NANO LETTERS
Mesoporous Membrane Templated by a Polymeric Bicontinuous Microemulsion
2006 Vol. 6, No. 10 2354-2357
Ning Zhou,† Frank S. Bates,*,† and Timothy P. Lodge*,†,‡ Department of Chemical Engineering & Materials Science and Department of Chemistry, UniVersity of Minnesota, Minneapolis, Minnesota 55455 Received July 27, 2006; Revised Manuscript Received August 16, 2006
ABSTRACT We demonstrate a facile method for preparing a novel nanoporous material with an isotropic, three-dimensionally continuous pore structure from a polymeric bicontinuous microemulsion precursor, which consists of two immiscible homopolymers and the corresponding diblock copolymer. The protocol for the generation of nanopores is selective cross-linking of one domain with the subsequent removal of the other domain by solvent dissolution.
The generation of materials with controlled porosity on the submicrometer scale has been one of the most fertile arenas in materials chemistry. Such structures can be of direct utility, or can serve as templates into which other, more functional components can be distributed. A rich variety of strategies have been developed on both the inorganic and organic sides of the chemical aisle, and indeed inorganic-organic hybrids represent an important class on their own. Altogether, these materials hold tremendous potential for applications as diverse as membranes for filtration and separation, catalyst supports and catalytically active materials, fuel cells, photovoltaics, and photonics. In this letter, we disclose a new approach to nanoporous or mesoporous materials that exploits a polymeric bicontinuous microemulsion. This equilibrium state is attained by judicious combination of two immiscible homopolymers (A and B) with a symmetric AB diblock copolymer.1-8 One of the interpenetrating networks, say A, is then permanently solidified (in this case by cross-linking), thereby allowing the B phase to be selectively and completely removed (in this case, by simple solvent extraction) (Figure 1). This approach should be quite general, with resulting porosities on the order of 50% and characteristic pore dimensions continuously tunable over the range from at least 10 to 100 nm. The self-assembly of amphiphilic molecules in general, and of block copolymers in particular, followed by selective removal of one block, has proven to be a fruitful route to nanoporous materials.9 Removal strategies have included ozonolysis,10 reactive ion etching,11 chemical etching,12-15 depolymerization,16 and dissolution.17 However, in most examples the samples have been thin films; only in select * Corresponding authors. E-mail:
[email protected]; bates@ cems.umn.edu. † Department of Chemical Engineering & Materials Science. ‡ Department of Chemistry. 10.1021/nl061765t CCC: $33.50 Published on Web 08/30/2006
© 2006 American Chemical Society
Figure 1. Schematic illustration of the preparation of nanoporous material from a polymeric bicontinuous microemulsion precursor: selective cross-linking of one domain (A) with the subsequent removal of the other domain (homopolymer B) by solvent dissolution.
cases have macroscopic nanoporous materials been realized.12-15 Among the four equilibrium morphologies formed by AB diblock copolymers, that is, lamellae, hexagonally packed cylinders, bicontinuous double-gyroid and bcc spheres, the cylindrical structure has been utilized the most for generating nanoporous materials. A crucial processing
step is the attainment of near-perfect alignment of cylinders along one axis in order to achieve uninterrupted connectivity of the pores across the entire material. In this respect, the bicontinuous double-gyroid precursor, having channel continuity in three dimensions, could be more appealing.13,18-21 However, the applicability of the nanoporous double gyroid suffers from the necessary precise control of block copolymer composition and molecular weight because this phase is only stable over a rather narrow window.22,23 Finally, the channel diameters in block copolymer cylinders or double gyroids are typically in the range of 5-20 nm. To achieve significantly larger pore sizes, such as those typically desired for ultrafiltration or for photonics, is challenging indeed. Polymeric bicontinuous microemulsions (BµE) offer an appealing complementary route to bicontinuous structures. The universality of this phase has been amply demonstrated.2,3,5,6,8 It is most readily achieved by blending equal volumes of two homopolymers of comparable molar volume; modest molar masses can be selected to bring the binary homopolymer blend close to its critical point. The addition of a symmetric copolymer will first reduce the phaseseparation temperature and then produce the BµE. Further addition of copolymer will generate a highly swollen lamellar phase. The detailed phase behavior1-8 and fascinating rheological response24-27 of these ternary blends has been documented extensively. In the current context, the salient attributes of the BµE are these: the three-dimensional connectivity obviates the need for domain alignment; the natural lengthscale falls in the range 10-100 nm, and is tunable by the amount of added copolymer; the amount of the more “expensive” copolymer ingredient is on the order of only 10%. This range of length scale serves as a bridge between (more restrictively defined28) mesoporous (2-50 nm) and macroporous (>50 nm) materials. The concept of using a BµE to generate a porous material dates back at least to the pioneering work of Cussler and co-workers.29,30 They cross-linked BµEs formed in oil/water/ surfactant systems by polymerization of the oil. Although porous materials could be produced, it was found that the original structure of the BµE could not be preserved on fixation; the appearance of much larger scale heterogeneities was apparently unavoidable. This illustrates a rather general point, namely that “soft” material templates made by selfassembly are delicately poised, and it is often tricky to fix one nanodomain while maintaining the overall structure. In this respect, the polymeric form of the BµE has the advantage of much greater mechanical and dimensional robustness. The BµE precursor used here was prepared from ternary blends of polyisoprene (PI), polystyrene (PS), and a PI-PS diblock copolymer with molecular weights (g/mol) of 3100, 3400, and 16 000 (volume fraction fPI ) 0.50), respectively, and low polydispersities (PDI < 1.1). The polymers were synthesized by living anionic polymerization using standard procedures. To locate the composition window of the BµE phase, a combination of small-angle X-ray scattering, rheology, and optical microscopy was employed to map out the phase diagram along the isopleth (Figure 2), where the two homopolymers are mixed in equal volumes. The ternary Nano Lett., Vol. 6, No. 10, 2006
Figure 2. Phase diagram along the isopleth for the ternary blends of PI/PS/PI-PS. Φh denotes the total volume fraction of the two homopolymers. The phase boundaries were determined by various techniques: (filled squares) order-disorder transition investigated by rheology and SAXS, where there are discontinuous changes in the dynamic elastic modulus (rheology) and peak scattering intensity (SAXS); (filled circles) macrophase separation investigated by optical microscopy, where phase separation is revealed by opacity. The narrow bicontinuous microemulsion channel, spanning about 3% wide in block copolymer composition, exists between the lamellar and macrophase-separated region.
polymer blends were mixed by co-dissolution in benzene followed by vacuum-drying to constant weight. The molecular weights of the two homopolymers and diblock copolymer were particularly designed to avoid the possible complex phase behavior below the BµE phase channel.2,8 Here, the BµE channel extends down at least to the Tg of the PS component (ca. 72 °C). Experiments were carried out on a BµE precursor containing 10% block copolymer (by volume). The synchrotron SAXS pattern at room temperature is shown in Figure 3. The scattering profile was described well by the Teubner-Strey microemulsion model,31 and the characteristic domain size and amphiphilicity factor were calculated to be 80 nm and -0.85, respectively, consistent with a highly structured microemulsion.32 The bulk BµE samples (1 mm thick) were annealed at 100 °C for 10 min and then rapidly cooled to room temperature, thereby vitrifying the PS phase. Sulfur monochloride (S2Cl2) was used to cross-link the continuous PI phase. To preserve the BµE morphology during the cross-linking process, we exposed the samples to S2Cl2 vapor at RT without any direct contact with the S2Cl2 liquid. (The S2Cl2 liquid is a good solvent for both PS and PI components and therefore can dissolve the blends immediately.) The S2Cl2 diffused into the samples slowly to undergo cross-linking reactions with the PI double bonds. A detailed description of the cross-linking mechanism can be found elsewhere.33,34 After approximately two weeks, the samples turned brown in color, indicating considerable uptake of S2Cl2. Then, PS homopolymers were removed by simple dissolution using hexane as solvent. The FTIR spectrum (see Supporting Information, Figure S1) of the nanoporous material indicated no residual double bonds from polyisoprene. 2355
Figure 3. Synchrotron SAXS pattern of the BµE sample containing 10% diblock copolymer at room temperature after being quenched from 100 °C. The solid curve is a fit to the Teubner-Strey microemulsion model. The bottom panel shows SANS measurements of the BµE sample, before (b) and after (9) processing. These results suggest some increase in the most probable pore dimension.
The resulting nanoporous samples were freeze-fractured for subsequent SEM analysis (Figure 4a and b, and Figure S2a and b), which clearly shows that the bicontinuous morphology is maintained after the aforementioned procedure. SEM images with low magnification (Figures 4a and S2a) suggest that these interconnected pores can span the entire material uniformly. In addition, it is worth mentioning that the pore walls are by default coated with the PS block from the diblock copolymer. The porous structures remained almost intact even after heating to 200 °C, although thermogravimetric analysis indicates that there is some thermal degradation35 of the matrix material above 160 °C (Figure S3). This superior thermal stability can be attributed to the high degree of PI cross-linking, rather than the PS block coating the pore walls. The cross-linking also confers excellent solvent resistance to this nanoporous material; for example, on exposure to benzene, acetone and THF, no visible swelling or tangible softening occurred. Preliminary mechanical testing indicates a substantial modulus (ca. 50 MPa) but significant embrittlement (strain to break > 2%), consistent with the extensive cross-linking. To establish that these materials were truly porous, we immersed macroscopic pieces in the ionic liquid butyl methyl imidazolium (bmim) phosphorus hexafluoride (PF6). After a few hours, the specimens sank to the bottom of the flask, 2356
Figure 4. SEM images of the freeze-fractured surface of nanoporous material from the BµE sample under two different magnifications: (a) × 10 000; (b) × 50 000.
indicating substantial solvent uptake within the pores. This is an interesting observation in itself, in that the PS blocks coating the pores are not themselves soluble in this ionic liquid. The electrical conductivity of the filled membrane was measured at RT to be 3.89 × 10-4 S/cm by AC impedance spectroscopy; in comparison, the ionic liquid alone had a conductivity of 1.40 × 10-3 S/cm. This result is extremely satisfying because it confirms the high permeability of the material; most of the original PS nanodomains must have percolated through the BµE. Given that the overall porosity is approximately 0.45, we may infer a “tortuosity factor” of 2 for this membrane, which is also fully consistent with the presumed structure. As a further characterization tool, we employed the Brunauer-Emmett-Teller (BET) method to assess the pore Nano Lett., Vol. 6, No. 10, 2006
Supporting Information Available: Material synthesis, cross-linking procedure, and IR, SEM, and TGA of the nanoporous material. This material is available free of charge via the Internet at http://pubs.acs.org. References
Figure 5. Nitrogen adsorption (0) - desorption (3) isotherms of the nanoporous material. Bottom panel: pore size distribution from desorption data.
size distribution. Figure 5 illustrates the nitrogen adsorptiondesorption isotherms (at T ) 77 K) of this nanoporous material. The desorption branch was used to calculate the pore size distribution in terms of the Barrett-JoynerHalenda model,36 giving a most probable pore diameter of 43 nm with a half-width at half-height of 17 nm. In conclusion, we have demonstrated a facile method for preparing a novel nanoporous material with an isotropic, three-dimensionally continuous pore structure from a polymeric BµE precursor. Macroscopic pieces (> millimeters in the small dimension) can be fabricated readily. The spontaneous formation of interconnected pores, without any alignment by external force, could offer a significant advantage over other types of pores (e.g., cylinders) with respect to processing. More significantly, the pore sizes accessible by this protocol are substantially larger than those achievable from pure block copolymers. Also, the considerable thermal stability and solvent resistance of this nanoporous material is an adventitious attribute of the cross-linking protocol. Acknowledgment. This work was supported primarily by the MRSEC program of the National Science Foundation under Award No. DMR-0212302. We thank Qiang Lan and Lifeng Wu for providing PS and PS-PI, Yiyong He for help with the conductivity measurement, and Huiming Mao for help with the BET measurement. Use of the Advanced Photon Source at Argonne National Labs was supported by the U.S. Department of Energy, Basic Energy Sciences, Office of Science, under Contract No. W-31-109-Eng-38. Nano Lett., Vol. 6, No. 10, 2006
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