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Mixed Micelles of Nonionic Surfactants and Uncharged Block Copolymers in Aqueous Solutions: Microstructure Seen by Cryo-TEM Y. Zheng and H. T. Davis* Department of Chemical Engineering and Materials Science and Center for Interfacial Engineering, University of Minnesota, Minneapolis, Minnesota 55455 Received February 16, 2000. In Final Form: May 22, 2000 While mixtures of ionic polymers and ionic surfactants have been extensively studied, mixed systems of nonionic surfactants and nonionic amphiphilic copolymers have largely been ignored. Direct microstructure imaging by cryotransmission electron microscopy of aqueous mixtures of a nonionic surfactant C12E5 and two uncharged block copolymerssdiblock copolymer poly(ethyleneoxide)-polybutadiene (EO126-B45) and triblock copolymer poly(ethyleneoxide)-poly(ethylethylene)-poly(ethyleneoxide) (EO21-EE35-EO21)revealed that the shapes and sizes of mixed micelles change as functions of the surfactant-to-copolymer concentration ratio. Cylindrical micelles of EO21-EE35-EO21 are transformed to spherical micelles upon the addition of C12E5, while large spherical micelles of EO126-B45 change into smaller spheres when mixed with the nonionic surfactant. The range of surfactant-to-copolymer concentration ratios in which the micelle microstructure changes was correlated with the solution surface-tension variation. The triblock copolymer showed different effects than did the diblock counterpart on mixed micellar behavior. The nonionic mixed amphiphile system, with the near-monodispersity of components and the simple chemical structure, is a good candidate for theoretical modeling.
Introduction Mixed amphiphile systems (including surfactants, polymers, and copolymers) are fascinating from a scientific standpoint because of the complex ways they associate into “supramolecular”, “nanoscale”, and “self-assembled” structures. They are technically important because mixture systems provide a way of tailoring microdomain properties through simple composition variations; new structures may thus be obtained by changing the system composition, rather than through synthesis of new materials. Moreover, the phase diagrams of mixed surfactants have been shown to be distinctly different from those of single-component systems.1 While single surfactants are not known to form thermodynamically stable vesicles, an equilibrium vesicle phase spontaneously appears in mixtures of oppositely charged surfactants.2 Mixtures of various surfactants in water have been studied experimentally and theoretically.3,4 Until recently, relatively little was known about ternary systems of amphiphilic block copolymers and surfactants in water despite the widespread use of block copolymers as emulsifiers and solubilizers of hydrophobic compounds in aqueous solutions. Amphiphilic block copolymers, with an aliphatic chain and a polar chain chemically bonded together, can be viewed in many aspects as the macromolecular counterparts of ordinary, low-molecular-weight surfactants for their close resemblance in micelle formation and liquid-crystalline phase formation. Yet these polymeric “surfactants” exhibit featuressnotably, special aggregate/solvent interactions and interaggregate inter* Corresponding author. (1) Kaler, E. W.; Herrington, K. L.; Miller, D. D.; Zasadzinski, J. A. NATO ASI Ser. C 1991, 369, 571. (2) Kaler, E. W.; Murthy, K.; Rodriguez, B. E.; Zasadzinski, J. A. N. Science 1989, 245, 1371-1374. (3) In Mixed Surfactant Systems; Holland, P. M., Rubingh, D. N., Eds.; American Chemical Society: Washington, DC, 1992; Vol. 501. (4) In Mixed Surfactant Systems; Ogino, K., Abe, M., Eds.; Marcel Dekker: New York, 1993; Vol. 46.
actionssthat have no direct analogues in surfactants. These features can be largely attributed to the presence of the corona region in the self-assembled polymer microstructures rather than to the comparatively sharp boundary of the self-assembled surfactant aggregates. Moreover, block copolymers can be synthesized into a variety of molecular architectures such as di-, tri-, or multiblocks in various sequences. These features can result in many new self-assembled structures and properties that cannot be attained by common surfactants. Some recent investigations5-8 focusing on the relatively dilute micellar behavior have documented the mixedmicellar behavior of triblock copolymers of the poly(ethyleneoxide)-poly(propyleneoxide)-poly(ethyleneoxide) (PEO-PPO-PEO, which is commercially named Pluronic by BASF or Synperonic by ICI C&P). Using 13C NMR and fluorescence quenching measurements, Almgren et al.6 have investigated the interaction of Pluronic copolymers EO13-PO30-EO13 (L64) and EO78-PO30-EO78 (F68) with sodium dodecyl sulfate (SDS) in the dilute regime. The experiments demonstrated that Pluronic copolymers form mixed micelles with SDS at a concentration well below the critical micelle concentration (CMC) of SDS and that addition of SDS reduces the size of Pluronic micelles markedly. The shape of the mixed micelles was assumed to be spherical, with coiled PO blocks solubilized in the interior of a SDS micelle. In a later study on an EO97-PO69-EO97 (F127) and SDS system, Hecht et al.,7,8 using small-angle neutron scattering, light scattering, and differential scanning calorimetry, found that SDS binds to block copolymer molecules and suppresses the formation of block copolymer micelles. Hecht et al.7,8 concluded that surfactant molecules bind to the (5) Contractor, K.; Bahadur, P. Eur. Polym. J. 1998, 34, 225-228. (6) Almgren, M.; van Stam, J.; Lindblad, C.; Li, P.; Stilbs, P.; Bahadur, P. J. Phys. Chem. 1991, 95, 5677-5684. (7) Hecht, E.; Hoffmann, H. Langmuir 1994, 10, 86-91. (8) Hecht, E.; Mortensen, K.; Gradzielski, M.; Hoffmann, H. J. Phys. Chem. 1995, 99, 4866-4874.
10.1021/la000230r CCC: $19.00 © 2000 American Chemical Society Published on Web 07/12/2000
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PO block, which renders the block hydrophilic and disaggregates the block copolymer micelles. Each polymer molecule was estimated to be saturated with four to five binding SDS molecules. The study on the PEO-PPOPEO and SDS system has also been extended to the highconcentration regime, and it was found that addition of SDS induces the transition of anisotropic liquid-crystalline phases to isotropic bicontinuous solutions and, furthermore, to micellar solutions.9 The reported studies have been limited to the commercial triblock copolymers with broad size distributions that complicate data interpretation. The essential information on the aggregate microstructure is also lacking, since only average information can be obtained from the reported techniques. There seem to be no studies on interactions between nonionic surfactant and the uncharged block copolymers perhaps because the interaction is expected to be weak and, thus, uninteresting. In this paper, we apply direct-imaging cryotransmission electron microscopy (cryo-TEM) to mixed micelles of the nonionic surfactant C12E5 and the nearly monodispersed block copolymers with different block architectures (diblock and triblock). The polymers are diblock copolymer poly(ethyleneoxide)-polybutadiene (EO126-B45) and triblock copolymer poly(ethyleneoxide)-poly(ethylethylene)-poly(ethyleneoxide) (EO21-EE35-EO21). They were synthesized via anionic polymerization.10 From a theoretical point of view, these systems are attractive because (i) the block copolymers are chemically similar to CiEj surfactants (note that ethylethylene is a saturated 1,2-butadiene group) so that they can be treated by well-known procedures of polymer statistics while avoiding the complication of headgroup interaction between ionic surfactant and PEO chains and (ii) the block copolymers have narrow molecular weight distributions compared with those of commercial copolymers so that molecular-weight dependence can be deduced. As a characterization technique, TEM is a good local structural probe, and it is the only technique that allows real-space imaging at high resolution; only rapidvitrification cryogenic TEM (cryo-TEM) enables modelindependent direct imaging of microstructures in liquid solutions at ambient temperatures. Recently, the corecorona of block copolymer micelles has been directly imaged.11
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Figure 1. Surface tension measurement of aqueous solutions of neat diblock copolymer poly(ethyleneoxide)-polybutadiene (EO126-B45) and triblock copolymer poly(ethyleneoxide)-poly(ethylethylene)-poly(ethyleneoxide) (EO21-EE35-EO21). Data of C12E5 in water is also shown for comparison. Critical micelle concentrations (CMCs) of the two block copolymers in water derived from the intercepts of the two straight lines fitting each set of data points are 0.0285 mM (1.1 × 10-5 wt %) for EO126-B45 and 0.016 mM (1.3 × 10-5 wt %) for EO21-EE35EO21. vessel containing the fluid, we used a Petri dish with the plate placed in its center. The accuracy of measurements was checked by frequent calibration measurements of pure water. Cryo-TEM experiments were performed on a JEOL1210 transmission electron microscope using the minimum beam-dose operating system. A 10-200 nm thick liquid film of sample solution was prepared at 25 °C in air saturated with water vapor on a perforated carbon film (holes ranging from 1 to 10 µm in diameter) supported on a 200-mesh TEM copper grid. The specimen was then vitrified by being plunged into liquid ethane at its freezing point. The vitrified samples were mounted on a Gatan-626 cryoholder and transferred into the TEM. Specimens were visualized at nominal underfocuses of 0.7-4 µm; the underfocuses provided maximum phase contrast at spatial frequencies of 10-52 Å. Specimen temperature was maintained below -170 °C during sample observation. All images were recorded with a Gatan 724 multiscan digital camera and processed with DigitalMicrograph 3.1. Digital imaging leads to a cleaner vacuum because wet film is not introduced into the microscope, and so no contaminate from this source condenses on the specimen. High-magnification images were recorded with as low electron doses as practicable in order to minimize electron beam radiation damage to the specimen.
Experimental Details Materials. EO126-B45 (molecular weight 8100, polydispersity 1.09) and EO21-EE35-EO21 (molecular weight 3860, polydispersity 1.10) were generously provided by Y.-Y. Won and F. S. Bates at the University of Minnesota. C12E5 was obtained from Nakino (Japan) and was used as received. Sample Preparation. Aqueous solutions of varying surfactant-to-polymer ratios with a fixed total concentration of 1 wt % were prepared through mixing appropriate amounts of polymer and surfactant stock solutions. All the water was purified by a Millipore UV apparatus coupled with a Millipore Q water purification system. Thoroughly mixed samples were kept at room temperature for several days before the measurements were performed. Methods. Surface tensions of dilute aqueous solutions were measured by the Wilhelmy method using a digital tensiometer (Kru¨ss, model K10ST) with a platinum plate. To avoid contamination, we cleaned the platinum plate with a flame before each measurement. To avoid any effects from the meniscus in the (9) Zhang, K.; Lindman, B.; Coppola, L. Langmuir 1995, 11, 538542. (10) Won, Y.-Y., Ph.D. Dissertation, University of Minnesota, Minneapolis, MN, 2000. (11) Zheng, Y.; Won, Y.-Y.; Bates, F. S.; Scriven, L. E.; Davis, H. T.; Talmon, Y. J. Phys. Chem. B 1999, 103, 10331-10334.
Results and Discussion Since the changes in surface tension are related to the activities of the monomeric surfactant species in solution, interactions of the surfactant with other species which result in changes in monomer activity can be detected from a surface tension measurement. Surface tension has been widely used in monitoring interactions between surfactants and polymers.12 Plots of surface tension against logarithms of concentration are shown in Figure 1. A C12E5 curve is also plotted for comparison. The CMCs of the two block copolymers, derived from the intercepts of the two straight lines fitting each set of data points, are 0.0285 mM (1.1 × 10-5 wt %) for EO126-B45 and 0.016 mM (1.3 × 10-5 wt %) for EO21-EE35-EO21. In contrast to the reported results of commercial block copolymer Pluronics (PEO-PPO-PEO), which often showed multibreak points on the curves of surface tension versus concentration,13,14 the appearance of only one break in our nearly mono(12) Jones, M. N. J. Colloid Interface Sci. 1967, 23, 36. (13) Anderson, R. A. Pharm. Acta Helv. 1972, 47, 304-309. (14) Alexandridis, P.; Athanassiou, V.; Fukuda, S.; Hatton, T. A. Langmuir 1994, 10, 2604-2612.
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Figure 2. Surface tension measurement of aqueous solutions of C12E5/EO126-B45 and C12E5/EO21-EE35-EO21 mixtures. Copolymer concentrations are constant in each curve, while the C12E5 concentration is varied. (a) Both EO126-B45 and EO21-EE35-EO21 concentrations are fixed at 0.01 wt %, which is lower than their CMCs. (b) Both EO126-B45 and EO21-EE35-EO21 concentrations are fixed at 0.1 wt %, which is higher than their CMCs.
dispersed copolymer indicates that the other breaks may be the effects of polydispersity. Surface tension curves of mixed solutions of C12E5 and block copolymers are shown in Figure 2, where the polymer concentrations were fixed and surfactant concentrations varied. Concentrations above and below the CMCs (shown in panels a and b, respectively, of Figure 2) were chosen for each block copolymer. There is no discernible change in the C12E5 surface tension curve when polymer concentrations are below the CMCs. Considering that PEO associates with SDS even at PEO concentrations as low as 10-2 g/L according to the PEO/SDS “phase diagram”,15 we believe that the absence of change in Figure 2a suggests no association of polymers with the surfactant (otherwise, association-induced surfactant monomer concentration changes can be easily detected by the surface tension measurements). This suggests that the block copolymer is dispersed in single-molecule state. Because of the strong hydrophobicity of PEE and PB blocks, each molecule must form a “unimer micelle” (a micelle consists of one molecule), in which coiled PEE or PB blocks form the core and are partially shielded from contact with water by the hydrophilic PEO block wrapping around. When the polymer concentration is increased to 0.1 wt %, about 10 times higher than its CMC, a large deviation from Figure 2a is seen upon the gradual addition of C12E5 (Figure 2b). Two break points can be seen that divide the surface tension curve into three distinct regions. In region I, the curve remains similar to that of the neat polymer solution (Figure 1), with surface tension values higher than those of the C12E5 solutions, indicating the absence of C12E5 molecules on the solution/air interface. The added C12E5 must be associated with the polymeric micelles in the solutions. When the C12E5 concentration increases to a certain point (R1) entering into region II, however, the solution surface tension starts to fall dramatically, indicating the increase of the monomer activity of C12E5 at the solution/air interface. Thus, R1 marks the onset of a change in the state of the copolymer micelles, which, we believe, corresponds to the saturation of C12E5 binding to the copolymer micelles in region I. The C12E5-to-polymer molar ratio at R1 is 2:1 for the EO126-B45 diblock copolymer and 1:1 for the EO21-EE35-EO21 triblock copolymer. Further increases of C12E5 concentration continues to lower the surface tension until a second break point R2 is reached (15) Cabane, B.; Duplessix, R. J. Phys. (Paris) 1982, 43, 1529.
where the surface tension levels off, marking the starting point of region III, a plateau in surface tension values. The flat surface tension represents fixed monomeric concentrations of surfactant and polymer. That results from the formation of stable micelles in the bulk solution. The C12E5-to-polymer molar ratio at R2 is 20:1 for EO126B45 and 10:1 for EO21-EE35-EO21. It is natural to compare the block copolymer/surfactant interaction with the well-studied polymer/surfactant interactions. It is well-known that surfactants interact with polymers in many systems.16 The generally observed behavior is that the specific association between surfactants and polymers leads to a decrease in free surfactant monomer concentration in solution and induces surfactant/ polymer complex formation. Several distinct surfactantpolymer binding stages have been reported experimentally upon increased surfactant concentration in a polymer solution: very little surfactant-polymer binding in stage I (different from the notations of regions I, II, and III in the previous paragraph), specific stoichiometric binding of surfactants onto polymer in the form of clusters of premicellar aggregates in stage II, and true micellization in stage III. The boundary between stages I and II denotes the effective CMC of the surfactant with the addition of polymer. The effective CMC is always smaller than that of pure surfactant, suggesting the formation of mixed micelles in stage II. The structure of mixed micelles in stage II has been described as a stoichiometric aggregate containing a single macromolecule with associated SDS molecule cluster.15 The generation of such a stoichiometric aggregate continues through stage II until stage III, where the stoichiometric binding saturates. In stage III, the excess surfactant forms regular micelles in equilibrium with the stoichiometric aggregates of stage II. In our block copolymer/surfactant system stage I, in which there is very little association between surfactant and polymer, seems to be absent. This is attributed to the high hydrophobicity of PEE and PB blocks in the copolymers which strongly associate with surfactant molecules even at very low surfactant concentrations. The ready association of surfactant monomers with polymer, as indicated in region I in Figure 2b, for concentrations up to the surfactant CMC suggests little resistance experienced by the surfactant molecules in diffusing through (16) In Interactions of surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, Fl, 1993.
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Figure 3. Cryo-TEM micrographs of the triblock copolymer EO21-EE35-EO2mixed with C12E5. The total concentration of the solutions is kept at 1 wt %, while the surfactant-to-copolymer molar ratio is varied. (a) Neat 1 wt % EO21-EE35-EO2 block copolymer solution forms long cylindrical micelles. Surfactant-to-copolymer molar ratios are (b) 1.5:1 (“F” denotes frost), (c) 3:1, (d) 5:1, (e) 7:1, (f) 10:1, and (g) 15:1 (arrows point to the second kinds of micelles. (h) Same sample as in (g), but a slightly long electron beam exposure was applied to demonstrate the two kinds of spherical micelles. The smaller micelles are more readily damaged by the electron beam than are the larger ones because of their higher surface-to-volume ratio.
the micellar corona region of block copolymer micelles in binding to the micellar core. At high surfactant concentrations, the surface tension change resembles that of typical surfactant/polymer systems showing a gradual change in aggregate structures until saturation (region II in Figure 2b). The association between surfactant and copolymers is probably stoichiometric, as suggested by the plateau in the surface tension (region III in Figure 2b). The microstructure of the micelles formed with varying surfactant-to-polymer ratios was directly visualized by cryo-TEM. Figure 3 shows micrographs of aqueous solutions of C12E5 and EO21-EE35-EO21. Figure 4 shows micrographs of aqueous solutions of C12E5 and EO126B45. A 1% EO21-EE35-EO21 aqueous solution forms long cylindrical micelles with a diameter of 12 nm and lengths of more than several micrometers (Figure 3a). When mixed with a small amount of C12E5 (1.5:1 C12E5-to-polymer molar ratio), the block copolymer forms spherical micelles (Figure 3b) with approximately the same diameter as that of the cylindrical micelles. This concentration corresponds to the saturation of C12E5 molecules to the cylindrical copolymer micelles and the onset of mixed micelle transformation as measured from the surface tension curve (Figure 2b). With increases of the surfactant-to-polymer ratio to 3:1, 5:1, and 7:1, the cylindrical micelles gradually disappear, while more and more spherical micelles are generated until few remains of cylindrical micelles can be detected (Figure 3c-e). When the ratio reaches 10:1 (Figure 3f), no cylindrical micelles can be observed, and all micelles are spherical and about 10 nm in diameter; the surfactantto-polymer ratio of about 10:1 is in good agreement with that of the breakpoint R2 on the surface tension curve (Figure 2b). Interestingly, during the micelle transformation process, all the spherical micelles are about the same size while the cylindrical micelles are being depleted. Adding more C12E5 leads to the formation of smaller spherical micelles (Figure 3g,h) that coexist with the larger ones. In Figure 3h, which used the same sample as Figure 3g, slightly longer electron beam exposure was applied to better demonstrate the existence of the smaller spherical micelles. The smaller micelles are more readily damaged by the electron beam than the larger ones, presumably
because of their higher surface-to-volume ratio.17 They are probably C12E5 micelles because C12E5 forms spherical micelles in aqueous solutions at concentrations smaller than 0.25 wt %, and the micelles grow to cylindrical at concentrations greater than 0.5 wt %.18 Thus, the trend of EO21-EE35-EO21/C12E5 mixed-micelle transformation with increasing C12E5 concentration is that cylindrical micelles are transformed to spherical shapes, and as the surfactant-to-polymer ratio rises, so does the sphericalto-cylindrical micelle population. During the process, spherical micelles are all the same size, and the cylindrical micelles also remain at the same diameter as their number diminishes. The possibility that the polymer concentration causes micellar shape change can be ruled out because solutions of polymer alone at concentrations of 1 and 0.1 wt % form cylindrical micelles with the same diameter. The shape transformation of the mixed micelles from cylindrical to spherical is perplexing in that dilute aqueous solutions (at concentrations greater than 0.5 wt %) of either C12E5 or EO21-EE35-EO21 form long cylindrical micelles and their mixtures form micelles with greater curvatures than those of either pure micelle, which is against the popular packing parameter model for surfactant aggregates predicting mixed micelles to have an intermediate curvature between those of each component micelle.19 This seeming abnormality, however, could be understood in terms of the unique feature of block copolymer micelles: the corona. Block copolymer micelles can be viewed as a polymeric colloidal particle (the hydrophobic core) grafted with polymer chains (hydrophilic corona in our case) that form the so-called “polymer brushes”. The chains are stretched because of the high grafting density at the core/ water interface. The stretched state is not the energetically favorable state for a polymer chain which tends to form a random coil. The added short-chain surfactant C12E5 (17) Talmon, Y. Electron Beam Radiation Damage to Organic and Biological Cryospecimens. In Cryotechniques in Biological Electron Microscopy; Steinbrecht, R. A., Zierold, K., Eds.; Springer-Verlag: Berlin, 1987; Chapter 3. (18) Bernheim-Groswasser, A.; Talmon, Y. To be published. (19) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525-1568.
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Figure 4. Cryo-TEM micrographs of the diblock copolymer EO126-B45 mixed with C12E5. The total concentration of the solutions is fixed at 1 wt % while the surfactant-to-copolymer molar ratio is varied. (a) Neat 1 wt % EO126-B45 block copolymer solution forms large spherical micelles. Core-corona structure can be readily observed. Surfactant-to-copolymer molar ratios are (b) 1:1, (c) 3:1 (“M” denotes mixed micelles), (d) 5:1, (e) 10:1, and (f) 20:1.
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molecules act as spacers between the polymer chains20 and thus allow more room for each chain to relax to the coiled state that occupies more area at the micelle surface. The “fatter” PEO headgroups thus favor spherical micelles, the structures with higher curvature. The C12E5/EO126-B45 system exhibits a different pattern of mixed-micelle transformation. The 1 wt % aqueous solution of EO126-B45 alone forms giant spherical micelles (Figure 4a) with very uniform micellar size. The corecorona structure is plainly visible under the high resolution, showing a total diameter of 47 nm, including a 16 nm thick corona. When C12E5 is added in a molar ratio of 1:1 to the EO126-B45 solution, the corona of the micelles disappears (Figure 4b), corresponding to the saturation of surfactant molecules binding to the polymer micelles and the onset of the mixed-micelle transformation, that is, the R1 in surface tension measurement. Further increase of the C12E5-to-polymer molar ratio to 3:1, 5:1, and 10:1 produces mixed micelles with smaller sizes and a broad size distribution (Figure 4c-e). The size distribution narrows after the C12E5-to-polymer ratio reaches 20:1 (Figure 4f), corresponding to the breakpoint R2 in the surface tension curve (Figure 2b). Now all the mixed micelles are about 5 nm in diameter. The micellar size change is not caused by the polymer concentration decrease, since solutions of neat polymer with concentrations of 1 and 0.1 wt % were observed to form spherical micelles of identical size. The feature of the EO126-B45/ C12E5 mixed-micelle transformation is that as the ratio of surfactant to copolymer rises, the mean size of the spherical micelles diminishes while the size distribution widens and then narrows again upon surfactant/copolymer complex saturation. The contrasting mixed-micellar transformation of the diblock and triblock copolymers with C12E5 is interesting. While the EO126-B45/C12E5 system resembles that of common mixed surfactants with good mutual miscibility in that the mixed micelles in the system have a wide range of size and composition, the EO21-EE35-EO21/C12E5 system forms only two kinds of micelles and seems to show limited miscibility, resembling the fluorocarbon/ hydrocarbon system. This phenomenon might stem from the effect of block structure (di- or triblock). For a BAB (where B refers to the hydrophilic and A to the hydrophobic blocks) triblock copolymer to form a micelle, the A blocks are subjected to backfolding or looping to exclude the B blocks from the micellar core. This looping free energy increases with the block copolymer molecular weight. Jacobsen and Stockmayer21 showed that the reduction in entropy is proportional to ln N for a linear chain of N segments in the same plane or on one side of a plane. Nagaragian compared the solubilization of organic molecules in the micelle core for triblock and diblock copolymer systems. He concluded that a triblock copolymer behaves almost as a diblock copolymer with the same composition but half its molecular weight. This might be true with (20) Milner, S. T. Science 1991, 251, 905-914. (21) Jacobsen, H.; Stockmayer, W. H. J. Chem. Phys. 1950, 18, 1600.
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respect to the CMC and micelle size, but in the case of mixed-micelle formation with surfactants, one triblock copolymer EO21-EE35-EO21 does not equal two smaller diblock copolymers of EO21-EE17 because C12E5 would readily mix with EO21-EE17 at any proportion as mixed nonionic surfactant systems.22,23 Thus, a detailed account of the chain packing in the interior core of the mixed micelles is needed, possibly based on the available model on surfactant tail packing.24 The aggregation number of a micelle cannot be directly determined from the cryo-TEM micrographs, but the micrographs reveal one pitfall of characterizing the mixed amphiphile systems with standard aggregation measuring techniques such as neutron scattering, X-ray scattering, and light scattering: the effect of the particle size distribution. Since most of the techniques are averaging techniquessthat is, the measured quantity is integrated over a certain sample volumesthey generally cannot distinguish the coexistence of different kinds of micelles. The core-corona structure of the block copolymer micelles further complicates the data analysis and interpretation. For example, the hydrodynamic radius of a micelle as measured by dynamic light scattering is actually close to the total diameter including the corona, which may lead to serious overestimation of the aggregation number. Therefore, both cryo-TEM and scattering techniques are needed for a quantitative characterization of micellar systems. Conclusion Direct imaging by cryo-TEM demonstrates that there is strong association between nonionic surfactants and uncharged amphiphilic block copolymers. They form mixed micelles whose microstructures depend on the block copolymer architecture. Whereas the diblock copolymer exhibits complete micellar miscibility with the surfactant, the triblock copolymer structure limits the micellar miscibility. It is also shown that the micellar microstructure transformation is correlated with the surface tension of the mixed amphiphile system. Surface tension is a good index of mixed aggregate formation and of saturation. Acknowledgment. The authors thank Y.-Y. Won and F. S. Bates for their generous supply of the block copolymers used in the study. This work was supported in part by the National Science Foundation Center for Interfacial Engineering (CIE) at the University of Minnesota. Also, the authors are grateful to Professor Y. Talmon, in whose laboratory some of the micrographs of the paper were prepared. LA000230R (22) Nagarajan, R. Adv. Colloid Interface Sci. 1986, 26, 205-264. (23) Puvvada, S.; Blankschtein, D. J. Phys. Chem. 1992, 96, 55795592. (24) Ben-Shaul, A.; Gelbart, W. M. Annu. Rev. Phys. Chem. 1985, 36, 179.
