Novel Fabrication of 2D and 3D Inverted Opals and their Application

Transcription

1 Inverted Opals Novel Fabrication of 2D and 3D Inverted Opals and their Application Jae Ah Lee, Son Tung Ha, Hong Kyoon Choi, Dong Ok Shin, Sang Ouk Kim, Sang Hyuk Im, * and O Ok Park * Offering well-defined pore size, high surface area, and photonic bandgap tunability and having potential applications in membranes, [ 1 ] sensors, [ 2 ] superhydrophilic phobic films, [ 3 ] solar cells, [ 4 ] and photonic crystals, [ 5 ] 2D hexagonal structures and 3D inverted opal (IO) structures have recently been intensively studied. In order to fabricate uniform IO structures consisting of regular arrangements of spherical void spaces surrounded by solid walls, 2D and 3D colloidal crystals consisting of a regular arrangement of uniform spherical particles are required. To date, a number of techniques to assemble colloidal particles into 2D and 3D opals have been developed: 3D colloidal crystals have been fabricated by sedimentation, [ 6 ] convective assembly, [ 7 ] and electrophoretic deposition, [ 8 ] while 2D colloidal crystals are produced by confined convective assembly. [ 9 ] Generally, 2D colloidal crystals have been used to fabricate 2D structures through edge spreading lithography, [ 10 ] colloidal masking, and templating while 3D colloidal crystals have been independently utilized to construct 3D structures through infiltration and templating. [ 11, 12 ] The assembly of colloidal particles into 3D opals over large area through conventional convective assemblies is simpler and more convenient than the assembly of 2D colloidal crystals because the colloidal particles positioned at the air-medium substrate interface tend to be assembled into [+] [+] J. A. Lee, S. T. Ha, Dr. H. K. Choi, Prof. O. O. Park Department of Chemical & Biomolecular Engineering (BK21 Graduate Program) Korea Advanced Institute of Science and Technology (KAIST) 291 Daehak-ro, Yuseong-gu Daejeon , Republic of Korea ookpark@kaist.ac.kr D. O. Shin, Prof. S. O. Kim Department of Materials Science and Engineering Korea Advanced Institute of Science and Technology (KAIST) 291 Daehak-ro, Yuseong-gu Daejeon , Republic of Korea Dr. S. H. Im KRICT-EPFL Global Research Laboratory Advanced Materials Division Korea Research Institute of Chemical Technology 19 Sinseongno, Yuseong Daejeon , Republic of Korea imromy@krict.re.kr [+] These authors contributed equally to this work. DOI: /smll multiple layers by a combination of capillary force and convective flow due to spontaneous evaporation of the medium. [ 9 ] Hence, 2D colloidal crystals have been assembled over a large area by confined convective assembly. Using this approach, the assembly rate of colloidal particles toward the airmedium substrate interface can be controlled by the external air flux. Furthermore, the thinning rate of the meniscus can be adjusted by the lift-up speed of the substrate: a thinner meniscus leads to the assembly of colloidal particles into a thinner layer and ultimately into a single layer. [ 9 ] Therefore, the fabrication of uniform 2D IO structures (to notify that the 2D honeycomb structure is derived from 3D opal, hereafter we denoted it as 2D IO even though the opal means a 3D structure) conveniently and economically from a conventional 3D colloidal crystal template remains challenging. Herein, we report on a simple and convenient technique to construct 2D and 3D IO structures from 3D colloidal crystals by control of the solidification rate of partially hydrolyzed and condensed silica sol (hereafter silica sol). We also demonstrate that the 2D and 3D IO structures constructed from the 3D colloidal crystals can serve as templates for directing self-assembled nanostructures of polystyrene- block - polymethylmethacrylate (PS- b-pmma) copolymers. The fabrication method of 2D and 3D IO films is schematically illustrated in Scheme 1. First, physically interconnected 3D PS colloidal crystals were prepared on a glass plate by dipping the glass plate vertically into a 0.1 wt% PS colloidal suspension containing wt% of poly(vinyl pyrrolidone) (PVP). [ 13, 14 ] The PS colloidal particles interconnected by PVP bridges keep the structure of 3D colloidal template sufficiently stable during the subsequent infiltration process of silica sol (see the upper inset in Figure 1 a). Second, for the fabrication of 2D and 3D IO structures, we connected the 3D colloidal crystal template to a dip coating machine in order to control the lift-up speed. A glass plate fixed on the bottom of the table was attached to the top of the 3D colloidal template film, leaving a gap of 100 μ m, as shown in Scheme 1. Finally, silica sol was injected into the gap by capillary force and the back 3D colloidal template film was then lifted up at a specified rate while blowing warm air toward the meniscus in order to evaporate the solvent of the silica sol and thereby solidify the silica sol. Note that a top-opened IO structure could not be obtained unless the fixed glass plate first sweeps out the excess silica sol on top of the 3D colloidal template. For the fabrication of 2D and 3D IO structures by the same single process, we controlled the solidification rate of silica 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 2581

2 communications J. A. Lee et al. PS PVP Silica sol temp. (25 o C) Calcination (a) Lift-up Fixed Bending by shrinkage Silica sol (b) Hot air flux (~50 o C) Silica sol temp. (-4 o C) Selfdetachment Scheme 1. The systematic illustration for the fabrication of 2D and 3D IO structures. a,b) The fabrication procedure of the 3D (a) and 2D (b) IO structure. sol as illustrated in Scheme 1. Generally, the solidification rate of silica sol is dependent on the solvent evaporation rate because the silica sol begins to gelate and is then solidified through further volume shrinkage as it is concentrated by solvent evaporation. Based on this simple concept, we have tried to control the solidification rate of silica sol through control over the solution temperature and lift-up speed because the evaporation rate of the solvent depends on both of these parameters. As a model system for constructing 2D IO structures, we increased the evaporation rate of the solvent and used a precooled silica sol of 4 C because an increased evaporation rate will accelerate the solidification rate at the top layer and the reduced solution temperature will induce a sufficient temperature gradient between the bottom layer and the top layer in the 3D colloidal template retaining the precooled silica sol. In other words, to fabricate 2D IO structures from a 3D colloidal template, the infiltrated silica sol in the first bottom colloidal layer should remain in solid form and the remaining silica sol and PS colloidal particles should be detached from the back glass plate. To realize this concept, it is necessary to solidify the infiltrated silica sol from the top colloidal layer via a temperature gradient by solidifying the precooled silica sol under reduced lift-up speed. The temperature difference between the bottom and the top layers causes the infiltrated silica sol to be solidified from the top surface. The solidified silica sol then undergoes volume shrinkage, causing the solidified silica sol with PS colloidal particles to be bent in an outward direction, as shown in Scheme 1 b. The bent solidified silica sol with the 3D colloidal film then self-detaches from the original glass plate, leaving the 2D IO structure of solidified silica sol. The preferentially solidified silica sol at the interface between the top colloidal layer and the fixed glass plate tends to adhere to the fixed glass plate at the meniscus of the silica sol air glass interface. This appears to facilitate self-detachment from the mother glass plate by shear force. Also, the precooled silica sol prevents solidification of the reservoir silica sol because, if the reservoir silica sol is solidified, the two glass plates will adhere to each other. Accordingly, the precooled silica sol enables fabrication of 2D IO over a large area. At the same time, in order to detach the silica-sol-infiltrated 3D colloidal film from the mother glass plate, the silica sol should be sufficiently solidified via reduced lift-up speed. In addition, the PS colloidal particles tightly interconnected via PVP bridges facilitate complete detachment of the solidified silica-sol-infiltrated 3D colloidal film from the mother glass plate. Furthermore, the remaining PVP on the glass plate can aid the formation of clear 2D IO patterns (see the lower inset in Figure 1 a) because it can hold the colloidal particles and retain the solidifying silica sol until the solidified silica sol and 3D colloidal template are detached from the mother glass plate. Therefore, we infiltrated precooled silica sol of 4 C to the PVP incorporated 3D colloidal template and then lifted up the silica-sol-infiltrated 3D colloidal film at a rate of 1.5 mm/min while blowing warm air of 50 C. As a model system for constructing 3D IO structures, we reduced the solidification speed of the infiltrated silica sol by increasing the lift-up speed. Basically, it is much simpler and easier to fabricate 3D IO structures from 3D colloidal crystals because only the infiltrated silica sol at the top colloidal layer has to be eliminated before the silica sol is solidified through either faster lift-up speed or relatively cooler air (room temperature). After sweeping out the silica sol from the top colloidal film with the fixed glass plate, a 3D IO structure having an open-top structure is fabricated irrespective of the solidification of silica sol. Hence, we only increased the lift-up speed to 6 mm/min while the other experimental conditions were held constant. The structure and morphology of the 2D and 3D IO structures fabricated through this simple concept are shown in Figure 1. The top-view scanning electron microscopy (SEM) image of the 3D colloidal template in Figure 1 a shows that PS particles of 270 nm diameter are assembled into a 3D ordered structure through the vertical deposition method in the PVPcontained PS colloidal suspension. [ 14 ] The insets clearly show that the PS colloidal particles are interconnected through PVP bridges and the PS particles on the first bottom layer also adhere to each other through the PVP. Figure 1 b clearly shows that a 3D IO structure could be fabricated by a quick lift-up process followed by calcination at 500 C for 4 h, as the silica sol could be quickly swept out prior to being solidified. The reduced pore size (to 240 nm) is attributed to typical volume shrinkage of calcined silica. Figure 1 c indicates that a 2D IO structure could be produced from the 3D colloidal template by slowly lifting up the precooled silica-sol-infiltrated 3D colloidal film. Here, the key control parameter for self-detachment is the solidification speed of the silica sol under shear force because preferential solidification at the interface of the top colloidal layer, and the fixed glass plate causes the solidified silica-sol-infiltrated 3D colloidal layer to Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

3 2D and 3D Inverted Opals Figure 1. The structure and morphology of the fabricated 2D and 3D IOs. Scanning electron microscopy (SEM) surface image of a) pristine 3D colloidal crystal (270 nm PS colloid): upper inset = SEM cross-sectional image, lower inset = SEM surface image of glass plate after detaching 3D PS colloids by adhesive tape; b) 3D IO; and c) 2D IO: d) magnified image, e) magnified tilted image, f) AFM topology, and g) roughness. be bent outward and the fixed glass plate continuously pulls the sticky solidified layer from the mother glass plate. However, the PVP bridges in the 3D colloidal crystals also play an important role in completely detaching the solidified silicasol-infiltrated 3D colloidal film from the mother glass plate. This was demonstrated by a failure to detach all colloidal particles from the original glass plate when the same experiment was performed with 3D colloidal crystals not having PVP bridges, as shown in Figure S1 (in the Supporting Information (SI)). The magnified top and tilted view images of the 2D IO structure in Figure 1 d,e confirm the formation of 2D hexagonal structured IO. As shown in Figure 1 f,g, the topographic image and roughness of the 2D IO film measured by atomic force microscopy (AFM) also confirm the formation of hexagonal 2D IO, which has 94 nm hole depth and 285 nm hole diameter. In order to assess whether this simple concept can be extended to fabricate smaller and larger 2D and 3D IO structures from their 3D colloidal templates, we fabricated 2D and 3D IO structures with 180-nm- and 480-nm-sized PS colloidal crystals, as shown in Figure 2. While optimized conditions for the fabrication of 2D and 3D IO structures with differently sized PS colloids could be tuned according to the concept, we fabricated 2D and 3D IO structures with different diameters under the same experimental conditions as previously used for the 270-nm-sized IO structure. This implies that this concept is very versatile for the fabrication of 2D and 3D IO structures. Furthermore, we could reproducibly fabricate several 2D and 3D IO structures with this method, and the images in Figure 2 also confirm the reproducibility. Basically there is no limitation to fabricate inverted opals through the proposed scheme if the original colloidal crystals have sufficient physical stability during the next infiltration process. However, we only have obtained the original colloidal crystals under 100 μ m thickness by vertical deposition method because it is difficult to fabricate millimeter thick colloidal crystals without cracks or sufficient physical stability 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

4 communications J. A. Lee et al. Figure 2. The SEM images of 2D (a,c) and 3D (b,d) IOs fabricated from differently sized polystyrene colloids. a,b) 180 nm and c,d) 480 nm. In particular, these IO structures can be used as templates for directing self-assembled nanostructures of block copolymers. Therefore, we attempted to regulate the self-assembled nanostructures of PS- b -PMMA block copolymers as a model system. Figure 3 a c show that the 270 nm 2D IO films could confine the PS- b -PMMA (number-averaged molecular weights, M n : (48 kda)- b -(46 kda), polydispersity index, PDI = 1.09) within the unit holes and regulate the internal structure of PS- b -PMMA. Here, the morphology of the annealed PSb -PMMA (240 C for 3 days) confined within the 2D IO film is developed by the surface affinity toward specific block of the block copolymer. Therefore, the PMMA block in the annealed PS- b-pmma within the 2D IO structure tends to be located at the surface of the 2D IO film due to the hydrophilic nature of the solidified silica sol. The morphology of Figure 3 a might be formed by incomplete phase separation or dimensional incommensurateness between block length of PS- b -PMMA and a unit hole of 2D IO film. To enhance the mobility of PS- b -PMMA ((48 kda)- b -(46 kda)) in a 2D IO film without significant perturbation of structural dimension of block copolymer, we added 10% weight fraction of a low-molecular-weight PS- b-pmma ( M n : (5 kda)- b- (5 kda), PDI = 1.14) with a low χ N (χ : Flory Huggins interaction parameter, N : degree of polymerization) value in the PS- b -PMMA ((48 kda)- b - (46 kda)) [ 15, 16 ] and annealed the film under the same condition. Figure 3 b shows that the morphology of PS- b-pmma could be fully developed like onion structure with the assist of small block copolymer molecules. To check the morphology of vertically aligned PS- b -PMMA lamella in a confined geometry of 2D IO film, we neutralized the surface of the 2D IO film with a hydroxyl terminated PS- r-pmma (polystyrene- random -polymethylmethacrylate copolymer (PS- r-pmma: weight-averaged molecular weight, M w = g/mol, PDI = 1.45, PS fraction = 0.6). [ 17 ] Figure 3c clearly shows that the PS- b -PMMA ((48 kda)- b -(46 kda)) lamella are vertically aligned within the 2D IO film, similar to the morphology on a flat substrate (see Figure S2 in the SI), and there are no directional order between adjacent unit holes due to the physical separation of each hole unlike the morphology developed on a flat substrate. Finally, we infiltrated PS- b-pmma ((48 kda)- b -(46 kda)) into the 270 nm 3D IO film and annealed it at 240 C for 3 days (see the SEM images of each processing step for 3D IO film and templated PS- b-pmma colloidal particles in Figure S3 in the SI). Figure 3 d shows the SEM image of templated PS- b -PMMA particles from 3D IO film after annealing of infiltrated PS- b -PMMA in 3D IO film and consecutive etching of silica frame by HF solution. The templated PS- b -PMMA particles have ellipsoid in shape due to volume shrinkage of 3D silica IO film during calcination. To see the internal morphology of templated PS- b-pmma particles, we stained the PS blocks in the particles with RuO 4 Figure 3. The SEM images of annealed block copolymer on 270 nm 2D and 3D IO films. a) PS- b -PMMA ((48 kda)- b -(46 kda)), b) mixture of PS- b -PMMA ((48 kda)- b -(46 kda)) and PS- b -PMMA ((5 kda)- b -(5 kda)) by 10:1 (wt:wt), c) PS- b -PMMS ((48 kda)- b-(46 kda)) on PS- r -PMMA brush-coated 2D IO films, and d) PS- b -PMMA ((48 kda)- b -(46 kda)) templated particles from 3D IO film: inset = transmission electron microscopy (TEM) image Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

5 2D and 3D Inverted Opals to make contrast. The TEM image of cross-sectionally mircotomed PS- b -PMMA particles in the inset of Figure 3 d clearly shows that the morphology of PS- b-pmma copolymers is fully developed to an onion structure. In summary, we could fabricate silica-based 2D and 3D IO structures from a PVP-incorporated 3D colloidal crystal template by controlling the solidification rate of infiltrated silica sol. 2D IO structures were fabricated by quickly solidifying precooled silica sol ( 4 C) infiltrated in a 3D colloidal template from the top colloidal layer under slow lift-up speed (lift-up at a rate of 1.5 mm/min while blowing 50 C warm air). The precooled silica sol prevented solidification of the reservoir silica sol separating two glass plates with a gap during the process. The increased evaporation rate by slow lift-up speed solidified the precooled silica sol retained in the 3D colloidal template preferentially from the top colloidal layer. Accordingly, the fixed glass plate caused the solidified silica-sol-infiltrated 3D colloidal layer to be bent outward and the fixed glass plate continuously detached the sticky solidified layer from the mother glass plate. Silica-based 3D IO structures could be simply fabricated by reducing the solidification rate (which was achieved by fast lift-up speed of 6 mm/min) because the over-infiltrated silica sol on the top colloidal layer only has to be swept out before being solidified. In addition, the 2D and 3D IO films could be used as template for directing self-assembled nanostructures of PSb -PMMA. These 2D and 3D IO films can be useful to make well-ordered complex morphology or combined mixed structure of short range internal structure and relatively long range external structure. Experimental Section Materials : Styrene monomer (Aldrich), sodium styrene sulfonate (Aldrich), potassium persulpate (Aldrich), PVP (Aldrich, M w = g/mol), ethanol (Merck), tetraethyl orthosilicate (TEOS, Aldrich), hydrochloric acid (Merck), PS- b -PMMA ((48 kda)- b - (46 kda), PDI = 1.09, Polymer source) and PS- b -PMMA ((5 kda)- b -(5 kda), PDI = 1.14, Polymer source) were used as received. Preparation of PS Colloids : PS colloidal particles of 270 nm diameter were prepared by surfactant-free emulsion polymerization. [ 12 ] We poured deionized water (350 g) into a reactor and kept the water at a temperature of 70 C while stirring at 350 rpm. We then added sodium styrene sulfonate (0.1 g) as an emulsifier and styrene monomer (50 g) into the solution. After 30 min, we added potassium persulfate (0.25 g) as an initiator with deionized water (100 g) to the solution. Finally, we performed polymerization under a nitrogen atmosphere for 18 h. The same procedure was used to prepare 180 and 480 nm PS colloidal particles, and the content of sodium styrene sulfonate was varied (0.5 and 0.05 g, respectively). Preparation of 3D Colloidal Template: To assemble 3D colloidal crystals, we dipped the glass substrates vertically into a 0.1 wt% PS colloidal suspension (40 ml) containing 1.9 mwt% of PVP. The colloidal suspensions were then placed in a furnace at 70 C for 18 h to deposit the 3D colloidal crystals onto a glass substrate. [ 14 ] Preparation of Silica Sol: Ethanol (4 ml), TEOS (6 ml), and deionized water (3 ml) were poured into a 20 ml round flask under magnetic stirring. Hydrochloric acid (1 ml) was then added slowly as an acidic catalyst. After 10 min, we kept the silica solution in a freezer at 4 C. Preparation of the 2D IO Structure : We connected the glass substrate with deposited 3D colloidal crystals to a dip coating machine (Sigma Koki, Japan) to control the lift up speed. The glass substrate pretreated by oxygen plasma for 5 min was then attached to the 3D colloidal crystal film, leaving a gap of approximately 100 μ m. We injected precooled silica sol of 4 C into the gap via capillary force. Lastly, we lifted up the back substrate at a rate of 1.5 mm/min while blowing 50 C warm air from a distance of 30 cm towards the meniscus in order to solidify the silica sol by evaporation of the solvent. Preparation of the 3D IO Structure: To fabricate 3D IO structures, we used silica sol of 25 C instead of precooled silica sol of 4 C. We then lifted up the silica sol infiltrated 3D colloidal template film at a rate of 6 mm/min while the other conditions were identical to those employed in the 2D IO fabrication procedure. We next calcined the silica sol infiltrated film in a furnace to remove the PS particles. The temperature was ramped up at 2 C/min from room temperature to 500 C and held at 500 C for 4 h. The furnace was subsequently cooled gradually to room temperature. Morphology Regulation of PS-b-PMMA in 2D and 3D IO Film: To check the morphology difference of PS- b -PMMA ((48 kda)- b-(46 kda)) in 2D silica IO film, we spin-coated 0.1 wt% of PS- b-pmma toluene solution on a 270 nm 2D IO film at 2500 rpm for 40 s. The coated film was then thermally annealed in a vacuum oven at 240 C for 3 days to develop morphology. To enhance the mobility of PSb -PMMA ((48 kda)- b -(46 kda)), we mixed PS- b -PMMA ((48 kda)- b -(46 kda)) and PS- b -PMMA ((5 kda)- b -(5 kda)) by 10:1 (wt:wt) in toluene. The 0.1 wt% of block copolymer mixture in toluene was then spin-coated on a 2D IO film and then was thermally annealed at the same condition to develop morphology. For the preparation of neutral surface of 2D IO film, we spin-coated a hydroxylterminated random copolymer of PS and PMMA (PS- r-pmma: M w = g/mol, PDI = 1.45, PS fraction = 0.6) on a 2D IO film and placed it at 160 C for 3 days in a vacuum oven to thermally anchor the PS- r -PMMA brush copolymers on the surface of 2D IO film. The PS- r -PMMA copolymers anchored 2D IO film was then washed with toluene to remove the unreacted PS- r-pmma copolymers. The 0.1 wt% of PS- b -PMMA ((48 kda)- b-(46 kda)) solution was then spin-coated on the surface neutralized 2D IO film and was annealed at the same condition. For the regulation of internal structure of PS- b -PMMA ((48 kda)- b -(46 kda)) in 3D IO film, we dropped PS- b -PMMA solution in the 270 nm 3D IO film to infiltrate the PS- b -PMMA in the hole of 3D IO film (see Figure S3c in the SI). The PS- b-pmma-infiltrated 3D IO film was then annealed at 160 C for 3 days in a vacuum oven. To remove the silica frame in the PS- b-pmma-infiltrated 3D IO film, it was chemically treated by HF solution (CAUTION: very corrosive). To check the internal structure of PS- b -PMMA particles, we stained them with RuO 4 and sliced the film with a microtome. Supporting Information Supporting Information is available from the Wiley Online Library or from the author Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

6 communications J. A. Lee et al. Acknowledgements This work was supported by an ERC grant of the National Research Foundation of Korea NRF) funded by the Korea Ministry of Education, Science and Technology (MEST) (No. R (2009)), partially supported by a WCU grant from MEST (R ), and partially supported by National Research Laboratory (NRL) Program (R0A ). [1 ] B. Gates, Y. Yin, Y. Xia, Chem. Mater. 1999, 11, [2 ] A. C. Arsenault, H. Miguez, V. Kitaev, G. A. Ozin, I. Manners, Adv. Mater. 2003, 15, 503. [3 ] H. Li, J. Wang, L. Yang, Y. Song, Adv. Funct. Mater. 2008, 18, [4 ] S. Guldin, S. Hüttner, M. Kolle, M. E. Welland, P. Müller-Buschbaum, R. H. Friend, U. Stiner, N. Tétreault, Nano Lett. 2010, 10, [5 ] Y. A. Vlasov, X. Z. Bo, J. C. Sturm, D. J. Norris, Nature 2001, 414, 289. [6 ] R. Mayoral, J. Requena, J. S. Moya, C. López, A. Cintas, H. Mıguez, F. Meseguer, A. Vàzquez, M. Holgado, A. Blanco, Adv. Mater. 1997, 9, 257. [7 ] P. Jiang, J. F. Bertone, K. S. Hwang, V. L. Colvin, Chem. Mater. 1999, 11, [8 ] M. Holgado, F. García-Santamaría, A. Blanco, M. Ibisate, A. Cintas, H. Miguez, C. J. Serna, C. Molpeceres, J. Requena, A. Mifsud, F. Meseguer, C. López, Langmuir 1999, 15, [9 ] M. H. Kim, S. H. Im, O. O. Park, Adv. Funct. Mater. 2005, 15, [10 ] J. M. McLellan, M. Geissler, Y. Xia, J. Am. Chem. Soc. 2004, 126, [11 ] S. M. Yang, S. G. Jang, D. G. Choi, S. Kim, H. K. Yu, Small 2006, 2, 458. [ 12 ] A. Stein, F. Li, N. R. Denny, Chem. Mater. 2008, 20, 549. [13 ] J. H. Kim, M. Chainey, M. S. El-Aasser, J. W. Vanderhoff, J. Polym. Sci., Polym. Chem. 1992, 30, 171. [14 ] H. K. Choi, M. H. Kim, S. H. Im, O. O. Park, Adv. Funct. Mater. 2009, 19, [15 ] T. Xu, H. C. Kim, J. DeRouchey, C. Seney, C. Levesque, P. Martin, C. M. Stafford, T. P. Russell, Polymer 2001, 42, [16 ] D. O. Shin, J. R. Jeong, T. H. Han, J. M. Koo, H. J. Park, Y. T. Lim, S. O. Kim, J. Mater. Chem. 2010, 20, [17 ] P. Mansky, Y. Liu, E. Huang, T. P. Russell, C. Hawker, Science 1997, 275, Received: April 27, 2011 Revised: June 8, 2011 Published online: August 2, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim