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Polymer-Silica Hybrid Monolayers as Precursors for Ultrathin Free-Standing Porous Membranes Hui Xu and Werner A. Goedel* Department of Organic and Macromolecular Chemistry, OC III, University of Ulm, D-89069 Ulm, Germany Received July 6, 2001. In Final Form: October 21, 2001 Forty-nanometer-thin free-standing membranes with 55-nm-wide pores have been prepared via Langmuir-Blodgett transfer. Hydrophobized silica colloids and polyisoprenes with quaternary ammonium headgroups were jointly spread onto a water surface to form a hybrid monolayer in which two-dimensional ordered domains of particles are embedded in a polymer monolayer matrix. Photochemical cross-linking of the polyisoprene, followed by transfer to gold grids with 100-µm wide openings yielded free-standing hybrid membranes. Removal of the colloids gives rise to a free-standing membrane with pores of uniform size. These porous membranes might find application as improved filtration media or as masks or molds for surface patterning and nanostructure fabrication.
Introduction Artificial porous materials have been fabricated by a number of techniques on the basis of selective removal of components from material that is spatially structured. The spatial structuring has been achieved by ion bombardment,1 spinodal demixing of glasses,2 self-assembly of block copolymers,3 and replica molding using various kinds of templates such as surfactant arrays,4 emulsion droplets,5 and solid particles.6 More recently, well-ordered porous materials with uniform holes sizes have been prepared using colloidal crystal templates.7 However, most of the porous materials produced by these methods have three-dimensional structures or thicknesses significantly larger than the pore sizes. If porous materials are used as filtration membranes, a complicated three-dimensional pore structure and a comparatively large membrane thickness easily lead to the trapping or adsorption of particles and large molecules within the membranes,8 and * Corresponding author. E-mail:
[email protected]. (1) Yoshida, M.; Asano, M.; Suwa, T.; Reber, N.; Spohr, R.; Katakai, R. Adv. Mater. 1997, 9, 757. (2) (a) Tonucci, R. J.; Justus, B. L.; Campillo, A. J.; Ford, C. E. Science 1992, 258, 783. (b) Pearson, D. H.; Tonucci, R. J. Adv. Mater. (Weinheim, Ger.) 1996, 8, 1031. (3) Chan, V. Z. -H.; Hoffman, J.; Lee, V. Y.; Iatrou, H.; Avgeropoulos, A.; Hadjichristidis, N.; Miller, R. D.; Thomas, E. L. Science 1999, 286, 1716. (4) (a) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (b) Monnier, A.; Schu¨th, F.; Huo, Q.; Kumar, D.; Margolese, D.; Maxwell, R. S.; Stucky, G. D.; Krishnamurty, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Science 1993, 261, 1299. (c) Huo, Q.; Margolese, D. I.; Ciesla, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schu¨th, F.; Stucky, G. D. Nature 1994, 368, 317. (d) Imhof, A.; Pine, D. J. Nature 1997, 389, 948. (5) Widawski, G.; Rawiso, M.; Francois, B. Nature 1994, 369, 387. (6) (a) LeMay, D. J.; Hopper, R. W.; Hrubesh, L. W.; Pekara, R. W. MRS Bull. 1990, 15, 19. (b) Even, W. R., Jr.; Gregory, D. P. MRS Bull. 1994, 19, 29. (c) Walsh, D.; Mann, S. Nature 1995, 377, 320. (d) Davis, S. A.; Burkett, S. L.; Mendelson, N. H.; Mann, S. Nature 1997, 385, 420. (e) Ozin, G. A. Acc. Chem. Res. 1997, 30, 17. (7) (a) Velev, O. D.; Jede, T. A.; Lobo, R. F.; Lenhoff, A. M. Nature 1997, 389, 447. (b) Park, S. H.; Xia, Y. N. Chem. Mater. 1998, 10, 1745. (c) Holland, B. T.; Blanford, C. F.; Stein, A. Science 1998, 281, 538. (d) Wijnhoven, J. E. G. J.; Vos, W. L. Science 1998, 281, 802. (e) Zakhidov, A. A.; Baughman, R. H.; Iqbal, Z.; Cui, C. X.; Khayrullin, I.; Dantas, S. O.; Marti, J.; Ralchenko, V. G. Science 1998, 282, 897. (f) Johnson, S. A.; Ollivier, P. J.; Mallouk, T. E. Science 1999, 283, 963. (g) Jiang, P.; Hwang, K. S.; Mittleman, D. M.; Bertone, J. F.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 11630. (8) Pusch, W.; Walch, A. Angew. Chem., Int. Ed. Engl. 1982, 21, 660.
increase the diffusion time of permeating substances without improving the retention of larger particles. Therefore, it is highly desirable to reduce the thickness of the porous media and to make it comparable to the size of the pores. The Langmuir-Blodgett (LB) technique9 offers the opportunity to prepare nanometer-thin membranes or coatings by assembling a monolayer at the air-water interface and then transferring it to almost any desired substrate. Compared to other techniques such as spincasting, vapor deposition, or monolayer self-assembly, the LB technique allows us to cover holes in solid substrates with a monolayer and thus proves particularly suitable for generating free-standing, ultrathin membranes. Stable and homogeneous free-standing membranes with a thickness of 10-50 nm have been obtained this way by using polymeric monolayers stabilized by vitrification10,11 or chemical 12,13 or physical cross-linking.14,15 The goal of the current contribution is to introduce into these membranes monodisperse pores and thereby make it possible to use them as filtration media or as masks or molds for surface patterning and nanostructure fabrication. The technique proposed here is based on the fact that hydrophobic colloids, like surface-active polymers, are able to form stable monolayers on a water surface. Thus, we investigate here the possibility of generating hybrid monolayers composed of hydrophobic colloids and surface active polymers, cross-linking the polymer, and then removing the colloids to generate thin membranes with uniform holes, as schematically depicted in Figure 1. (9) For an introduction to the characterization and transfer of insoluble films see: Gaines, G. L., Jr. Insoluble Monolayers at LiquidGas Interfaces; Interscience Publishers: New York, 1966. LangmuirBlodgett Films; Roberts, G., Ed.; Plenum Press: New York, 1990. Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: San Diego, CA, 1991. (10) Albrecht, O.; Matsuda, H.; Eguchi, K.; Nakagiri, T. Thin Solid Films 1996, 285, 152. (11) Goedel, W. A.; Peyratout, C.; Ouali, L.; Schadler, V. Adv. Mater. (Weinheim, Ger.) 1999, 11, 213. (12) Seufert, M.; Fakirov, C.; Wegner, G. Adv. Mater. (Weinheim, Ger.) 1995, 7, 52. (13) Goedel, W. A.; Heger, R. Langmuir 1998, 14, 3470. (14) Kunitake, M.; Nishi, T.; Yamamoto, H.; Nasu, K.; Manabe, O.; Nakashima, N. Langmuir 1994, 10, 3207. (15) Mallwitz, F.; Goedel, W. A. Angew. Chem., Int. Ed. 2001, 40, 2645.
10.1021/la011026m CCC: $22.00 © 2002 American Chemical Society Published on Web 02/01/2002
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Figure 2. Structure of polyisoprenes with ionic headgroups and anthracene side groups (top) and silica colloids of 140 nm diameter hydrophobized with polyisobutylene amphiphiles (bottom) used in this investigation.
Figure 1. Schematic representation of the formation of a freestanding porous membrane via cross-linking of a polymercolloid hybrid monolayer on a water surface, followed by transfer and removal of the colloidal particles.
Results and Discussion To build up hybrid monolayers, we used polyisoprenes with ionic headgroups and anthracene side groups and silica colloids of 140 nm diameter that were hydrophobized with polyisobutylene amphiphiles (Figure 2). As previously reported, the polyisoprene gives rise to a smooth, continuous lateral pressure-area isotherm (see Figure 3a) and forms 20-40-nm-thin liquid monolayers on a water surface,16 which can be photochemically crosslinked and transferred to form freely suspended membranes.13 The hydrophobized silica colloids, when spread onto a water surface, give rise to a smooth,continuous lateral pressure-area isotherm(Figure 3b). The area (A) of the steep region of curve b estimated by extrapolating to zero pressure (A ) 0.01 m2/mg) is much larger than the expected area of a close-packed two-dimensional array of colloids (A ) 0.0066 m2/mg). Scanning electron microscopy (16) (a) Heger, R.; Goedel, W. A. Macromolecules 1996, 29, 8912. (b) Baltes, H.; Schwendler, M.; Helm, C. A.; Heger, R.; Goedel, W. A. Macromolecules 1997, 30, 6633.
Figure 3. Lateral pressure-area isotherms at 20 °C. (a) Pure polymeric monolayer of polyisoprene with ionic headgroups. (b) Pure silica colloidal monolayer. (c) Hybrid monolayer composed of silica colloids and polyisoprene with ionic headgroups. (d) Hypothetical isotherm of a mixed monolayer assuming complete incompatibility between colloids and polymer (weighted average of the areas of (a) and (b)).
images of monolayers transferred to a mica substrate (Figure 4) show that the colloids form circular twodimensional domains; they do not rearrange into a closepacked layer even at high lateral pressures and thus occupy a significantly larger area than does a close-packed layer. Hybrid monolayers of silica colloids and polyisoprene were formed by spreading a chloroform solution containing these two components onto a water surface, followed by lateral compression (silica colloids-polyisoprene amphiphiles 47:53 by weight). The surface pressure-area isotherm of the hybrid monolayer is presented in Figure 3c in comparison with the isotherms of a pure colloidal monolayer and a pure polymeric monolayer. The colloidal monolayer and polymeric monolayer both have only one clearly identifiable collapse pressure, at 60 and 37 mN/m, respectively, while the hybrid monolayer has two collapse pressures at 35 and 62 mN/m that are close to the collapse pressures of the polymeric monolayer and the colloidal monolayer, respectively. The existence of two collapse pressures in the hybrid monolayer indicates that
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Figure 4. Scanning electron micrographs (SEM) of a silicacolloidal monolayer deposited on a mica surface at a surface pressure 20 mN/m. (a) Low magnification. (b) High magnification.
the two components are not homogeneously mixed and that domains of different stability coexist.17 This interpretation is in accordance with the scanning electron microscopy (SEM) pictures of cross-linked membranes discussed below (Figure 5) that indeed show twodimensional domains of particles imbedded in a polymer matrix. After spreading and lateral compression, the hybrid monolayers were cross-linked by UV light (λ ≈ 360 nm). Then, the cross-linked monolayers were transferred to gold grids with 100-µm-wide openings or to continuous, flat graphite substrates. (The non-cross-linked liquid monolayers have a tendency to dewet from continuous substrates after transfer, either forming droplets on the substrate or aggregating around the colloidal domains; if transferred as freely suspended membranes onto grids, they quickly rupture. Cross-linked monolayers do not dewet from continuous substrates after transfer, and the cross-linked membrane is mechanically stable enough to be transferred as a freely suspended membrane.) The freestanding cross-linked hybrid membranes supported on gold grids obtained by this procedure are shown in Figure 5a and b. As in the case of the pure colloidal monolayers, the silica colloids form circular domains that are embedded in a continuous polymeric membrane. To remove the silica colloids, the transferred monolayers were exposed to the vapor of hydrofluoric acid (HF) for 2-3 min. (Compared to other chemical reactions, the dissolution of SiO2 in HF has the unique advantage that all products are volatile.) As can be seen from Figure 5c, this method gives rise to a free-standing porous membrane. To measure the size of the holes and the thickness of the membrane, a cross-linked hybrid monolayer was transferred to a continuous graphite substrate and investigated by atomic force microscopy (AFM) before and after removal (17) Langmuir-Blodgett Films; Roberts, G., Ed.; Plenum Press: New York, 1990; p 26.
Figure 5. Scanning electron micrographs (SEM) of freestanding cross-linked hybrid monolayers and resulting porous membranes. (a) SEM picture of a hybrid monolayer at low magnification. (b) Representative part of (a) at high magnification. (c) Free-standing porous membranes obtained by removing silica colloids by exposure to HF vapor.
of the particles. As can be seen from the AFM picture shown in Figure 6b, the holes are approximately 40 nm deep, 55 nm wide, and 140 nm apart from each other. To generate a porous membrane via colloidal templating as depicted in Figure 1, it is important that (i) the polymer infiltrates the colloidal domains and fills the space between the individual colloids and that (ii) the colloids penetrate the hybrid layer and stick out of the membrane at the top and the bottom surfaces. Figure 5b gives clear evidence that the approach used here was successful. In addition, we would like to highlight a few details further illustrating the above-mentioned points: (i) The area occupied by the hybrid monolayer as depicted in Figure 3 is smaller than the weighted-average area (dotted line in Figure 3). The height of the polymer ridges between the holes remaining after removal of the particles (Figure 6b) is higher than the smooth polymeric layer in the area between the circular
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Figure 6. Atomic force microscopy (AFM) of (a) a cross-linked hybrid membrane transferred to a continuous graphite substrate and (b) a porous monolayer resulting from (a) after removal of the particles.
domains. Both observations indicate that preferential wetting of the colloids by the polymer occurs and that the polymer is attracted into the space between the colloids. (ii) As can be seen from Figure 6a, the colloids stick out of the top surface much further in regions supported by a solid substrate than in the freely suspended membrane. This behavior is due to the fact that in the freely suspended membrane the colloids stick out of the membrane at the top and bottom surface but are forced up when the bottom surface is flattened by a solid substrate. The approach presented here allows, in principle, the tuning of pore size and mechanical properties of the membrane by varying the size of the particle and the crosslinking density of the polymer. Particles are available in a size range of 1000 nm, and the thickness of the polymeric monolayer can be tuned from 10 to 100 nm by varying the chain length and surface concentration of the polymeric amphiphiles.16 The polymer used here was designed to cross-link via the dimerization of the anthracene side groups. The relatively low grafting density of such side groups (∼1.5%) limited the cross-linking density, and sometimes large holes of ∼2 µm diameter appeared. We learned that the mechanical properties can be improved by addition of a photoinitiator to the spreading solution. Membranes prepared this way are surprisingly robust, and the existence of the pores does not significantly affect the mechanical properties of the membrane. To prove the feasibility of our concept, the membranes have been transferred to gold grids that have comparatively large openings. The membranes can also be transferred to other technically relevant macroporous substrates. The membrane size, which is limited only by the size of the water surface, may be unlimited if a continuous process is used.
of pores as well as the small membrane thickness and uniform pore size make such membranes especially suitable for separation processes with improved efficiencies. In addition, such two-dimensional porous membranes may find application as masks for surface-pattern formation or nanostructure fabrication. Experimental Section Linear polyisoprene (Mn ≈ 67 200 g/mol) with a quaternary ammonium headgroup and anthracene side groups (about 1.8%) has been synthesized via anionic polymerization of isoprene,18-20 followed by platinum-catalyzed hydrosilylation of the quaternary ammonium-terminated parent polyisoprene.13 Silica colloids coated with polyisobutene amphiphiles (mean radius ) 70 nm, polydispersity, σ ) 11%, suspended in cyclohexane) were obtained from Utrecht Colloid Synthesis Facility, Van’t Hoff Laboratory for Physical and Colloid Chemistry, Utrecht University, The Netherlands. Trichloromethane (Aldrich, 99.99% pure) and ethanol (Aldrich, 99.99+% pure) were used as received. Water (resistivity ) 18.2 × 106 Ω/cm, total dissolved organic carbon
