pubs.acs.org/Langmuir © 2009 American Chemical Society
Block Copolymer Strands with Internal Microphase Separation Structure via Self-Assembly at the Air-Water Interface Eric W. Price, Yunyong Guo, C.-W. Wang, and Matthew G. Moffitt* Department of Chemistry, University of Victoria, P.O. Box 3065, Victoria, BC V8W 3V6, Canada Received December 30, 2008. Revised Manuscript Received March 13, 2009 Block copolymer microphase separation in the bulk is coupled to amphiphilic block copolymer self-assembly at the air-water interface to yield hierarchical Langmuir-Blodgett (LB) structures combining organization at the meso- and nanoscales. A blend of polystyrene-b-poly(ethylene oxide) (PS-b-PEO) (Mn = 141K, 11.4 wt % PEO) and polystyrene-b-poly(butadiene) (PS-b-PB) (Mn = 31.9K, 28.5 wt % PB) containing a PS-b-PB weight fraction of f = 0.75 was deposited at the air-water interface, resulting in the spontaneous generation of aggregates with multiscale organization, including nanoscale cylinders in mesoscale strands, via evaporation of the spreading solvent. The resulting features were characterized in LB films via AFM and TEM and at the air-water interface via Langmuir compression isotherms. Blends containing lower PS-b-PB contents formed mesoscale aggregate morphologies of continents and strands ( f = 0.50) or mesoscale continents with holes ( f = 0.25), but without the internal nanoscale organization found in the f = 0.75 blend. The interfacial self-assembly of pure PS-b-PB at the airwater interface ( f = 1) yielded taller and more irregularly shaped aggregates than blends containing PS-b-PEO, indicating the integral role of the amphiphilic copolymer in regulating the mesoscale organization of the hierarchically structured features.
Introduction The generation of structural hierarchy via the spontaneous organization of organic and inorganic building blocks on multiple length scales opens up new routes to tunable function in photonic, electronic, and magnetic materials. Self-assembling systems which combine tunable morphologies with complex internal structure provide intriguing biomimetic routes to highly organized materials with desired functionality. Block copolymer microphase separation in the solid state is well-known to give rise to various nanostructured domain morphologies, including lamellae, gyroids, cylinders, and spheres, depending on the relative volume fractions of the incompatible blocks.1 The resulting features arise due to a minimization of free energy contributions from segment-segment interactions (enthalpic) and chain stretching (entropic), with characteristic length scales governed by the polymer chain dimensions, ranging from ∼5 nm to tens of nanometers. Various examples of hierarchical structures have been produced by integrating nanoscale block copolymer domains into materials with additional features at the mesoscale or microscale. In some cases, the larger length scale is defined by a preformed lithographic pattern or template, which can serve to direct the formation of the block copolymer domains: for example, block copolymer microphase separation has been investigated on *To whom correspondence should be addressed. (1) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: New York, 1998. (2) Cheng, J. Y.; Ross, C. A.; Smith, H. I.; Thomas, E. L. Adv. Mater. 2006, 18, 2505. (3) Sundrani, D.; Darling, S. B.; Sibener, S. J. Nano Lett. 2004, 4, 273. (4) La, Y.-H.; Edwards, E. W.; Park, S.-M.; Nealey, P. F. Nano Lett. 2005, 5, 1379. (5) Kim, T. H.; Hwang, J.; Hwang, W. S.; Huh, J.; Kim, H.-C.; Kim, S. H.; Hong, J. M.; Thomas, E. L.; Park, C. Adv. Mater. 2008, 20, 522. (6) Chai, J.; Buriak, J. M. ACS Nano 2008, 2, 489. (7) Chen, X.; Knez, M.; Berger, A.; Nielsch, K.; Gosele, U.; Steinhart, M. Angew. Chem., Int. Ed. 2007, 46, 6829.
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surfaces with periodic topographically or chemically defined patterns,2-6 in microporous templates,7 in colloidal crystals,8 and in mesoscale lamellar structures formed by holographic polymerization.9 Alternatively, hierarchical structures have been prepared by coupling block copolymer microphase separation with spontaneous processes that generate features on additional, larger length scales, including polymer-polymer macrophase separation,10,11 emulsion formation,12-17 liquid crystal ordering,18 electrospinning,19,20 and recently block copolymer micellization to form “compartmentalized micelles” in three dimensions (3D).21-25 The two-dimensional (2D) self-assembly of amphiphilic block copolymers at the air-water interface has been widely studied in (8) Arsenault, A. C.; Rider, D. A.; Tetreault, N.; Chen, J. I.-L.; Coombs, N.; Ozin, G. A.; Manners, I. J. Am. Chem. Soc. 2005, 127, 9954. (9) Birnkrant, M. J.; Li, C. Y.; Natarajan, L. V.; Tondiglia, V. P.; Sutherland, R. L.; Lloyd, P. F.; Bunning, T. J. Nano Lett. 2007, 7, 3128. (10) Hashimoto, T.; Kimishima, K.; Hasegawa, H. Macromolecules 1991, 24, 5704. (11) Iizuka, N.; Bodycomb, J.; Hasegawa, H.; Hashimoto, T. Macromolecules 1998, 31, 7256. (12) Lu, Z.; Liu, G.; Liu, F. Macromolecules 2001, 34, 8814. (13) Lu, Z.; Liu, G.; Phillips, H.; Hill, J. M.; Chang, J.; Kydd, R. A. Nano Lett. 2001, 1, 683. (14) Zheng, R.; Liu, G.; Yan, X. J. Am. Chem. Soc. 2005, 127, 15358. (15) Jeon, S.-J.; Yi, G.-R.; Koo, C. M.; Yang, S.-M. Macromolecules 2007, 40, 8430. (16) Higuchi, T.; Tajima, A.; Yabu, H.; Shimomura, M. Soft Matter 2008, 4, 1302. (17) Hu, J.; Liu, G.; Nijkang, G. J. Am. Chem. Soc. 2008, 130, 3236. (18) Park, J.-W.; Thomas, E. L. Adv. Mater. 2003, 15, 585. (19) Kalra, V.; Kakad, P. A.; Mendez, S.; Ivannikov, T.; Kamperman, M.; Joo, Y. L. Macromolecules 2006, 39, 5453. (20) Ma, M.; Krikorian, V.; Yu, J. H.; Thomas, E. L.; Rutledge, G. C. Nano Lett. 2006, 6, 2969. (21) Li, Z.; Kesselman, E.; Talmon, Y.; Hillmyer, M. A.; Lodge, T. P. Science 2004, 306, 98. (22) Li, Z.; Hillmyer, M. A.; Lodge, T. P. Nano Lett. 2006, 6, 1245. (23) Li, Z.; Hillmyer, M. A.; Lodge, T. P. Macromolecules 2006, 39, 765. (24) Cui, H.; Chen, Z.; Zhong, S.; Wooley, K. L.; Pochan, D. J. Science 2007, 317, 647. (25) Zhu, J.; Hayward, R. C. Macromolecules 2008, 41, 7794.
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recent years.26-44 For example, solutions of polystyrene-b-poly (ethylene oxide) (PS-b-PEO) copolymers in chloroform (a good solvent for both blocks) deposited onto the surface of water spontaneously form surface aggregates with a range of morphologies upon solvent evaporation, including dots, strands (or spaghetti), network structures, planar “continents”, rings, and chains, depending on the relative molecular weights of the blocks and the concentration of the spreading solution.31,33,36,37 Interfacial self-assembly of PS-b-PEO is the result of the interplay of attractive interactions between the water surface and the hydrophilic PEO blocks, and the repulsive interactions between the water surface and the hydrophobic PS blocks, with kinetic factors such as chain entanglements and polymer vitrification strongly influencing the final morphologies.33,36,37 Unlike block copolymer microphase separation in the bulk, the lateral dimensions of block copolymer aggregates obtained at the air-water interface can be orders of magnitude larger than the polymer chain dimensions, ranging from ∼100 nm to tens of microns. We previously incorporated PS-coated CdS quantum dots (QDs) into mesoscale PS-b-PEO strands and rings at the air-water interface, demonstrating the potential for using interfacial selfassembly of block copolymers to confine nanostructures.45,46 In related work, Kumaki has demonstrated 2D microphase separation of a block copolymer within a continuous LangmuirBlodgett (LB) film in which both blocks spread on the water surface, although no surface aggregates were formed in that case due to the absence of a nonspreading hydrophobic block.47 To our knowledge, only one previous study has investigated blends of a hydrophobic block copolymer with an amphiphilic block copolymer in LB films.48 In that work, poly(styrene-b-ferrocenylsilane) was blended with poly(styrene-b-2 vinylpyridine) and deposited at the air-water interface, where the hydrophobic diblock had a plasticizing effect on the films which significantly influenced the surface morphologies; internal bicontinuous structures arising from phase separation were also shown within the resulting surface micelles via reactive ion etching of the LB films. (26) Zhu, J.; Eisenberg, A.; Lennox, R. B. J. Am. Chem. Soc. 1991, 113, 5583. (27) Goncalves da Silva, A. M.; Filipe, E. J. M.; d’Oliveira, J. M. R.; Martinho, J. M. G. Langmuir 1996, 12, 6547. (28) Goncalves da Silva, A. M.; Simoes Gamboa, A. L.; Martinho, J. M. G. Langmuir 1998, 14, 5327. (29) Faure, M. C.; Bassereau, P.; Carignano, M.; Szleifer, I.; Gallot, Y.; Andelman, D. Eur. Phys. J. B 1998, 3, 365. (30) Faure, M. C.; Bassereau, P.; Lee, L. T.; Menelle, A.; Lheveder, C. Macromolecules 1999, 32, 8538. (31) Cox, J. K.; Yu, K.; Constantine, B.; Eisenberg, A.; Lennox, R. B. Langmuir 1999, 15, 7714. (32) Cox, J. K.; Yu, K.; Eisenberg, A.; Lennox, R. B. Phys. Chem. Chem. Phys. 1999, 1, 4417. (33) Devereaux, C. A.; Baker, S. M. Macromolecules 2002, 35, 1921. (34) Seo, Y.; Im, J.-H.; Lee, J.-S.; Kim, J.-H. Macromolecules 2001, 34, 4842. (35) Rivillon, S.; Munoz, M. G.; Monroy, F.; Ortega, F.; Rubio, R. G. Macromolecules 2003, 36, 4068. (36) Cheyne, R. B.; Moffitt, M. G. Langmuir 2005, 21, 5453. (37) Cheyne, R. B.; Moffitt, M. G. Langmuir 2006, 22, 8387. (38) Kim, Y.; Pyun, J.; Frechet, J. M. J.; Hawker, C. J.; Frank, C. W. Langmuir 2005, 21, 10444. (39) Lu, Q.; Bazuin, C. G. Nano Lett. 2005, 5, 1309. (40) Chung, B.; Choi, M.; Ree, M.; Jung, J. C.; Zin, W. C.; Chang, T. Macromolecules 2006, 39, 684. (41) Wen, G.; Chung, B.; Chang, T. Polymer 2006, 47, 8575. (42) Lopes, S. I. C.; Goncalves da Silva, A. M. P. S.; Brogueira, P.; Picarra, S.; Martinho, J. M. G. Langmuir 2007, 23, 9310. (43) Matmour, R.; Joncherary, T. J.; Gnanou, Y.; Duran, R. S. Langmuir 2007, 23, 649. (44) Joncherary, T. J.; Denoncourt, K. M.; Meier, M. A. R.; Schubert, U. S.; Duran, R. S. Langmuir 2007, 23, 2423. (45) Cheyne, R. B.; Moffitt, M. G. Langmuir 2005, 21, 10297. (46) Cheyne, R. B.; Moffitt, M. G. Macromolecules 2007, 40, 2046. (47) Kumaki, J.; Hashimoto, T. J. Am. Chem. Soc. 1998, 120, 423. (48) Seo, Y.-S.; Kim, K. S.; Galambos, A.; Lammertink, R. G. H.; Vancso, G. J.; Sokolov, J.; Rafailovich, M. Nano Lett. 2004, 4, 483.
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In this paper, we demonstrate spontaneous generation of structural hierarchy in polymeric LB films by coupling block copolymer microphase separation in the bulk with block copolymer self-assembly at the air-water interface. This is achieved by blending in chloroform an amphiphilic block copolymer, PS-bPEO (Mn = 141K, 11.4 wt % PEO), with a microphase-separating hydrophobic block copolymer, polystyrene-b-polybutadiene (PS-b-PB) (Mn = 31.9K, 28.5 wt % PB), and spreading the resulting solutions at the air-water interface of an LB trough. Upon solvent evaporation, the two copolymer components spontaneously form hierarchically structured aggregates, including “cylinders in strands”, with mesoscale structure directed by the interfacial self-assembly of PS-b-PEO and internal nanoscale features arising from the microphase separation PS-b-PB (Figure 1). The resulting multiscale assemblies are then easily transferred to solid substrates via the LB method. This work highlights the potential for coupling block copolymer self-assembly processes on different length scales at the air-water interface, demonstrating an extremely efficient route to obtain polymeric thin films with unique structural complexity and potentially diverse function.
Experimental Section Materials. The two diblock copolymers used in this study, polystyrene-b-poly(ethylene oxide) (PS-b-PEO) (Mn = 141K, 11.4 wt % PEO, Mw/Mn = 1.04) and polystyrene-b-poly(butadiene) (PS-b-PB) (Mn = 31.9K, 28.5 wt % PB, Mw/Mn = 1.04), were purchased from Polymer Source and used without further purification. Preparation of Polymer Blend Solutions. A series of PS-b-PB/PS-b-PEO blend solutions in chloroform, each containing a total polymer concentration of 2.0 mg/mL, with PS-bPB weight fractions relative to total polymer ( f ) of f = 0, 0.25, 0.50, 0.75, and 1, were prepared in the following manner. All solutions were made in amber-colored glass screw-top vials that had been rinsed three times with methanol and three times with chloroform. Separate stock solutions of PS-b-PB and PS-b-PEO at 5.0 mg/mL were prepared using spectroscopic grade chloroform (99.8% Caledon). The stock solutions were stirred for ∼4 h after solvent addition and then left to equilibrate overnight. After equilibration, the stock solutions were mixed in various ratios to obtain the desired blend solution compositions, followed by dilution of each blend solution with chloroform to 2.0 mg/mL and stirring for 30 min. Stock and blend solutions were given a maximum shelf life of 7 days for LB film preparation, with isotherms being run within 4 days of solution preparation. All stock and blend solutions were kept in the freezer when not in use to prevent PEO degradation. Blend solutions were allowed to equilibrate to room temperature prior to LB or isotherm experiments. Surface Pressure-Area Isotherms of PS-b-PB/PS-b-PEO Langmuir Films. Surface pressure-area (π-A) isotherms and LB films were obtained using a KSV 3000 Langmuir trough (KSV Instruments Ltd.) secured inside a dust shield. The total trough surface area was 150 515 mm2, and the total trough volume was ∼1 L. The trough area was robotically controlled by two hydrophobic paddles, which compressed the spread film symmetrically and bilaterally at a rate of 10 mm/min. House-distilled, deionized water (Barnstead NANOpure Diamond, 18.2 mΩ 3 cm) was used as the subphase in all trials. For all experiments, the subphase temperature was controlled at 25 ( 1 °C. Prior to each trial, the water surface was cleaned by aspirating off any debris such that the surface pressure remained 100 nm) and more irregular in width and height than the polymeric strands shown in Figure 3. The internal structure of the various aggregates via TEM (Figure 5) shows Langmuir 2009, 25(11), 6398–6406
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Figure 4. TEM images of rectangular (a) and tear-shaped (b, c) continent features with internal microphase separation structure in LB film of PS-b-PB/PS-b-PEO blend ( f = 0.75). Higher-magnification image in (c) is taken from the indicated region in (b). Arrows in (c) indicate boundaries between region containing microphaseseparated PS-b-PB and featureless peripheral and interior regions.
clear evidence of PS-b-PB microphase separation. The surface features formed in the f = 1 case are consistent with the hydrophobic nature of the PS-b-PB copolymer, which tends to “pile up” to form tall and irregular aggregates due to unmediated repulsive interactions with the water surface. This points to the important role of amphiphilic PS-b-PEO in regulating the formation of mesoscale morphologies such as strands and Langmuir 2009, 25(11), 6398–6406
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continents from PS-b-PB/PS-b-PEO blends at the air-water interface. We also investigated the effect of blend composition and found that decreasing the PS-b-PB content in the PS-b-PB/PS-b-PEO blends strongly influences the interfacial self-assembly at both critical length scales. Figure 6 shows TEM images of f = 0.50 (a, c) and f = 0.25 (b, d) PS-b-PB/PS-b-PEO blends. From the low-magnification image of the f = 0.50 blend (Figure 6a), it can be seen that the aggregate morphologies at the air-water interface are similar to those found at f = 0.75, a mixture of strands and continents with some dots, although the mean strand width for the lower PS-b-PB content is smaller (∼100 nm, Figure 6c) compared to the f = 0.75 case (∼200 nm). Decreasing the PS-bPB content even further to f = 0.25, the aggregate morphologies change significantly, with very few strands and dots and a predominance of large, interconnected continents perforated with multiple micron-scale holes (Figure 6b). To understand these observed differences, we consider how aggregates of predominantly hydrophobic PS-b-PEO copolymers develop upon spreading chloroform solutions at the air-water interface.37 On the basis of AFM measurements of various kinetically trapped states of the surface aggregates,37 we have shown that mesoscale aggregation at the air-water interface begins with spinodal-like hole formation in spreading and evaporating films, with raised rims of accumulated material around the holes; the rims ultimately merge as the holes grow, forming networks of strands which then break into strandlike aggregates.37 Aggregates become trapped at various stages of this process by an interplay of chain entanglements and solvent evaporation, such that the observed morphologies depend strongly on the spreading concentration.33,36,37 On the basis of these earlier results, the perforated continent morphology formed from the f = 0.25 blend appears to be trapped at an earlier stage of the aggregation process, compared to the continents and strands formed from the f = 0.50 and f = 0.75 blends. This trend may be explained by the effect of variable PS-b-PB content on chain mobility for a constant weight concentration of blend solution. A greater content of longer PS blocks from PS-b-PEO (1200 repeat units), relative to shorter PS blocks from PS-b-PB (220 repeat units), may result in increased chain entanglements and decreased lateral chain mobility in the f = 0.25 blend during the stage of solvent evaporation while the hydrophobic blocks are still swollen with chloroform. We note that the same trend is not continued for the pure PS-b-PEO ( f = 0) case, which shows mainly strands rather than continent-like features, as discussed previously (Figure 2a). It therefore appears that chain mobility decreases with decreasing PS-b-PB content in the blends but then increases when no PS-bPB is dissolved in the hydrophobic layer. This suggests that chain entanglement effects are quite different in the pure PS-b-PEO film compared to the blends. Along with influencing the mesoscale aggregate morphologies, the blend composition was also found to be an important factor in the extent of nanoscale organization of PS and PB blocks within the aggregates. Parts c and d of Figure 6 show high-magnification TEM images of continent and strand features in the f = 0.50 blend and continent features in the f = 0.25 blend, respectively; the samples have been stained with OsO4 vapor, indicating the distribution of darker PB blocks. From the location of dark regions, therefore, we find that PS-b-PB copolymer is localized within the strands (Figure 6c) and along the edges of continents (Figure 6c,d), similar to their distribution within aggregates of the f = 0.75 blend. However, unlike the f = 0.75 case, no distinct microphase separation structure is observed in either of the blends with lower PS-b-PB content, as revealed by the absence of DOI: 10.1021/la804317s
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Figure 5. TEM images of circular droplets of pure PS-b-PB ( f = 1) with internal microphase separation structure in LB film following selfassembly at the air-water interface.
fingerprint patterns in high-magnification TEM of the PS-b-PBcontaining regions (Figure 6c,d, insets). This suggests that microphase separation between PS and PB blocks is impeded above a certain weight fraction of PS-b-PEO, which points to mixing of PS-b-PB and PS-b-PEO during self-assembly. The effect of blend composition on the observed microphase separation therefore provides some evidence that the PS-b-PB chains are intimately mixed with the PS blocks of PS-b-PEO within the surface aggregates, rather than sitting on top of a monolayer PS-b-PEO. The interfacial behavior of self-assembled PS-b-PB/PS-b-PEO blends was investigated via π-A compression isotherms of the surface aggregates following evaporation of the spreading solvent. Several studies on compression isotherms of PS-b-PEO monolayers have been reported.27-33,36,37 Based on AFM measurements of transferred LB films, recent interpretation of these compression isotherms have accounted for the existence of aggregates at the air-water interface, consisting of kinetically frozen PS cores surrounded by dynamic PEO coronae.31-33,36,37 We take the view of Eisenberg and co-workers31,32 that isotherms of PS-b-PEO films (and PS-b-PEO/PS-b-PB blend films in the present case) must be interpreted in terms of lateral rearrangement and packing of aggregates, mediated by dynamic PEO blocks, rather than reorganization of individual copolymer chains within the aggregates. Prior to compression, PEO blocks will radiate from the periphery of well-separated PS (or PS/PB) cores, forming 2D coronae at the water surface (the so-called “pancake” conformation);31,33,36,37 it has been proposed that some PEO blocks are also located underneath the aggregates, forming a buffer layer between the hydropobic cores and the 6404
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water surface.32,37 Lateral compression of the separated aggregates decreases the distances between the cores, resulting in eventual overlap of neighboring PEO coronae and an initial gradual increase in the surface pressure. Above surface pressures of ∼10 mN m-1, the strongly overlapping PEO chains submerge into the subphase (“pancake-to-brush” transition), allowing the cores to come into direct contact, as supported by AFM of PS-bPEO aggregates transferred at surface pressures g10 mN/m.37 Further compression of the relatively incompressible cores results in a steep increase in π at high surface densities. Compression isotherms were obtained for the pure PS-b-PEO ( f = 0) and PS-b-PB ( f = 1) components, along with blends of various compositions ( f = 0.25, 0.50, 0.75), with the total mass of deposited polymer (0.10 mg) and the spreading concentration (2.0 mg/mL) being held constant for all experiments. In Figure 7, we only show isotherms for the f = 0, 0.75, and 1 cases for simplicity, although determined limiting areas are reported for all compositions (inset). Since mean molecular areas cannot be easily defined for blends of two different copolymers, we report isotherms as plots of π vs the total trough area, which allow direct comparison of different blend compositions considering that the total film mass is constant in all cases. For the pure PS-b-PEO copolymer ( f = 0), the compression isotherm shows a gradual increase in π as the trough area is decreased, consistent with the initial overlap of coronae consisting of flexible PEO chains, followed by a sharp rise in π due to direct contact between the rigid PS cores. In contrast, the isotherm for the pure PS-b-PB copolymer ( f = 1) shows no “soft” increase in π upon compression, with only a single sharp rise in surface pressure at low trough area attributed to unmediated Langmuir 2009, 25(11), 6398–6406
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Figure 6. TEM images of surface aggregates in LB film of PS-b-PB/PS-b-PEO blends f = 0.50 (a, c) and f = 0.25 (b, d). Darker regions in strands (c) and at the edges of continents (c, d) indicate the location of PS-b-PB within the aggregates. Insets to (c) and (d) reveal the absence of internal microphase separation structure at these blend compositions.
Figure 7. Langmuir compression isotherms of pure PS-b-PEO ( f = 0), pure PS-b-PB ( f = 1), and a representative PS-b-PB/PSb-PEO blend ( f = 0.75). Determination of the limiting area (A0) is shown for the f = 0.75 case. Inset shows plot of A0 vs f determined from isotherms obtained for the f = 0, 0.25, 0.50, 0.75, and 1 cases.
contact between the purely hydrophobic PS/PB aggregates. Not unexpectedly, isotherms of the blend films (e.g., f = 0.75) show intermediate behavior, increasingly resembling pure PS-b-PB as the hydrophobic polymer component is increased. The limiting area, A0,33,36,37 represents the average trough area occupied by a constant mass of copolymer when surface aggregates are in a condensed state (i.e., π > 10 mN/m). Unlike the large distances between aggregates observed at lower transfer pressures (e.g., Figure 2), condensed aggregates at surface Langmuir 2009, 25(11), 6398–6406
pressures g10 mN/m show close-packed cores with negligible free area,37 such that A0 can be directly related to the areal density of copolymer within the various aggregates. For each blend composition, A0 was determined from extrapolation of the steepest part of the isotherm to π = 0, as shown in Figure 7 for the f = 0.75 case. Comparing the two extreme values, the higher A0 for pure PS-b-PEO ( f = 0) compared to pure PS-b-PB ( f = 1) can be explained by the areal contribution of surface-active PEO chains underneath the aggregates of PS-b-PEO37 compared to the relatively dense aggregates of purely hydrophobic PS-b-PB. We note that although the general trend in A0 is decreasing with increasing PS-b-PB content, the plot of A0 vs f (Figure 7, inset) is clearly nonlinear, suggesting that the presence of one component influences the surface conformation and areal contribution of the other. This supports the conclusion from TEM and AFM data that the two components combine during interfacial self-assembly to form hybrid, hierarchically structured aggregates. An interesting feature of the plot of A0 vs f is an increase in A0 with the first addition of PS-b-PB, resulting in a maximum at f = 0.25 followed by a decrease in A0 as the PS-b-PB content is further increased (Figure 7, inset). This can be explained by an initial diluting effect of the PS-b-PB copolymer among PS chains of the PS-b-PEO monolayer, which decreases the packing density and increases the mean molecular area relative to pure PS-b-PEO. We have previously reported an identical effect in blends of PS-b-PEO copolymers with various amounts of added PS-coated CdS QDs.46 It appears reasonable to suggest that synergistic self-assembly of PS-b-PEO and PS-b-PB components at the air-water interface DOI: 10.1021/la804317s
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although recent theoretical work reproduces experimental surface features such as strands using entanglement-vitrification forces between polymer chains.52 In regions of the film where continent structures become kinetically trapped, PS-b-PB is found to be localized near the continent edges (Figures 4 and 6); from the typical undulating structure of continent edges at the bottom of Figure 6d, these edges appear to form via the merging of rims around multiple holes. From this, we conclude that PS-b-PB chains accumulate within the rims during hole growth (Figure 8b). Finally, in the late stages of chloroform evaporation, the increasing repulsion between PS and PB blocks appears to trigger phase separation at the nanoscale within the mesoscale aggregates, provided the PS-b-PB content is sufficiently high (Figure 8d).
Figure 8. Schematic showing side view (not to scale) of formation of surface aggregates with internal microphase separation structure. As discussed in the text, solvent evaporation induces mesoscale formation of PS-b-PB/PS-b-PEO surface aggregates (a-c) followed by nanoscale microphase separation of PS-b-PB chains within the aggregates (d). Arrows in (b) indicate lateral movement of rims arising from hole growth in unstable films.
involves two separate phase-separation processes on different length scales, both driven by solvent evaporation and ultimately frozen by vitrification. Figure 8 shows a side-view schematic of the proposed process, where the larger scale features represent cross sections of continent (Figure 8b) and strandlike (Figure 8c,d) aggregates. Initial spreading of blend solutions at the air-water interface should result in a film with bilamellate structure, with PEO blocks of PS-b-PEO adsorbed at the water surface and the connected PS blocks dissolved in the solution layer; in addition, the solution layer will likely contain solubilized free chains of PSb-PB, along with some PS-b-PEO in equilibrium with adsorbed chains (Figure 8a). As described in Figure 8b,c, we propose that continued chloroform evaporation eventually results in destabilization of the polymer solution layer at the water surface, with the development of mesoscale continents (Figure 8b) and then isolated strands (Figure 8c) via the formation and growth of holes; experimental evidence for this mechanism is provided by previous AFM data for kinetically trapped surface aggregates of the same PS-b-PEO copolymer employed in this study (i.e., without the PS-b-PB component).37 Film rupture and surface feature formation may be attributed to spinodal dewetting of the evaporating solution from the water surface due to van der Waals forces,50,51 (50) Reiter, G. Langmuir 1993, 9, 1344. (51) Reiter, G.; Sharma, A.; Casoli, A.; David, M.-O.; Khanna, R.; Auroy, P. Langmuir 1999, 15, 2551.
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Conclusions By combining block copolymer microphase separation in the bulk with block copolymer self-assembly at the air-water interface, we have demonstrated the spontaneous generation of mesoscale polymeric aggregates with internal nanoscale organization. LB films with structural hierarchy are prepared by depositing blends of PS-b-PB and PS-b-PEO copolymers codissolved in chloroform at the air-water interface. For a blend composition in which PS-b-PB is the major component ( f = 0.75), solvent evaporation gives rise to mesoscale strands and continents with internal nanoscale cylinders consisting of PB blocks, as determined by LB transfer of the aggregates to solid substrates followed by AFM and TEM analysis. Blends with lower PS-b-PB contents ( f = 0.50 and 0.25) were found to form various mesoscale aggregates, although without internal nanoscale organization, suggesting mixing of PS-b-PB chains with PS blocks of PS-b-PEO within the aggregates. Isothermal compression experiments at the air-water interface also point to mixing of copolymer components during self-assembly at the air-water interface, with the amount of one component influencing the conformation and areal contribution of the other. It is proposed that the observed formation of hierarchical surface features is a unique example of two sequential block copolymer phase-separation processes on different length scales driven by a single solvent evaporation step. Considering the vast structural diversity previously demonstrated for block copolymer self-assembly both at the air-water interface26,31-33,36-44 and in bulk films,1 the simple and efficient combination of these processes should offer an immense range of multiscale polymeric materials with tunable function. Acknowledgment. The authors gratefully acknowledge the Natural Science and Engineering Research Council (NSERC, Canada), the Canadian Foundation for Innovation (CFI), and the British Columbia Knowledge Development Fund (BCKDF) for their generous support of the research. E.P. also appreciates the financial support of an NSERC USRA summer research scholarship. (52) Hosoi, A. E.; Kogan, D.; Devereaux, C. E.; Bernoff, A. J.; Baker, S. M. Phys. Rev. Lett. 2005, 95, 037801.
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