7960
Langmuir 2006, 22, 7960-7963
Block Copolymer Film with Sponge-Like Nanoporous Strucutre for Antireflection Coating Wonchul Joo, Min Soo Park, and Jin Kon Kim* National CreatiVe Research InitiatiVe Center for Block Copolymer Self-Assembly, Department of Chemical Engineering and Polymer Research Institute, Pohang UniVersity of Science and Technology, Pohang, Kyungbuk 790-784, Korea ReceiVed May 22, 2006. In Final Form: July 10, 2006 We prepared nanoporous films by spin-coating of polystyrene-block-poly(methyl methacrylate) copolymers (PSb-PMMA) to a glass and irradiating by ultra-violet source followed by selective removal of PMMA blocks with acetic acid. When spin-coated PS-b-PMMA was no longer annealed at high temperatures, microphase separation between two blocks occurred only in the short-range scale. The porous films prepared from PS-b-PMMA with the volume fraction of PMMA block of 0.69 exhibited a spongelike nanoporous structure over the entire film thickness and showed excellent antireflection with a minimum reflection less than 0.1% at visible and near-infrared wavelengths. The observed reflectances were in good agreement with the predictions based on the characteristic matrix theory.
1. Introduction Porous thin films have been extensively employed for cell culture media,1,2 templates for inorganic growth masks,3-5 membranes6 and dielectric materials for electronic divices,7,8 and optical materials.9 Recently, a block copolymer (BCP) thin film was used to fabricate a porous thin film for nanotemplates for high-density storage media,10-14 optical materials for photonic crystal,15 and filtration membranes for viruses with nanometer sizes.16 Nanoporous BCP thin films can be used as an antireflection (AR) coating because the refractive index (n) decreases with an increase in the amount of pores generated by the selective removal of one block in the BCP. To reduce completely the reflection at the interface between air and the film, two requirements should be satisfied:17nf ) (nsn0)1/2, with nf, ns, and n0 being the refractive indices of the AR film, substrate, and transmitted medium, respectively. Also, the film thickness should be a quarter of a specific wavelength in the optical medium. For a glass substrate * To whom correspondence should be addressed. Fax: +82-54-2798298. E-mail:
[email protected]. (1) Nishikawa, T.; Nishida, J.; Ookura, R.; Nishimura, S. I.; Wada, S.; Karino, T.; Shimomura, M. Mater. Sci. Eng., C 1999, 10, 141. (2) Yabu, H.; Tanaka, M.; Ijiro, K.; Shimomura, M. Langmuir 2003, 19, 6297. (3) Park, M.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Adamson, D. H. Science 1997, 276, 1401. (4) Li, R. R.; Dapkus, P. D.; Thompson, E. E.; Jeong, W. G.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Adamson, D. H. Appl. Phys. Lett. 2000, 76, 1689. (5) Thurn-Albrecht, T.; Schotter, J.; Ka¨stle, G. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290, 2126. (6) Ynag, G.; Siong, X.; Zhang, L. J. Membr. Sci. 2002, 201, 161. (7) Ding, S.; Wang, P.; Wan, X.; Zhang, D. W.; Wang, J.; Lee, W. W. Mater. Sci. Eng., B 1999, 83, 130. (8) Schwo¨diauer, R.; Bauer, S. ReV. Sci. Instrum. 2002, 73, 1845. (9) Park, M. S.; Lee, Y.; Kim, J. K. Chem. Mater. 2005, 17, 3994. (10) Naito, K.; Hieda, H.; Sakurai, M.; Kamata, Y.; Asakawa, K. IEEE Trans. Magn. 2002, 38, 1949. (11) 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. (12) Mansky, P.; Harrison, C. K.; Chaikin, P. M.; Register, R. A.; Yao, N. Appl. Phys. Lett. 1996, 68, 2586. (13) Park, C.; Yoon, J.; Thomas, E. L. Polymer 2003, 44, 6725. (14) Kim, S. O.; Solak, H. H.; Stoykovich, M. P.; Ferrier, N. J.; DePablo, J. J.; Nealy, P. F. Nature (London) 2003, 424, 411. (15) 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. (16) Yang, S. Y.; Ryu, I.; Kim, H. Y.; Jang, S. K.; Kim, J. K.; Russell, T. P. AdV. Mater. 2006, 18, 709. (17) Macleod, H. A. Thin-Film Optical Filters; Hilger: Bristol, 1986.
with n ) 1.52, n of an AR film must be 1.23 to achieve zero reflectance. However, since the lowest value of n for most dielectric inorganic or organic materials is ∼1.35, an AR coating has usually been achieved by introducing porous structure.18-20 Among several methods to achieve good AR coating, polymer materials would be attractive because of the ease of processing. Fluoropolymers with n ∼ 1.34 have been used for AR coating, but these have limitations due to the lack of a suitable solvent and a high melting temperature, in addition to a high price.21 Steiner and co-workers22 showed that the blend of polystyrene (PS) and poly(methyl methacrylate) (PMMA), followed by removing selectively the PS homopolymer, exhibited excellent AR. Kim and co-workers developed an AR coating by using a polymer/solvent/nonsolvent mixture,9 as well as by preparing porous films based on the breath figure.23 The above-mentioned AR coating processes, however, have some limitations in controlling film thicknesses less than a few hundreds of nanometers and pore sizes to avoid light scattering at visible wavelengths because of the size of macrophase-separated domains. Rubner and co-workers showed that excellent AR at visible light wavelengths was achieved by using pH-sensitive polyelectrolyte layers, although the preparation needs multiple layer-by-layer coating steps.24,25 On the other hand, BCP exhibits various microdomains with 10∼50 nm.26 Once these microdomains are selectively removed by ultra-violet (UV) irradiation or ozone treatment, a porous structure is generated.12,16,27-30 Because the size of generated (18) Uhlmann, D. R.; Suratwala, T.; Davidson, K.; Boulton, J. M.; Teowee, G. J. Non-Cryst. Solids 1997, 218, 113. (19) Hattori, H. AdV. Mater. 2001, 13, 51. (20) Zhang, X.; Sato, O.; Tauchi, M.; Einaga, Y.; Murakami, T.; Fujishima, A. Chem. Mater. 2005, 17, 696. (21) Castner, D. G.; Grainger, D. W.; Pellerite, M.; Anton, D. Fluorinated Surfaces, Coatings, and Films; Oxford University Press: New York, 2001. (22) Walheim, S.; Schaffer, E.; Mlynek, J.; Steiner, U. Science 1999, 283, 520. (23) Park, M. S.; Kim, J. K. Langmuir 2005, 21, 11404. (24) Hiller, J.; Mendelsohn, J. D.; Rubner, M. F. Nat. Mater. 2002, 1, 59. (25) Cebeci, F. C.; Wu, Z.; Zhai, L.; Cohen, R. E.; Rubner, M. F. Langmuir 2006, 22, 2856. (26) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: New York, 1998. (27) 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, 787. (28) Shin, K.; Leach, A.; Goldbach, J. T.; Kim, D. H.; Jho, J. Y.; Tuominen, M.; Hawker, C. J.; Russell, T. P. Nano Lett. 2002, 2, 933. (29) Hashimoto, T.; Tsutsumi, K.; Funaki, Y. Langmuir 1997, 13, 6869.
10.1021/la061441k CCC: $33.50 © 2006 American Chemical Society Published on Web 08/12/2006
Letters
Langmuir, Vol. 22, No. 19, 2006 7961
pores is much smaller than the visible light wavelength, light scattering by these pores is completely avoided. The pore volume in a BCP film which determines the final n could be easily controlled due to easy change in the volume fraction of one block in a block copolymer. It is known that a nanoporous BCP film with n ) 1.23 coated onto a glass could exhibit zero reflectance. In this situation, the optimum pore volume in the film with zero reflectance can be estimated31
n2 ) npolymer2(1 - fpore) + nair2fpore
(1)
where npolymer, nair, and fpore are n of the polymer and air and the pore volume fraction in the film, respectively. Since npolymer is ∼1.6 (which is the case of polystyrene) and nair )1, n becomes 1.23 for a porous film with a pore volume of ∼0.68. This indicates that, when the volume fraction of one block in a block copolymer, which could be selectively removed and becomes pores, becomes ∼0.68, the porous film prepared by this BCP film could exhibit zero reflectance at a specific wavelength. In this study, we introduce an AR coating film prepared by spin-coating of polystyrene-block-poly(methyl methacrylate) copolymer (PS-b-PMMA) onto a glass. When the film was irradiated by UV, PS chains are cross-linked, whereas PMMA chains are degraded.32 The degraded PMMA chain was selectively removed by acetic acid, generating porous structures.12,16,27,30 Since the volume fraction of PMMA block in the PS-b-PMMA is 0.69, the pore volume in the film is almost the same as the optimum value for the zero reflectance. We found that, without further annealing at temperatures higher than the glass transitions of PMMA and PS blocks, the film showed microphase separation in a short-range scale because of the rapid evaporation of the solvent. When this film was irradiated by UV followed by rinsing with acetic acid, the film exhibited a spongelike co-continuous nanoporous structure over the entire film thickness. We found that the adhesion between the substrate and the film was strong enough, since the film could not be peeled off from the substrate under very harsh condition, for instance, the immersion of the film into any solvent for a long time (say several days). The nanoporous films with various thicknesses showed excellent AR with a minimum reflectance less than 0.1% at visible and nearinfrared wavelengths. The observed reflectance curves were in good agreement with the predictions based on the characteristic matrix theory (CMT).17 2. Experimental Section PS-b-PMMA, purchased from Polymer Source Inc. (Lot No. P2406-SMMA), was synthesized by using anionic polymerization. The total number-average molecular weight (Mn), the polydispersity, and the weight fraction of the PMMA block in the block copolymer are 94 200, 1.15, and 0.72, respectively. To convert the weight fraction to the volume fraction, we used the mass densities of PS (1.05 g/cm3) and PMMA (1.18 g/cm3).33 Thus, the volume fraction of PMMA block (fPMMA) in the block copolymer is 0.69. This block copolymer exhibited PS cylindrical microdomains when annealed at 170 °C for 48 h, confirmed by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). Glass slide was purchased from Corning Glass Works (Corning brand, Plain (Product #2947)), which was soda lime glass which has refractive index of 1.52. The glass was cleaned by dipping in the (30) Jeong, U.; Ryu, D. Y.; Kim, J. K.; D. H.; Kim, D. H.; Wu, X.; Russell, T. P. Macromolecules 2003, 36, 10126. (31) Choy, T. C. EffectiVe Medium Theory; Oxford University Press: New York, 1999. (32) Reiser, A. PhotoreactiVe Polymers; John Wiley & Sons: New York, 1989. (33) Brandup, J., Immergut, E. H., Eds.; Polymer Handbook; John Wiley & Sons: New York, 1989.
Figure 1. (a) AFM images (2 × 2 µm) and (b) cross-sectional TEM image of the PS-b-PMMA film after spin-coating without further thermal annealing at high temperatures. mixture of sulfuric acid and hydrogen peroxide for 30 min at 80 °C. PS-b-PMMA in toluene (2∼3 wt %) was spin-coated on a glass substrate with a rotating speed of 2000∼4000 rpm. The film was irradiated with an ultraviolet lamp with a maximum intensity at 253.7 nm (Sankyo Denki, G15T8) for 1 h in a vacuum chamber, which degraded PMMA chains, but cross-links PS chains. After the UV irradiation, the films were dipped into acetic acid for 30 min followed by washing with distilled water. Finally, the films were dried for 6 h in a vacuum. Morphologies of the BCP porous films were investigated by atomic force microscope (AFM; Digital Instrument, Nanoscope IIIa) in tapping mode and by a field emission scanning electron microscope (FE-SEM; Hitachi S-4800) with an accelerating voltage of 10 kV. The samples for FE-SEM were prepared by coating the films with osmium tetraoxide (Meiwa Shoji Co., NEOC-ST) and dried under vacuum for 12 h. A cross-sectional image of the film before UV irradiation was investigated by TEM (Hitachi, S-7600) with an accelerating voltage of 100 kV. For this purpose, a thin layer of carbon was first evaporated onto the film, covered with epoxy (Polysicences, Inc., Araldite, 502 Kit), and then cured at 60 °C for 12 h. Finally, the film was peeled off from the substrate in liquid nitrogen. Thin sections of the film were obtained by using an Ultramicrotome (RMC MT-7000) with a diamond knife and stained by ruthenium tetraoxide (RuO4). PS phase appeared dark in TEM images because of selective staining of RuO4. The reflectances of porous thin films were measured by a UVvis-NIR spectrophotometer (Varian, Cary-5000) in specular reflection mode with an incidence angle of 12°. To eliminate the backside reflection of glass, the other side of glass was attached by black tape. Film thickness and refractive index of porous films are characterized by the CMT.17 The film thickness was also measured by AFM after the film was scratched by a razor.9
3. Results and Discussion Figure 1 gives height and phase AFM images and a crosssectional TEM image of the PS-b-PMMA film spin-coated on a glass without any further thermal annealing at high temperatures. Randomly distributed nanodomains were observed in AFM images (Figure 1a) without exhibiting a sharp boundary between PS and PMMA blocks. These nanostructures were formed by fast evaporation of toluene during spin-coating. Because further annealing was not carried out at high temperatures, PS and PMMA
7962 Langmuir, Vol. 22, No. 19, 2006
Letters
Figure 3. Reflectances of PS-b-PMMA film with three thicknesses ((0) 126 nm, (O) 169 nm, and (4) 200 nm) after UV irradiation followed by rinsing with acetic acid. The predicted reflectances by the CMT were given as solid curves (1∼3).
Figure 2. (a) Three-dimensional AFM height image, (b) height profile, and (c) cross-sectional FE-SEM image of PS-b-PMMA film after UV irradiation followed by rinsing with acetic acid.
chains were frozen immediately after solvent evaporation. Thus, nanophase separation could occur in a short-range scale over the entire film thickness, as shown in Figure 1b, without showing hexagonal packed cylinder microdomains, the equilibrium morphology when annealed at high temperatures. Figure 2a-c gives an AFM height image, a height profile, and a cross-sectional FE-SEM image, respectively, of the PS-bPMMA film after UV irradiation followed by rinsing with acetic acid. It is seen from Figure 2a,b, the film showed a rough surface with an rms roughness of ∼15 nm. The spongelike nanoporous structures spanning the entire film thickness were observed (Figure 2c). It is noted that the porous structures were formed by removing the PMMA block that is confirmed by Fourier transform infrared spectroscopy, as shown in Figure S1 in the Supporting Information. The formation of the spongelike nanoporous structure could be explained as follows. When PS-b-PMMA in toluene solution is spin-coated, the solution induces nanophase separation with increasing concentration of the BCP. The effective segregation power between two blocks, given by φχN, in which φ, χ, and N are the volume fraction of the BCP in the solution, the Flory segmental interaction parameter between PS and PMMA (∼ 0.04 at room temperature34), and the total segments (N ∼ 940) of the PS-b-PMMA, becomes larger than 10.5 for φ g 0.3. Thus, during short times of spin coating, microphase-separation between PS and PMMA block could occur in a short range scale. However, the full development into PS cylindrical microdomains with distinct boundary between PS and PMMA blocks is severely restricted by fast solidification of both blocks at room temperature, which is far below than the glass transition temperature (∼100 °C) for both PS and PMMA blocks. In this situation, the removal (34) Russell, T. P.; Hjelm, R. P., Jr.; Seeger, P. A. Macromolecules 1990, 23, 890.
of PMMA chains by UV irradiation followed by rinsing with acetic acid could give a co-continuous spongelike porous structure, by merging several neighboring PS domains in a short-range scale. Figure 3 gives the reflectances of the porous PS-b-PMMA films with three thicknesses (126, 169, and 200 nm). The film thickness was measured by AFM after scratching the films. With increasing film thickness, the wavelength at which the minimum reflectance is observed becomes larger (590 to 940 nm with increasing film thickness from 126 to 200 nm). The minimum reflectance for three films was less than ∼0.1%, indicating that all films showed excellent AR. The measured reflectances are compared with prediction by the CMT for one layer film on the glass17
( (
) )
2 n0ns δ δ - nf sin2 + 2 nf 2 R) 2 n n δ 0 s δ (n0 + ns)2 cos2 + + nf sin2 2 nf 2
(n0 - ns)2 cos2
(2)
where R is the reflectance of an incident light, n0 is the refractive index (1.0) of the incident medium, ns is the refractive index (1.52) of the glass, nf is the refractive index of the porous film coated on the glass, and δ is the phase angle given by (4πnfh)/(λ cos θ) in which h is the thickness of porous film and λ and θ are the wavelength and the incidence angle (12°) of light. Here, CMT for a one-layer model was used because of the co-continuous and uniform porous structure along the thickness direction, as shown in Figure 2c. Predicted reflectances based on the CMT, which are shown as solid lines in Figure 3, are in good agreement with measured ones. The thicknesses and n for three porous films characterized by the CMT were (1) 135 nm and 1.205; (2) 175 nm and 1.195; and (3) 198 nm and 1.205. The calculated film thicknesses are in good agreement with measured ones (126, 169, and 200 nm, respectively). Now, we estimate the values of n for porous thin films by eq 1. Here, we used npolymer (1.61) of cross-linked PS determined by ellipsometer (J. A. Woollam Co., Inc.), which is slightly larger than that (1.59) for un-cross-linked PS.35 Since the volume fraction (35) Kim, D. H.; Lau, K. H. A.; Robertson, J. W. F.; Lee, O. J.; Jeong, U.; Lee, J. I.; Hawker, C. J.; D. H.; Russell, T. P.; Kim, J. K.; Knoll, W. AdV. Mater. 2005, 17, 2442.
Letters
of PMMA (fPMMA) in PS-b-PMMA (and thus the pore volume in the film) is 0.69, predicted n of the porous PS-b-PMMA film by eq 1 is 1.22, which is in good agreement with determined n (∼ 1.2) by the CMT. Since eq 2 is derived for the one-dimensional porous structure, we conclude that the porosity in the film does not change with thickness direction, consistent with the result given in Figure 2c. When the fPMMA in the PS-b-PMMA copolymer is changed, the pore volume is also changed, which in turn affects the reflectance of the nanoporous BCP film. For this purpose, we employed two different values of fPMMA (0.46 and 0.30). Both films without being further annealed at higher temperatures were irradiated by UV and followed by rinsing with acetic acid. The reflectances of these two films are shown in Figure S2 in the Supporting Information. The minimum reflectance of the PSb-PMMA with fPMMA ) 0.46 was 0.4% at 500 nm, whereas that of the PS-b-PMMA with fPMMA ) 0.30 was 1.4% at 570 nm. This result clearly indicates that with increasing fPMMA (thus, the pore volume) in PS-b-PMMA copolymer thin film, the minimum reflectance decreased and it became less than ∼0.1% for a film with fPMMA ∼ 0.69. Finally, it is noted that the AR coating film prepared in this study was obtained from the nonequilibrium morphology of PSb-PMMA copolymer. Thus, it would be of interest to compare the reflectance of the nanoporous BCP film prepared by equilibrium morphology which could be obtained by long annealing at higher temperatures. For this purpose, the PS cylindrical nanodomains should be aligned vertically to the substrate. Otherwise, the film could not maintain the shape after the UV irradiation followed by rinsing with acetic acid. Unfortunately, we found that, when the film thickness was over 50 nm, we could not orient the PS cylindrical nanodomains of PS-b-PMMA vertically to the glass. Thus, we could not directly compare the reflectance of the nanoporous film of PS-b-PMMA having fPMMA ) 0.69 by employing the equilibrium morphology. However, we succeeded in preparing the vertical orientation of PMMA cylindrical nanodomains of the PS-b-PMMA film. When the nanoporous film was prepared by removing vertical orientation of PMMA cylindrical nanodomains, the cylindrical pores spanned over the entire thickness even when the film thickness was higher than 100 nm (See Figure S3 in the Supporting Information).35,36 The nanoporous film prepared by equilibrium morphology of (36) Jeong, U.; Ryu, D. Y.; Kho, D. H.; Kim, J. K.; Goldbach, T.; D. H.; Kim, D. H.; Russell, T. P. AdV. Mater. 2004, 16, 533.
Langmuir, Vol. 22, No. 19, 2006 7963
PMMA cylindrical nanodomains showed the minimum reflectance of 1.5% at 580 nm, as shown in Figure S3. Interestingly, this value is essentially the same as that obtained from the same PS-b-PMMA film under nonequilibrium condition (that is without further annealing at high temperatures). Furthermore, this value is very similar to that (1.4% at 570 nm) prepared by PS-b-PMMA with fPMMA ) 0.30 (see Figure S2). This indicated that, once fPMMA in the PS-b-PMMA is similar, the reflectance of the nanoporous film prepared by PS-b-PMMA would be similar irrespective of the total molecular weights and further thermal treatments. The above results indicate that nanoporous films prepared by PS-b-PMMA without being annealed at high temperatures exhibited excellent AR with a minimum reflectance less than 0.1% when the fPMMA ∼ 0.69. This process employed in this study seems very simple and economic; that is, only spin coating and UV irradiation followed by rinsing acetic acid are needed to have excellent AR. This novel spongelike nanoporous structure could be employed not only for AR coating but also for functional nanomaterials such as separation membranes of biomaterials with high selectivity.
4. Conclusion We prepared a nanoporous thin film using PS-b-PMMA with fPMMA ∼ 0.69. When the film was spin-coated without further annealing, microphase separation occurs in a short range scale. When the films were irradiated by UV light and rinsing with acetic acid, nanoporous film was prepared. The films with various thicknesses exhibited a spongelike co-continuous nanoporous structure over the entire film thickness and showed excellent AR with a minimum reflectance less than 0.1% reflectance at visible and near infrared wavelengths. The wavelength at which the lowest reflectance was observed was easily adjusted by controlling the film thickness. Acknowledgment. This work was supported by the National Creative Research Initiative Program by the Korea Organization of Science and Engineering Foundation (KOSEF). Supporting Information Available: The characterization of the nanoporous film by FTIR, the reflectances of nanoporous films prepared by PS-b-PMMA with two different values of fPMMA (0.46 and 0.30), and nanoporous films prepared by the mixture of PS-b-PMMA with fPMMA of 0.28 and a PMMA homopolymer. This material is available free of charge via the Internet at http://pubs.acs.org. LA061441K
