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Swollen Poly(dimethylsiloxane) (PDMS) as a Template for Inorganic Morphologies Daniel P. Brennan,† Arthur Dobley,‡ Paul J. Sideris,§ and Scott R. J. Oliver*,† Department of Chemistry and Biochemistry, University of California, Santa Cruz, 1156 High Street, Santa Cruz, California 95064 Received June 3, 2005. In Final Form: September 25, 2005 We report a series of silica, titania, and zirconia microstructures synthesized within swollen poly(dimethylsiloxane) (PDMS). Voids created by solvent-swelling the polymer are used to template the product. The inorganic morphologies range from spheres to networks, depending upon the nature of the polymer, its degree of swelling, and the synthetic conditions. Organic solvents as well as pure metal alkoxide liquids have been used to swell the polymer. Once the alkoxide precursor is inside the swollen polymer, water is introduced to bring about hydrolysis and condensation polymerization. The product is a textured metal oxide within a PDMS matrix. Scanning electron microscopy (SEM), optical microscopy, nuclear magnetic resonance (NMR), and powder X-ray diffraction (PXRD) were used to characterize the products. Microstructures formed in this manner have potential use as an inexpensive route to catalysts, fillers, capsules, or membranes for separations.
Introduction We have devised a synthetic strategy to shape inorganic materials into various microscopic morphologies and networks.1 Polymers such as poly(dimethylsiloxane) (PDMS) are known to swell in a wide variety of organic solvents. We use the void space of this swollen elastomer to direct the growth of inorganic materials, such as metal oxides. This method, through its use of a flexible template, can be conceptualized as the reverse of the rigid templating method utilized by Miele and co-workers.2 In our case, the swollen polymer is loaded with a reactive metal oxide precursor, and the precursor is transformed into the corresponding metal oxide. A continuous metal oxide network or a distinct morphology is thereby created, depending on the synthetic conditions and nature of the polymer. This approach allows for the formation of a variety of morphologies from the same polymer host rather than requiring specific templates for each desired product type. In this way, the work resembles the so-called “template-free” approach of Seshadri and co-workers.3-5 Chemical or thermal removal of the original polymer gives the molded metal oxide. There is much interest in titania morphologies for their use as fillers,6 catalysts,7,8 and catalytic supports.9,10 * Corresponding author. E-mail:
[email protected]. Tel: (831) 459-5448. Fax: (831) 459-2935. † University of California. ‡ Current Address: Yardney, Inc., Pawcatuck, Connecticut. § Current Address: SUNY Stony Brook, Department of Chemistry. (1) Brennan, D. P.; Dobley, A. D.; Oliver, S. R. J. Microscale Titania Shapes Grown Within Swollen PDMS. In Solid-State Chemistry of Inorganic Materials IV; Alario-Franco, M. A Ä . et al., Eds.; Materials Research Society 2002 Fall Meeting Proceedings, Boston, MA, 2003, p 755. (2) Dibandjo, P.; Chassagneux, F.; Bois, L.; Sigala, C.; Miele, P. J. Mater. Chem. 2005, 15, 1917-1923. (3) Panda, M.; Rajamathi, M.; Seshadri, R. Chem. Mater. 2002, 14, 4762-4767. (4) Toberer, E. S.; Weaver, J. C.; Ramesha, K.; Seshadri, R. Chem. Mater. 2004, 16, 2194-2200. (5) Toberer, E. S.; Joshi, A.; Seshadri, R. Chem. Mater. 2005, 17, 2142-2147. (6) Kocman, V.; Bruno, P. TAPPI J. 1996, 79, 303-306. (7) Obee, T. N.; Hay, S. O. Environ. Sci. Technol. 1997, 31, 20342038.
Hollow spheres in particular have large surface areas, low densities, and high surface permeabilities, enabling them to be used as release capsules for medication, cosmetics, and dyes.11 Caruso et al. have molded titanium oxide into macroscale shapes by several different methods.12-14 The majority of compounds they synthesized were porous titania networks grown around an organic template. They rely upon an organic membrane or polymer gel as a scaffold. The gels are porous but lack long-range order, yielding network morphologies that are not very well defined. They extended this basic method to shape titania around polystyrene spheres, which resulted in the formation of hollow nanosized TiO2 spheres.15,16 Reacting precursors within swollen polymer networks has been studied but only to a limited degree and only for partial filling of the polymer. Biffis et al. grew nanoclusters of palladium inside swollen resins.17 In addition, work has been focused on growing single crystals within nonaqueous swollen polymers. Doxsee et al. reported the growth of silver bromide crystals with varying morphology within poly(vinyl chloride) swollen by dimethyl sulfoxide.18 In each case, however, the products were small clusters or crystals grown in isolated regions rather than completely filling the polymer voids. Yin et al. used polymer beads as templates to prepare hollow spheres of titania.19 Solid titania spheres of (8) Sirisuk, A.; Hill, C. G.; Anderson, M. A. Catal. Today 1999, 54, 159-164. (9) Hayashi, T.; Tanaka, K.; Haruta, M. J. Catal. 1998, 178, 566575. (10) Chary, K. V. R.; Kishan, G.; Sri Lakshmi, K.; Ramesh, K. Langmuir 2000, 16, 7192-7199. (11) Hu, J.-S.; Guo, Y.-G.; Liang, H.-P.; Wan, L.-J.; Bai, C.-L.; Wang, Y.-G. J. Phys. Chem. B 2004, 108, 9734-9738, and references therein. (12) Caruso, R. A.; Schattka, J. H. Adv. Mater. 2000, 12, 1921-1923. (13) Caruso, R. A.; Antonietti, M.; Giersig, M.; Hentze, H.-P.; Jia, J. Chem. Mater. 2001, 13, 1114-1123. (14) Caruso, R. A.; Giersig, M.; Willig, F.; Antonietti, M. Langmuir 1998, 14, 6333-6336. (15) Caruso, R. A.; Susha, A.; Caruso, F. Chem. Mater. 2001, 13, 400-409. (16) Caruso, F.; Shi, X.; Caruso, R. A.; Susha, A. Adv. Mater. 2001, 13, 740-744. (17) Biffis, A.; D’Archivio, A. A.; Jerabek, K.; Schmid, G.; Corain, B. Adv. Mater. 2000, 12, 1909-1912. (18) Doxsee, K. M.; Chang, R. C.; Chen, E.; Myerson, A. S.; Huang, D. P. J. Am. Chem. Soc. 1998, 120, 585-586.
10.1021/la051468o CCC: $30.25 © 2005 American Chemical Society Published on Web 11/03/2005
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Figure 1. Simplified depiction of the swollen polymer templating method for creating (a) discrete particles and (b) continuous networks.
submicrometer size were also synthesized by Rubio et al. by gas-phase hydrolysis of titanium tetrabutoxide.20 Titania solid spheres, once synthesized, were hot pressed by Yanagisaw et al. to form a porous titania network.21 All of these reports use the same basic principle of shaping the titanium oxide, though the methods require a special apparatus to form the desired material. It would be beneficial to have a simpler approach to these morphologies. Here, we show that a variety of microscale inorganic shapes can be synthesized reproducibly in swollen PDMS. These shapes include spheres, bowls, networks, and webs and depend on the synthesis conditions and the nature of the polymer and inorganic precursor. In all cases, the synthesis involves simple methods under ambient conditions and inexpensive materials. In essence, we create a replica of the internal void space of the swollen polymer (Figure 1). Experimental Section Materials. The following reagents were used as received from the manufacturer: hexanes (99.9%, Fisher Scientific), diethyl ether (99.99%, J. T. Baker), 2-propanol (99.9%, Fisher Scientific), tetrahydrofuran (99.9%, EM Industries Inc.), ethanol (99.5%, Pharmco), acetone (99.5%, EM Industries Inc.), dichloromethane (99.8%, Aldrich), titanium isopropoxide (95% in 2-propanol, Gelest), titanium n-propoxide (95% in n-propanol, Gelest), titanium ethoxide (95% in ethanol, Gelest), titanium diisopropoxide bis(acetylacetonate) (75% in 2-propanol, Aldrich), titanium diisopropoxide bis(ethylacetoacetate) (95% in 2-propanol, Gelest), titanium n-butoxide (95% in n-butanol, Gelest), zirconium n-propoxide (70% in n-propanol, Gelest), silicon dioxide (99.9%, Alfa Aesar), 1.0 M tetrabutylammonium fluoride solution in tetrahydrofuran (Aldrich), Dicone NC9 (Prosoco), Dicone NC15 (Prosoco) and Amtex-CCR (Amtex Chemical Co.). Reaction Conditions. PDMS was polymerized using a Sylgard 184 silicone elastomer kit (Dow Corning, Inc.) with a monomer-to-curing agent weight ratio of 19:1, 15:1, 10:1, or 5:1. The precursor and hardener were gently mixed together manually using a Teflon spatula for approximately 3 min in a plastic container until homogeneous and then degassed for 15 min under vacuum. The viscous prepolymer was then poured into 100 mm × 15 mm polystyrene Petri dishes and degassed again for 10 min to remove any visible air bubbles. The elastomer was cured at 85°C for at least 1 h but no longer than 12 h. The resulting cross-linked PDMS rubber was designed to be approximately 1.0 mm thick. In preparation for the swelling reactions, the elastomer sheet was cut with a razor blade into square pieces ca. 1.0 cm along each side. (19) Yin, Y.; Lu, Y.; Gates, B.; Xia, Y. Chem. Mater. 2001, 13, 11461148. (20) Rubio, J.; Oteo J. L.; Villegas, P.; Duran, P. J. Mater. Sci. 1997, 32, 643-652. (21) Yanagisawa, K.; Ioku, K.; Yamasaki, N. J. Am. Ceram. Soc. 1997, 80, 1303-1306.
Swelling reactions were performed by placing polymer pieces of each of the above elastomer/cross-linker ratios in a sealed glass vial containing the desired solvent(s). Organic solvents tested were hexane, diethyl ether, 2-propanol, tetrahydrofuran, ethanol, acetone, dichloromethane, and solvent mixtures as well as deionized water. In addition, neat metal alkoxide liquid was likewise used to swell the polymer samples. Approximately 5 to 40 mL of solvent was sufficient to submerse the polymer, even when swollen. The soaking duration varied from several hours to 7 days. Inorganic growth was carried out for the samples swelled with organic solvents by adding a metal alkoxide to the solvent in which the swollen polymer has previously been submerged, followed by 30 min of mixing with a magnetic stir bar. The sample was kept in this solution for another 2 days to allow for diffusion of the precursor into the swollen polymer. In the neat metal alkoxide filling method, no additional alkoxide was added. For all cases, samples were removed from their solutions, and hydrolysis and subsequent condensation were initiated by one of the three following methods: (i) submersing into a beaker of water, (ii) submersing into an alcohol-water mixture, or (iii) simply exposing to air. The duration of this step varied from 1 to 60 days. Samples possessing an extended network morphology were observed by cross section, where the center of the edge of the PDMS-metal oxide hybrid was cut with a razor blade and the sample was then manually pried in half. For morphologies consisting of discrete particles, it was often possible to remove the inorganic shapes mechanically after cutting the PDMS. When removal was not possible, as for the bowl-shaped clusters, the morphologies were observed by optical microscopy while still embedded in the PDMS matrix.22 Instrumentation and Techniques. Environmental scanning electron microscopy (ESEM) images and electron microprobe analyses (EMPA) were obtained using a JEOL 8900 Superprobe. SEM data was collected on a Hitachi S570-LB SEM, run at 20 kV and a working distance of 11 mm. Specimens were first coated with ca. 20 nm of gold using a Denton Vacuum Inc. desk 1 sputter coater. The transmission electron microscope was a Hitachi H-7000 125 kV, run at 100 kV, with samples mounted on a laceycarbon-coated copper grid. PXRD data was collected on a Scintag XDS 2000 powder diffractometer using Cu KR radiation (λ ) 1.5418Å), a solid-state detector, a scan range of 2 to 45° (2θ), a step size of 0.02°, and a scan rate of 4.0°/min. Solid-state 29Si nuclear magnetic resonance (NMR) was performed on a Bruker AC 300.
Results and Discussion Swelling data of the PDMS itself for the solvents tested (Figure 2) were similar to those reported by Favre.23 Two of our mixed solvent systems were acetone-water (4:1 v/v) and dichloromethane-2-propanol (2:1 v/v). Even though Favre did not use exactly the same solvent ratios, their swelling numbers were similar. The method of (22) PDMS is transparent, allowing direct observation of the inorganic morphology. (23) Favre, E. Eur. Polym. J. 1996, 32, 1183-1188.
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Figure 2. After submerging 10:1 PDMS for 1 week, metal alkoxides, organic solvents, and metal organics swell the polymer to varying degrees, whereas water has no effect.
weighing and calculating the degree of polymer swelling was based upon a slight variation in the method used by Favre. The routine experiment involved removing the swollen polymer from the solvent, blotting the polymer dry with filter paper, and weighing the polymer once the weight stabilized. The degree of swelling was calculated as the mass ratio of the polymer plus absorbed solvent to the subsequently deswelled polymer (which was dried overnight at 85°C). As detailed below, in initial experiments the solventswelled polymer was placed in a bath of metal alkoxide before the addition of water to yield the corresponding metal oxide. In each case, however, only small amounts of discrete spheres were formed within the polymer itself. For this reason, a second approach was then used, in which the polymer was swelled directly in neat metal alkoxide. Despite a lesser degree of swelling (Figure 2), the product displayed considerably more metal oxide within the polymer. All morphologies reported in this manuscript for the swollen polymer method were synthesized by this second filling method. In both the solvent preswelling and neat alkoxide filling methods, we noted a larger degree of swelling and presumably a greater liquid pore diameter for PDMS with higher elastomer/cross-linker ratios. This finding is supported by the morphologies obtained for the samples produced by the neat alkoxide filling method. Larger discrete particles were observed when higher elastomer/cross-linker ratios were used, as opposed to thinner extended morphologies from lower ratios. To develop a better understanding of the role of PDMS in directing the morphologies observed, 29Si NMR and PXRD analyses were performed on representative samples to determine whether the polymer was acting simply as a physical support in which the microstructures formed or if the PDMS was undergoing chemical change during metal oxide formation. NMR experiments showed that the local environment around the Si atom remained relatively unchanged, with the most intense peak occurring at ca. -21 ppm [-20.87 ppm for the product from Ti(OnPr)4 vs -20.86 ppm in the case of Ti(OnBu)4]. These values are very close to the theoretical value of -22 ppm
Brennan et al.
Figure 3. PXRD patterns from 10:1 PDMS reacted with Ti(OnBu)4 showing (a) the as-synthesized product and (b) after heating the product in air to 800 °C.
for Si in PDMS.24 This close agreement in chemical shift, over multiple experiments, suggests that no reaction occurred between the polymer matrix and inorganic precursor. The PDMS is a passive support in which the morphologies form. The degree of cross linking influences the morphology obtained through control of swelling, which is determined by the rigidity of the polymer host. Other support for the chemical inertness of the PDMS scaffold is provided by the PXRD of the samples before and after heating. Figure 3 shows the PXRD pattern for the as-synthesized product (Figure 3a) and calcination product (Figure 3b) for the composite from the swelling of 10:1 PDMS with Ti(OnBu)4 for 1 week, followed by hydrolysis in air. The room-temperature product is an amorphous composite with isolated microspheres throughout. The calcined product is a poorly crystalline sample possessing the anatase form of TiO2. If a chemical reaction had occurred between the titanium precursor and the polymer or unreacted polymer precursors, then a titanium silicate would be expected. This finding further strengthens our assertion that the PDMS functions to support and direct the formation of microstructures without itself becoming a chemically bound member of the morphology. Chemical removal of the PDMS was attempted with 1.0 M TBAF in THF on the basis of previous success in dissolving PDMS by this method.25 Initial attempts, however, also resulted in the decomposition of the metal oxide structure along with the removal of the PDMS polymer. In other attempts to chemically remove the PDMS, the hybrids were treated with Dicone NC9, Dicone NC15, and Amtex-CCR commercial silicone dissolvers. No appreciable change in the samples was observed after treatment with any of these substances. The microspheres produced by our method could be removed mechanically, whereas the rest of the products were analyzed as the polymer/metal oxide composite. In all cases, the assynthesized products formed were X-ray amorphous. Microspheres and Bowls. The sphere morphology, which was characterized by the presence of white, opaque spheres in the clear, colorless PDMS matrix, was present as the sole product for all experiments where the polymer (24) Casserly, T. B.; Gleason, K. K. J. Phys. Chem. B 2005, 109, 13605-13610. (25) Arias, F.; Oliver, S.; Xu, B.; Holmlin, R. E.; Whitesides, G. M. J. Microelectromech. Syst. 2001, 10, 107-112.
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Table 1. Predominant Morphologies Observed for the Metal Sources and Polymer Crosslinking Ratios Explored Using the Neat Metal Alkoxide or SiO2 Filling Method inorganic precursor(s) SiO2 Ti(OEt)4 and Ti(OiPr)2(Et(acac))2 Ti(OiPr)4 Ti(OnPr)4 Ti(OnBu)4 Zr(OnPr)4
5:1 PDMS
10:1 PDMS
15:1 PDMS
19:1 PDMS
no ordered product no ordered product
no ordered product network
no ordered product spheres
webs
spheres no ordered product no ordered product network
spheres bowls
spheres bowls
spheres spheres
spheres
spheres
spheres
spheres
no ordered product
no ordered product
spheres
was first swelled in an organic solvent and then submerged in metal alkoxide liquid. The sphere morphology was also commonly found in samples prepared by the neat metal alkoxide filling method and could be observed for nearly all PDMS cross-linking ratios tested and alkoxides used in this approach (Table 1). The best results for the microspheres were 19:1 PDMS samples swelled in neat Ti(OiPr)4, with an average diameter of 100 µm.1 This is the highest elastomer-to-crosslinker ratio used (the PDMS has a gummy texture due to the small amount of cross linker); therefore, the swelling of this PDMS sample creates the largest voids. Because of the discrete nature of these particles and the inertness of the PDMS, removal of the larger (>50 µm diameter) metal oxide spheres could be performed mechanically without damaging the product. As mentioned above, attempts to remove the metal oxide spheres by dissolving the PDMS host in TBAF or commercial silicone removers were unsuccessful. In addition, calcination of the PDMS/metal oxide composite material resulted in poorly crystalline anatase titania embedded in amorphous silica. The objective of this research was not only to template interesting and useful morphologies but also to allow a facile means of controlling the morphology simply through the degree of polymer cross linking and/or swelling conditions. By reducing the elastomer/cross-linking ratio of the polymer template, bowl-shaped products were instead observed as the major product.1 The ideal condition for the formation of this shape was 10:1 PDMS swelled in neat Ti(OnPr)4. The morphology consists of a large spherical base (average diameter 75 µm) and a wall composed of small fused microspheres (average diameter 5 µm).1 Some discrete microspheres could still be observed as a minor product (∼5%). Networks. Along with discrete spheres or bowls, our method also produced extended network morphologies. The majority of the discrete shapes described above are composed of titania. The networks, however, were made with either titania or zirconia. A zirconia network (Figure 4a) was obtained by swelling a piece of 5:1 PDMS in a bath of Zr(OnPr)4 for 7 days, followed by reaction with atmospheric moisture. A titania network (Figure 4b) was also obtained but was formed in a bath of mixed titanium alkoxides. The sample was prepared by swelling a sample of 10:1 PDMS in a bath containing both Ti(OEt)4 and Tibis-ethylacetylacetanato diisopropoxide, each comprising half the volume of the bath. The reason we believe we observed an extended network in the 10:1 sample is due to the known slower reaction rate of the Ti-bis-ethyl-
Figure 4. (a) ZrO2 network formed within a sample of PDMS (scale bar ) 10 µm). (b) TiO2 network grown in PDMS from a mixture of Ti alkoxide sources (scale bar ) 150 µm).
Figure 5. Image of the web morphology obtained from burning a sample of PDMS preloaded with silica powder (scale bar ) 2 µm).
acetylacetanato diisopropoxide compared to the rates of the other alkoxides used.26 Webs. Another morphology we obtained was a delicate weblike silica network (Figure 5). The method was different from that of the previous shapes in that silica powder was incorporated directly into the prepolymer before curing. Silica powder (325 mesh) was dispersed into 19:1 PDMS prepolymer, followed by thorough manual mixing to obtain an evenly dispersed, opaque sample. The weight ratio of PDMS to silica was 1:2. The cured sample was heated to 1100°C to decompose the polymer and then investigated by examining a cross section of the sample under SEM. The web features were present in isolated areas but were a minor morphology in an otherwise amorphous sample. The feature was reproduced in subsequent samples but was still a very small component of the sample (
