Mesoscale Pincushions, Microrings, and Microdots Prepared by

We describe here a preparation of pincushion structures with holes, hexagonally arranged microrings, and microdots by simple heating and peeling of ...
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Langmuir 2006, 22, 4992-4997

Mesoscale Pincushions, Microrings, and Microdots Prepared by Heating and Peeling of Self-Organized Honeycomb-Patterned Films Deposited on a Solid Substrate Hiroshi Yabu*,†,‡ and Masatsugu Shimomura*,†,‡,§ Nanotechnology Research Center, Research Institute for Electronic Science, Hokkaido UniVersity, N12W6, Sapporo, 001-0021, Japan, Frontier Research System, The Institute of Physical and Chemical Research (RIKEN Institute), 1-12, Hirosawa, Wako, Saitama, 351-0198, Japan, and CREST, Japan Science and Technology Agency (JST), Hon-machi, Kawaguchi, Japan ReceiVed December 26, 2005. In Final Form: March 27, 2006 We describe here a preparation of pincushion structures with holes, hexagonally arranged microrings, and microdots by simple heating and peeling of self-organized honeycomb-patterned films. We have reported that the honeycombpatterned films can be prepared by casting the solution of an amphiphilic polymer and a hydrophobic polymer under humid conditions. When annealing the honeycomb-patterned films prepared from an amphiphilic copolymer and poly(bisphenol A carbonate), we obtained a variety of mesoscale structures, depending on the heating temperatures. We revealed that these microstructures were formed by using the phase-separation structures in the self-organized honeycomb-patterned films. These micropatterns can be utilized for the template for microelectrodes, superhydrophobic surfaces, photonic crystals, and as a substrate for tissue engineering.

1. Introduction The patterning of two-dimensional periodic structures ranging from the submicron to the micron scale is one of the significant issues in electronics,1 photonics2 and biotechnology.3 Photolithography and related technologies (soft lithography,4 nanoimprint lithography,5 etc.) have been used to prepare periodic twodimensional structures. Alternative preparation methods for twodimensional periodic structures involving the assembly of colloidal particles,6 dewetting of polymer materials,7 and phaseseparation of block-copolymers8 have also been reported. Recently, the preparation of honeycomb-patterned polymer films by casting solutions under humid conditions has been reported.9-13 After casting a polymer solution onto a solid substrate, hexagonally packed water microdroplets were formed by evaporative cooling on the solution surface. The water droplets * To whom correspondence should be addressed. Tel/Fax: +81-11706-9369. E-mail: [email protected] (H.Y.); shimo@ poly.es.hokudai.ac.jp (M.S.). † Hokkaido University. ‡ The Institute of Physical and Chemical Research (RIKEN Institute). § Japan Science and Technology Agency (JST). (1) Heiko, O. J.; Whitesides, G. M. Science 2001, 291, 1763. (2) Joannopoulos, J. D.; Meade, R. D.; Winn, J. N. Photonic Crystals - Molding the Flow of Light; Princeton University Press: Princeton, NJ, 1995. (3) Ostuni, E.; Chen, C. S.; Ingber, D. E.; Whitesides, G. M. Langmuir 2001, 17 (9), 2828. (4) (a) Xia, Y.; Whitesides, G. M. Annu. ReV. Mater. Sci. 1998, 28, 153. (b) Odom, T. W.; Love, J. C.; Wolfe, D. B.; Paul, K. E.; Whitesides, G. M. Langmuir 2002, 18, 5314. (c) McDonald, J. C.; Duffy, D. C.; Anderson, J. R.; Chiu, D. T.; Wu, H.; Schueller, O. J. A.; Whitesides, G. M. Electrophoresis 2000, 21, 27. (d) Rogers, J. A.; Paul, K. E.; Jackmann, R. J.; Whitesides, G. M. Appl. Phys. Lett. 1997, 70 (20), 2658. (5) Zhang, W.; Chou, S. Y. Appl. Phys. Lett. 2003, 83 (9), 1632. (6) (a) Cassagneau, T.; Caruso, F. AdV. Mater. 2002, 14 (24), 1837. (b) Mı´guez, H.; Yang, S. M.; Te´treault, N.; Ozin, G. A. AdV. Mater. 2002, 14 (24), 1805. (c) Yu, J.-S.; Kang, S.; Yoon, S. B.; Chai, G. J. Am. Chem. Soc. 2002, 124, 9382. (7) (a) Karthaus, O.; Ijiro, K.; Shimomura, M. Chem. Lett. 1996, 821. (b) Karthaus, O.; Maruyama, N.; Yabu, H.; Koito, T.; Akagi, K.; Shimomura, M. Macromol. Symp. 2000, 160, 137. (c) Karthaus, O.; Yabu, H.; Akagi, K.; Shimomura, M. Mol. Cryst. Liq. Cryst. 2001, 364, 395. (d) Karthaus, O.; Yabu, H.; Koito, T.; Akagi, T.; Shimomura, M. Mol. Cryst. Liq. Cryst. 2001, 370, 353. (e) Yabu, H.; Shimomura, M. AdV. Funct. Mater. 2005, 15 (4), 575. (8) (a) Thurn-Albrecht, T.; Steiner, R.; DeRouchey, J.; Stafford, C. M.; Huang, E.; Bal, M.; Tuominen, M.; Hawker, C. J.; Russell, T. P. AdV. Mater. 2000, 12 (11), 787. (b) Templin, M.; Frank, A.; Du Chesne, A.; Leist, A.; Zhang, A.; Ulrich, R.; Scha¨lder, V.; Wiesner, U. Science 1997, 278, 1795.

thus formed were transferred to the solution front by convectional flow or capillary force. After solvent evaporation, a honeycombpatterned film was formed with the water droplet array as a template. The template water droplets evaporated soon after the solvent did. Novel microstructures were subsequently fabricated from the honeycomb-patterned films. One method of fabrication was the thermal annealing of these films. The honeycomb-patterned films had a double-layered structure supported by pillars at the vertexes of the honeycomb hexagons, and thermal annealing collapsed the pillars to form a single-layer honeycomb structure14 (Figure 1a). Mechanical deformation of the honeycomb-patterned films was another technique for preparing new structures. Stretching these elastic films produced microporous films with ellipsoidal or rectangular pores15 (Figure 1b). A negative mold could be obtained by filling the pores of the patterned films with various materials after removing the honeycomb template (Figure 1c). We have reported that a negative mold of poly(dimethylsiloxane) (PDMS) elastomer could be utilized for microlens arrays.16 Furthermore, by peeling off the upper layer of the honeycombpatterned film with adhesive tape, pincushion-like structures could be formed on the substrate surface and on the adhesive tape17 (Figure 1d). The structure of the bottom layer of the honeycombpatterned films was different from that of the upper half layer (9) (a) Widawski, G.; Rawiso, M.; Franc¸ ois, B. Nature 1994, 369, 387. (b) Pitois, O.; Franc¸ ois, B. Eur. Phys. J. B 1999, 8, 225. (c) Pitois, O.; Franc¸ ois, B. Colloid Polym. Sci. 1999, 277, 574. (10) (a) Karthaus, O.; Maruyama, N.; Cieren, X.; Shimomura, M.; Hasegawa, H.; Hashimoto, T. Langmuir 2000, 16 (15), 6071. (d) Karthaus, O.; Cieren, X.; Maruyama, N.; Shimomura M. Mater. Sci. Eng. 1999, 10C, 103-106. (11) (a) Govor, L. V.; Bashmakov, I. A.; Kaputski, F. N.; Pientka, M.; Parisi, J. Macromol. Chem. Phys. 2000, 201, 2721. (b) Govor, L. V.; Bashmakov, I. A.; Kiebooms, R.; Dyakonov, V.; Parisi, J. AdV. Mater. 2001, 13 (8), 588. (12) Srinibasarao, M.; Collings, D.; Philips, A.; Patel, S. Science 2001, 292, 79. (13) Stenzel, M. H. Aust. J. Chem. 2002, 55, 239. (14) Erdogan, B.; Song, L.; Wilson, J. N.; Park, J. O.; Srinivasarao, M.; Bunz, U. H. J. Am. Chem. Soc. 2004, 126 (12), 3678. (15) Nishikawa, T.; Nonomura, M.; Arai, K.; Hayashi, J.; Sawadaishi, T.; Nishiura Y.; Hara, M.; Shimomura, M. Langmuir 2003, 19 (15), 6193. (16) Yabu, H.; Shimomura, M.; Langmuir 2005, 21 (5), 1709. (17) Yabu, H.; Takebayashi, M.; Tanaka, M.; Shimomura, M. Langmuir 2005, 21 (8), 3235.

10.1021/la053486b CCC: $33.50 © 2006 American Chemical Society Published on Web 04/28/2006

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Figure 1. Schematic illustration of novel microstructures fabricated from self-organized honeycomb-patterned films. The cartoon shows the preparation of a single-layer honeycomb-patterned film (a), a stretched honeycomb-patterned film (b), a micro lens array (c), and a pincushion structure (d). Chart 1

stuck on the adhesive tape. There were uniformly sized holes in the upper layer and regular dimples on the bottom layer. To fix the upper layer on the substrate, the honeycombpatterned film was turned upside down onto another solid substrate before peeling. The honeycomb-patterned film was removed from the substrate in water using tweezers, and placed on another substrate (Figure 2). After drying at room temperature (RT), the honeycomb-patterned film was annealed to immobilize it securely on the substrate. Peeling of the inverted honeycomb-patterned film yielded a pillar structure with holes on the solid substrate. In this paper, we report the effects of varying the annealing temperature on the surface structures of the inverted honeycombpatterned film after peeling. 2. Experimental Section 2.1. Preparation of Honeycomb-Patterned Films. The synthesis and characterization of the amphiphilic copolymer 1 used in this work (see Chart 1) have been reported elsewhere.18 A chloroform solution of the amphiphilic copolymer 1 and poly(bisphenol A carbonate) (Mw ) ∼64 000 g/mol, Aldrich, USA) was prepared, with a 1:9 weight ratio of copolymer 1 and poly(bisphenol A carbonate) and a 2.5 mg/mL concentration. This homogeneous solution was dropped on a glass substrate (a glass Petri dish of 90 mm diameter and 10 mm height), after which humid air ( ∼40-70% relative humidity) was applied vertically on the solution surface. The morphology of the prepared film was observed by an optical Figure 2. Schematic illustration of transferring the honeycombpatterned film onto a solid substrate.

(18) Nishida, J.; Nishikawa, K.; Nishimura, S.-I.; Wada, S.; Karino, N.; Nishikawa, T.; Ijiro, K.; Shimomura, M. Polym. J. 2002, 34 (3), 166-174.

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Figure 3. Schematic illustration of the pattern formation process by annealing honeycomb-patterned films on solid substrates (Black: poly(bisphenol A carbonate); Red: 1). The left column shows the result of annealing the sample at 100 °C and peeling after immersion in liquid nitrogen. A pincushion structure could be clearly imaged by SEM. The center column shows the results of annealing at 130 °C. An SEM image of a peeled honeycomb-patterned film and an AFM image of the structure remaining on the ITO electrode are also shown. The right column shows the results of annealing at 190 °C. An SEM image of a peeled honeycomb-patterned film and an AFM image of the structure remaining on the ITO substrate after annealing are also shown. microscope (OM; BH-2, Olympus, Japan), and a scanning electron microscope (SEM; S-3500N, Hitachi, Japan). 2.2. Transferring the Honeycomb-Patterned Film onto a Solid Substrate, Annealing, and Peeling Off the Honeycomb-Patterned Film. A schematic illustration of the process of transferring the honeycomb-patterned film onto a solid substrate and annealing is shown in Figure 2. The surface of a glass or indium-tin-oxide (ITO) electrode was cleaned by UV-ozone treatment for 30 min. using an ozone cleaner (NL-UV253, Nippon Laser Denshi, Japan). The substrate was washed with chloroform and ethanol with sonication for 30 min and dried at RT. After washing, the surface became highly hydrophilic (the water contact angle became less than 10°). The prepared honeycomb-patterned film was cut into ∼10 × 20 mm sheets using a surgical knife. A sheet of the cut film was peeled off from the glass substrate in water and floated on the water surface. The floating film was turned upside down with tweezers and placed on the surface of another solid substrate (cleaned glass, ITO, or gold electrode) immersed in the water. The sample thus prepared was dried in vacuo.

The film on the substrate was annealed at temperatures ranging from 100 to 200 °C for about 5-60 min on a hot stage (RINCAM600, Japan Hi-tech, Japan) under dry N2. After cooling to RT, the film was peeled off with a sheet of adhesive tape (Scotch tape, 3M, Japan). The surface structures of the film on the substrate and on the adhesive tape were observed using an OM, an SEM, and atomic force microscopy (AFM; SPI400, Seiko Instruments, Japan). By using a gold-coated AFM tip, electric current mapping and topography were simultaneously imaged. The patterned film electrode was fixed on the AFM sample stage with conductive silver paste (Dotite, Doujin Chemical Industry, Japan), and a bias voltage of 5.00-9.99 V was applied between the sample stage and the AFM tip. 2.3. Fourier Transform Infrared Reflection Absorption Spectroscopy (FT-IR RAS). Gold-coated ITO substrates were used for FT-IR RAS measurements. A cleaned ITO electrode was coated with Au by an ion sputtering apparatus (VPS-020, Sinku Kiko, Japan). FT-IR RAS measurement was carried out using an FT-IR200 spectrometer (JASCO, Japan) with a polarizing reflection unit (PR500).

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Figure 5. Plot of the pore size of the original honeycomb-patterned films versus the diameters (red closed circles) and heights (blue closed squares) of transferred rings (a) and dots (b).

Figure 4. FT-IR RAS spectra of the honeycomb-patterned films (a), microrings (b), and microdots (c), on a gold substrate. 2.4. Differential Scanning Calorimetry (DSC) Measurement. Ten milligrams each of poly(bisphenol A carbonate) and amphiphilic copolymer 1 and a 2.5 mg sheet of the honeycomb-patterned film were sealed in separate aluminum pans (d ) 5 mm). DSC curves of these materials were measured using a DSC8230 (Rigaku, Japan) with an empty aluminum pan as a reference. The heating and cooling rates were 10 and 5 °C/min, respectively.

3. Results and Discussion 3.1. Preparation of Honeycomb-Patterned Films. The chloroform solution of 1 and poly(bisphenol A carbonate) was cast on an ITO substrate. After complete evaporation of the solvent, a white turbid polymer film with interference colors was observed. A typical scanning electron micrograph of this honeycomb-patterned film is shown in Figure 2.1. Well-arranged and uniformly sized micropores were formed. The pore size was controlled from 4 to 10 µm by changing the casting volume in this experiment. The solvent evaporation time, which was equal to the water condensation time, was the main controlling factor

of pore size in all cases. Larger amounts of casting solution required longer times for complete solvent evaporation, and the water droplets grew larger.19 A cross-section of the film is shown in the inset of Figure 2.1, and this micrograph indicates that the honeycomb-patterned film consisted of double layers supported by pillar structures. Pillars at the vertexes of the hexagons connected the hexagonal lattices and indentations vertically. 3.2. Formation of Pincushions, Hexagonally Arranged Microrings, and Microdots. After annealing at 100 °C, there was no structure remaining after just peeling with adhesive tape at RT (Figure 3.6, left column). Poly(bisphenol A carbonate) is a mechanically stable polymer, and therefore it was difficult to break the pillars. To reduce the mechanical stability of poly(bisphenol A carbonate), the inverted honeycombpatterned film placed on a glass substrate was immersed in liquid nitrogen. After removing the film from the liquid nitrogen, the frozen honeycomb-patterned film was peeled with adhesive tape under dry atmosphere. Interference colors were observed from the surface of this substrate (Figure 3.7), and a uniform pincushion structure with microscopic holes on the substrate was observed by SEM (Figure 3.8, left column). A pincushion structure with dimples was formed on the adhesive tape. (19) (a) Beysens, D. Atmos. Res. 1995, 39, 215. (b) Yabu, H.; Tanaka, M.; Ijiro, K.; Shimomura, M. Langmuir 2003, 19 (15), 6297. (c) Yabu, H.; Shimomura, M. Chem. Mater. 2005, 17 (21), 5231.

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Figure 6. (a) Schematic illustration of conductive AFM measurements. Topography (b,c) and current images (d,e) of microrings and microdots, respectively.

After annealing at 130 °C, SEM and AFM clearly revealed that a hexagonally arranged ring structure was formed on the ITO substrate (Figure 3.6, center column). This honeycomb structure was transferred onto adhesive tape. The average diameter, height, and width of these microrings were 3.3 µm, 180 nm, and 540 nm, respectively, when the honeycomb-patterned film with 4 µm pores was used. After annealing at temperatures above 190 °C, the structure formed on the substrate was drastically changed. AFM measurements showed that micron-sized dots were formed on the substrate surface (Figure 3.8, right column). The arrangement of the dot structure was reflected in the hexagonal arrangement of honeycomb pores. There were hexagonally arranged dents on the adhesive tape surface. It was noted that the template honeycomb-patterned film was deformed, indicating that the original honeycomb structure was melted and deformed during the annealing process. 3.3. Formation Mechanism of Micropatterns. To reveal the chemical composition of the ring and dot structures, FT-IR RAS spectroscopy was performed. Figure 4a-c shows the FT-IR spectra of the original honeycomb-patterned film, the hexagonal microring structure, and the hexagonal microdot structure prepared on a gold-coated ITO electrode, respectively. The Cd O stretching absorption of the carbonate group of poly(bisphenol A carbonate) was measured at 1790 cm-1. The microring and microdot structures did not exhibit this absorption. Absorptions at 1648 and 1708 cm-1 attributed to the stretching of amide CdO and N-H groups were observed in the samples of microrings and microdots, as well as C-H stretching absorptions at 2930 and 2853 cm-1. These absorptions were not clearly observed in the absorption spectra of the honeycomb-patterned film. The foregoing results indicated that the main component of the honeycomb-patterned films was poly(bisphenol A carbon-

Yabu and Shimomura

Figure 7. DSC curves of bulk poly(bisphenol A carbonate) (blue), bulk 1 (red), and the honeycomb-patterned films (purple).

ate). Surprisingly, the major component of the hexagonal rings and dots was the acrylamide polymer, 1. In the process of formation of the honeycomb-patterned film, a temporary water-solution interface was formed, and the amphiphilic polymer 1 condensed at this interface. After evaporation of solvent and water, 1 was localized at the edge of the honeycomb pore, and the resulting regular arrangement of microrings reflected the regularity of the pore arrangement in the original honeycomb-patterned film. The diameter of the microrings was identical to that of the honeycomb pores. Figure 5 shows the diameters and changes in height of microrings and microdots when the dimensions of the original honeycombpatterned films were changed. When the original pore size increased, the diameters of the microrings also increased. This diameter was almost the same as the pore size of the original honeycomb-patterned films. However, the rings sometimes stretched slightly. Because the film was handled by tweezers, some stresses were applied to the film. As the result, the pores of the film were also stretched. The size distributions of the rings in the case of the average pore size of 6.0 and 9.0 µm were caused by this stretching. On the other hand, the ring heights were ∼200 nm in all cases. A change in the size of the microdot structure was also observed (Figure 5b). The heights of the dots increased with increasing pore and microring diameter. Topography and current images of microrings and microdots were simultaneously observed using AFM (Figure 6). The inverted topography image was observed in the current images, so no polymers remained on the substrate except for microrings or microdots. Since the microring and microdot structures were prepared by thermal annealing of the honeycomb-patterned films, their thermal properties played a significant role in the formation of these

Preparation of Pincushins, Microrings, & Microdots

structures. DSC measurements shed light on the thermal properties of poly(bisphenol A carbonate), 1, and the honeycomb-patterned film (Figure 7). From the DSC curve of poly(bisphenol A carbonate), its glass transition temperature (Tg) was determined to be 144.6 °C. The amphiphilic copolymer 1 showed multiple transitions ranging from ∼45 to 140 °C. It is well-known that some amphiphilic molecules form liquid crystalline phases. These transition points corresponded to these mesophases in the bulk of the amphiphilic copolymer. Moreover, the melting point was clearly observed at 194.3 °C. The DSC curve of the honeycomb-patterned film composed of the 9:1 mixture of poly(bisphenol A carbonate) and 1 is shown as the purple line in Figure 7. The two clear transition points observed at 116.1 and 181.3 °C indicated the formation of a polymer blend. These transition temperatures corresponded respectively to the formation temperatures of the two microstructures, namely, microrings and microdots. Below the first transition point (116.1 °C), the amphiphilic polymer 1 did not diffuse to a solid substrate because the polymer blend was still too rigid for polymer diffusion. Between the two transition points, the mobility of the honeycomb-patterned film increased, and therefore the amphiphilic copolymer 1, which forms a mesophase, diffused to the hydrophilic surface of the ITO substrate to form microrings. This may have occurred because the affinity between 1 and the ITO surface might have been higher than that between 1 and poly(bisphenol A carbonate). When annealing was carried out above the second transition temperature (181.3 °C), the honeycomb matrix of poly(bisphenol A carbonate) started to plasticize. The spherical pores collapsed into hemispherical shapes, after which the amphiphilic copolymer 1 melted in the hemispherical dents formed between the substrate and the deformed honeycomb matrix of poly(bisphenol A

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carbonate). After cooling, hemispheres of 1 remained on the hydrophilic surface. When the honeycomb-patterned film was peeled in the frozen state, the double-layered structure was cleaved to form two pincushion structures because the polymer blend was in the glassy state.

4. Conclusion Hexagonally arranged microrings and microdots were formed by thermal treatment of honeycomb-patterned films on ITO electrodes. The diameters of the features and the spacing and periodicity of the above-mentioned structures depended on the dimensions of the honeycomb-patterned films, regardless of the height and width of the structures. FT-IR RAS spectroscopy and staining experiments showed that only the amphiphilic copolymer 1 was transferred onto the electrode surface. This was caused by the difference between the Tg values of poly(bisphenol A carbonate) and 1. Other amphiphilic polymers with Tg values lower than those of matrix compounds could be used to prepare these patterns. Furthermore, these patterns could be useful for versatile applications in electronics, photonics, and biotechnology. Microelectrodes, two-dimensional photonic crystals,20 and cell culture substrates21 could be prepared by utilizing these patterns. Acknowledgment. This work was partly supported by a Grantin-Aid for Scientific Research, MEXT, Japan. LA053486B (20) Noda, S.; Cutinan, A.; Imada, M. Nature 2000, 407, 608. (21) (a) Brock, A.; Chang, E.; Ho, C.-C.; LeDuc, P.; Jiang, X.; Whitesides, G. M.; Ingber, D. E. Langmuir 2003, 19 (5), 1611. (b) Chen, C. S.; Mrksich, M.; Huang, S.; Whitesides, G. M.; Ingber, D. E. Biotechnol. Prog. 1998, 14, 356.