AFM Study of Micelle Chaining in Surface Films ... - ACS Publications

These stars were prepared following a method developed by Francis et al.17 and based ...... Jennifer L. Logan, Pascal Masse, Yves Gnanou, Daniel Taton...
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Langmuir 2005, 21, 3424-3431

AFM Study of Micelle Chaining in Surface Films of Polystyrene-block-Poly(ethylene oxide) Stars at the Air/Water Interface Jennifer L. Logan,† Pascal Masse,† Brian Dorvel,† Andrew M. Skolnik,† Sergei S. Sheiko,¥ Raju Francis,†,#,§ Daniel Taton,# Yves Gnanou,# and Randolph S. Duran*,† The George and Josephine Butler Polymer Research Laboratory, Department of Chemistry, University of Florida, P.O. Box 117200, Gainesville, Florida 32611-7200, Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, and Laboratoire de Chimie des Polyme` res Organiques, ENSCPB, Universite´ Bordeaux I, 16 Avenue Pey-Berland, 33607 Pessac Cedex, France Received December 21, 2004 A series of three-arm star block copolymers were examined using atomic force microscopy (AFM). These stars consisted of a polystyrene core composed of ca. 111 styrene units/branch with poly(ethylene oxide) (PEO) chains at the star periphery. Each star contained different amounts of PEO, varying from 107 to 415 ethylene oxide units/branch. The stars were spread as thin films at the air/water interface on a Langmuir trough and transferred onto mica at various surface pressures. Circular domains representing 2D micelle-like aggregated molecules were observed at low pressures. Upon further compression, these domains underwent additional aggregation in a systematic manner, including micellar chaining. At this point, domain area and the number of molecules/domain increased with increasing pressure. In addition, it was found that longer PEO chains led to greater intermolecular separation and less aggregation. These AFM results correspond to attributes seen in the surface pressure-area isotherms of the stars. In addition, they demonstrate the viability of AFM as a quantitative characterization technique.

Introduction We are interested in understanding how well-defined amphiphilic star block copolymers, containing a hydrophilic and hydrophobic portion, assemble at air/water interfaces. Stars represent the most basic branched architecture by which macromolecules can be attached to one central branching point. When applied to an interface, such architecture proves interesting, especially when compared to the linear homologue. In the case of the biologically relevant air/water interface, polymers must be amphiphilic, containing separate hydrophobic and hydrophilic portions. Structurally speaking, this criterion is efficiently fulfilled by diblock copolymers. A classic example of a simpler linear block copolymer that is amphiphilic is polystyrene-b-poly(ethylene oxide) (PS-b-PEO). While the hydrophobic PS does not produce a stable monolayer, a combination of these two polymers allows stable surface films to form.1-8 The stability results from the anchoring effect of the PS block and the steric (excluded volume) repulsion of the PEO chains in the * Author to whom correspondence should be addressed. E-mail: [email protected]. Phone: 352-392-2011. Fax: 352-392-9741. † University of Florida. ¥ University of North Carolina at Chapel Hill. # Universite ´ Bordeaux. § Permanent Address: Department of Chemistry, St. Joseph’s College, Devagiri, University of Calicut, Calicut, Kerala-673 008, India. (1) Gragson, D. E.; Jenson, J. M.; Baker, S. M. Langmuir 1999, 15, 6127-6131. (2) (a) Faure´, M. C.; Bassereau, P.; Lee, L. T.; Menelle, A.; Lheveder, C. Macromolecules 1999, 32, 8538-8550. (b) Faure´, M. C.; Bassereau, P.; Carignano, M. A.; Szleifer, I.; Gallot, Y.; Andelman, D. Eur. Phys. J. B 1998, 3, 365-375. (c) Goncalves da Silva, A. M.; Simoes Gamboa, A. L.; Martinho, J. M. G. Langmuir 1998, 14, 5327-5330. (d) Richards,

corona. Lateral compression changes the balance between the PS attraction and PEO repulsion. As the pressure increases, the PEO chains desorb downward, reducing the distance between the attractive PS blocks. Aggregation then results. Numerous studies of PS-b-PEO copolymers have investigated the monolayer formed from linear chains at the air/water interface. In general, it was found that upon compression of the PS-b-PEO, a characteristic rearrangement of the monolayer occurs. The most common interpretation of the data involves the formation of a pancakelike structure, evolving to that of a brush or cigar structure. In the former, compact hydrophobic PS globules sit atop flat hydrophilic PEO “pancakes”. When compressed, the PEO pancakes interact resulting in an increase in pressure. At a pressure of ca. 10 mN/m, the PEO chains gradually desorb, leaving only the PS anchored at the surface. With continued compression, the PEO chains further dangle and stretch into the water subphase to form the brush structure. R. W.; Rochford, B. R.; Webster, J. R. P. Polymer 1997, 38, 1169-1177. (e) 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, 4467-4473. (3) Cox, J. K.; Yu, K.; Constantino, B.; Eisenberg, A.; Lennox, R. B. Langmuir 1999, 15, 7714-7718. (4) Francis, R.; Skolnik, A. M.; Carino, S. R.; Logan, J. L.; Underhill, R. S.; Angot, S.; Taton, D.; Gnanou, Y.; Duran, R. S. Macromolecules 2002, 35, 6483-6485. (5) Goncalves da Silva, A. M.; Filipe, E. J. M.; d’Oliveira, J. M. R.; Martinho, J. M. G. Langmuir 1996, 12, 6547-6553. (6) Cox, J. K.; Yu, K.; Eisenberg, A.; Lennox, R. B. Phys. Chem. Chem. Phys. 1999, 1, 4417-4421. (7) Baker, S. M.; Leach, K. A.; Devereaux, C. E.; Gragson, D. E. Macromolecules 2000, 33, 5432-5436. (8) Devereaux, C. A.; Baker, S. M. Macromolecules 2002, 35, 19211927.

10.1021/la0468242 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/15/2005

AFM Study of Micelle Chaining in Surface Films

The PS-b-PEO films are easily transferred as Langmuir-Blodgett (LB) films and are studied through atomic force microscopy (AFM). The nanoscale resolution of AFM allows visual examination of the response of the amphiphilic copolymers to surface pressure. Several studies have been done on linear systems.3,6-8 Cox et al.,3 for example, found that linear PS-b-PEO spontaneously forms surface aggregates. Such aggregation was neither compression-induced, associated with micellization in the spreading solvent, nor due to LB film transfer. The aggregates formed by block copolymers in monolayers have been described as surface micelles by Lennox et al.9 For spherical micelles, two types are described. Star micelles refer to those systems containing a relatively small core and large corona, while crew-cut micelles represent the opposite. A general discussion of work done by Eisenberg and collaborators10 notes that these aggregated micelles result from a balance between corechain stretching, corona-chain repulsion, and interfacial tension between the core and outside solution. In studying the formation of these 2D micelles, Seo et al.11 used AFM to examine diblock copolymers composed of PS and poly(methyl methacrylate) (PMMA). They noted the formation of small circular micelles consisting of aggregated molecules. Seo et al.12 also found that aggregation depends on the relative amount of PMMA compared to PS. In a related study, Zhang et al.13 experimented with blends of random copolymers with PS and PMMA homopolymers. They used AFM data to determine the extent of compatibility and its dependence on functionality. In addition to imaging changes in morphology, the combination of microscopy and LB films provides quantitative analysis of polymer behavior at the molecular scale. Matyjaszewski et al.,14 for example, used AFM to examine the polydispersity of three- and four-arm star brushes. They demonstrated how quantitative analysis of AFM images could be used to determine the effects of different reaction conditions. In a similar fashion, Kiriy et al.15 used AFM to examine the role of solvent in molecular association of polystyrene/poly(2-vinyl-pyridine) heteroarm star copolymers. Through surface-supported metallization, they were able to directly count the number of arms in single molecule images, as well as to determine the amount of molecules aggregated in 2D micelles. Sheiko et al.16 also obtained AFM images of individual molecules, showing how the number-average molecular weight and molecular weight distribution could then be calculated. Most work on PS-b-PEO amphiphilic monolayers has focused on linear systems. Exceptions include two recent articles from our group examining PS-b-PEO stars. One gave preliminary results on the ability of the stars to form reproducible, compressible surface films,17 while the other (9) Zhu, J.; Eisenberg, A.; Lennox, R. B. J. Am. Chem. Soc. 1991, 113, 5583-5588. (10) Choucair, A.; Eisenberg, A. Eur. Phys. J. E 2003, 10, 37-44. (11) Seo, Y.; Im, J.-H.; Lee, J.-S.; Kim, J.-H. Macromolecules 2001, 34, 4842-4851. (12) Seo, Y.; Paeng, K.; Park, S. Macromolecules 2001, 34, 87358744. (13) Zhang, W.; Fu, B. X.; Seo, Y.; Schrag, E.; Hsiao, B.; Mather, P. T.; Yang, N.-L.; Xu, D.; Ade, H.; Rafailovich, M.; Sokolov, J. Macromolecules 2002, 35, 8029-8038. (14) Matyjaszewski, K.; Qin, S.; Boyce, J. R.; Shirvanyants, D.; Sheiko, S. S. Macromolecules 2003, 36, 1843-1849. (15) Kiriy, A.; Gorodyska, G.; Minko, S.; Stamm, M.; Tsitsilianis, C. Macromolecules 2003, 36, 8704-8711. (16) Sheiko, S. S.; da Silva, M.; Shirvaniants, D.; LaRue, I.; Prokhorova, S.; Mo¨ller, M.; Beers, K.; Matyjaszewski, K. J. Am. Chem. Soc. 2003, 125, 6725-6728. (17) Francis, R.; Taton, D.; Logan, J. L.; Masse, P.; Gnanou, Y.; Duran, R. S. Macromolecules 2003, 36, 8253-8259.

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Figure 1. Model of the three-arm (PS-b-PEO)3 star architecture. Table 1. Composition, Including the Amounts of PS and PEO, of the Three-Arm Star Copolymers Depicted in Figure 1 star

PS units/branch (mass %)

PEO units/branch (mass %)

total Mna (g/mol)

Mw/Mnb

1 2 3 4

111 (71%) 111 (57%) 111 (52%) 111 (39%)

107 (29%) 195 (42%) 237 (47%) 415 (61%)

49 000 60 600 66 200 89 600

1.2 1.1 1.1 1.2

a Total molecular weight (M ) was calculated using 1H NMR.17 n Polydispersity indices (Mw/Mn) were determined using size exclusion chromatography (SEC).17

b

investigated the aggregation and surface morphology of a star through AFM.4 This latter work showed spontaneous aggregation among domains, similar to behavior described in linear systems.3 Another exception is a very recent study by Peleshanko et al.18 who compared asymmetric PS-bPEO stars to their linear homologues. Their discussion of heteroarm stars (ABn) provides an interesting comparison to our own (AB)n stars (where n represents the number of arms in stars containing A and B polymers). In this study, we examine a series of PS-b-PEO threearm stars ((PS-b-PEO)3) containing the same PS core but differing PEO lengths. These stars were prepared following a method developed by Francis et al.17 and based on atom transfer radical polymerization of styrene from a trifunctional initiator, chain end transformation, and anionic polymerization of ethylene oxide. The resulting macromolecules are both well-defined and of controlled molar mass, providing novel architectures suitable for characterization at the air/water interface. These stars allow us to show how the combination of Langmuir and AFM techniques is very powerful for investigating surface micelles. It provides quantitative characterization of the aggregation number, as well as morphology. Experimental Section Materials. A series of star block copolymers were synthesized according to a previously reported procedure.17 Using the corefirst method, a three-arm PS star was prepared from a benzyl halide core by atom transfer radical polymerization (ATRP). The chain ends were then modified, enabling the growth of a poly(ethylene oxide) (PEO) chain through anionic polymerization. The resulting stars consist of a polystyrene (PS) core followed by a PEO corona (Figure 1). The characteristics of the (PS-b-PEO)3 star block copolymers are presented in Table 1. Composed of the same PS core, they differ in the number of units of poly(ethylene oxide) per arm. The number-average molar mass (Mn) was determined through NMR, and the polydispersity index (PDI) was obtained with SEC.17 The mass percentages of each block are also shown. Langmuir Films. Isotherm characterization of these copolymers was accomplished using a Teflon Langmuir trough system (KSV Ltd., Finland) equipped with two moving barriers and a Wilhelmy plate for measuring surface pressure. Between runs, (18) Peleshanko, S.; Jeong, J.; Gunawidjaja, R.; Tsukruk, V. V. Macromolecules 2004, 37, 6511-6522.

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Figure 2. Three main regions are apparent in the (PS-b-PEO)3 star isotherms. Region I represents an expanded area commonly referred to as the pancake region. A pseudoplateau denotes Region II, while Region III occurs at smaller areas where the surface film is the least compressible. the trough was cleaned with ethanol and rinsed several times with Millipore filtered water of a resistivity g 18.2 MΩ/cm. Samples were typically prepared by dissolving 1 mg of polymer in 1 mL of chloroform. Approximately 70 µL of the sample solution were spread dropwise on a layer of Millipore water with a gastight Hamilton syringe. The chloroform was allowed to evaporate for 30 min to ensure no residual solvent remained. When not in use, solutions were wrapped with Teflon tape followed by Parafilm and stored at 10 °C in order to prevent changes in concentration due to chloroform evaporation. Langmuir studies yielded plots of surface pressure (π) versus mean molecular area (MMA) (the average occupied area per molecule). This calculation is made using Mn and assumes all molecules remain at the surface. Barrier movement did not exceed (10 mm/min, while compression occurred in a linear fashion at a rate of 0.5 mN/m‚min. The temperature of the subphase was controlled at 25 °C using a water bath circulator. Atomic Force Microscopy (AFM). Surface films of the star copolymers were transferred onto freshly cleaved mica at various pressures (25 °C). The desired surface pressure was attained using linear compression of 0.5 mN/m‚min at rates of (10 mm/ min. Once the film had equilibrated at a constant π for at least 15 min, the mica was then pulled out at a rate of 1 mm/min. The transferred film was air-dried in a dust-free environment for 24 h and subsequently scanned in tapping mode with a Nanoscope III AFM (Digital Instruments, Inc., Santa Barbara, CA) using silicon probes (Nanosensor dimensions: T ) 3.8-4.5 µm, W ) 26-27 µm, L ) 128 µm). Through Digital Instruments software, the images were processed with a second-order flattening routine. Domain areas and number were analyzed using software developed by D. Shirvaniants at the University of North Carolina, Chapel Hill.

Results and Discussion Pressure-Area Isotherms. The surface pressurearea (π-A) isotherms of several star copolymers are presented in Figure 2, with a log scale on the x axis for convenient visualization. The isotherms showed that the surface films were reproducible (for instance, within 1% at π ) 15 mN/m for Star 1) and could be compressed up to surface pressures as high as 55 mN/m. For stars of higher molar mass, their surface films initially occupied larger areas, as expected. Within the isotherms in Figure 2, three distinct regions can be seen. At large molecular areas, the surface film is expanded (I). This region is typically known as the “pancake” region due to the pancakelike shape that the PEO is believed to adopt when absorbed at the air/water interface.5 As compression continues, the increasing surface pressure plateaus at pressures of 8-10 mN/m (II). Since the pressure still increases (albeit slightly), this

Logan et al.

region represents a pseudoplateau. Qualitatively, the pseudoplateau pressure is in the range of that observed for PEO homopolymers. The third region (III) occurs beyond the pseudoplateau where the surface pressure sharply increases at low areas, indicating the film is more rigid toward compression. Here, extrapolation of the linear portion of the isotherm to π ) 0 mN/m yields Ao, the theoretical area that a perfectly compact surface film would occupy at zero pressure. Repeat trials confirmed this area to be reproducible. Considering the affinity of PEO for the air/water interface19 and the similarity of our isotherms to those for linear copolymers in the literature,1-3,5-7,20 we propose that the expanded region (I) of the star isotherms represents a film of PEO at the surface. Due to its hydrophobicity, the PS in turn exists as small globules atop the PEO film. This interpretation is based on that of Cox et al. and da Silva et al. in their work on linear PS-b-PEO.3,5,6 Due to the transitionlike nature of region II, we believe the pseudoplateau represents a biphasic state. This region is observed in all of the stars (Figure 2). The fact that the pseudoplateau pressures correspond to the collapse pressure observed in a monolayer of pure PEO21 indicates that PEO again plays a role in this region. In addition, careful observation shows that as PEO chain length increases, the pseudoplateau pressures increase as well. Star 1, with only 107 EO units/branch, demonstrates a pseudoplateau beginning at a pressure of ca. 5 mN/m while Star 4 (415 EO units/branch) starts at ca. 9 mN/m. As compression continues, the PEO is progressively forced into the aqueous subphase. At this point, a pure PEO homopolymer monolayer of such chains would be lost to submersion below the interface or dissolution to the bulk aqueous phase. The presence of the PS, however, anchors the PEO, leading to higher surface pressures (III). As a result, this region primarily reflects PS. The fact that, here, the isotherms of the four samples are much closer to each other indicates that the different chain lengths of PEO have a negligible effect on packing at higher pressures. Instead, the same PS core of each star leads to similar behavior in a highly compressed state. The overall behavior of the three-arm stars as surface films compares to that of linear PS-b-PEO systems. Like linear chains, the stars form reproducible, compressible films. While some comparison between the given architectures is possible, more work is needed to better determine the role that architecture plays. Regardless, both linear and star PS-b-PEO architectures exhibit various compression-induced conformations. The extent of these regions depends on PEO at higher molecular areas and on PS at lower ones. Atomic Force Microscopy. The use of AFM is a valuable technique in visualizing the morphology of a surface film at the air/water interface. Lennox et al.3 remarked that both AFM and transmission electron microscopy (TEM) provide complementary evidence of phase separation not readily available by other techniques such as Brewster angle microscopy (BAM), specular X-ray reflectivity, or neutron reflectivity due to domain size. In (19) Glass, J. E. J. Phys. Chem. 1968, 72, 4459-4467. (20) (a) Rivillon, S.; Mun˜oz, M. G.; Monroy, F.; Ortega, F.; Rubio, R. G. Macromolecules 2003, 36, 4068-4077. (b) Noskov, B. A.; Akentiev, A. V.; Miller, R. J. Colloid Interface Sci. 2002, 247, 117-124. (c) Sauer, B. B.; Yu, H.; Tien, C. F.; Hager, D. F. Macromolecules 1987, 20, 393400. (d) Rother, G.; Findenegg, G. H. Colloid Polym. Sci. 1998, 276, 496-502. (21) Sauer, B. B.; Yu, H. Macromolecules 1989, 22, 786-791.

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Table 2. Positive Transfer Ratios Obtained for Stars 1-4 Indicate a Successful LB Transfer in Each Casea pressure

Star 1

Star 2

Star 3

Star 4

π < 1.0 mN/m 1.0 2.0 3.0 4.0 5.0

3.33 1.12 0.74 1.25 1.49 1.75

3.05 1.06 1.71 1.64 2.32 1.32

3.56 1.07 1.26 1.17 1.12 1.52

2.02 1.31 1.34 1.36 1.24 1.28

6.0 7.0 8.0 9.0 10.0

2.13 4.47

1.52 3.24 6.37 6.08 1.17

1.21 1.05 1.71 1.66 1.16

1.50 1.34 1.50 5.29 6.58

11.0 12.0 15.0 20.0 30.0 50.0

1.40 0.87

0.34

0.58

0.33 0.50

0.45

0.75 1.15 0.33 0.35 0.31

0.32

In general, low pressures (π < 1.0 mN/m) yielded higher ratios that then decreased to values between 0.7 and 2.3 within the pancake region. Near the pseudoplateau, transfer ratios increased again (boxed values) while brush area pressures resulted in low ones. A transfer ratio of 1.0 represents a uniform transfer in which the surface film at the air/water interface resembles that transferred onto the substrate.

Figure 4. AFM images of the (PS-b-PEO)3 stars at various transfer pressures. The first row represents Star 1 (the shortest PEO chains) followed by Stars 2, 3, and 4 (the latter possessing the longest PEO chains). Aggregation between the circular domains can be seen for all four stars at different pressures (denoted by the boxed images). All films are scanned on mica at a scale of 1 µm × 1 µm.

a

Figure 3. Transfer ratios obtained for the four stars show some dependence on pressure. While initially high, the transfer ratio drops to between 1 and 2 within the pancake region. Once pressures within the pseudoplateau are reached (7-10 mN/m, depending on the star), higher transfer ratios occur. These then drop to 11.0 mN/ m), transfer ratios were significantly lower (0.3-0.7). The pressures at which these variations occurred differed depending on the star. Star 1, for example, demonstrates a pseudoplateau at a lower pressure than the others and so shows an increase in transfer ratio at a lower pressure (Figure 3). Star 4, however, does not indicate this change in transfer ratio until a higher pressure of π ) 9 mN/m. Such high pseudoplateau transfer ratios are to be expected. When LB film transfer occurs within the pseudoplateau region, some of the PEO is pushed into the water subphase. The displacement of these chains requires further barrier compression in order to maintain the same pressure throughout transfer. The barriers thus move more than would be expected for a substrate of a certain size, resulting in a transfer ratio greater than one. As the observed transfer ratios do not usually equal one, the morphology observed in the scans may not perfectly represent the surface film at the air/water interface. Nonetheless, qualitative trends may be observed. At lower pressures corresponding to the pancake region, transfer ratios tend to be >1. This phenomenon implies that the surface film densifies upon transfer, resulting in an AFM image showing domains to be denser than they actually are at the air/water interface. The transferred (PS-b-PEO)3 stars are overly compressed. In contrast, higher pressures lead to lower transfer ratios. The surface films are now less dense on the mica than they were at the air/water interface. As a result, the domains expand since each molecule has more space to spread out on the mica. At even higher pressures (>11 mN/m), the transfer becomes inhomogeneous with only parts of the mica possessing any (PS-b-PEO)3 surface film. Quantitative Analysis of AFM Scans. Typical AFM scans obtained for Stars 1-4 are shown in Figure 4. AFM images of the (PS-b-PEO)3 stars clearly demonstrated a systematic dependence of morphology on the surface pressure. For all of the stars, lower pressures yielded a semi-ordered array of small circular domains. These domains were counted, and average domain areas were measured and analyzed. Another characteristic feature

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of the AFM images can be seen in the z or height information. While not as accurate as the x-y measurements, the height of the circular domains was ca. 4 nm. This height is far greater than a monolayer thickness and implies one block of the copolymer aggregated into a 3D structure. In comparison to previous studies, our films are similar to those described by Cox et al. (PS-b-PEO, ca. 5 nm through TEM).6 Overall, the domains are consistent with 2D micelle structures seen in other AFM studies.4,7,11,12,18,22-25 Due to the hydrophilic nature of the mica, we suppose PEO transfers as the bottom layer while the hydrophobic PS occupies the top portion of the film. As a result, we deduce that the white higher elevation domains consist of PS aggregates, repulsed by the hydrophilicity of the PEO and mica. Since PEO actively absorbs at the air/ water interface,19 the continuous dark phase in the scanned images represents a thin layer of PEO. The circular domains thus illustrate micellar structures with a white PS core surrounded by a dark PEO corona. Zhu et al.9 made a similar observation, noting that the strongly hydrophobic PS would aggregate to reduce PS/water interactions. Similar to our (PS-b-PEO)3 stars, their linear poly-4-vinylpyridine-b-polystyrene system results in a radial micelle-like aggregate. Ordering of Surface Micelles. Within the pancake region, a characteristic packing of domains occurs. Local ordering resembles hexagonal packing with each domain surrounded by six regularly spaced neighbors. Over longer distances, the hexagonal packing clearly dissipates. Overall, the ordering is reminiscent of a 2D analogue of hexatic phases seen in liquid crystals.26 These small aggregates of molecules occur even at the low pressure of π ) 0.1 mN/m, indicating that initial aggregation of 2D micelles takes place spontaneously at the air/water interface. Hexagonal ordering has also been observed by Boyce et al.27 in multi-arm molecular brushes with grafted poly(n-butyl acrylate) (PBA). When compressed, these brushes undergo a transition from a starlike to a disklike conformation that, in turn, attain local hexagonal order. Overall, the observed aggregates are consistent with an idea proposed by Lennox et al.3 in which surface aggregation of block copolymers occurs spontaneously, dependent on neither compression nor spreading solvent. In addition, spontaneous aggregation explains the vertical heights of the films. The 2-5 nm heights observed are much larger than that expected for PS chains lying in a monolayer. Due to spontaneous aggregation upon deposition, we suppose the stars never form the monolayer of individual dispersed molecules generally seen in smaller amphiphiles. Rather, the mesoscopic micelle unit is the building block of this Langmuir monolayer. While Peleshanko et al.18 believe that aggregation occurs as a result of compression, even a very slight one, we were unable to obtain reproducible transfer at less than 0.1 mN/m to produce reliable AFM. The formation of globular domains can also be seen in work by Sheiko et al.28 In examining a series of PBA (22) Mateˇjı´cˇek, P.; Humpolı´cˇkova´, J.; Procha´zka, K.; Tuzar, Z.; ×b3Sˇ pı´rkova´, M.; Hof, M.; Webber, S. E. J. Phys. Chem. B. 2003, 107, 8232-8240. (23) Li, Z.; Zhao, W.; Liu, Y.; Rafailovich, M. H.; Sokolov, J.; Khougaz, K.; Eisenberg, A.; Lennox, R. B.; Krausch, G. J. Am. Chem. Soc. 1996, 118, 10892-10893. (24) Seo, Y.; Esker, A. R.; Sohn, D.; Kim, H.-J.; Park, S.; Yu, H. Langmuir 2003, 19, 3313-3322. (25) Spatz, J. P.; Sheiko, S. S.; Mo¨ller, M. Adv. Mater. 1996, 8, 513517. (26) Pershan, P. S. Structure of Liquid Crystal Phases; World Scientific Publishing Co., Inc.: Teaneck, NJ, 1988.

Logan et al.

Figure 5. Gaussian distributions of the domain areas for AFM images obtained at π ) 1 mN/m. As PEO chain length increases, the average domain area decreases.

brushes of large molecular weight (>1.6 × 108 g/mol), they were able to observe single molecules as LB films through AFM. As the molecular area of these brushes decreased, a transition from rodlike to globular structures occurs. The formation of the globules results from desorption of the side chains. For brushes with lower grafting density, the rod-to-globule transition was not observed.29 Though these globules represent single molecules, their shape resembles that of our 2D micelles. In addition, the importance of peripheral chains (PEO in our case, PBA in theirs) is demonstrated. At low pressures, the average domain areas for the four stars remain relatively constant. Star 1 (710 ( 340 nm2) yields the largest domains, while Stars 2 (600 ( 250 nm2), 3 (470 ( 230 nm2), and 4 (430 ( 180 nm2) result in smaller and smaller domain areas. As the stars increase in size, their domain areas decrease. Each star, however, demonstrates a critical pressure at which a steep increase in domain area occurs. This pressure is denoted in Figure 4 for Stars 1-4 as 5, 7, 9, and 10 mN/m, respectively. The domain areas in each scan yielded a Gaussian-like distribution. Figure 5 illustrates such distributions for films of the four stars transferred at π ) 1 mN/m. Again, the domain area is observed to decrease with longer PEO chain lengths. These distributions are typically broad, indicating the presence of both smaller and larger domain populations. The center of each distribution, as well as the standard deviation in domain area, is as follows 800 ( 300 nm2 (Star 1), 620 ( 210 nm2 (Star 2), 500 ( 170 nm2 (Star 3), and 380 ( 190 nm2 (Star 4). At this particular pressure (π ) 1 mN/m), the distribution of domain areas becomes broader as the length of PEO decreases. The pressure at which the domain areas significantly increase also results in broader distributions for each star. Measuring the Aggregation Number. In addition to domain area, we are also interested in aggregation number (Γ) or the number of molecules present in each AFM domain. By considering the mean molecular area of the film during transfer, we are able to calculate the number of molecules per domain (Γ) using eq 1: Γ ) A/(Nd σ) (1), where A refers to the scanned area of the image, Nd is the number of domains, and σ is the mean molecular area of the film during transfer. (27) Boyce, J. R.; Shirvanyants, D.; Sheiko, S. S.; Ivanov, D. A.; Qin, S.; Bo¨rner, H.; Matyjaszewski, K. Langmuir 2004, 20, 6005-6011. (28) Sheiko, S. S.; Prokhorova, S. A.; Beers, K. L.; Matyjaszewski, K.; Potemkin, I. I.; Khokhlov, A. R.; Mo¨ller, M. Macromolecules 2001, 34, 8354-8360. (29) Lord, S. J.; Sheiko, S. S.; LaRue, I.; Lee, H.-I.; Matyjaszewski, K. Macromolecules 2004, 37, 4235-4240.

AFM Study of Micelle Chaining in Surface Films

Figure 6. AFM data shows a dependence of the number of molecules/domain (Γ) on pressure. Included are results for a reverse architecture star, comprising a PEO core (45 EO units/ branch) and a PS corona (77 Styrene units/branch).4 The general trend indicates that as pressure increases, more molecules aggregate to form the observed circular domains.

The importance in determining an aggregation number in the analysis of AFM images was first demonstrated in detail by Zhu et al.30 on polyelectrolyte-containing copolymers. They developed several possible methods for calculating the number of molecules/domain based on the assumption of a 1:1 correspondence between the LB film and the film adsorbed at the air/water interface. Figure 6 shows the dependence of the aggregation number on pressures below 10 mN/m. Each star possesses ca. 10 molecules/domain at a pressure of 0.1 mN/m. At lower pressures, this aggregation number is relatively constant. Upon examining the images in Figure 4, the number of domains clearly increases with surface pressure. This value is offset, however, by a decrease in the mean molecular area, thus explaining the constant aggregation number. At a certain pressure, however, the number of molecules/domain dramatically increases. The same trend thus results where shorter PEO chains exhibit a jump in Γ at a lower pressure than for longer chains. In fact, the domain area has a linear dependence on the aggregation number. This correlation is expected; more aggregated molecules result in larger domains. In addition to the (PS-b-PEO)3 stars, a star of reverse architecture also appears in Figure 6. This star, comprised of a PEO core (45 EO units/branch) and a PS corona (77 PS units/branch), was previously studied by our group.4 While this structure demonstrates similar circular domains at low pressures, the star contains a higher number of aggregated molecules within each domain. Not only does this star represent the reverse architecture, it also contains a significantly lower mass percentage of PEO (20% compared to a 29-61% mass for Stars 1-4). Both factors probably contribute to the higher number of molecules/domain. Another comparison in architecture can be made between the linear PS-b-PEO systems of Baker et al.7 and our stars. Here, LB films of PS-b-PEO were prepared on silicon wafers and examined through AFM. Baker et al. observed that, for pressures e 10 mN/m, the number of molecules per domain remained constant. As the mass percentage of PEO increased, however, the aggregation number decreased. One sample containing 60% PEO compares to Star 4 (61%). For pressures at which the circular domains occur (π < 10 mN/m), the aggregation number for Star 4 ranges from 8 to 13. In contrast, Baker et al. calculated an aggregation number of 95 for their (30) Zhu, J.; Lennox, R. B.; Eisenberg, A. Langmuir. 1991, 7, 15791584.

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sample, a number significantly higher than that observed for our star. In fact, Baker et al. determined the aggregation number to equal 250-2.4× (% PEO). This equation yields values of Γ much higher than any we found for our stars. Therefore, star architecture leads to smaller domain formation. While the linear PS-b-PEO showed similar trends in which increasing PEO led to less aggregation and smaller domains, they differ from our stars in terms of magnitude. Compared to a given linear system, these stars have significantly different entropy and triple the chain density of PS/PEO junction points. These junctions should align themselves at the interface due to incompatibility between PS and PEO, resulting in the formation of smaller domains. The radius of gyration () for an ideal chain in 2D depends on the square root of N. Here, it is predicted to vary according to an exponent between 0.5 and 0.75,31,32 while in 3D, this exponent is 0.5.33 In this case, we have neither a fully 3D, nor a fully 2D system. Nonetheless, with three junction points, our three-arm stars have 3N/ 3-length corona chains and their radii of gyration sum to give a significantly larger value than that of a single copolymer chain of length N. To accommodate this higher chain density at the interface, we expect a much different splay elastic energy. The entropy of conformation of the star block copolymer should also be much less than the corresponding linear block copolymer.34 Both factors would lead to a much smaller domain size. These observations are also in agreement with the work of Milner et al.35 who predicted that the morphology of branched systems should be different from linear analogues. Comparison can also be made to micelles composed of PS-b-PEO formed in solution. Brown et al.,36 for example, prepared aqueous dispersions of a PS-b-PEO chains consisting of an average of 13 styrene units and 123-160 EO units. Determining that the aggregation number depended on PS alone, they calculated 170 polymer chains/ micelle. This value lies within experimental error of the aggregation number determined by Seo et al.37 In this latter work, Seo et al. formed aqueous micelles with ca. 190 ( 40 PS-b-PEO chains (59 St units, 1066 EO units). Both of these aggregation numbers are greater than those determined for our stars by a factor of 10 or more. This discrepancy illustrates the smaller extent to which our stars aggregate. Our “unimolecular micelles” aggregate less than traditional micelles composed of linear chains. Such behavior could lead to better control over micellar size by using branched architectures. Domain Aggregation and Chaining. In the AFM images of our series of (PS-b-PEO)3 stars, both similarities and differences occur. All four stars possess circular domains representing aggregated molecules at low pressures. Upon compression, the number of domains increases. Eventually, the domains are pushed close enough to undergo chaining in which both the domain area and aggregation number dramatically increase. Differences arise in the pressure at which this chaining occurs, as well as in actual domain size and separation. These (31) Flory, P. J. Science 1975, 188, 1268-1276. (32) Tobochnik, J.; Webman, I.; Lebowitz, J. L.; Kalos, M. H. Macromolecules 1982, 15, 549-553. (33) Rubinstein, R.; Colby R. L. Polymer Physics; Oxford University Press: Oxford, 2003. (34) (a) Zimm, B. H.; Stockmayer, W. H. J. Chem. Phys. 1949, 17, 1301-1314. (b) Orofino, T. A. Polymer 1962, 2, 305-314. (35) Millner, S. T. Macromolecules 1994, 27, 2333-2335. (36) Brown, G. J.; Richards, R. W.; Heenan, R. K. Polymer 2001, 42, 7663-7673. (37) Seo, Y. S.; Kim, M. W.; Ou-Yang, D. H.; Peiffer, D. G. Polymer 2002, 43, 5629-5638.

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differences, in fact, depend on PEO, as discussed later in this section. Zhu et al.38 also noted that the morphologies of 2D aggregates depend greatly on the ratio of hydrophobic and hydrophilic block sizes. An additional effect of PEO is observed in the type of micelle chaining seen in each star. Star 1, containing the smallest amount of PEO, forms pearl-necklace-type chains while Star 4, with the highest amount of PEO, produces more irregular micelle aggregates. Baker et al.7 described a similar PEO effect in noting that PS-b-PEO consisting of 15.5% (mass) PEO led to strings while higher percentages (60% and 92%) did not. These strings, however, differ from ours in that the circular domains remain separated, forming only the semblance of a chain. Considering that our smallest star still contains 29% PEO, micelle chaining occurs more readily in stars than in linear systems. A similar type of chaining is observed by Peleshanko et al.18 for an asymmetric PS-b-PEO star. Consisting of one PEO branch and two PS ones, this three-arm star contains 27% (mass) PEO, resembling our Star 1 (29%). Their AFM images compare to those of Star 1, demonstrating small circular morphology at a low pressure (π ) 5 mN/m) and chaining at a higher one (π ) 10 mN/m). In contrast, other asymmetric stars produced AFM images of less uniform morphology. These stars were more hydrophobic, however, with less than 20% (mass) PEO. Another example of the chaining phenomenon is the polyelectrolyte necklaces described by Dobrynin et al.39 The necklacelike morphology was also noted by Bo¨rner et al.40 in brush macromolecules containing PS and PBA blocks. They observed that PS segregates into globules due to incompatibility with the PBA and air. When PS formed the outer block of the PBA-b-PS graft, the PS globules formed pearl necklaces. When, however, PS was the inner part, the globules remained separate as individual entities. The chaining in the block copolymer brushes thus results from the attraction of desorbed PS blocks and repulsion between the adsorbed PBA blocks. This observation contrasts with our own work in which both PS core and corona stars4 demonstrate chaining. Compression-induced aggregation presumably occurs due to PEO being pushed into the water subphase. The PEO in the vicinity of contact, however, may also be pushed aside, densifying nearby PEO chains. At higher pressures, less PEO remains at the interface to separate the hydrophobic PS domains. In comparing Star 2 with Star 1, the former contains longer PEO chains than the latter (195 vs 107 PEO units/branch). As a result, the shorter PEO chains of Star 1 would submerge into the water at a lower pressure than that required by the longer chains of Star 2. Observing greater aggregation in Star 1 at lower pressures and over a smaller range of π is therefore expected. Stars 3 and 4 follow this same trend. Star 3 demonstrates further aggregation between the circular domains at a pressure of 9 mN/m, while Star 4 does so at 10 mN/m. The longer the PEO chain, the higher the pressure at which aggregation occurs. Based on this trend, the picture that emerges appears in Figure 7. Longer PEO chains lead to greater separation between molecules, leading to smaller circular domains. The opposite holds true for shorter PEO chains where more closely packed, larger domains result. In addition, this model explains the high aggregation number observed (38) Zhu, J.; Lennox, R. B.; Eisenberg, A. J. Phys. Chem. 1992, 96, 4727-4730. (39) Dobrynin, A. V.; Rubinstein, M. Macromolecules 2000, 33, 80978105. (40) Bo¨rner, H. G.; Beers, K.; Matyjaszewski, K.; Sheiko, S. S.; Mo¨ller, M. Macromolecules 2001, 34, 4375-4383.

Logan et al.

Figure 7. The models depicted above demonstrate the role of PEO in domain size and separation. As the chain length of PEO increases, the stars remain farther apart, resulting in smaller domains (lower aggregation number). Both images were transferred at 1 mN/m and are 1 µm × 1 µm.

in the star of reverse architecture (referred to in Figure 6). A PEO core surrounded by a significantly larger PS corona would be less able to separate molecules, leading to domains containing more molecules. The formation of these micelles results from a balance between the surface and stretching energies, as described by Kramarenko et al.41 Our model resembles that of Seo et al.,24 who investigated linear polystyrene-b-poly(methyl methacrylate) (PSb-PMMA) at the air/water interface. They proposed that PS-PMMA forms micelles resembling a fried egg. PS represents the yolk, while PMMA resembles the egg white. A general observation in their series of three PS-b-PMMA linear chains was that the longest PMMA chains gave a small core size, while the shorter ones led to larger core sizes. This trend matches our own observations. For Seo et al., however, the PS-b-PMMA films were less uniform at low pressures. Uniform distribution of domains did not appear until 7 mN/m. In our case, domains appear throughout the pancake region. While surface active, PMMA is hydrophobic. In contrast, the water-soluble PEO would be more sensitive to surface pressure since compression induces hydration. As a result, PEO would be expected to exert greater control than PMMA over film morphology at areas below the brush structure. Another point of interest in the AFM scans is the phenomenon of micelle chaining introduced above. While this super-aggregation is most visible at a certain critical pressure range within the pseudoplateau region, evidence for the phenomenon can be observed at lower pressures. Star 1 in Figure 4, for example, consists mainly of unimer domains at π ) 0.1 mN/m. A small number of domains coalesces to double their length, forming “dimers” at π ) 1 mN/m. By π ) 5 mN/m, a significant fraction of the population exists as dimers or trimers, and by 7 mN/m, longer micelle chains are observed and few individual domains remain. Chaining may, in fact, reflect an intermediate phase of an “ultimately chained” system. Currently, a corresponding isotherm feature is not apparent, rendering quantification of this possible phase difficult. In general, pearl chaining results from short-range attraction and long-range repulsion. Figure 8 shows a model we propose to explain the process at the molecular level. The circular domains, consisting of aggregated (41) Kramarenko, E. Y.; Potemkin, I. I.; Khokhlov, A. R.; Winkler, R. G.; Reineker, P. Macromolecules 1999, 32, 3495-3501.

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Figure 8. This model proposes one mechanism by which chaining may occur. PS globules are separated by a corona of PEO. Upon compression, the PEO shifts such that PS aggregation occurs through only one point of contact. This selectivity results in a chainlike structure. Further domain aggregation is most likely to occur in areas of lowest PEO density. Part (a) represents a side view, while part (b) denotes the top view. Part (c) illustrates the distorted hexagonal geometry around one domain when a dimer forms. Possible chaining points are indicated at 2π/6 positions surrounding the central micelle. The positions labeled R, β, and β′ are most probable due to lower PEO chain density. In contrast, γ and γ′ would be least likely.

molecules, exist as PS cores separated by PEO coronas (Figure 8a). Upon compression, the PS cores move closer together. The PEO chains between these cores shift to the side, resulting in a higher local density. Eventually, the PS cores move close enough to chain with each other, pushing the PEO chains to either side of their point of contact (Figure 8b). Further chaining is now less likely to occur in the areas of high PEO density. Instead, the area opposite the point of chaining (or at an angle of 180°) would be the most probable location for additional aggregation (Figure 8c, position R). Once trimers or longer species form adjacent to each other, steric factors hinder interior units within a chain from further aggregation. Figure 8c illustrates alternative chaining positions β and β′, which account for the many chain bends observed (for example, in Star 1 at 7 mN/m), as well as the occasional three-point junctions. In contrast, γ and γ′ represent angles at which chaining is least likely to occur due to a high density of PEO chains. Overall, a total of 2π/6 possible chaining points exist, as indicated at positions surrounding the central micelle. Conclusion The formation of stable, reproducible surface films of novel (PS-b-PEO)3 star copolymers was demonstrated. The stars responded to compression through rearrangement in structure, as observed in distinct isotherm regions. At

smaller areas, the surface film is highly compressed. Here, PEO no longer played as large of a role as these areas related to packing of PS domains. Such behavior implies that fundamental rearrangement in the surface film occurs within the pseudoplateau. Corresponding changes in morphology were observed via AFM studies. While spontaneous aggregation of the star molecules appears to occur upon film deposition, AFM demonstrated systematic additional aggregation upon an increase in pressure. Also, the aggregation number and domain size were much smaller than in linear block copolymers. The most dramatic change occurred at pressures corresponding to the pseudoplateau region seen in the isotherms. As the area in this region of the isotherm depends on the amount of PEO, the importance of composition in determining (PS-b-PEO)3 star morphology is shown. In addition, the presence of PEO in the corona led to an interesting micelle-chaining phenomenon. Acknowledgment. Support from DOE-BES and NSF/ CNRS Grant No. DE-FG02-01ER45933 and NSF Grant No. INT-9816175 is acknowledged. J.L. received support from the NSF Graduate Fellowship Program, while P.M. participated in the NSF REU Program at the University of Florida. LA0468242