Temperature Tunable Micellization of Polystyrene-block-poly (2

Oct 25, 2010 - Highly ordered and stable micelles formed from both symmetric and asymmetric block copolymers of polystyrene- block-poly(2-vinylpyridin...
0 downloads 0 Views 1MB Size
Download PDF
pubs.acs.org/Langmuir © 2010 American Chemical Society

Temperature Tunable Micellization of Polystyrene-block-poly(2-vinylpyridine) at Si-Ionic Liquid Interface Haiyun Lu, Dong Hyun Lee, and Thomas P. Russell* Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003, United States Received July 20, 2010. Revised Manuscript Received September 30, 2010 Highly ordered and stable micelles formed from both symmetric and asymmetric block copolymers of polystyreneblock-poly(2-vinylpyridine) (PS-b-P2VP) at the Si-ionic liquid (IL) interface have been investigated by scanning force microscopy (SFM) and transmission electron microscopy (TEM). The 1-butyl-3-methylimidazolium trifluoromethanesulfonate IL, a selective and temperature-tunable solvent for the P2VP block, was used and gave rise to block copolymer micelles having different morphologies that strongly depended on the annealing temperature. The effects of film thickness, molecular weight of block copolymers, and experimental conditions, such as preannealing, rinsing, and substrate properties, on the morphologies of block copolymer micelles were also studied. In addition, spherical micelles consisting of PS core and P2VP shell could also be obtained by core-corona inversion by annealing the as-coated micellar film in the IL at high temperatures. The possible mechanism for micelle formation is discussed.

Introduction Block copolymers have received considerable attention due to their ability to self-assemble into a variety of nanostructures depending on the volume fraction of components, chain architecture, and experimental conditions, such as solvent type and substrate properties.1-9 Block copolymer micelles are of particular interest due to their well-defined phase-separated core-corona morphology10,11 which offers good opportunities for a diverse range of applications, like delivery vehicles for drugs,12 nanocontainers and nanoreactors in material science,13-15 pharmaceutical formulations, and other dispersant technologies.16 In a selective solvent, the soluble block forms a swollen corona that extends into the solvent so as to shield the insoluble block that forms a highly condensed core.1,17 These micelles can be spherical, cylindrical, or worm-like, depending on the block ratios, the interfacial energy between the blocks, and the solvent quality. The direct adsorption of block copolymer micelles from organic solvents onto solid substrates, due to the kinetic stability of the micellar aggregates, affords a general way of generating *To whom correspondence should be addressed. E-mail: Russell@ mail.pse.umass.edu. Telephone: þ1 (413) 577-1516. Fax: þ1 (413) 577-1510.

(1) Nagarajan, R.; Ganesh, K. J. Chem. Phys. 1989, 90(10), 5843–5856. (2) Halperin, A.; Tirrell, M.; Lodge, T. P. Adv. Polym. Sci. 1992, 100, 31–71. (3) Zhan, Y. J.; Mattice, W. L. Macromolecules 1994, 27(3), 677–682. (4) Krausch, G. Mater. Sci. Eng., R 1995, 14(1-2), 1–94. (5) Zhang, L. F.; Eisenberg, A. Science 1995, 268(5218), 1728–1731. (6) Zhang, L. F.; Yu, K.; Eisenberg, A. Science 1996, 272(5269), 1777–1779. (7) Segalman, R. A. Mater. Sci. Eng., R 2005, 48(6), 191–226. (8) Kim, H. C.; Hinsberg, W. D. J. Vac. Sci. Technol., A 2008, 26(6), 1369–1382. (9) Krishnamoorthy, S.; Hinderling, C.; Heinzelmann, H. Mater. Today 2006, 9(9), 40–47. (10) Webber, S. E. J. Phys. Chem. B 1998, 102(15), 2618–2626. (11) Riess, G. Prog. Polym. Sci. 2003, 28(7), 1107–1170. (12) Savic, R.; Eisenberg, A.; Maysinger, D. J. Drug Targeting 2006, 14(6), 343–355. (13) Forster, S.; Antonietti, M. Adv. Mater. 1998, 10(3), 195–217. (14) Glass, R.; Moller, M.; Spatz, J. P. Nanotechnology 2003, 14(10), 1153–1160. (15) O’Reilly, R. K. Philos. Trans. R. Soc. London, Ser. A 2007, 365(1861), 2863– 2878. (16) Alexandridis, P.; Lindman, B. Amphiphilic Block Copolymers: Self-Assembly and Applications; Elsevier: Amsterdam, 2000. (17) Moffitt, M.; Khougaz, K.; Eisenberg, A. Acc. Chem. Res. 1996, 29(2), 95–102.

17126 DOI: 10.1021/la102890s

surface nanostructures.18 Meiners et al. first showed that the adsorption of polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) micelles onto mica from toluene solutions resulted in the spontaneous formation of spherical micelles with a high degree of longrange lateral order.19 In situ scanning force microscopy (SFM) images showed that the micellar morphology remained more or less intact after deposition from solution onto the substrate.20 By changing the copolymer concentration, spin-coating speeds, and solvents, the periodicity of the block copolymer micellar arrays could be finely tuned.21 The resultant film morphology, a hexagonally ordered array of micelles, is not at thermodynamical equilibrium. Consequently, with a symmetric diblock copolymer thermal annealing leads to a transformation of the ordered micelles to alternating lamellae oriented parallel to the substrate, due to the preferential interaction of one block with the substrate and the lower surface energy of one of the blocks.22 Surface-induced laterally ordered microdomain structures can be found in ultrathin films when there is a strong preferential interaction of one of the blocks with the surface, forming a tightly bound monomolecular layer on the surface.23,24 This type of surface micellar pattern was first observed experimentally in thin films of PS-b-P2VP copolymers from a dilute nonselective solvent on a mica substrate where the substrate was covered by a thin layer of P2VP blocks and micelles with a PS core were arranged in nearly a hexagonal order.23,24 Potemkin and co-workers proposed a simple model to describe the physical factors leading to the (18) Krystyna, A.; Mourran, A.; Moeller, M. Surface micelles and surfaceinduced nanopatterns formed by block copolymers. In Ordered Polymeric Nanostructures at Surfaces; Springer-Verlag: Berlin, 2006; Vol. 200, pp 57-70. (19) Meiners, J. C.; Ritzi, A.; Rafailovich, M. H.; Sokolov, J.; Mlynek, J.; Krausch, G. Appl. Phys. A: Mater. Sci. Process. 1995, 61(5), 519–524. (20) Hamley, I. W.; Connell, S. D.; Collins, S. Macromolecules 2004, 37(14), 5337–5351. (21) Krishnamoorthy, S.; Pugin, R.; Brugger, J.; Heinzelmann, H.; Hinderling, C. Adv. Funct. Mater. 2006, 16(11), 1469–1475. (22) 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(44), 10892– 10893. (23) Spatz, J. P.; Roescher, A.; Sheiko, S.; Krausch, G.; Moller, M. Adv. Mater. 1995, 7(8), 731–735. (24) Spatz, J. P.; Sheiko, S.; Moller, M. Adv. Mater. 1996, 8(6), 513–517.

Published on Web 10/25/2010

Langmuir 2010, 26(22), 17126–17132

Lu et al.

surface micellar patterns,25 and in a later study they calculated the free energy of the worm-like surface aggregates and globular micelles and discussed the conditions of their stability and the observed transitions.26 Amphiphilic block copolymers can also self-assemble into various surface micellar patterns when spread at the air-water interface. Since the first report on the surface micellar morphology was presented by Eisenberg and co-workers,27 the twodimensional self-assembly of amphiphilic block copolymers at the air-water interface has been widely studied.28-37 Various morphologies of surface micelles, including dots, strands, network structures, planar continents, rings, and chains, were described where the morphology observed depended on the relative size of the two blocks, the concentration of the spreading solution, and surface pressure.38-42 In this work, we investigated the micellization of PS-b-P2VP block copolymers at the Si substrate-ionic liquid (IL) interface. Owing to their extremely low vapor pressure and excellent chemical and thermal stabilities, ILs have received increasing attention recently and have been used as a “green solvent” industrially.43-45 The self-assembly of amphiphilic diblock copolymer micelles in 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM PF6) ionic liquid was investigated by Lodge and co-workers, who used cryogenic transmission electron microscopy to directly visualize the micellar structures (spherical micelle, worm-like micelle, and bilayered vesicle) formed in the IL.46,47 They also reported an interesting phenomenon termed the “micelle shuttle”, where the block copolymer micelles transferred reversibly and with preservation of micelle structure from an aqueous phase at room temperature to a hydrophobic IL at high (25) Kramarenko, E. Y.; Potemkin, I. I.; Khokhlov, A. R.; Winkler, R. G.; Reineker, P. Macromolecules 1999, 32(10), 3495–3501. (26) Potemkin, I. I.; Kramarenko, E. Y.; Khokhlov, A. R.; Winkler, R. G.; Reineker, P.; Eibeck, P.; Spatz, J. P.; Moller, M. Langmuir 1999, 15(21), 7290– 7298. (27) Zhu, J. Y.; Eisenberg, A.; Lennox, R. B. J. Am. Chem. Soc. 1991, 113(15), 5583–5588. (28) Li, S.; Hanley, S.; Khan, I.; Varshney, S. K.; Eisenberg, A.; Lennox, R. B. Langmuir 1993, 9(8), 2243–2246. (29) Cox, J. K.; Yu, K.; Constantine, B.; Eisenberg, A.; Lennox, R. B. Langmuir 1999, 15(22), 7714–7718. (30) daSilva, A. M. G.; Filipe, E. J. M.; dOliveira, J. M. R.; Martinho, J. M. G. Langmuir 1996, 12(26), 6547–6553. (31) da Silva, A. M. G.; Gamboa, A. L. S.; Martinho, J. M. G. Langmuir 1998, 14(18), 5327–5330. (32) Faure, M. C.; Bassereau, P.; Lee, L. T.; Menelle, A.; Lheveder, C. Macromolecules 1999, 32(25), 8538–8550. (33) Faure, M. C.; Bassereau, P.; Carignano, M. A.; Szleifer, I.; Gallot, Y.; Andelman, D. Eur. Phys. J. B 1998, 3(3), 365–375. (34) Hosoi, A. E.; Kogan, D.; Devereaux, C. E.; Bernoff, A. J.; Baker, S. M. Phys. Rev. Lett. 2005, 95(3), 037801. (35) Choi, M.; Chung, B.; Chun, B.; Chang, T. Macromol. Res. 2004, 12(1), 127–133. (36) Chung, B.; Choi, M.; Ree, M.; Jung, J. C.; Zin, W. C.; Chang, T. H. Macromolecules 2006, 39(2), 684–689. (37) Joncheray, T. J.; Denoncourt, K. M.; Meier, M. A. R.; Schubert, U. S.; Duran, R. S. Langmuir 2007, 23(5), 2423–2429. (38) Seo, Y.; Im, J. H.; Lee, J. S.; Kim, J. H. Macromolecules 2001, 34(14), 4842– 4851. (39) Seo, Y. S.; Kim, K. S.; Galambos, A.; Lammertink, R. G. H.; Vancso, G. J.; Sokolov, J.; Rafailovich, M. Nano Lett. 2004, 4(3), 483–486. (40) Cheyne, R. B.; Moffitt, M. G. Langmuir 2005, 21(12), 5453–5460. (41) Cheyne, R. B.; Moffitt, M. G. Langmuir 2006, 22(20), 8387–8396. (42) Price, E. W.; Guo, Y. Y.; Wang, C. W.; Moffitt, M. G. Langmuir 2009, 25 (11), 6398–6406. (43) Earle, M. J.; Seddon, K. R. Pure Appl. Chem. 2000, 72(7), 1391–1398. (44) Rogers, R. D.; Seddon, K. R. Science 2003, 302(5646), 792–793. (45) Lu, J. M.; Yan, F.; Texter, J. Prog. Polym. Sci. 2009, 34(5), 431–448. (46) He, Y. Y.; Li, Z. B.; Simone, P.; Lodge, T. P. J. Am. Chem. Soc. 2006, 128 (8), 2745–2750. (47) Simone, P. M.; Lodge, T. P. Macromol. Chem. Phys. 2007, 208(4), 339–348. (48) He, Y. Y.; Lodge, T. P. J. Am. Chem. Soc. 2006, 128(39), 12666–12667. (49) Bai, Z. F.; He, Y. Y.; Young, N. P.; Lodge, T. P. Macromolecules 2008, 41(18), 6615–6617. (50) Bai, Z. F.; He, Y. Y.; Lodge, T. P. Langmuir 2008, 24(10), 5284–5290.

Langmuir 2010, 26(22), 17126–17132

Article Table 1. Molecular Characteristics of PS-b-P2VP Diblock Copolymers sample code

PS Mn (kg/mol)

P2VP Mn (kg/mol)

fPS

Mw/Mn

S2VP200K S2VP77K S2VP80K

102 56 50.9

97 21 29.1

0.51 0.73 0.64

1.12 1.06 1.06

temperature.48-50 This thermally controlled micelle shuttling could be repeated many times between the two “green” media: water and the ionic liquid, triggered simply by temperature change.51 Moreover, the micellization-unimer-inverse micellization self-assembly of doubly thermosensitive block copolymers in ILs was observed during heating and cooling cycles.52 ILs, due to immiscibility with many organic liquids, high vapor pressure, and polarity, are a very attractive medium to study the selfassembly of various diblock and triblock copolymers.53-55 By annealing PS-b-P2VP thin films on a silicon wafer in 1-butyl3-methylimidazolium trifluoromethanesulfonate ([BMIM][CF3SO3]) ionic liquid at different temperatures, we have observed micelles with various shapes on the substrate by using scanning force microscopy (SFM) and transmission electron microscopy (TEM). [BMIM][CF3SO3], an air- and water-stable hydrophilic room temperature ionic liquid, was chosen here because it is a selective and temperature-tunable solvent for the P2VP block but a nonsolvent for the PS block.56 The effects of film thickness, molecular weight of polymers, and experimental conditions, such as preannealing, rinsing, substrate properties, and interaction time, on the micellar structures have also been studied.

Experimental Section Materials. Diblock copolymers of polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) having three different molecular weights were used as received from Polymer Source Inc. The molecular weights, fractions of PS, and polydispersity are listed in Table 1. S2VP200K was used in most experiments, unless otherwise particularly mentioned. PS-b-P2VP copolymers were dissolved in 1,1,2-trichloroethane (TCE, Aldrich) at room temperature for 12 h to yield polymer solutions with different concentrations ranging from 0.2 to 1.2 wt %. The polymer solutions were then filtered through Millipore 0.45 μm PTFE filters. Preparation of Block Copolymer Micelles. PS-b-P2VP films were spin-coated from TCE solutions at 2000-3000 rpm onto Si Æ100æ wafers (International Wafer Service Inc.), which were precleaned by acetone and oxygen plasma. The as-spun or preannealed (heating to 180 C for 1 h in a high vacuum oven with pressure e10-6 bar) samples were then annealed at different temperatures in 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIM][CF3SO3], Aldrich). So far, we do not have any information on the boiling point of this ionic liquid, but we found it was quite stable even when heated to temperatures as high as 230 C, which is far above our experimental temperatures. After quenching with cold deionized water and continuous rinsing for at least 60 s, the samples were then dried under a stream of nitrogen. Characterization of Micellar Structures. The surface topographies of PS-b-P2VP thin films on the silicon wafers were (51) Bai, Z. F.; Lodge, T. P. J. Phys. Chem. B 2009, 113(43), 14151–14157. (52) Ueki, T.; Watanabe, M.; Lodge, T. P. Macromolecules 2009, 42(4), 1315– 1320. (53) Zhang, S. H.; Li, N.; Zheng, L. Q.; Li, X. W.; Gao, Y. A.; Yu, L. J. Phys. Chem. B 2008, 112(33), 10228–10233. (54) Tamura, S.; Ueki, T.; Ueno, K.; Kodama, K.; Watanabe, M. Macromolecules 2009, 42(16), 6239–6244. (55) Virgili, J. M.; Hexemer, A.; Pople, J. A.; Balsara, N. P.; Segalman, R. A. Macromolecules 2009, 42(13), 4604–4613. (56) Lu, H.; Chen, W.; Russell, T. P. Macromolecules 2009, 42(22), 9111– 9117.

DOI: 10.1021/la102890s

17127

Article

Lu et al.

Figure 1. SFM 3-D height images of PS-b-P2VP micelles formed from (a) as-spun and (b) preannealed 16 nm thick S2VP200K films annealed in the IL at 150 C for 1 h. measured with a scanning force microscope (Digital Instruments Inc., Nanoscope III) in the tapping mode. The film thickness was measured by optical reflectometry and X-ray reflectivity. Transmission electron microscopy (TEM) samples were prepared on silicon nitride membranes (DuraSiN from Electron Microscopy Sciences) using the same procedure as described above. Brightfield TEM was performed on a JEOL-1200EX TEM operated at an accelerating voltage of 100 kV. The contact angle measurements were performed by the static sessile drop method on an OCA20 interfacial tension measuring device (Future Digital Scientific Co.).

Results and Discussion Figure 1a shows scanning force microscopy (SFM) images of the micellar structure formed from an as-spun PS-b-P2VP film annealed in the IL at 150 C for 1 h followed by quenching with cold water. Scattered “pancake”-like micelles (∼160 nm in diameter and ∼100 nm in height) were seen on the substrate. The featureless areas outside of the micelles were found to be the bare Si substrate surface by comparison to regions scratched with a razor blade. Because of the strong insolubility of PS and increased solubility of P2VP in the ionic liquid at elevated temperatures, the P2VP blocks formed a corona around a core of PS to minimize energetically nonfavorable interactions with both the solvent and the substrate. After removal from the hot IL bath, the film was immediately rinsed with cold water, which is termed “quenching”. In this case, stretched P2VP chains contracted and the corona became more compact, and the structure was frozen after drying with nitrogen gas. Since [BMIM][CF3SO3] IL is miscible with water, continuous water rinsing (no less than 60 s) could remove the IL in the film, which was confirmed by X-ray photoelectron spectroscopy. If we slowly cooled the whole system to room temperature before rinsing with water, which is termed “slow cooling”, the micelles obtained were slightly collapsed and attached to each other. The micellar structures on the substrate formed by quenching and slow cooling were compared in Figure S1 of the Supporting Information. Consequently, quenching was used in the present work, since this more accurately portrays the morphology of the micelles deposited on the surface from solution. In addition, the surface topographies should not be influenced by the SFM tip pressure since we applied 17128 DOI: 10.1021/la102890s

a normal experimental force on the sample surface without any changes in the images. The effect of water or IL in the samples on the morphology could also be neglected. The preannealed films gave rise to more compact micelles with a much higher degree of lateral ordering (Figure 1b). Upon thermal annealing, the symmetric PS-b-P2VP film microphaseseparated into well-aligned lamellae parallel to the Si substrate, with P2VP preferentially segregating to the substrate and the PS domains to the air interface.57 When this lamellar film was immersed in the IL and heated to 150 C, micellization also occurred at the Si-IL interface as demonstrated by SFM and transmission electron microscopy (TEM) (see Supporting Information Figure S2). The micelles formed from the as-spun film and preannealed films are essentially the same. Experimentally, for the as-spun sample, the film was immersed into the ionic liquid right after spin-coating, where the P2VP and PS blocks are in a disordered, kinetically trapped state. For the preannealed sample, the film was annealed under high vacuum at temperatures far above the glass transition temperatures of both blocks, where the P2VP domains orient parallel to the substrate-polymer interface. The geometry or orientation of the polymer chains is different in both cases. After annealing in the ionic liquid at 150 C for 1 h, the micelles formed on the substrate are quite similar, except that the preannealed film gave rise to more compact micelles. Figure 2 shows the temperature evolution of the surface morphologies of the spin-coated PS-b-P2VP films annealed in the ionic liquid. The as-spun film showed a cylindrical morphology having ∼28 nm diameter cylinders with a ∼76 nm center-to-center distance between adjacent cylinders (Figure 2a). TCE, a good solvent for both blocks,58 was used to spin-coat the samples, and the morphology observed here is in agreement with that observed by Jiang and co-workers, who found a cylindrical morphology for PS-b-P4VP spin-coated from chloroform, a nonselective solvent for both blocks.59 The absence of a well-defined morphology after spin-coating from a homogeneous solution of the block copolymer is not surprising, since the system is kinetically trapped far (57) Fasolka, M. J.; Mayes, A. M. Annu. Rev. Mater. Res. 2001, 31, 323–355. (58) Moller, M.; Lenz, R. W. Makromol. Chem. 1989, 190(5), 1153–1168. (59) Zhao, J. C.; Tian, S. Z.; Wang, Q.; Liu, X. B.; Jiang, S. C.; Ji, X. L.; An, L. J.; Jiang, B. Z. Eur. Phys. J. E 2005, 16(1), 49–56.

Langmuir 2010, 26(22), 17126–17132

Lu et al.

Article

Figure 2. SFM images (height mode) of a 16 nm thick PS-b-P2VP (S2VP200K) film (a) as-spun and after annealing in the IL at different temperatures for 1 h: (b) 70, (c) 90, (d) 110, (e) 130, and (f) 150 C.

from equilibrium. While chloroform is also a good solvent for PSb-P2VP, here 1,1,2-trichloroethane is used as the casting solvent, instead of chloroform, since the boiling point of the former (110;115 C) is higher which could give rise to smoother films in comparison to chloroform. Annealing this film in the IL at room temperature and 50 C did not affect an obvious change in the morphologies (data not shown), which, more than likely, is due to the poor solubility of P2VP in the IL at low temperatures and the fact that the experiments were done well below the glass transition temperature (Tg) of the components. As the annealing temperature was increased to 70 C, the diameter and spacing of the cylinders increased to ∼38 and 113 nm, respectively (Figure 2b). A further increase in the annealing temperature to 90 C led to a further increase in the center-to-center distance between adjacent cylinders (Figure 2c). When annealed in the IL at 110 C, slightly higher than the Tg of the both blocks, a network of interconnected and loosely packed strands or cylinders, with diameters of ∼125 nm, is seen (Figure 2d). This annealing temperature allows for a limited rearrangement of the block copolymer chains over the time scale of the annealing. TEM images, using the same condition as that used to obtain the results in Figure 2d, showed that some cylinders broke up into smaller cylindrical domains due to Rayleigh instabilities,60 driven by the reduction in surface area or total surface energy (see Supporting Information Figure S3). This effect was more obviously seen at 130 C where the surface morphology was dominated by strings of “pearls” (spherical micelles) coexisting with a few “ribbons” (cylindrical micelles) (Figure 2e). In comparison to cylinders, a row of spheres have a smaller surface area and thus smaller free energy, which is the dominant driving force to bring about the transition from cylindrical to spherical micelles. When the sample was annealed at higher temperatures, e.g. 150 C, the most stable morphology, spherical micelles, was fully developed (Figure 2f). The same experiments were done on the preannealed films, and the results are shown in Supporting Information Figure S4. No significant change in the film morphology was observed until after increasing the annealing temperature to 70 C, in which condition (60) Rayleigh, L. Philos. Mag. 1882, 5 (14), 184-186.

Langmuir 2010, 26(22), 17126–17132

Scheme 1. Schematic Illustration of Different Micellar Structures Formed at the Si-IL Interface

the P2VP domains were swollen by the IL, causing an increase in the volume fraction of the P2VP block, which led to the formation of cylindrical micelles. A further increase in the annealing temperature to 90 C led to a widening of the cylinders and an increase in the spacing. As the temperature was increased above the bulk Tg, like 110 and 130 C, random particles with irregular shapes were formed. After a further increase in annealing temperature to 150 C, the “particles” aggregated into larger spherical micelles, at least 3 times the size of the diameter on average. The micellization of PS-b-P2VP films in the IL was influenced significantly by the annealing temperature due to the increase in the solubility of P2VP block and the increase in the chain mobility of both blocks. The solubility of P2VP homopolymer in the IL was measured at different temperatures. At room temperature, P2VP was barely soluble in the IL over a 24 h period, while at 70 C and higher temperatures the P2VP solubility increased significantly (see Supporting Information Figure S5). Micellization can only occur at and above temperatures where the P2VP DOI: 10.1021/la102890s

17129

Article

Lu et al.

Figure 3. (a) Static contact angle of a water droplet on the surface of 16 nm thick S2VP200K films before and after annealing in the IL at different temperatures. Right: optical images of static water contact angles on the surface of the preannealed S2VP200K films (b) before and (c) after annealing in the IL at 150 C for 1 h.

can be dissolved in the IL. Elevated temperatures also enhance the mobility of the insoluble PS block, enabling a reorganization of the block copolymer. Scheme 1 summarizes possible micellization steps involved in the self-assembly of PS-b-P2VP copolymers at the Si-IL interface. The as-spun film undergoes microphase separation upon thermal annealing to form lamellar microdomains, as shown in Scheme 1b. When this lamellar film is brought into contact with the IL at elevated temperatures, the soluble P2VP blocks are swollen by the IL, and the nonsoluble PS chains aggregate to minimize contact with the IL and form the cylindrical micelles (Scheme 1c). At higher temperatures, as shown in Scheme 1d,e, the cylinders break up into rows of spheres to minimize the total free energy. The driving force for this transition from cylindrical micelles to spherical micelles is that the free energy of the whole system tends to be minimized. In this particular case, Rayleigh instabilities dominate, even though other factors cannot be eliminated completely. Contact angle measurements of a water droplet on the film surface were used to evaluate the hydrophilicity or hydrophobicity of the film (Figure 3). The contact angle on the preannealed film surface was 95 ( 1, indicating that the film surface is hydrophobic (Figure 3b). This is understandable since the more hydrophobic PS segregates to the free surface after microphase separation. After annealing in the ionic liquid, the film surface became increasingly more hydrophilic with increasing annealing temperature, as evidenced by the decrease of the contact angle as a function of annealing temperature. Annealing at 150 C yields a surface that is hydrophilic with a contact angle of 47 (Figure 3c). The reduction in the contact angle can arise from one of two different origins: either the formation of micelles with a PS core and a more hydrophilic P2VP corona or the exposure of the bare silicon substrate, which is hydrophilic because of the native silicon oxide layer. The changes in the elemental compositions at the surface were also studied by X-ray photoelectron spectroscopy analysis before and after micellization. The results showed that only PS was on the surface for the preannealed film, while after micellization in the IL, the surface was dominant by P2VP and bare Si substrate (see Supporting Information Figure S6). The binding energy of the pyridine nitrogen in the micellar film (401.5 eV) was higher than that of free N atom of pyridine group (398 eV),61 which may be due to the quaternization of pyridine nitrogen in the IL at 150 C. As-spun films gave rise to a similar trend in the temperature dependence of the contact angle but did not change much above 90 C. The dependence of the contact angle on (61) Mathew, J. P.; Srinivasan, M. Eur. Polym. J. 1995, 31(9), 835–839.

17130 DOI: 10.1021/la102890s

Figure 4. SFM images (height and phase mode) of (a) 16 nm thick S2VP200K film on PS-brush surface after thermal annealing in a high-vacuum oven with pressure e10-6 bar at 180 C for 1 h and (b) micelles formed after being annealed in the IL at 150 C for 1 h (insets: magnified height and phase images of a cluster of micelles).

annealing temperature may be due to the initial structure of the film. To study the effect of the annealing time on micellization of the PS-b-P2VP films, annealing was carried out in the IL at room temperature and 150 C for different times. Spherical micelles did not form with the as-spun films in the IL after 24 h at room temperature, and no morphological transitions were observed. At 150 C, isolated spherical micelles appeared within 5 min, and an increase in the annealing time did not change the surface morphology. This indicated that the annealing temperature of thin film in the IL is much more important than the annealing time for inducing a transformation of morphology. The effect of film thickness on surface morphologies of the micellar films was also studied (see Supporting Information Figure S7). Under the same conditions, thicker films gave rise to more closely packed spherical micelles, while the size of the micelles did not show an obvious dependence on film thickness. When the film thickness was increased to ∼78 nm, close to the Langmuir 2010, 26(22), 17126–17132

Lu et al.

Article

Figure 5. Height mode SFM images of an 18 nm thick S2VP200K film (a) as-spun from toluene solution and (b) after being annealed in the IL at 150 C for 1 h. Scale bar: 500 nm.

lamellar period (L0) of the PS-b-P2VP copolymer (based on the small-angle X-ray scattering measurements, see Supporting Information Figure S8), the surface morphology was changed from a monolayer to stacked multiple micelle layers, and the “pancake”-like micelles were squeezed into sheets. PS-b-P2VP micelles were also prepared on hydrophobic PSbrush surface to investigate the effect of the substrate. PS-brush surface was prepared by spin-coating a thin film of hydroxylterminated polystyrene (PS-OH, Mn = 10 kg/mol, from Polymer Source Inc.) on a Si wafer from a solution in toluene, followed by annealing at 170 C under vacuum for 50 h and subsequent rinsing with toluene to remove the unattached chains.62 The symmetric S2VP200K film was then spin-coated onto the PS-brush-coated substrate, and sparse island structures, ∼60 nm in height, were seen after thermal annealing, which might be due to a dewetting of the block copolymer film on the PS-brush surface (Figure 4a). After 1 h annealing in the IL at 150 C, the film surface was decorated by isolated clusters of spherical micelles, with each cluster consisting of a few closely packed micelles (Figure 4b). PS-b-P2VP micelles with PS cores and P2VP coronae could also be prepared by a core-corona inversion. First, the PSb-P2VP micelles with P2VP core and PS corona were prepared by spin-coating a 0.5 wt % S2VP200K solution from toluene on the Si wafer. Because of the selectivity of toluene for the PS block, highly ordered spherical micelles consisting of insoluble P2VP cores surrounded by a stretched corona of soluble PS blocks were formed on the substrate,19 as shown in Figure 5a. Sohn and coworkers reported a core-corona transition for free-standing micellar films of PS-b-P4VP, where the inversion was induced simply by dipping the free-standing monolayer film in ethanol for less than 3 min.63 Arrays of nanoscale bumps in PS-b-P2VP films could be transformed into arrays of nanoscale holes by exposure to methanol.64 By introducing the 1,3-propane sultone into the P4VP block, fast switching between core and corona was induced in situ with the introduction of water in the PS-b-P4VP films.65 Upon exposure to supercritical CO2, the morphology of polystyrene-b-poly(1,10 ,2,20 -tetrahydroperfluorooctyl methacrylate) (PS-b-PFOMA) micellar films was inverted, where the PS coronae (62) Kim, E.; Ahn, H.; Ryu, D. Y.; Kim, J.; Cho, J. Macromolecules 2009, 42(21), 8385–8391. (63) Sohn, B. H.; Yoo, S. I.; Seo, B. W.; Yun, S. H.; Park, S. M. J. Am. Chem. Soc. 2001, 123(50), 12734–12735. (64) Krishnamoorthy, S.; Pugin, R.; Brugger, J.; Heinzelmann, H.; Hoogerwerf, A. C.; Hinderling, C. Langmuir 2006, 22(8), 3450–3452. (65) Song, L. X.; Lam, Y. M. Nanotechnology 2007, 18(7), 075304. (66) Li, Y.; Meli, L.; Lim, K. T.; Johnston, K. P.; Green, P. F. Macromolecules 2006, 39(20), 7044–7054.

Langmuir 2010, 26(22), 17126–17132

became the cores.66 However, the P2VP, P4VP, or PFOMA corona obtained after core-corona inversion was a continuous matrix, and it was hard to differentiate between individual micelles. In our case, the IL, a selective but temperature-tunable solvent for P2VP, was introduced to the micellar PS-b-P2VP films and heated at 150 C. Isolated spherical micelles (∼135 nm in diameter on average and ∼105 nm in height) formed on the Si substrate (Figure 5b). The high annealing temperature allowed the polymer blocks within the film to reorganize. Because of the solubility of P2VP in the IL, the original P2VP cores swelled and coalesced to form the corona, while the nonsoluble PS chains segregated to reduce the contact area with the solvent, thereby forming the cores. After washing with cold water, the stretched coronae contracted to form spherical shells to reduce the surface energy. This structure inversion was also confirmed by TEM images (see Supporting Information Figure S9). Micellization at the Si-IL interface was also observed in asymmetric S2VP77K (56 kg/mol-b-21 kg/mol) and S2VP80K (51 kg/mol-b-29 kg/mol) films, and the temperature effects are shown in Figure 6 and the Supporting Information Figure S10, respectively. Preannealed S2VP77K films showed island-like morphologies (Figure 6a). Annealing in the IL at 110 C led to the formation of worm-like micelles (lower area in Figure 6b). An increase in the annealing temperature to 130 C extended the micelle formation to the “island” area (Figure 6c). Very uniform worm-like micelles were distributed on the substrate surface if the film was annealed in the IL at 150 C (Figure 6d). For the preannealed S2VP80K films, annealing in the IL at 110 C gave rise to the spherical micelles coexisting with cylindrical micelles, and at higher temperatures, the surface morphology was dominated by spherical micelles (Supporting Information Figure S10), which was quite similar to the symmetric S2VP200K films. At the highest annealing temperature used in the experiments, e.g. 150 C, pancake-like or worm-like micelles formed at the SiIL interface from PS-b-P2VP thin films with different molecular weights. According to Potemkin, the equilibrium structure of the micelles is determined by the interplay between the surface energy, which favors the aggregation of the PS segments, and the energy of stretching of the P2VP blocks, which depends essentially on the relative length of the two blocks.25,26 The chain length ratios of PS block to the whole block copolymer for S2VP200K and S2VP80K are 0.51 and 0.64, respectively, while that of S2VP77K is 0.73 (see Table 1). When the length of the two blocks is comparable, the spherical or more suitably speaking, island-like structures are favored. With an increase in the length of the PS block, this morphology transforms into stripe-like structures. DOI: 10.1021/la102890s

17131

Article

Lu et al.

Figure 6. SFM images (height mode) of (a) preannealed 27 nm thick S2VP77K film and (b-d) micelles formed after annealing in the IL at different temperatures for 1 h: (b) 110, (c) 130, and (d) 150 C.

Temperature is also important in the micellization process and the morphology of the formed micelles. Rapid formation of stable micelles was observed at elevated temperatures due to the enhanced solubility of the P2VP block and mobility of the PS block. Micelles formed at the Si-IL interface were quite stable under rigorous rinsing with water due to the strong interaction between P2VP segments and the oxide on the silicon substrate. The micellar structures were also resistant to treatment with hydrogen fluoride (HF). Initially, HF was used to etch the Si wafer, which freed the micellar films. But we found that the micelles formed on the Si wafer were very stable and could not be detached. We intended to show that the micelles are very stable under rigorous rinsing with water or even HF. No obvious morphological changes, except for a decrease in micelle size, were observed after exposure the micellar film to 5% aqueous solution of HF for 10 min (see the Supporting Information Figure S11). This decrease in micelle size might be due to the protonation of P2VP blocks by HF, which may lead to an increase in electrostatic interactions between the adjacent P2VP chains and, thus, a closer chain packing in the P2VP-rich corona. The interactions between the PS-b-P2VP micelles and HF are beyond the scope of this work and will be discussed in future studies.

Conclusions As a selective and temperature-tunable solvent for the P2VP block, the ionic liquid 1-butyl-3-methylimidazolium trifluoromethanesulfonate could penetrate into PS-b-P2VP films, swell the P2VP domain, and generate stable micelles with different shapes, which strongly depend on the annealing temperature and the molecular weight of the block copolymer. Micellization

17132 DOI: 10.1021/la102890s

occurs, not only in as-spun films, in which the P2VP and PS blocks are in a disordered, kinetically trapped state, but also in preannealed films, in which the P2VP domains orient parallel to the substrate-polymer interface. In addition, PS-b-P2VP micelles with PS core and P2VP shell could also be prepared by corecorona inversion from the as-spun micellar films. In the presence of the IL and at suitable temperatures, the polymer chains are mobile and can reorganize into the specific micelle structure that minimizes the total free energy. Acknowledgment. This work is funded by the US Department of Energy (DOE) Office of Basic Energy Science and NSFsupported Materials Research Science and Engineering Center (DMR-0213695) at the University of Massachusetts Amherst. Use of the Advanced Light Source, Lawrence Berkeley National Laboratory, was supported by the DOE, Office of Science, Office of Basic Energy Science. Supporting Information Available: SFM images of S2VP200K micelles from quenching and slow cooling, TEM images of micelles obtained at 150 C, TEM images of micelles obtained at 110 C, temperature effect of the micellization of the preannealed film, UV-vis absorption spectra of P2VP in the IL, X-ray photoelectron spectra, the effect of film thickness, small-angle X-ray scattering data, TEM images of corecorona inversion, SFM images of S2VP80K micelles, and SFM images of micellar structures after HF treatment. This material is available free of charge via the Internet at http:// pubs.acs.org.

Langmuir 2010, 26(22), 17126–17132