Langmuir 1997, 13, 6869-6872
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Nanoprocessing Based on Bicontinuous Microdomains of Block Copolymers: Nanochannels Coated with Metals Takeji Hashimoto,*,†,‡ Kiyoharu Tsutsumi,† and Yoshinori Funaki† Hashimoto Polymer Phasing Project, ERATO, JST, 15 Morimoto-cho, Shimogamo, Sakyo-ku, Kyoto 606, Japan, and Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 606-01, Japan Received August 27, 1997. In Final Form: October 24, 1997X A novel process to produce a nanochannel comprised of a void in a polymer matrix is reported. The processing involves a selective degradation of one of the bicontinuous microdomain phases of microphaseseparated block copolymers to create a continuous, tortuous hole having a diameter of nanometer scale (nanochannel) and plating of the surfaces with nickel metal. The nanochannel and the metal plating were confirmed by the transmission and scanning electron microscopies.
In this Letter, we report the creation of a nanochannel through processing of a bicontinuous microdomain structure formed by microphase separation of a block copolymer. The process, designated hereafter as nanoprocessing, involves a selective degradation of one of the bicontinuous microdomain phases of microphase-separated block copolymers to create a continuous, tortuous hole having a diameter of nanometer scale (designated hereafter as “nanochannel” for the sake of convenience) and plating of the surfaces of the nanochannel with nickel metal. It is well-known that block copolymers, e.g., A-B type diblock copolymers, form ordered nanostructures such as body centered cubic (bcc) spheres, hexagonal cylinders, and bicontinuous structures (ordered double network structure composed of a tripod unit with Ia3d symmetry, the so-called gyroid phase, etc.) rich in A-block chains in a matrix rich in B-block chains, alternating lamellae composed of A-rich and B-rich block chains, and their phase-inverted structures.1-4 The size and shape of the nanostructures can be “tailor-made” by their molecular weights and compositions. The structure units and their spacings are of the order of the radii of gyrations of block copolymers and hence of nanometer scale. In this study, we used a block copolymer system composed of a binary mixture of polystyrene-blockpolyisoprene (SI) block copolymer and homopolystyrene (HPS) containing 62 wt % of SI. The block copolymer was polymerized by living anionic polymerization with secbutyllithium as an initiator and cyclohexane as a solvent. Polyisoprene block chains have a microstructure with 1,4and 3,4-linkages of 93 and 7%, respectively, as determined by 1H NMR. The high 1,4-content enhances an efficiency of ozonolysis to create the nanochannel. The SI diblock has a total number (Mn) and weight average molecular weights (Mw) of 8.8 × 104 and 9.1 × 104, respectively, with a weight fraction of polystyrene block chains of 0.51. The HPS has Mn ) 7.5 × 103 and Mw ) 7.9 × 103. The SI and HPS were mixed so that the mixture had an overall volume fraction of styrene units of 0.66.
Figure 1. TEM micrograph showing the bicontinuous microdomain structure of a SI/HPS mixture cast from toluene solution. Ultrathin sections were stained with OsO4.
* To whom all correspondence should be addressed at Kyoto University. † Hashimoto Polymer Phasing Project. ‡ Kyoto University. X Abstract published in Advance ACS Abstracts, December 1, 1997. (1) Sadron, C.; Gallot, B. Makromol. Chem. 1973, 164, 301. (2) Hashimoto, T.; Shibayama, M.; Fujimura, M.; Kawai, H. In Block Copolymers-Science and Technology; Meier, D. J., Ed.; MMI Press/ Harwood Academic: New York, 1983; pp 63-108. (3) Thomas, E. L.; Alward, D. B.; Kinning, D. J.; Martin, D. C.; Handlin, D. L.; Fetters, L. J. Macromolecules 1986, 19, 2197. Hasegawa, H.; Tanaka, H.; Yamasaki, K.; Hashimoto, T. Macromolecules 1987, 20, 1651. (4) Hajduk, D. A.; Harper, P. E.; Gruner, S. M.; Honeker, C. C.; Kim, G.; Thomas, E. L.; Fetters, L. J. Macromolecules 1994, 27, 4063.
S0743-7463(97)00967-0 CCC: $14.00
Figure 2. SEM micrograph showing bicontinuous nanochannels formed in the matrix of PS through the ozone degradation of the PI network phase. © 1997 American Chemical Society
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Figure 3. SEM micrographs showing a bicontinuous nanochannel in the matrix of PS with two different magnifications (parts a and b) and computer graphics of a double gyroid network: (c) a three-dimensional view and (d) a two-dimensional intersection cut along the (211) direction. Part c shows a solid model in which only the matrix phase, corresponding to the PS matrix in our specimens, is shown. In part d, the bright domain corresponds to the PS matrix and the gray and dark phases correspond to the degraded PI phase.
The mixture was dissolved into ca. 5 wt % total polymer concentration solution with toluene as a neutrally good solvent for both polystyrene (PS) and polyisoprene (PI). Thin films about 100-300 µm thick were prepared from the solution in a Petri dish by slowly evaporating solvent over approximately a week. The as-cast films were further dried until a constant weight was attained. The microdomain structure in the as-cast films was observed under a transmission electron microscope (TEM, JEOL JEM2000FXZ) operated at 200 kV on the ultrathin sections obtained with a Reichert-Nissei Ultracut-S ultramicrotome stained with OsO4 vapor for 30 min. The as-cast films were subjected to the ozonolysis, as detailed later, by which the PI domains were selectively degraded and transformed into holes. The nanochannels thus created in the PS matrix were investigated under a scanning electron microscope (SEM, Hitachi S-900S) operated at 20 kV. The films subjected to the ozonolysis
were freeze-fractured according to a conventional method, and the fractured surfaces were subsequently sputtercoated with platinum (by Hitachi E-1030 ion sputter) for SEM examination. The materials with the nanochannels were then subjected to nonelectrolytic plating with nickel metal as also detailed later below. The materials having the nanochannels whose surfaces are coated with nickel metal were subjected to the ultrathin sectioning and to the TEM observations without staining, as described above for the transmission microscopy. The ozonolysis was used to selectively degrade the PI domains. To cleave the carbon-carbon double bond of the polyisoprene, the film was exposed to an atmosphere of ozone at room temperature for 24 h. The cleaved compounds were leached out from the micropore by soaking the film in ethanol at room temperature for 24 h. The extent of isoprene removal was 100% for the films of ca.
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100 µm, which was determined from a NMR measurement and a weight measurement before and after the ozonolysis, as will be detailed elsewhere.5,6 Lee et al. already studied the selective degradation of the polyisoprene domain by the ozonolysis.7 They prepared a block copolymer comprised of PI and cross-linkable PS derivative and made the porous film of cross-linked PS by selective degradation of the PI domains of the cast film having lamellar or cylinder structure. In this study, we focused on making a film having three-dimensional continuous nanochannels. For this purpose, we attempted ozonolysis of the cast film having an ordered bicontinuous structure called the gyroid structure. To the best of our knowledge, no work has been reported along this line. The nonelectrolytic plating was done by using commercial reagents for the plating (Okuno Chemical Industries CO., Ltd., Japan). First, the nanoprocessed films were soaked in “TMP sensitizer” (an aqueous solution of SnCl2) so that the surface of the nanochannel adsorbs Sn2+. Next, the sensitized films were soaked in “TMP activator” (the aqueous solution of PdCl2) so that the surface adsorbs Pd0 through the reaction of Sn2+ + Pd2+ f Sn4+ + Pd0, which exchanges Sn2+ adsorbed on the surface into Pd0. Finally the activated films were soaked in the 1:4 mixture of “TMP chemical nickel A” (the aqueous solution of NiCl2) and “TMP chemical nickel B” (the aqueous solution of reductant) for the nickel plating. Pd0 was used as the catalyst of the reduction of Ni2+. All the commercial solutions were diluted with water to avoid rapidly clogging of the nanochannels by the nickel metal.8 Usually, the nonelectrolytic plating is used to decorate the surface of the polymer materials or to add the corrosion resistance to the materials. In those cases the outer surfaces of the polymer materials were completely covered by the metal layer. In this study, we aim to add a catalytic functionality to the film having the nanochannels and then to utilize the film as a membrane reactor. Consequently, we attempted to introduce the metal, which could be used as a catalyst, by plating on the surface of the nanochannels in the polymer films. Figure 1 shows the TEM micrograph of the as-cast specimen first ultrathin-sectioned and then stained with OsO4 in which the dark and bright phases correspond, respectively, to the stained PI and unstained PS phases. The micrograph shows the ordered bicontinuous phases with spacings of 50-60 nm. Detailed analysis based on a series of the TEM pictures with varying tilt angles and ultra-small-angle X-ray scattering revealed that the nanostructure can be well approximated by a double PI network in the PS matrix, identified by the so-called gyroid phase with Ia3d symmetry,4,9 as will be detailed elsewhere.5,6 Figure 2 represents the SEM micrograph obtained on the specimen subjected to the ozonolysis. The micrograph (5) Tsutsumi, K.; Funaki, Y.; Kanazawa, Y.; Hashimoto, T. In preparation. (6) Nishikawa, Y.; Tsutsumi, K.; Koga, T.; Hashimoto, T. In preparation. (7) Lee, J.-S.; Hirao, A.; Nakahama, S. Macromolecules 1988, 21, 276. (8) One may suspect that the aqueous solution may not contact the exposed area of the nanochannels in the materials, because PS is the matrix of the materials with hydrophobicity and the nanochannels are on the order of 20-30 nm in diameter. However, after the ozonolysis the surfaces of nanochannels will become quite polar, having hydroxyl and carbonyl groups, and hence wet with the aqueous solution, as evidenced clearly by an almost complete coverage with nickel metals, as will be shown later in Figure 4. (9) The microdomain in Figure 1 (or Figure 4) is close to the image shown in part b or d in Figure 3 and hence to the image obtained with the ultrathin section cut nearly parallel to the (211) plane of the cubic phase. It appears to be tilted about 30° off the vertical, which may be due to deformation induced by the sectioning.
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Figure 4. TEM micrograph of the unstained ultrathinsectioned nanohybrid showing a continuous, regular nanochannel plated with nickel metal. The nanochannel appears dark in the micrograph due to the nickel plating.
revealed a characteristic topological feature of the regularly spaced nanochannels with diameter of 20-30 nm, essentially identical to the features seen in the TEM shown in Figure 1. This result suggests that the PI phase is selectively degraded into the nanochannel in the PS matrix without significant perturbations of original symmetry of the structure in the as-cast films. To the best of our knowledge, it is for the first time that the threedimensionally continuous and periodic nanochannels with long-range spatial order are created in the polymer matrix by selective degradation of the gyroid phase and observed by the SEM method, though there have been some reports of selective degradation of one of the phases, of the microdomains with some discrete morphologies such as lamellae or cylinder.7 Figure 3 shows another set of SEM micrographs of the freeze-fractured specimens obtained with the degraded specimens. The micrograph (a) with low magnification shows the uniformity of the structure and one of the most frequently observed images on the freeze-fractured specimens. The micrograph reveals a high degree of uniformity of the nanostructure. The micrograph (b) with a higher magnification shows that the fracture surface corresponds approximately to the (211) plane of the double network of the gyroid structure whose three-dimensional (3D) computer graphic and a sliced section are shown, respectively, in parts c and d. The selective etching of one of the phases is proposed here to be quite useful for exploration of 3D structures of the bicontinuous phases, as will be described separately elsewhere.6 Figure 4 shows the TEM micrograph of the ultrathin sections of the PS specimens having 3D continuous nanochannels whose surfaces (or interfaces) are plated with nickel metal.10 It should be noted that the TEM was (10) It might seem difficult to prepare ultrathin sections from the unsupported porous structure without crumbling. However, it was actually possible to prepare the sections, partly because the Ni coating on the surface of the nanochannels may reinforce the materials and the PS matrix is continuous, and occupying 66% of volume of the materials. We deliberately used thick samples, thicker than 100 µm, for the ultrathin sectioning of the nanoprocessed materials. Here a undegraded part of the specimen may help to prevent the degraded part from being severely crumbled in the sectioning process.
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observed with the specimens unstained by OsO4: the metal-coating of the channel surfaces give rise to the absorption contrast for the TEM without staining. A comparison between Figures 1 and 4 clearly reveals that the symmetry, image contrast, and size of the nanopattern observed after the nanoprocessing are in complete agreement with those of the nanopattern observed in the ascast specimens:9 the dark PI phase stained with OsO4 in the as-cast films in Figure 1 corresponds to the dark channels phase in Figure 4 whose surface is coated with nickel metal. This evidence shows a conservation of the nanostructure through the nanoprocessing and the fact that the PI phase is transformed into a channel with its surface coated by the metal. The inset of Figure 4 shows an enlarged image representing the image viewed along the axis of the channel. The image highlights a deposition of the metal particles with a diameter of ca. 10 nm on the surface of the channel with a diameter 25 nm corresponding to a part in the original PI gyroid network phase, thus leaving an empty space with diameter of ca. 5 nm in the middle of the
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channel. The fine metal particles appear to contact each other and are welded more or less into the coating layer on the surface of the channels. In conclusion, we are able to prepare nanochannels regularly spaced in a glassy polymer matrix whose surfaces are coated by nickel metals. This nanohybrid is expected to be useful as a high-performance membrane reactor because of the high surface area of the metal nanoparticles and the easy handling of the polymer films. In this study, we used nickel for the plating metal. Plating reagents for palladium or gold plating are also commercially available. If palladium plating is used instead of nickel plating, the membrane can be used as a membrane reactor for wide range of reactions. Detailed X-ray analysis before and after the ozonolysis and plating will be presented elsewhere.5 The details of the nickel loading as to control of thickness and uniformity as well as electrical conductivity of the membrane are now under investigation in our laboratory. LA970967P
