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Langmuir 2008, 24, 12612-12617
Nanohole Structure Prepared by a Polystyrene-block-poly(methyl methacrylate)/poly(methyl methacrylate) Mixture Film Wonchul Joo,† Seung Yun Yang,† Jin Kon Kim,*,† and Hiroshi Jinnai*,‡ National CreatiVe Research InitiatiVe Center for Block Copolymer Self-Assembly, Department of Chemical Engineering and EnVironmental Science and Engineering, and Polymer Research Institute, Pohang UniVersity of Science and Technology, Pohang, Kyungbuk 790-784, Korea, and Department of Macromolecular Science and Engineering, Graduate School of Science and Engineering, Kyoto Institute of Technology, Kyoto 606-8585, Japan ReceiVed July 4, 2008. ReVised Manuscript ReceiVed August 29, 2008 Cylindrical nanoporous structures were prepared by using a mixture film of polystyrene-block-poly(methyl methacrylate) copolymer (PS-b-PMMA) and PMMA homopolymer (hPMMA), and they were analyzed by transmission electron microtomography (TEMT), X-ray reflectivity (XR), and grazing incidence small-angle X-ray scattering. For this purpose, the mixture film was spin-coated onto a silicon wafer modified by a neutral brush for PS and PMMA blocks, which generates PMMA cylindrical microdomains oriented normal to the substrate. Two methods were employed to prepare nanoporous structures: (1) all of the PMMA phase (PMMA block and PMMA homopolymer) in the film was removed by UV irradiation, followed by rinsing with a selective solvent (acetic acid) to PMMA and (2) only PMMA homopolymer was removed by selective solvent etching without UV irradiation. We found via TEMT and XR that the nanoporous structure in the film prepared by UV irradiation exhibited almost perfect cylindrical shape throughout the entire film thickness. However, when the film was rinsed with a selective solvent, nanoporous structures were not straight cylinders but had a funnel shape in which the diameter of nanopores located near the top of the film was larger than that located near the bottom of the film.
1. Introduction Block copolymers (BCP) consisting of different polymer blocks connected by chemical covalent bonding exhibit various selfassembled mesoscopic structures.1 Recently, nanoporous templates based on polystyrene-block-poly(methyl methacrylate) copolymers (PS-b-PMMA) have been employed to fabricate highdensity storage materials,2-7 nanoporous membranes,8 optical materials for photonic crystal,9 and antireflection coating materials.10 To prepare nanoporous templates, the cylindrical microdomains should be oriented normal to a substrate.11-14 The vertical orientation of PMMA cylindrical microdomains in PS-b-PMMA was achieved by using either an energetically neutral * To whom correspondence should be addressed. E-mail: jkkim@ postech.ac.kr and
[email protected]. † Pohang University of Science and Technology. ‡ Kyoto Institute of Technology. (1) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: New York, 1998. (2) Naito, K.; Hieda, H.; Sakurai, M.; Kamata, Y.; Asakawa, K. IEEE Trans. Magn. 2002, 38, 1949. (3) Cheng, J. Y.; Ross, C. A.; Chan, V. Z. H.; Thomas, E. L.; Lammertink, R. G. H.; Vancso, G. J. AdV. Mater. 2001, 13, 1174. (4) Mansky, P.; Harrison, C. K.; Chaikin, P. M.; Register, R. A.; Yao, N. Appl. Phys. Lett. 1996, 68, 2586. (5) Park, C.; Yoon, J.; Thomas, E. L. Polymer 2003, 44, 6725. (6) Kim, S. O.; Solak, H. H.; Stoykovich, M. P.; Ferrier, N. J.; DePablo, J. J.; Nealy, P. F. Nature 2003, 424, 411. (7) Kim, J. K.; Lee, J. I.; Lee, D. H. Macromol. Res. 2008, 16, 267. (8) Yang, S. Y.; Ryu, I.; Kim, H. Y.; Jang, S. K.; Kim, J. K.; Russell, T. P. AdV. Mater. 2006, 18, 709. (9) Edrington, A. C.; Urbas, A. M.; DeRege, P.; Chen, C. X.; Swager, T. M.; Hadjichristidis, N.; Xenidou, M.; Fetters, L. J.; Joannopoulos, J. D.; Fink, Y.; Thomas, E. L. AdV. Mater. 2001, 13, 421. (10) Joo, W.; Park, M. S.; Kim, J. K. Langmuir 2006, 22, 7960. (11) Thurn-Albrecht, T.; Steiner, R.; Derouchey, J.; Saford, C. M.; Huang, E.; Bal, M.; Tuominen, M.; Hawker, C. J.; Russell, T. P. AdV. Mater. 2000, 12, 787. (12) Jeong, U.; Kim, H. C.; Rodriguez, R. L.; Tsai, I. Y.; Stafford, C. M.; Kim, J. K.; Hawker, C. J.; Russell, T. P. AdV. Mater. 2002, 14, 274. (13) Jeong, U.; Ryu, D. Y.; Kho, D. H.; Kim, J. K.; Goldbach, J. T.; Kim, D. H.; Russell, T. P. AdV. Mater. 2004, 16, 533. (14) Kim, S. H.; Misner, M. J.; Russell, T. P. AdV. Mater. 2004, 16, 2119.
surface for thin films with a thickness of less than 50 nm11 or a mixture of PS-b-PMMA with PMMA homopolymer (hPMMA) for thicker films with a thickness of up to ∼300 nm.13 The nanoporous films are obtained by either the removal of all PMMA phases (PMMA block and hPMMA) by UV irradiation, following by rinsing with acetic acid, or the selective removal of only hPMMA by using acetic acid. An exact knowledge of the inner structure of the nanoporous film is important because it determines the final shape of nanomaterials when it is used as a nanotemplate for the preparation of various nanorods. Some research groups have investigated the inner structure of nanoporous BCP films.15-18 Russell and co-workers showed that the nanopores could be formed only by a selective solvent in a PS-b-PMMA thin film with a thickness of ∼30 nm, which is verified by using grazing incidence smallangle X-ray scattering (GI-SAXS), X-ray reflectivity (XR), and scanning probe microcopy (SPM).15,16 Yoon et al. investigated vertically oriented nanoporous film of PS-b-PMMA with a thickness of ∼70 nm by using GI-SAXS with model equations containing various structure factors such as the thickness, domain spacing, and hole radius.17 Kim et al.18 also analyzed the hole structure in a very thick film (∼400 nm) of a PS-b-PMMA/ hPMMA mixture by using optical waveguide spectroscopy.18 However, there has been no direct visualization and analysis for exact nanohole shape, especially when the nanoporous film thickness is larger than ∼50 nm. (15) Xu, T.; Stevens, J.; Villa, J. A.; Goldbach, J. T.; Guarini, K. W.; Black, C. T.; Hawker, C. J.; Russell, T. P. AdV. Funct. Mater. 2003, 13, 698. (16) Xu, T.; Goldbach, J. T.; Misner, M. J.; Kim, S.; Gibaud, A.; Gang, O.; Ocko, B.; Guarini, K. W.; Black, C. T.; Hawker, C. J.; Russell, T. P. Macromolecules 2004, 37, 2972. (17) Yoon, J.; Yang, S. Y.; Lee, B.; Joo, W.; Heo, K.; Kim, J. K.; Ree, M. J. Appl. Crystallogr. 2007, 40, 305. (18) Kim, D. H.; Aaron Lau, K. H.; Joo, W.; Peng, J.; Jeong, U.; Hawker, C. J.; Kim, J. K.; Russell, T. P.; Knoll, W. J. Phys. Chem. B 2006, 110, 15381.
10.1021/la8021134 CCC: $40.75 2008 American Chemical Society Published on Web 10/07/2008
Nanohole Structure Prepared by a Mixture Film
In this work, the inner structures of nanopores prepared by PS-b-PMMA/hPMMA mixture films were investigated by using transmission electron microtomography (TEMT) and XR. Because TEMT allows one to observe the 3D image directly,19 this technique has been widely used to investigate the complicated structures in BCP,20-24 nanoparticles on porous polymer substrates,25 and polymer/clay nanocomposites.26,27 Even though GI-SAXS is very powerful in investigating the nanoporous structure, the model equations for the structure parameters such as the diameter of the cylinder are needed to fit the scattering intensity.17 Moreover, it cannot give a direct visualization of nanoporous structures. We found that the nanoporous structure in the film prepared by UV irradiation exhibited an almost perfect cylindrical shape throughout the entire film thickness. However, when nanopores were generated by the removal of hPMMA by using a selective solvent, they were not straight cylinders but funnel-shaped, where the diameter near the top of the film was larger than that near the bottom of the film. We also obtained the electron density profile of nanoporous films by using XR, from which the shape of nanopores in the thin films is decided. The result obtained from XR is consistent with the TEMT image.
2. Experimental Section Materials and Sample Preparation. Asymmetric PS-b-PMMA synthesized in this laboratory by atomic-transfer radical polymerization13 has a weight-average molecular weight (Mw) of 73 000 g/mol and a polydispersity index (PDI) of 1.06 characterized by gel permeation chromatography (Waters). The volume fraction of the PMMA block in the BCP was 0.28, and thus the BCP exhibited cylindrical microdomains confirmed by small-angle X-ray scattering. The hPMMA was purchased from Polymer Source Inc. and used as received. The Mw and PDI of hPMMA are 31 800 g/mol and 1.08, respectively. A hydroxyl end-functionalized random copolymer of styrene and methyl methacrylate, denoted PS-r-PMMA with Mw ) 13 100 and PDI ) 1.47 was purchased from Polymer Source Inc. (lot no. P3437-SMMAran-OH) and used as received. The weight fraction of PS in PS-r-PMMA was 0.59. Thin films (∼70 nm) of the mixture of PS-b-PMMA and 10 wt % hPMMA relative to the PMMA block were prepared by spincoating from a 2 wt % toluene solution onto a silicon surface modified by a neutral brush of PS-r-PMMA and annealed at 170 °C for 2 days in vacuum, followed by quenching to room temperature. Then, the cylindrical nanodomains with hPMMA located near the center of the domains are oriented vertically to the substrate, as shown in Figure 1a. We prepared nanoporous structures by using two different methods: (1) After using UV irradiation with a maximum intensity at 253.7 nm (Sankyo Denki, G15T8) for 90 min, the entire PMMA phase was removed by rinsing with acetic acid, a selective solvent for PMMA, for 30 min and washing with distilled water. Then, the film was dried for 6 h in vacuum at room temperature. This method is referred to as UV etching, as shown in Figure 1b. (2) The film was immersed in acetic acid for 1 day at room temperature, followed by rinsing with distilled water. Then, the film was dried for 6 h in vacuum at room temperature. No further annealing at high temperature
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Figure 1. Schematic of the generation of nanoporous structures from (a) a neat PS-PMMA/PMMA mixture film by using two different methods: (b) UV etching and (c) solvent etching.
was carried out. In this situation, only hPMMA was removed with some swelling of the PS-b-PMMA chains. This method is referred to as solvent etching, as shown in Figure 1c. Analysis of Nanoporous Structure. To prepare the sample for transmission electron microscope (TEM: JEM-2200FS, JEOL Ltd.) images, the mixture film and films with nanoporous structures were floated onto a 5 wt % hydrofluoric acid solution and transferred to a copper grid. The samples were stained with ruthenium tetroxide (RuO4), which is a selective staining agent for PS.28 The TEM experiment was carried with a 200 kV accelerating voltage with 200 µA. To construct 3D images of TEM (namely, TEMT), a projection image at each tilt angle was acquired with a frame size of 1024 × 1024 pixels. A TEM sample holder was scanned over a tilt angle from -60° to +60° in 1° increments. Each tilt series was aligned separately by using 10 nm gold particles placed on the samples.20 The surface morphology of the thin films was investigated by scanning probe microscopy (SPM: Nanoscope IIIa, Digital Instrument) in height and tapping modes with silicon nitride tips (Veeco). The tip scan rate was 0.7 Hz with 256 resolution on a 2 µm × 2 µm surface. GI-SAXS measurements were carried out at the 4C2 beamline of the Pohang Accelerator Laboratory with an X-ray radiation source of λ ) 0.1542 nm and a 2D charge-coupled detector (2D CCD: Roper Scientific, Trenton). Samples were located on a four-circle goniometer (Huber, Rimsting) with a distance of 2152 mm between the sample and detector. The beam angle of incidence was ∼0.2°, which is between the critical angles of the films and the silicon substrates.17 X-ray reflectivity (XR) was carried out at the 3C2 beamline of the Pohang Accelerator Laboratory with an X-ray radiation source of λ ) 0.1542 nm and a scintillation counter with an enhanced dynamic range (Bede Scientific, EDR). Samples were located on a four-circle goniometer. The electron density profiles across the film thickness were obtained by using Parratt 1.6 software.29
3. Results and Discussion (19) Jinnai, H.; Nishikawa, Y.; Ikehara, T.; Nishi, T. AdV. Polym. Sci. 2004, 170, 115. (20) Jinnai, H.; Nishikawa, Y.; Ito, M.; Smith, S. D.; Agard, D. A.; Spontak, R. J. AdV. Mater. 2002, 14, 1615. (21) Sugimori, H.; Nishi, T.; Jinnai, H. Macromolecules 2005, 38, 10226. (22) Jinnai, H; Nishikawa, Y.; Spoontak, R. J.; Smith, D. A.; Agard, D. A.; Hashimoto, T. Phys. ReV. Lett. 2000, 84, 518. (23) Xu, T.; Zvelindovsky, A. V.; Sevink, G. J. A.; Lyakhova, K. S.; Jinnai, H.; Russell, T. P. Macromolecules 2005, 38, 10788. (24) Niihara, K.; Matsuwaki, U.; Torikai, N.; Atarashi, H.; Tanaka, K.; Jinnai, H. Macromolecules 2007, 40, 6758. (25) Jinnai, H.; Kaneko, T.; Nishioka, H.; Hasegawa, H.; Nishi, T. Chem. Rec. 2006, 6, 267. (26) Nishioka, H.; Niihara, K.; Kaneko, T.; Yamanaka, J.; Inoue, T.; Nishi, T.; Jinnai, H. Compos. Interfaces 2006, 13, 589.
Figure 2a gives the phase SPM image of the top surface of the PS-b-PMMA/hPMMA mixture film, from which hexagonally packed PMMA cylindrical microdomains are oriented vertically to the substrate. From the 2D GI-SAXS pattern (Figure 2A), the peaks in the qxy plane (in-plane X-ray scattering) were observed at q* and 3q*, although the intensity of the 3q* peak is much (27) Jinnai, H.; Shinbori, Y.; Kitaoka, T.; Akutagawa, K.; Mashita, N.; Nishi, T. Macromolecules 2007, 41, 6758. (28) Trent, J. S.; Scheinbeim, J. I.; Couchman, P. R. Macromolecules 1983, 16, 589. (29) Parratt 32 1.6, http://www.hmi.de.
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Figure 2. AFM images (left panels) and 2D GI-SAXS patterns (right panels) for a neat film (a and A), a UV-etched sample (b and B), and a solvent-etched sample (c and C). (D) One-dimensional SAXS profiles in the qxy plane at qz ) 0 for three samples.
smaller than that of q*. Here, q is the scattering vector given by 4π sin θ/λ, in which 2θ is the scattering angle. We also observed this peak from the 1D plot at qz ) 0. q* is the first peak position observed at 0.184 nm-1, corresponding to a domain spacing (D ) 2π/q*) of 34.2 nm. The diameter (d) of PMMA microdomains was determined to be 20 nm by SPM. This result indicates that PMMA cylindrical microdomains were oriented normal to the substrate. Figure 2b,B shows the SPM height image and 2D GI-SAXS patterns for the UV-etched sample, respectively. In this situation, all of the PMMA phases, including the PMMA block and hPMMA, were completely removed. From qxy peak positions of the 2D GI-SAXS pattern and the 1D SAXS profile given in Figure 2D, D of the UV-etched sample is essentially the same as that of the neat sample. Furthermore, d of the nanopores is the same as that of PMMA microdomains in the neat sample; thus the entire PMMA phase was completely removed by UV
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irradiation, followed by rinsing with acetic acid. This result indicates that the UV-etched sample exhibits vertical orientation of the nanoholes. Figure 2c,C shows the SPM height image and GI-SAXS 2D pattern for the solvent-etched sample, respectively. Nanopores were formed by the removal of only hPMMA, which was located near the center of PMMA cylindrical microdomains. Interestingly, the d and D of nanopores located near the surface measured by SPM are very similar to those in the UV-etched film, even though the volume of hPMMA is just 10% of that of the entire PMMA phase. A plausible reason is that some of the PMMA block chains migrated on the film surface during solvent etching. A similar observation was reported in ref 15, where only a neat PS-bPMMA thin film with a thickness of ∼30 nm was swollen by acetic acid. When some of the PMMA chains move on the film surface, the final film thickness of the solvent-etched sample should be larger than that of the neat film. This is verified by TEMT and XR results that will be discussed later. Furthermore, D measured by GI-SAXS and the 1D SAXS profile of the solvent-etched sample is the same as that of the neat sample and the UV-etched sample. However, we could not imagine the exact shape of microdomains or nanoholes of three samples by GI-SAXS profiles. Figure 3 gives conventional TEM images of the neat film, the UV-etched sample, and the solvent-etched sample. It is known that the PMMA microdomains or nanoholes look like bright spots. The black dots indicate gold nanoparticles used as the position markers for TEMT image reconstruction. It is seen that PMMA microdomains or nanoholes are well ordered in hexagonal packing. The bright spots in TEM images given in Figure 3b,c correspond to nanoholes. Interestingly, the TEM image of the UV-etched sample is different from that of the solvent-etched sample. For instance, bright circles exist near the center of the nanohole, but rims that are not as bright are seen near the boundary in Figure 3c, whereas uniform brightness is observed in Figure 3b. However, we do not clearly demonstrate the difference between these two cases by TEM imaging. Thus, we used the TEMT technique to verify the 3D image of nanopores in the UV-etched and solvent-etched samples. The movies for the 3D images for these two cases are given in the Supporting Information. On the basis of TEMT images, we reconstructed the nanoholes in both the UV-etched and solvent-etched samples as well as the cylindrical nanodomains in the neat film, as shown in Figure 4. The characteristic parameters are summarized in Table 1. It is seen that the diameter (d) of nanoholes near the top surface for both the UV-etched and solvent-etched samples is the same as that of PMMA cylindrical microdomains in the neat mixture film. The d of nanoholes of the UV-etched sample is maintained throughout the entire film thickness, as expected. However, the d of nanoholes located near the bottom of the solvent-etched sample is ∼7 nm, which is distinctly smaller than that (∼20 nm) of nanoholes located near the top of the sample. Thus, the nanoholes in the solvent-etched sample are not straight cylinders but funnel-shaped structures. Now, we calculate the diameter of hPMMA in the PMMA microdomains. When hPMMA is localized at the center of PMMA microdomains, the diameter of hPMMA (dh) is easily obtained from (d2 - dh2)/dh2 ) 1/fhPMMA, in which d is the diameter (20 nm) of the cylindrical microdomain for the total PMMA phases, and fhPMMA is the volume fraction of hPMMA for the total PMMA phases. Because fhPMMA ) 0.1, dh is estimated to be 6 nm, which is similar to that (7 nm) observed at the bottom surface by TEMT. This indicates that during the swelling process the PMMA block
Nanohole Structure Prepared by a Mixture Film
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Figure 4. Reconstructed three-dimensional TEM images of (a) the neat film, (b) the UV-etched sample, and (c) the solvent-etched sample. Table 1. Thicknesses and Diameters for the Neat Film, the UV-Etched Sample, and the Solvent-Etched Sample Determined by TEMT diameter (nm)
Figure 3. TEM images of (a) the neat film, (b) the UV-etched sample, and (c) the solvent-etched sample.
chains on the bottom surface of the film have little chance to migrate onto the film surface or into the neutral brush layer. It was reported that a uniform hole size was observed over the entire film thickness when the PS-b-PMMA thin film swelled in acetic acid.15,16 However, because the film thickness employed in refs 14 and 15 was ∼30 nm, PMMA block chains located even near the bottom of the film could be pulled out to the film surface. However, the film thickness employed in this study was ∼70 nm, which is much larger than the unperturbed end-to-end distance (〈R2〉1/2 ≈ 9.3 nm) of the PMMA block chains. Even though PMMA chains could swell more than 〈R2〉1/2 in a good solvent (acetic acid), the PMMA block chains located near the bottom of the film should be much too extended to be pulled out to the film surface. Therefore, with increasing film thickness, the diameter of the nanoholes observed near the film surface would not be the same as that near the bottom of the film; thus the straight cylindrical nanopores could not be obtained by the solvent-
sample
top surface
bottom surface
thickness (nm)
neat film UV-etched film solvent-etched film
20 21 20
20 20 7
67 65 71
etched sample. However, the UV-etched sample provides straight cylindrical nanopores because all of the PMMA phases are completely removed. Because TEMT images represent nanopores in only a limited area, we employed an XR experiment that gives us information about the nanopores over a much larger area. From XR, the change in the electron density is easily obtained throughout the film thickness. From Figure 4, we expect that the electron density should decrease from the top to bottom surface for the solventetched sample, whereas it should be constant through the entire film thickness for the neat sample and the UV-etched sample. Figure 5 gives XR profiles for three different films. The sample consists of two layers of the BCP film and the random brush layer. In Figure 5, however, we observe only high-frequency oscillation, even though two distinct oscillations (a high-frequency one for the BCP film and a low-frequency one for the random brush layer) should be present. This is because of the much thicker BCP film layer (more than ∼10 times) compared to that of the random brush and a small difference in the electron density between the BCP film and the random brush. For the solventetched film, the oscillation of the reflectivity disappeared at larger qz compared with that in neat and UV-etched samples. This is due to the large roughness of the film surface for the solventetched sample as a result of the movement of PMMA block chains to the film surface.
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Figure 5. X-ray reflectivity profiles for three different films (O): (a) the neat film, (b) the UV-etched sample, and (c) the solvent-etched sample. The solid lines of each curve represent the calculated reflectivity obtained from the best fit using Parratt 32 1.6 software.
Electron density profiles for three different films are shown in Figure 6. As shown in Figure 6a, the layer is assumed to have two layers of polymers (BCP and random brush) and a silicon wafer with a SiO2 layer with a thickness of 2 nm (which was verified by ellipsometry). From Figure 6b, the electron density becomes constant through the thickness direction for the neat film, indicating that PMMA microdomains are straight cylinders over the entire film thickness. Two distinct layers of the BCP film and the random brush layer were obtained, where the BCP film has a slightly larger electron density than that of the random brush. This is consistent with the result of ref 16. The film thickness of the BCP film was estimated to be 69.57 nm. The UV-etched sample has constant electron density through the thickness direction as shown in Figure 6c, indicating that nanoholes formed by the removal of the entire PMMA phase are straight and cylindrical in shape. Because of the cross-linking of PS block chains during UV irradiation, the film thickness (64.22 nm) was slightly decreased.13 Interestingly, the electron density profile for the solvent-etched film did not become constant; rather, a gradual increase with film thickness was observed, as shown in Figure 6d. This suggests that the size of nanoholes located near the top of the film surface should be larger than that of nanoholes located near the bottom of the film, indicating funnel-like hole structure, which is
Figure 6. (a) Cross-sectional view of the films and the electron density profiles of each layer: (b) the neat film, (c) the UV-etched sample, and (d) the solvent-etched sample.
consistent with TEMT results. Also, a slight increase in the film thickness (72.77 nm) was found as a result of the chain movement of PMMA block chains onto the film surface during swelling. It is noted that the random brush did not swell much during solvent etching as a result of the predicted thickness of the random brush layer (5.62 nm). In this study, the solvent-etched sample was prepared after the film was immersed in acetic acid for 1 day to induce the equilibrium morphology. Although a detailed investigation of the formation of the funnel-like hole structure during the entire immersion is an interesting subject, it is beyond of the scope of this study. However, we speculate that during short immersion times the PMMA block chains near the film surface can migrate on the top of the film and cover the PS matrix in addition to the removal of hPMMA chains near the surface. Both effects can generate nanoholes only near the surface; thus the electron density near the film surface is definitely smaller than that of the inner film that might have an electron density similar to that of the film without solvent etching. With increasing immersion time, the
Nanohole Structure Prepared by a Mixture Film
hPMMA chains in the inner film are continuously removed, whereas it is less possible for the PMMA block chains in the inner film to migrate on top of the film. In this case, the electron density near the middle of the film begins to decrease, but it is still higher than that near the film surface. Finally, as the immersion time approaches equilibrium, the funnel-like hole structure is formed, and the electron density of the film gradually decreases from the surface to the bottom, as shown in Figure 6d.
4. Conclusions We investigated, by using TEMT and XR, cylindrical nanoporous structures prepared by using a PS-b-PMMA/hPMMA mixture film. Two types of nanoporous structures were obtained from UV etching and solvent swelling. We found via TEMT and XR that the nanoporous structure in the film prepared by UV irradiation exhibited an almost perfect cylindrical shape throughout the entire film. However, when the film was rinsed with
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acetic acid, nanoporous structures were not straight cylinders but were funnel-shaped, where the diameter of nanopores located near the top of the film is larger than for those located near the bottom of the film. Acknowledgment. This work was supported by the National Creative Research Initiative Program of KOSEF and the second stage of the BK 21 program of Korea. H.J. is grateful to NEDO for support through the Japanese National Project “NanoStructured Polymer Project” by the Ministry of Economy, Trade and Industry and for support from the Ministry of Education, Science, Sports and Culture through Grants-in-Aid nos. 1855019 and 19031016. Supporting Information Available: Full movies of 3D TEMT for three different films. This material is available free of charge via the Internet at http://pubs.acs.org. LA8021134
