 120). The onset (30) Kaku, M.; Hsiung, H.; Sogah, D. Y.; Levy, M.; Rodriguez-Parada, J. M. Langmuir 1992, 8, 1239. (31) Rodriguez-Parada, J. M.; Kaku, M.; Sogah, D. Y. Macromolecules 1994, 27, 1571.
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Figure 3. Proposed conformation of E60N60 at the air-water interface at surface pressures below (left) and above (right) the plateaulike region.
areas are 0.39 nm2/oxazoline unit (total of 50) for PNOX, 0.54 nm2/oxazoline unit (total of 50) for PEOX, and 0.52 nm2/oxazoline unit for the equimolar mixture of both. For long-chain fatty acids onset areas are approximately 0.55 nm2/molecule for the expanded phase and 0.25 nm2/ molecule for the condensed phase.12 This comparison suggests that at low pressures, below approximately 3 mN/m, all oxazoline monomers reside directly at the airwater interface, irrespective of the nature of the side chains. For E60N60 a region of steep increase in the isotherm was found above about 50 nm2/molecule. This is followed (between 29 and 23 nm2 /molecule) by a region of somewhat slower increasing pressure (“plateaulike” region). Below areas of 23 nm2/molecule the pressure increases again steeply up to film breakdown at roughly 35 mN/m. If the ordinate is normalized in units of area per hydrophobic group (i.e., nonyl side chains), the isotherms of the poly(2-nonyl-2-oxazoline) homopolymer and that of the E60N60 copolymer coincide in this highpressure regime. This indicates that at high pressures the ethyloxazoline monomers do not occupy any area at the interface. At the degrees of polymerization used in this study (≈50), end-group effects are usually negligible and isotherms of polymers with different lengths collapse on a single master curve if normalized by the degree of polymerization. One example of this is the abovementioned comparison between our data for PNOX and those of other groups for poly(2-octyl-2-oxazoline) and poly(2-decyl-2-oxazoline).30,31 The isotherm of PEOX exhibits an onset area comparable to those of all other polymers studied here. However, expansion of the monolayer of PEOX after compression to areas below 0.2 nm2/monomer showed marked loss of material. Therefore, PEOX desorbs from the interface at pressures of about 5 mN/m and higher. Taken together, this indicates that below the plateaulike region the watersoluble block is located directly at the interface and is expelled from it during the region of enhanced compressibility (see Figure 3). This interpretation is further supported by the fact that the isotherms of the equimolar mixture and of E60N60 coincide after correction of the ordinate to compensate for the different degrees of polymerization. The only marked discrepancy between both isotherms is that for the mixture the plateaulike region is somewhat flatter and located at slightly lower pressures. This indicates that desorption of the ethyloxazoline block in E60N60 is hindered compared to pure PEOX by the covalent linkage to the water-insoluble nonyloxazoline block. The same scenario, an isotherm exhibiting an intermediate region of enhanced compressibility during which desorption takes place, was found for tethered chains of poly(ethylene oxide).13,32 For E60N60
the smallest area per polymer that was reached before film breakdown was 16 nm2. As the PEOX block is desorbed from the interface in this high lateral pressure regime, this corresponds to an area per hydrophobic group (i.e., nonyloxazoline) of 0.27 nm2. This is still well above the area occupied by a long-chain fatty acid in the crystalline phase.12 From the crystal structure data for poly(2-oxazoline)s33 we expect an area per hydrophobic group of 0.16 nm2 at closest packing. To calculate this, we assumed the polymer to be oriented with the hydrocarbon chains roughly perpendicular to the interface. Therefore, even at high pressures there is ample space to accommodate all the hydrophobic chains directly at the interface. No formation of quasi-three-dimensional aggregates occurs contrary to the case of polystyrene.27 Due to its amphiphilic structure, the water-insoluble block is confined to the interface. The hydrophilic polymer backbone remains in contact with water, whereas the hydrophobic side chains are oriented away from it. Electron Micrographs. First we shall describe the results for the diblock copolymer E60N60. Neither without copolymer (data not shown) nor at a transfer pressure of 5 mN/m (Figure 4) were any structures observed. In fact, micrographs of samples without copolymer and samples with copolymer transferred at a pressure of 5 mN/m cannot be distinguished. At a transfer pressure of 12.5 mN/m two different structures appeared. At a lower compression rate (2 mm2/s) (Figure 5) rounded structures with diameters from 20 to 100 nm and heights from 2.5 to 12 nm were formed. At a higher compression rate (20 mm2/s) (Figure 6) many smaller, elongated structures were observed instead. Most of them were of elliptical shape with a ratio of long to short perimeter of 2:1 but round structures existed as well. Most strikingly, the structures were much shallower, with a mean height of 2.5 nm. In the experiments reported in Figures 5 and 6 the monolayers were transferred at identical lateral pressure, surface density, and transfer speed. At the highest pressure of 25 mN/m (Figure 7) coalesced and elongated structures were present irrespective of
(32) Faure, M. C.; Bassereau, P.; Cariagnano, M. A.; Szleifer, I.; Gallot, Y.; Andelman, D. Eur. Phys. J. B 1998, in press.
(33) Litt, M.; Rahl, F.; Roldan, L. G. J. Polym. Sci., Polym. Phys. Ed. 1969, 7, 463.
Figure 4. Electron micrograph of a monolayer of E60N60 transferred onto mica at a surface pressure of 5.5 mN/m. Scale bar, 200 nm.
Compression-Induced Formation of Surface Micelles
Figure 5. Electron micrograph of a monolayer of E60N60 transferred onto mica at a surface pressure of 12.5 mN/m and a compression rate of 2 mm2/s. Scale bar, 200 nm.
Langmuir, Vol. 14, No. 15, 1998 4225
Figure 7. Electron micrograph of a monolayer of E60N60 transferred onto mica at a surface pressure of 25 mN/m. Scale bar, 200 nm.
using mica substrates. Monolayers of E60N60 were also transferred onto freshly prepared carbon films at surface pressures of 5, 12, and 25 mN/m. With these substrates the surface roughness of the carbon-coated glass posed serious problems. After several attempts we found a glass batch that was smooth enough to allow unequivocal identification of the surface micelles at 12 and 25 mN/m. Discussion
Figure 6. Electron micrograph of a monolayer of E60N60 transferred onto mica at a surface pressure of 12.5 mN/m and a compression rate of 20 mm2/s. Scale bar, 200 nm.
compression rate. Most of the structures had a diameter of 30 nm and a height of approximately 2 nm. The figures present the results of the unidirectional shadowing. We observed identical structures but with less contrast using rotary shadowing. Electron micrographs of the PEOX transferred onto mica at pressures of 5 and 7 mN/m showed no structures. This holds also for monolayers of PNOX transferred at pressures of 5, 12, and 25 mN/m. In fact, no micrograph of the homopolymers could be distinguished from micrographs of blank mica substrates. The only exception was PNOX transferred at 25 mN/m where the grain size of the platinum/carbon layer was somewhat enlarged, indicating a higher roughness of the substrate. Nevertheless, no structures besides the platinum/carbon clusters could be detected. All data presented up to this point were gained
Electron micrographs of E60N60 monolayers transferred onto mica showed lateral structures when the surface density was high enough. For several reasons we believe that these structures were already present at the airwater interface and were not created during the transfer. (1) It has been shown that Langmuir monolayers of fatty acids and phospholipids can be transferred from the airwater interface onto mica and freshly prepared silicon monoxide without artifacts.34,35 (2) Various diblock copolymers on a polystyrene basis can be transferred onto silicon monoxide without artifacts18-25. (3) In our experiments transfer ratios (i.e., the ratio of the area lost from the monolayer during transfer to the area of the substrate) were close to 1. This usually indicates a “good” transfer. (4) We observed different structures depending on compression rates at otherwise identical conditions (see Figures 5 and 6). This clearly proves that the observed structures were not determined by the transfer process but by the compression history of the monolayer. (5) The monolayers were very viscous. This should slow all possible restructuring processes. (6) Finally and most convincingly we observed similar structures on different substrates, mica and carbon-coated glass. Thus, all evidence points toward the formation of the surface micelles at the air-water interface and not during transfer. The micelles are formed during compression of the monolayer in the plateaulike region. This region of enhanced compressibility is caused by the desorption of (34) Fischer, A.; Sackmann, E. Nature 1985, 313, 299. (35) Fereshtehkhou, S.; Neuman, R. D.; Ovalle, R. J. Colloid Interface Sci. 1986, 109, 385.
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the only moderately water-soluble PEOX block from the surface. Monolayers of surface-grafted, weakly adsorbing polymers were treated theoretically by scaling analysis36,37 and self-consistent mean-field theory.13,32 A first-order phase transition was predicted by scaling analysis for such desorption phenomena. Phase separation accompanies first-order phase transitions, and this might be thought to explain the observed lateral structuring. Nevertheless, no phase transition in monolayers of surface-grafted, weakly adsorbing polymers was found using self-consistent mean-field theory, only a region of increased compressibility in the isotherm.13,32 As the experimentally observed plateau was not horizontal and the micelles were also observed at a higher pressure well outside the plateaulike region, our data do not indicate that the desorption process is a first-order phase transition. In light scattering studies of PEOX in water, a low second virial coefficient and indications for the formation of small aggregates were found.38 This shows that PEOX is only moderately water-soluble. Due to the low solvent quality, reduced contact of PEOX with water is energetically favorable. The energetic cost of a locally varying polymer concentration is outweighed by the energy gained by exposing less PEOX to water. The moderate solubility of the PEOX block in water does not play a role as long as this block resides directly at the surface. Therefore, no micelles are observed at a transfer pressure of 5 mN/ m, below the plateaulike region. The case of surface-grafted polymers in poor solvent with fixed grafting points has been studied theoretically and experimentally. Random-phase approximation,39,40 scaling theory41-43, Monte Carlo simulations44,45 and molecular dynamics calculations46 resulted in a coherent scenario. At very low grafting densities the surface is covered partially with individual collapsed polymers. At intermediate grafting densities many polymer molecules collapse cooperatively and form so-called “octopus” or “pinned micelles”. In contrast, at high grafting densities the osmotic pressure of the polymer chains enforces homogeneity of the brush. The transition between the different regimes is not a thermodynamic phase transition.42,44 Relaxation was found to be very slow.43 This general picture has been confirmed by AFM experiments on diblock copolymers adsorbed onto solid substrates47-51 and chemically end-grafted polymers.52 In all the mentioned studies, fixed grafting points were assumed. Lai (36) Alexander, S. J. Phys. (France) 1977, 38, 983. (37) Ligoure, C. J. Phys. (France) II 1993, 3, 1607. (38) Chen, C. H.; Wilson, J.; Chen, W.; Davis, R. M.; Riffle, J. S. Polymer 1994, 35, 3587. (39) Ross, R. S.; Pincus, P. Europhys. Lett. 1992, 19, 79. (40) Yeung, C.; Balazs, A. C.; Jasnow, D. Macromolecules 1993, 26, 1914. (41) Williams, D. R. M. J. Phys. (France) II 1993, 3, 1313. (42) Tang, H.; Szleifer, I. Europhys. Lett. 1994, 28, 19. (43) Zhulina, E. B.; Birshtein, T. M.; Priamitsyn, V. A.; Klushin, L. I. Macromolecules 1995, 28, 8612. (44) Lai, P.-Y.; Binder, K. J. Chem. Phys. 1992, 97, 586. (45) Soga, K. G.; Guo, H.; Zuckermann, M. J. Europhys. Lett. 1995, 29, 531. (46) Grest, G. S.; Murat, M. Macromolecules 1993, 26, 1914. (47) O’Shea, S. J.; Welland, M. E.; Rayment, T. Langmuir 1993, 9, 1826. (48) Siqueira, D. F.; Ko¨hler, K.; Stamm, M. Langmuir 1995, 11, 3092. (49) Meiners, J. C.; Ritzi, A.; Rafailovich, M. H.; Sokolov, J.; Mlynek, J.; Krausch, G. Appl. Phys. A 1995, 61, 519. (50) Spatz, J. P.; Sheiko, S.; Mo¨ller, M. Adv. Mater. 1996, 8, 513. (51) Stamouli, A.; Pelletier, E.; Koutsos, V.; van der Vegte, E.; Hadziioannou, G. Langmuir 1996, 12, 3221.
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and Binder44 discussed the case of mobile grafting points, albeit without performing a detailed calculation. These authors expect a macroscopic phase separation in this case, whereas for fixed grafting points the structures were found to be limited in size. The transition from the homogeneous to the segregated state should be accompanied by density fluctuations and an increased compressibility, but it is not a true thermodynamic phase transition. This agrees nicely with our results. In our case the water-soluble block is grafted to the interface by the amphiphilic PNOX block. This block covers a finite area of approximately 20 nm2, which ensures that only moderate grafting densities were realized in our experiments. The shape of the surface area covered by the insoluble block and the exact lateral position where the water-soluble block emerges into the solvent is variable. Moreover, the monolayer is compressible. All these factors complicate the process of pattern formation in our case compared to the case of grafting points, covering no area at the interface. Nevertheless, we expect that the basic conclusions for fixed grafting points hold in our case as well and that the lateral structuring observed is the result of poor solvent quality and moderate grafting density. Finally, we wish to point out that exceedingly long relaxation times are common with monolayers of polymeric substances and we do not expect that the observed structures were in complete thermodynamical equilibrium. This is demonstrated by the different structures of the surface micelles in the plateaulike region when different compression rates were used. Concluding Remarks We identified a novel mechanism for the formation of surface micelles in Langmuir monolayers of diblock copolymers, where micelle formation is driven by the low solvent quality of water for the soluble block. This driving force is only available when the water-soluble block is exposed to water, contrary to the known examples of surface micelle formation in polystyrene-containing block copolymers at the air-water interface. There the entirely hydrophobic polystyrene block yields the driving force for micelle formation and condenses into a three-dimensional conformation. This process takes place at vanishing surface pressure (i.e., during spreading of the monolayer). In our case micelle formation and desorption of the water-soluble block from the air-water interface are coupled processes that take place at elevated surface pressures. Due to the amphiphilic nature of the waterinsoluble block, its hydrophobic side chains remain anchored to the interface at all pressures. The conformation of the polymer backbone in the water-insoluble block is strictly two-dimensional, irrespective of surface pressure. Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft via Sonderforschungsbereich 266. T.R.B. thanks the Danish Research Academy for financial support. We thank Erich Sackmann for the possibility of performing this study in his laboratory and for many helpful and encouraging discussions. LA970601C (52) Zhao, W.; Krausch, G.; Rafailovich, M. H.; Sokolov, J. Macromolecules 1994, 27, 2933.
