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Strands, Networks, and Continents from Polystyrene Dewetting at the Air-Water Interface: Implications for Amphiphilic Block Copolymer Self-Assembly† Eric W. Price, Saman Harirchian-Saei, and Matthew G. Moffitt* Department of Chemistry, University of Victoria, P.O. Box 3065, Victoria, BC V8W 3 V6, Canada Received October 8, 2010. Revised Manuscript Received December 1, 2010 We demonstrate that nanoscale aggregates similar to those formed via amphiphilic block copolymer self-assembly at the air-water interface, including strands, networks, and continents, can be generated by the simple spreading of PS homopolymer solutions on water. Two different PS homopolymers of different molecular weight (PS-405k, Mn = 405 000 g mol-1 and PS-33k, Mn=33 000 g mol-1) are spread at the air-water interface at various spreading concentrations ranging from 0.25 to 3.0 mg/mL. Aggregate formation is driven by PS dewetting from water as the spreading solvent evaporates. We propose that a high spreading concentration or a high molecular weight lead to chain entanglements that restrict macromolecular mobility in the solution, enabling the kinetic trapping of nanostructures associated with early and intermediate stages of PS dewetting. Comparison of PS-405k with a mainly hydrophobic PS-b-PEO block copolymer of similar molecular weight (PSEO-392k, Mn = 392 000 g mol-1, 2.0 wt % PEO) allows the effect of a relatively short surface active block on aggregate formation to be investigated. We show that whereas the PEO block is not a required component for the formation of strands and other nonglobular aggregates, it does increase the number of these aggregates at a given spreading concentration and decreases the minimum spreading concentration at which these aggregates are observed, along with decreasing the dimensions and polydispersity of specific surface features. The results provide supporting evidence for the role of PS dewetting in the generation of multiple PS-b-PEO aggregate morphologies at the air-water interface, as originally described in earlier paper from our group.
Introduction The 2D self-assembly of amphiphilic block copolymers at the air-water interface has been widely studied as an interesting nonlithographic route to generating various nanoscale polymeric features that can be transferred to solid surfaces using Langmuir†
Part of the Supramolecular Chemistry at Interfaces special issue.
(1) Zhu, J.; Eisenberg, A.; Lennox, R. B. J. Am. Chem. Soc. 1991, 113, 5583. (2) Zhu, J.; Eisenberg, A.; Lennox, R. B. Langmuir 1991, 7, 1579. (3) Zhu, J.; Eisenberg, A.; Lennox, R. B. J. Phys. Chem. 1992, 96, 4727. (4) Zhu, J.; Eisenberg, A.; Lennox, R. B. Macromolecules 1992, 25, 6547. (5) Li, S.; Hanley, S.; Khan, I.; Varshney, S. K.; Eisenberg, A.; Lennox, R. B. Langmuir 1993, 9, 2243. (6) Meszaros, M.; Eisenberg, A.; Lennox, R. B. Faraday Discuss. 1994, 98, 283. (7) Li, Z.; Zhao, W.; Quinn, J.; Rafailovich, M. H.; Sokolov, J.; Lennox, R. B.; Eisenberg, A.; Wu, X. Z.; Kim, M. W.; Sinha, S. K.; Tolan, M. Langmuir 1995, 11, 4785. (8) Cox, J.; Eisenberg, A.; Lennox, R. B. Curr. Opin. Colloid Interface Sci. 1999, 4, 52. (9) Gragson, D. E.; Jensen, J. M.; Baker, S. M. Langmuir 1999, 15, 6127. (10) Cox, J. K.; Yu, K.; Constantine, B.; Eisenberg, A.; Lennox, R. B. Langmuir 1999, 15, 7714. (11) Cox, J. K.; Yu, K.; Eisenberg, A.; Lennox, R. B. Phys. Chem. Chem. Phys. 1999, 1, 4417. (12) Dewhurst, P. F.; Lovell, M. R.; Jones, J. L.; Richards, R. W.; Webster, J. R. P. Macromolecules 1998, 31, 7851. (13) Goncalves da Silva, A. M.; Filipe, E. J. M.; d’Oliveira, J. M. R.; Martinho, J. M. G. Langmuir 1996, 12, 6547. (14) Faure, M. C.; Bassereau, P.; Carignano, M.; Szleifer, I.; Gallot, Y.; Andelman, D. Eur. Phys. J. B 1998, 3, 365. (15) Faure, M. C.; Bassereau, P.; Lee, L. T.; Menelle, A.; Lheveder, C. Macromolecules 1999, 32, 8538. (16) Baker, S. M.; Leach, K. A.; Devereaux, C. E.; Gragson, D. E. Macromolecules 2000, 33, 5432. (17) Devereaux, C. A.; Baker, S. M. Macromolecules 2002, 35, 1921. (18) Cheyne, R. B.; Moffitt, M. G. Langmuir 2005, 21, 5453. (19) Cheyne, R. B.; Moffitt, M. G. Langmuir 2005, 21, 10297. (20) Cheyne, R. B.; Moffitt, M. G. Langmuir 2006, 22, 8387. (21) Cheyne, R. B.; Moffitt, M. G. Macromolecules 2007, 40, 2046. (22) Price, E. W.; Guo, Y.; Wang, C.-W.; Moffitt, M. G. Langmuir 2009, 25, 6398.
1364 DOI: 10.1021/la1040618
Blodgett (LB) techniques.1-23 When dissolved in a nonselective solvent and deposited onto the surface of water, polymeric amphiphiles such as polystyrene-block-poly(ethylene oxide) (PS-bPEO) spontaneously form aggregates of various morphologies upon solvent evaporation, including dots, long strands (or spaghetti), networks of strands, planar “continents”, rings, and chains. Although mixtures of morphologies are often formed, the predominant types of surface features can be varied by changing experimental parameters such as the nature and relative lengths of hydrophilic and hydrophobic blocks,4,10 the surface pressure,24-26 and the spreading solution concentration.17-22 Interfacial selfassembly is generally attributed to the interplay of attractive and repulsive interactions between the water surface and the hydrophilic and hydrophobic blocks, respectively.10 Also, recent experiments on the spreading concentration dependence of relatively high-molecular-weight and hydrophobic PS-b-PEO morphologies at the air-water interface suggest that interfacial copolymer aggregates can be strongly influenced by kinetic effects such as chain entanglements and polymer vitrification during solvent evaporation.17,18,20 In a previous paper, we presented a qualitative nonequilibrium mechanism for PS-b-PEO self-assembly at the air-water interface.20 This model was based on the widely studied phenomenon of dewetting of unstable liquid films from solid substrates27-35 and was supported by AFM data of various kinetically frozen (23) Harirchian-Saei, S.; Wang, M. C. P.; Gates, B. D.; Moffitt, M. G. Langmuir 2010, 26, 5998. (24) Lu, Q.; Bazuin, C. G. Nano Lett. 2005, 5, 1309. (25) Logan, J. L.; Masse, P.; Dorvel, B.; Skolnik, A. M.; Sheiko, S. S.; Francis, R.; Taton, D.; Gnanou, Y.; Duran, R. S. Langmuir 2005, 21, 3424. (26) Logan, J. L.; Masse, P.; Gnanou, Y.; Taton, D.; Duran, R. S. Langmuir 2005, 21, 7380. (27) Reiter, G. Phys. Rev. Lett. 1992, 68, 75. (28) Reiter, G. Langmuir 1993, 9, 1344.
Published on Web 12/29/2010
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Price et al.
transition structures formed from different spreading concentrations. Briefly, we proposed that morphology evolution begins with a continuous, homogeneous layer of copolymer solution spreading at the water surface, which ruptures via the formation and growth of holes as the solvent evaporates as a result of repulsive PS-water interactions. The raised rims of transferred material surrounding expanding holes eventually come into contact and merge to form networks of interconnected strands. Finally, the breaking of networks at junction points develops long strands that ultimately rupture into droplets via Rayleigh instabilities. Most of the various morphologies observed from PS-b-PEO copolymers described above can be explained by polymer vitrification at various stages of this process of film removal from the air-water interface, with more concentrated spreading solutions leading to the trapping of earlier kinetic states. Similar dewetting mechanisms for aggregate formation at the air-water interface were subsequently proposed for the 2D self-assembly of amphiphilic copolymers blended with cadmium sulfide nanoparticles19,21 or homopolystyrene,36,37 block copolymer blends,22 ultrahighmolecular-weight comb block copolymers,38 and cadmium telluride tetrapod-shaped nanoparticles.39 Because of its lack of amphiphilic character, the behavior of pure PS homopolymers at the air-water interface has attracted very little attention in the past. More than 20 years ago, Kumaki studied high-molecular-weight PS samples deposited on the water surface from extremely dilute solutions (∼10-3 mg/mL) and found that spreading and solvent evaporation gave rise to monomolecular PS particles.40,41 More recently, the da Silva group reported larger globules of PS arranged in chainlike structures at the air-water interface.42 In both cases, the globular shape of the resulting monomolecular or aggregate particles reflects the hydrophobic nature of PS and its unmediated repulsive interactions with the water surface. Our interest in investigating the aggregation behavior of PS homopolymers at the air-water interface stems from the apparent critical role of PS-water repulsive interactions in the formation of multiple surface features from PS-containing amphiphilic block copolymers. If the various 2D morphologies of amphiphilic block copolymers are different kinetically trapped states of PS block removal from the water surface, what is the role of the hydrophilic block in aggregate formation? In this article, we report the surprising result that two different high-molecular-weight PS homopolymers with no surface-active components form block-copolymer-like aggregates of multiple morphologies at the air-water interface when deposited from sufficiently concentrated spreading solutions, including strands, networks of strands, and continents. We also compare aggregate formation from a PS homopolymer and a mainly hydrophobic PS-b-PEO block copolymer with similar molecular weights in order to provide a conceptual bridge (29) Reiter, G.; Sharma, A.; Casoli, A.; David, M.-O.; Khanna, R.; Auroy, P. Langmuir 1999, 15, 2551. (30) Segalman, R. A.; Green, P. F. Macromolecules 1999, 32, 801. (31) Koplik, J.; Banavar, J. R. Phys. Rev. Lett. 2000, 84, 4401. (32) Kaya, H.; Jerome, B. Eur. Phys. J. E 2003, 12, 383. (33) Bollinne, C.; Cuenot, S.; Nysten, B.; Jonas, A. M. Eur. Phys. J. E 2003, 12, 389. (34) Muller-Buschbaum, P. J. Phys.: Condens. Matter 2003, 15, R1549. (35) Sharma, A.; Verma, R. Langmuir 2004, 20, 10337. (36) Wen, G.; Chung, B.; Chang, T. Macromol. Rapid Commun. 2008, 29, 1248. (37) Wen, G. J. Phys. Chem. B 2010, 114, 3827. (38) Zhao, L.; Goodman, M. D.; Bowden, N. B.; Lin, Z. Soft Mater. 2009, 5, 4698. (39) Goodman, M. D.; Zhao, L.; DeRocher, K. A.; Wang, J.; Mallapragada, S. K.; Lin, Z. ACS Nano 2010, 4, 2043. (40) Kumaki, J. Macromolecules 1986, 19, 2258. (41) Kumaki, J. Macromolecules 1988, 21, 749. (42) Lopes, S. I. C.; Goncalves da Silva, A. M. P. S.; Brogueira, P.; Picarra, S.; Martinho, J. M. G. Langmuir 2007, 23, 9310.
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Article
between PS and PS-b-PEO aggregation at the air-water interface. Our conclusion is that whereas short surface-active blocks appear to regulate the formation rate, dimensions, and polydispersities of the kinetic surface features, they are not a required molecular component in the generation of nanostructures generally associated with interfacial block copolymer self-assembly.
Experimental Section Materials. The two PS homopolymers (PS-405k: Mn = 405 000 g mol-1, PDI=1.09; PS-33k: Mn =33 000 g mol-1, PDI=1.04) and one PS-b-PEO block copolymer (PSEO-392k: Mn = 392 000 g mol-1, 2.0 wt % PEO, PDI=1.15) used in this study were purchased from Polymer Source Ltd. and used as received. Polymer Solution Preparation. All solution samples were contained in amber screw cap vials and were sealed with Teflon tape. The PS-b-PEO solution was additionally wrapped in foil to prevent light exposure. Stock solutions were prepared at 5 mg/mL with spectroscopic-grade chloroform (99.8% Caledon) using a Sartorius CP2245 analytical balance. The chloroform was filtered with Target nylon filters (1.2 μm nominal pore size) prior to solution preparation. The various stock solutions were prepared by weighing 100 mg of material and 20 mL of chloroform into a 30 mL amber vial, followed by stirring for 4 h and overnight equilibration in the freezer. Stock solutions were stored in the freezer when not in use to prevent PEO degradation and chloroform evaporation and were allowed to equilibrate to room temperature before use. The 5 mg/mL stock solutions for each sample were used to prepare 0.25, 0.50, 0.75, 1.0, 2.0, and 3.0 mg/mL working solutions by diluting with appropriate amounts of filtered chloroform. Following dilution, working solutions were stirred for 2 h and then placed in the freezer to equilibrate overnight. Langmuir-Blodgett Trough Preparation. A KSV 3000 Langmuir trough (KSV instruments with a trough volume of ∼1 L of deionized water at 150 515 mm2) was used to obtain both isotherm data and Langmuir-Blodgett films. The entire trough and dipping assembly was housed in a Plexiglas dust shield to prevent surface contamination, and the instrument was controlled using a Windows XP-based computer running KSV software. The subphase used throughout all experiments was house-distilled deionized water (Barnstead NANOpure Diamond, 18.2 mΩ cm) kept at 25 ( 1 °C using a temperature control unit (PolyScience). The trough surface, paddles, and contents of the dust shield were cleaned before and after use using deionized water, ethanol, and Kimwipes. The water surface was carefully cleaned of any debris by aspiration with a glass pipet until the surface pressure readings upon surface compression were constantly
