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Monitoring the Transformation of Colloidal Crystals by Styrene Vapor Using Atomic Force Microscopy Dongqi Qin, Susheng Tan, Shuhui Qin, and Warren T. Ford* Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078 Received September 15, 2003. In Final Form: January 23, 2004 The stages of transformation of a colloidal crystalline film of latex spheres to a new periodic structure were imaged by atomic force microscopy. Colloidal crystalline films were prepared with 320 nm diameter poly(styrene-co-2-hydroxyethyl methacrylate) (PSt/HEMA) spheres. The hexagonally ordered surfaces of the colloidal crystalline films were transformed with styrene vapor at room temperature to a new morphology having holes in the surface and the same periodicity as the original films. The surfaces of colloidal crystals and the transformed films have a raspberry-like texture superposed on the 320 nm hexagonal periodicity. Both height images and phase images reveal that the latex spheres shrink and the transformation proceeds by an order-disorder-order sequence. The final structure is an interconnected colloidal array with smaller polystyrene particles dispersed in a continuous PSt/HEMA matrix.
Introduction Colloidal particles have long been used in foods, inks, paints, coatings, cosmetics, and photographic films.1,2 Monodisperse colloidal spheres serve as models for fundamental physical investigations of the forces between particles in fluids and phase transitions in colloids.3-6 The particles studied most are silica7 and polymers.2,8 Monodisperse spheres can order into opalescent crystalline arrays that diffract visible or near-infrared light. Colloidal crystalline arrays can be made both from close-packed spheres and from dilute dispersions of spheres in a liquid, such as polystyrene latexes in water, in which the electrostatic repusions order the particles into facecentered cubic (fcc) or body-centered cubic (bcc) lattices.3,9,10 The dispersions in liquids can be locked into place by polymerization.11-13 Colloidal crystalline arrays have potential applications as chemical sensors and optical switches.14-19 The close-packed arrays can also be templates for inverse opals: filling the void spaces with polymerizable monomers, nanometer-sized colloidal met(1) Everett, D. H. Basic Principles of Colloid Science; Royal Society of Chemistry: London, 1992. (2) Emulsion Polymerization and Emulsion Polymers; Lovell, P. A., El-Aasser, M. S., Eds.; John Wiley & Sons: Chichester, 1997. (3) Clark, N. A.; Hurd, A. J.; Ackerson, B. J. Nature 1979, 281, 57. (4) Ackerson, B. J.; Schaetzel, K. Phys. Rev. E 1995, 52, 6448. (5) Grier, D. G. MRS Bull. 1998, 23, 21. (6) Ackerson, B. J.; Lei, X. L.; Tong, P. Pure Appl. Chem. 2001, 73, 1679. (7) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979. (8) Fitch, R. M. Polymer Colloids: A Comprehensive Introduction; Academic Press: San Diego, 1997. (9) Flaugh, P. L.; O’Donnell, S. E.; Asher, S. A. Appl. Spectrosc. 1984, 38, 847. (10) Schaetzel, K.; Ackerson, B. J. Phys. Rev. E 1993, 48, 3766. (11) Asher, S. A.; Holtz, J.; Liu, L.; Wu, Z. J. Am. Chem. Soc. 1994, 116, 4997. (12) Jethmalani, J. M.; Ford, W. T. Chem. Mater. 1996, 8, 2138. (13) Jethmalani, J. M.; Ford, W. T.; Beaucage, G. Langmuir 1997, 13, 3338. (14) Weissman, J. M.; Sunkara, H. B.; Tse, A. S.; Asher, S. A. Science 1996, 274, 959. (15) Pan, G.; Kesavamoorthy, R.; Asher, S. A. Phys. Rev. Lett. 1997, 78, 3860. (16) Holtz, J. H.; Asher, S. A. Nature 1997, 389, 829. (17) Asher, S. A.; Holtz, J.; Weissman, J.; Pan, G. MRS Bull. 1998, 23, 44. (18) Lee, K.; Asher, S. A. J. Am. Chem. Soc. 2000, 122, 9534. (19) Reese, C. E.; Baltusavich, M. E.; Keim, J. P.; Asher, S. A. Anal. Chem. 2001, 73, 5038.
als, metal oxides, sol-gel precursors, or semiconductors followed by solidifying the in-filled material and removing the parent spheres produces lattices with spherical holes.20-23 The three-dimensionally ordered porous materials have been proposed as collectors of solar energy, as supports for catalysts, and as low-dielectric materials for capacitors.21,23-25 During research on polymerized colloidal crystalline arrays, we discovered an intriguing transformation of colloidal crystals of close-packed monodisperse poly(styrene-co-2-hydroxyethyl methacrylate) (PSt/HEMA) spheres to interconnected colloidal arrays (ICAs) by a simple solvent vapor treatment.26,27 The colloidal crystalline arrays contain 10-20 layers of spheres coated on glass. Vapors of styrene or toluene, which are good solvents for polystyrene and poor solvents for PHEMA, permeate into the cores of the spheres and swell the polystyrene. The absorbed solvent lowers the glass transition temperature (Tg) of the polystyrene-rich core to subambient and makes the polystyrene phase mobile. The swollen polystyrene engulfs the PHEMA-rich shell to fill the air-polymer interface. Subsequent evaporation of the solvent leaves a film 0.72 times as thick as the original colloidal crystalline film. The transformed ICA film has holes in the surface with the same periodicity as the spheres in the original film. Recent work by Rugge shows that the transformation depends on solvent vapor pressure.28 Exposure of the colloidal crystal film to less than 16 Torr of toluene vapor swells the film, as shown by a shift of the optical diffraction peak to longer wavelength, and then as the toluene evaporates, the film contracts reversibly, and the diffraction peak returns to the original wavelength. After swelling (20) Imhof, A.; Pine, D. J. Nature 1997, 389, 948. (21) Velev, O. D.; Jede, T. A.; Lobo, R. F.; Lenhoff, A. M. Chem. Mater. 1998, 10, 3597. (22) Rogach, A.; Susha, A.; Caruso, F.; Sukhorukov, G.; Kornowski, A.; Kershaw, S.; Mo¨hwald, H.; Eychmuller, A.; Weller, H. Adv. Mater. 2000, 12, 333. (23) Xia, Y.; Gates, B.; Li, Z.-Y. Adv. Mater. 2001, 13, 409. (24) Yang, H.; Coombs, N.; Ozin, G. A. Nature 1997, 386, 692. (25) Imhof, A.; Pine, D. J. Adv. Mater. 1998, 10, 697. (26) Chen, Y.; Ford, W. T.; Materer, N. F.; Teeters, D. J. Am. Chem. Soc. 2000, 122, 10472. (27) Chen, Y.; Ford, W. T.; Materer, N. F.; Teeters, D. Chem. Mater. 2001, 13, 2697. (28) Rugge, A.; Ford, W. T.; Tolbert, S. H. Langmuir 2003, 19, 7852.
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at a higher vapor pressure of toluene, evaporation causes the same irreversible conversion to an ICA as reported before using styrene vapor, and a much weaker diffraction peak appears at shorter wavelength.26,27 The proposed mechanism is as follows. When a colloidal crystal is exposed to a low vapor pressure of toluene, the particle cores swell reversibly and expand the film in the direction perpendicular to the substrate. At higher vapor pressure, the swollen polystyrene fills the pores of the film and fills the air interface to minimize interfacial free energy. As the toluene evaporates, the film contracts to a nonporous solid polymer. In cross-sectional transmission electron microscopy (TEM) images, the ICA has smaller polystyrene-rich particles embedded in an ordered array in a continuous matrix of PSt/HEMA.28 To understand better the transformation of the colloidal crystal to an ICA, we have now monitored the surface topology of the films during the process by atomic force microscopy (AFM). Experimental Section Materials. Styrene (St) and 2-hydroxyethyl methacrylate (HEMA) from Aldrich Chemical Co. were passed through an alumina column to remove the inhibitor and stored at 3 °C. Potassium persulfate (KPS) from Aldrich was used as received. Water was deionized and treated with active carbon in a threecolumn Barnsted ultrapure system to a resistivity of >106 Ω cm after exposure to air. Scintillation vials for polymerization and glass slides for film deposition were soaked in a KOH/ethanol bath for 24 h, rinsed with water, and dried at 100 °C. Poly(styrene-co-HEMA). The latex was prepared by batch emulsion polymerization by the method of Cardoso.29 A scintillation vial was loaded with 90 mol % styrene (0.369 g, 3.55 × 10-3 mol), 10 mol % HEMA (0.049 g, 3.77 × 10-4 mol), and 5 mL of water. After purging with N2 for 5 min, the mixture was shaken at 60 °C with an orbit shaker (J-Kem, Inc.) at 300 rpm for 30 min. Then 0.10 mL of a 3.7 × 10-2 M aqueous solution of KPS (0.10 mol %) was added. The reaction was continued at 60 °C for 24 h. The polymerization mixture was filtered through cotton to remove any possible coagulum. The diameter of the latex particles was 320 nm, measured by AFM of colloidal crystalline arrays. Colloidal Crystals. A glass slide was placed vertically in a vial of latex of approximately 0.5% concentration under a large plastic box to protect from dust in the air. Water slowly evaporated from the latex solution over 3 days, and a 15 mm iridescent band of colloidal crystals deposited on the glass. To increase durability, the colloidal crystalline film was annealed at 80 °C for 48 h. (The polymer flowed and destroyed the colloidal crystal when annealed at 90 °C.) After cooling to room temperature, the samples were kept in the air until use. The colloidal arrays on glass can be stored for at least 6 months. Interconnected Colloidal Array. The colloidal crystalline films on glass were cut to a size of less than 10 × 10 mm and placed horizontally in a jar 10 mm above liquid styrene at 22 °C.26,27 The jar was sealed for 4, 5, 6, 8, 10, or 15 min for different samples. The iridescent colloidal crystalline film gradually changed to a more transparent polymer film. The film was dried in the draft of a fume hood before microscopy measurements. Measurements. AFM images were recorded on a DI NanoScope IIIa Multimode Scanning Probe Microscope with quadrex extender. Two types of noncontact silicon AFM cantilevers were used in the experiments. NSC15 cantilevers (standard tips, Mikromasch, www.spmtips.com) had a tip radius of less than 10 nm and a full tip cone angle of less than 20°. High aspect ratio cantilevers AR10-NCHR (sharper tips, Nano World, www.nanoworld.org) had a tip radius of less than 10 nm and a full tip cone angle of less than 10°. All AFM images in this paper were recorded using NSC15 cantilevers. Scanning electron microscopy (SEM) experiments were performed with a JEOL JXM 6400 (Tokyo, Japan) instrument at 25 (29) Cardoso, A. H.; Leite, C. A. P.; Galembeck, F. Langmuir 1999, 15, 4447.
Qin et al. kV accelerating voltage. Samples were coated with gold/palladium before imaging.
Results and Discussion AFM Height Images. Colloidal crystalline films of 320 nm diameter PSt/HEMA latexes were prepared by evaporative deposition on glass.26,27,30 Irreversible transformation of the films in styrene vapor at room temperature occurred too fast to isolate partly transformed films. However, after annealing at 80 °C for 48 h, the films transformed slower than the original ones. AFM images show that both the particles of PSt/HEMA latex and the transformed ICA film have bumpy surfaces on a length scale of about 50 nm, which we attribute to micro-phase separation of PHEMA-rich and polystyrene-rich regions. The bumps on the particles of the colloidal crystalline films are much larger in height than those on the surface of the ICA samples. Rough surfaces of PSt/HEMA latexes were reported before by Galembeck.29,31 Figure 1 shows that the AFM height images of an original colloidal crystalline film and an annealed colloidal crystalline film are nearly the same. The profiles from AFM height images measure the depth between two neighboring spheres in one layer along an axis connecting their centers to be 46-48 nm, while the depth along an axis perpendicular to the first axis is 54-55 nm. All films were analyzed with two types of cantilevers having full tip cone angles of
