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Localized Synthesis of Polypyrrole in the Nanopattern of Monolayer Films of Diblock Copolymer Micelles Seong Il Yoo,† Byeong-Hyeok Sohn,*,‡ Wang-Cheol Zin,† and Jin Chul Jung† Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea, and School of Chemistry, NANO System Institute, Seoul National University, Seoul 151-747, Korea Received July 21, 2004. In Final Form: September 7, 2004 A single-layered array of polystyrene-block-poly(4-vinylpyridine), PS-PVP, micelles in hexagonal order, fabricated by spin coating, was employed as a nanostructured template for synthesis of polypyrrole, a conducting polymer, in nanometer-sized domains. Oxidative catalysts of FeCl3 for the polymerization were selectively loaded in spherical PVP nanodamains so that they were hexagonally arranged over the film but confined in the nanometer range. The vapor-phase polymerization of pyrrole was localized in the PVP nanodomains, leading to a morphological transition from spherical to wormlike domains. In addition, the nanodomains containing polypyrrole were converted to open cavities by ethanol, a PVP block-selective solvent.
Introduction In recent years, there has been intensive research conducted on thin films of diblock copolymers and their micelles to utilize them as templates or etching masks for creating functional nanostructures.1-5 Diblock copolymers composed of two different polymers spontaneously selfassemble into periodic nanostructures, of which the size and morphology can be easily controlled by the molecular weight and composition of copolymers.6 In addition, the nanofabrication technique based on diblock copolymers is a parallel process, resulting in an easy creation of functional nanostructures.1-5 In a selective solvent for one of the blocks of diblock copolymers, for example, nanometer-sized micelles consisting of a soluble corona and an insoluble core are spontaneously formed.1,6 These micelles can be transferred onto solid substrates by Langmuir-Blodgett (LB), dip-coating, or spin-coating methods to form a variety of nanostructures, which have been used as nanostructured templates to synthesize nanoparticles or a lithographic mask to fabricate nanopattens.1,4,5,7,8 Moreover, the diblock copolymer approach for fabricating nanostructures is particularly advantageous for the functional materials difficult to be patterned with a conventional method.2-4 For instance, patterning of conducting polymers even in a micrometer scale with lithographic techniques is not a trivial process because of the poor processibility and possible damages in harsh lithographic processes involving irradiation and solvent * Corresponding author. Tel.: +82-2-883-2154. Fax: +82-2-8891568. E-mail:
[email protected]. † Pohang University of Science and Technology. ‡ Seoul National University. (1) Fo¨rster, S.; Antonietti, M. Adv. Mater. 1998, 10, 195. (2) Park, C.; Yoon, J.; Thomas, E. L. Polymer 2003, 44, 6725. (3) Hamley, I. W. Nanotechnology 2003, 14, R39. (4) Glass, R.; Mo¨ller, M.; Spatz, J. P. Nanotechnology 2003, 14, 1153. (5) Bronstein, L.; Antonietti, M.; Valetsky, P. In Nanoparticles and Nanostructured Films; Fendler, J. H., Ed.; Wiley-VCH: Weinheim, Germany, 1998. (6) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: New York, 1998. (7) Moffitt, M.; Eisenberg, A. Chem. Mater. 1995, 7, 1178. (8) Meiners, J. C.; Quintel-Ritzi, A.; Mlynek, J.; Elbs, H.; Krausch, G. Macromolecules 1997, 30, 4945.
dissolution.9-11 However, conducting polymers can be selectively incorporated or polymerized in situ in nanodomains of diblock copolymers without demanding processes.10-14 For example, it was possible to incorporate conducting polymers in nanometer-sized domains by their selective synthesis within the periodic nanostructure of diblock copolymers.12-14 Conducting polymers were also nanopatterned using diblock copolymer surface micelles, as templates, formed via the LB technique.10 Recently we reported formation of nanopatterns fabricated from single-layered films of diblock copolymer micelles on various substrates simply by spin coating, which served as nanotemplates for nanoparticle synthesis.15,16 In this article, application of a hexagonal array of diblock copolymer micelles as a nanostructured template was expanded to the synthesis of polypyrrole, a conducting polymer, as an example of organic functional materials, in the nanometer scale. Polypyrrole was effectively localized in the nanometer-sized domains of diblock copolymer micelles which were evenly distributed over the film. In addition, the nanodomains containing polypyrrole were converted to open cavities simply by a coreselective solvent. Experimental Section Polystyrene-block-poly(4-vinylpyridine), PS-PVP, diblock copolymers were purchased from Polymer Source, Inc. The numberaverage molecular weights of PS and PVP were 21 400 and 20 700 g/mol, respectively. The polydispersity index was 1.13. Glass slides and silicon wafers were cleaned in a piranha solution (70:30 v/v of concentrated H2SO4 and 30% H2O2; caution, piranha solution reacts violently with organic compounds and (9) Holdcroft, S. Adv. Mater. 2001, 13, 1753. (10) Goren, M.; Lennox, R. B. Nano Lett. 2001, 1, 735. (11) Seo, I.; Pyo, M.; Cho, G. Langmuir 2002, 18, 7253. (12) Ishizu, K.; Honda, K.; Kanbara, T.; Yamamoto, T. Polymer 1994, 35, 4901. (13) de Jesus, M. C.; Weiss, R. A.; Hahn, S. F. Macromolecules 1998, 31, 2230. (14) Selvan, S. T.; Spatz, J. P.; Klok, H. A.; Mo¨ller, M. Adv. Mater. 1998, 10, 132. (15) Sohn, B. H.; Yoo, S. I.; Seo, B. W.; Yun, S. H.; Park, S. M. J. Am. Chem. Soc. 2001, 123, 12734. (16) Sohn, B. H.; Choi, J. M.; Yoo, S. I.; Yun, S. H.; Zin, W.-C.; Jung, J. C.; Kanehara, M.; Hirata, T.; Teranishi, T. J. Am. Chem. Soc. 2003, 125, 6368.
10.1021/la048171v CCC: $27.50 © 2004 American Chemical Society Published on Web 10/26/2004
Localized Synthesis of Polypyrrole in the Nanopattern
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Results and Discussion
Figure 1. Plane-view TEM image of a spin-coated monolayer film of PS-PVP micelles containing FeCl3 in the PVP core. should not be stored in closed containers) at 90 °C for 20 min, thoroughly rinsed with deionized water several times, and then blown dry with nitrogen. Fresh mica substrates were prepared by cleaving a piece of mica. Substrates after cleaning or cleaving were immediately used for spin coating of diblock copolymer micelles. To make PS-PVP micelles in toluene that is a selective solvent for the PS block, a 0.5 wt % toluene solution of copolymers was prepared, stirred for 3 h at room temperature, and then stirred for an additional 2 h at 70 °C. FeCl3, an oxidative catalyst for polypyrrole synthesis,13 was added to the micellar solution (molar ratio of FeCl3/VP ) 0.1). The mixed solution was stirred for at least 24 h to ensure the complete loading of FeCl3 to the PVP core. From the PS-PVP solution with FeCl3, a micellar film was spin-coated on glass slides, silicon wafers, or mica substrates. A single-layered film of PS-PVP micelles was obtained typically at 2000 rpm.15,16 Polypyrrole was selectively synthesized in the PVP core blocks containing FeCl3 by exposing the monolayer film to saturated pyrrole vapor in a sealed glass vessel at room temperature for various periods. After polypyrrole synthesis, the film was washed with deionized water and dried in a vacuum. The monolayer film after polypyrrole synthesis was also dipped into ethanol for 1 h to induce a morphological change in the film. Fourier transformed infrared (FTIR) spectra of the monolayer film before and after polypyrrole synthesis were recorded on a Mattson Infinity Gold spectrophotometer in attenuated total reflection (ATR) mode. A monolayer film of PS-PVP micelles was directly coated on a zinc selenide crystal (25 mm × 10 mm × 3 mm). To obtain plane-view images in transmission electron microscopy (TEM), monolayer films of PS-PVP micelles were floated off from the mica substrate onto deionized water and collected on a carbon-coated TEM grid. TEM was performed on a JEOL 1200EX operating at 120 kV. The surface topography of the thin films was also imaged using an atomic force microscope (AFM, Digital Instruments Nanoscope IIIa) in a tapping mode with silicon nitride cantilevers.
In toluene, a selective solvent for the PS block, PSPVP diblock copolymers spontaneously associate into spherical micelles with soluble PS coronas and insoluble PVP cores.1,6 It is possible to coordinate various metal salts including FeCl3, a catalyst for polypyrrole synthesis,13 selectively to the PVP core.1,5 We first prepared a toluene solution of PS-PVP micelles containing FeCl3 in the PVP core and then fabricated a monolayer film of the micelles by spin-coating from this toluene solution as described in our previous report.15,16 Figure 1 is a plane-view TEM image of a single-layered film of PS-PVP micelles in a short-range hexagonal array. Because no staining was performed to enhance the contrast in the TEM images, the dark PVP core was associated with selectively loaded FeCl3. Although the molar ratio of FeCl3 to pyridine was 0.1, the contrast in the image was good enough to distinguish the core PVP blocks from the PS blocks. Thus, the catalyst for oxidative polymerization of pyrrole was isolated in the nanometer-sized core of the micelles (ca. 22.0 nm in diameter) in a short-range hexagonal array. The film shown in Figure 1 was exposed to pyrrole vapor to synthesize polypyrrole selectively in nanometer-sized domains by the oxidative catalyst. TEM images in the plane view after exposure for various periods are shown in Figure 2. The dark areas in the images correspond to the PVP domains containing polypyrrole molecules and FeCl3 catalysts. After exposing the hexagonal micellar film to pyrrole vapor for 20 min (Figure 2a), spherical PVP domains mostly became a wormlike morphology by means of interconnection between spherical domains, apparently due to inclusion of synthesized polypyrrole in the spherical PVP domain. When polypyrrole was synthesized in an aqueous polymerization condition with surface micelles of polystyrene-poly(2-vinylpyridine) fabricated by the LB method, deposition of polypyrrole in the PS domain was reported due to kinetically favored growth of polypyrrole in the hydrophobic PS surface.10 In our experimental condition, however, pyrrole monomers in vapor could be selectively incorporated into the PVP domain, presumably by the polar affinity of the monomer to the domain.13,14 Also, morphological changes by inclusion of additional molecules were observed in block copolymers blended with homopolymers.6,17 Although the domain morphology was altered after polypyrrole synthesis, each dark domain containing polypyrrole was still in the nanometer scale and also isolated with the bright continuous PS matrix. In the wormlike morphology, the size of originally spherical domains increased to about 28.0 nm and the thickness of interconnection was about 18.0 nm. Thus, polypyrrole was localized in nanometer-sized
Figure 2. Plane-view TEM images after exposure of a monolayer film of PS-PVP micelles containing FeCl3 to pyrrole vapor for various periods: (a) 20 min; (b) 3 h; and (c) 6 h.
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Figure 3. ATR-FTIR spectra of a monolayer film of PS-PVP micelles containing FeCl3: (a) before and (b) after exposure to pyrrole vapor for 20 min. Spectrum c was obtained after exposure of the monolayer film of PS-PVP micelles that did not contain FeCl3 to pyrrole vapor for 20 min.
domains using the template of a single-layered film of the copolymer micelles. After further exposure of the micellar film to pyrrole monomers (Figure 2b), the dark nanodomains became longer and smoother with the disappearance of apparent contour of interconnected spheres, due to inclusion of more polypyrrole molecules. As shown in Figure 2c, however, the boundary between the domains eventually became vague and individual domains were not discernible after long exposure to pyrrole vapor. Synthesis of polypyrrole after exposure of a film of PSPVP micelles containing FeCl3 to pyrrole vapor was confirmed by the characteristic bipolaron bands at 1220 and 910 cm-1 in the ATR-FTIR results, indicated by the solid arrows in Figure 3b.14,18 They were not observed in the spectrum before the synthesis as shown in Figure 3a. The peak at 1560 cm-1, indicated by the dashed arrow in Figure 3b, was also observed after the synthesis of polypyrrole. This band is associated with the ring stretching vibration of polypyrrole and a supportive evidence of the synthesis of polypyrrole.19 In addition, there were no bipolaron bands and no peak of the ring stretching vibration in the spectrum (Figure 3c) obtained after exposing the monolayer film of PS-PVP micelles, which did not contain FeCl3, to pyrrole vapor, indicating that FeCl3 catalysts were necessary to synthesize polypyrrole. To induce a morphological change in films containing polypyrrole in PVP domains, the film shown in Figure 2a was dipped into ethanol, a selective solvent for the PVP block.15 As shown in Figure 4a, the contrast was exactly inverted, compared with Figure 2a, that is, a bright wormlike morphology of interconnected spheres. The topological AFM image of the film treated with ethanol (Figure 4b) revealed that the bright area in the TEM image was mainly associated with the depressed region (the dark area) in the AFM image. The depth of cavities was at least 9.0 nm, which could be an underestimated value because the diameter of the AFM tip was comparable to the size of the cavity.20 The PVP domains containing polypyrrole were presumably swollen by the selective solvent of ethanol, and then subsequent removal of ethanol from the PVP domains could lead to the formation of cavities. The shape of cavities was not much changed from that of (17) Kinning, D. J.; Winey, K. I.; Thomas, E. L. Macromolecules 1988, 21, 3502. (18) Tian, B.; Zerbi, G. J. Chem. Phys. 1990, 92, 3892. (19) Mecerreyes, D.; Stevens, R.; Nguyen, C.; Pomposo, J. A.; Bengoetxea, M.; Grande, H. Synth. Met. 2002, 126, 171. (20) Boontongkong, Y.; Cohen, R. E. Macromolecules 2002, 35, 3647.
Yoo et al.
Figure 4. (a) Plane-view TEM and (b) AFM images after ethanol treatment on the film of PS-PVP micelles containing polypyrrole shown in Figure 2a.
the PVP domains before the ethanol treatment because the swelling and cavitation were possibly confined by the glassy PS domains. Thus, polypyrrole molecules synthesized in the PVP domain could still be located in the PVP cavities with a high possibility of being exposed to the air interface, which may result in different local surface conductivities before and after cavitation. However, inplane conductivities of the film containing polypyrrole before and after ethanol treatment were too low to be measurable by a four-point probe station available to us, mainly as a result of the isolated phase of polypyrrolecontaining PVP domains regardless of the ethanol treatment. Similar cavitation was reported in films of polystyrene-poly(acrylic acid) and polystyrene-poly(2-vinylpyridine) diblock copolymer micelles by hydration of spherical poly(acrylic acid) and poly(2-vinylpyridine) cores.10,20 However, no cavitation was observed when the film after longer exposure to pyrrole vapor shown in Figure 2b was dipped into ethanol. In this case, the amount of polypyrrole molecules in PVP domains could be large enough to suppress the swelling of the PVP domains by molecular entanglements. This result was analogous to the suppression of cavitation due to cross-linked poly(acrylic acid) cores by divalent cations in films of polystyrene-poly(acrylic acid) copolymer micelles.20 Conclusions We demonstrated a localized synthesis of polypyrrole in the nanodomains of PS-PVP diblock copolymer micelles containing FeCl3, an oxidative catalyst for polypyrrole synthesis, selectively in the PVP cores. A hexagonal array of PS-PVP micelles containing FeCl3 was fabricated simply by spin coating. Thus, FeCl3 was hexagonally arranged over the film but localized in the spherical PVP nanodomains. By exposing the micellar film to pyrrole vapor, the synthesis was directly carried out in the nanometer-sized pattern of the micellar film and the spherical domain turned to the wormlike domain. In addition, the morphology of the nanodomain containing polypyrrole was converted to the open cavity simply treated with a PVP core-selective solvent. Thus, a hexagonal array of diblock copolymer micelles on the solid substrate was successfully employed as a nanostructured template to synthesize organic functional materials in the nanometer scale. Acknowledgment. This work was supported by the National Core Research Center program of the Korea Science and Engineering Foundation (KOSEF) via the NANO Systems Institute at Seoul National University. LA048171V
