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Replication of Lyotropic Block Copolymer Mesophases into Porous Silica by Nanocasting: Learning about Finer Details of Polymer Self-Assembly Arne Thomas, Helmut Schlaad, Bernd Smarsly, and Markus Antonietti* Max Planck Institute of Colloids and Interfaces, Research Campus Golm, Am Mu¨ hlenberg 1, D-14476 Golm, Germany Received January 17, 2003. In Final Form: March 14, 2003 Block copolymers, easily tailor-made by growing poly(ethylene oxide) from a commercial hydroxylterminated poly(ethylene-co-butylene) (Kraton liquid), can self-assemble over a wide range of concentrations into mesophases constructed from spherical micelles with small polydispersity. These phases are analyzed in detail by transmission electron microscopy and small-angle X-ray scattering (SAXS) by their silica replicas, hollow negative replicas of those structures made by nanocasting. The influence of polymer concentration on the structural order of the porous silica and the size of the constituting micelles is examined. At high volume fractions, a glasslike, disordered structure is obtained, while close-packed structures with long-range order are found in an intermediate concentration range with high local mobility of the micelles but sufficiently strong intermicellar forces to drive colloidal crystallization.
1. Introduction Nanocasting, the 3D-transformation of self-assembled organic nanostructures into hollow inorganic replicas under preservation of fine structural details, has recently turned out to be a versatile tool, both for the synthesis of porous media with new pore topology as well as for the characterization of the assembled structures themselves. In the classical synthesis of mesoporous silica,1,2 a route which we call “synergistic precipitation”, the orders of the starting and end situations are not related. Nanocasting or the “true lyotropic liquid crystal approach”,3-6 as introduced by Attard and Go¨ltner, is different. Here, one starts from a high concentration ordered template phase, and the liquid continuous phase is solidified by chemical gelation reactions. In the early work already, it was shown that this technique offers the possibility to make an 1:1 imprint or negative copy of organic mesophases. X-ray measurements performed throughout the process showed that the solidified hybrid preserves all structural features throughout solidification of the matrix. The calcined or hollowed replica even possess the same structure, just slightly shrinked.3-5 To enable nanocasting, the casted structure must be compatible with both the liquid precursor phase as well as the final solidified replica. If this is not the case, the enormous interfaces involved (up to 103 m2/g) will add unfavorable energies, and replication breaks down. This is why most work is done on sol-gel silica,7 the surface chemistry of which is easy to address. In addition, the final product, amorphous SiO2, is able to (1) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834-10843. (2) Kresge, C. T.; Leonowicz, M.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710-712. (3) Attard, G. S.; Go¨ltner, C. G.; Corker, J. M.; Henke, S.; Templer, R. H. Angew. Chem. 1997, 109, 1372-1374. (4) Attard, G. S.; Glyde, J. G.; Go¨ltner, C. G. Nature 1995, 378, 366368. (5) Go¨ltner, C. G.; Henke, S.; Weiβenberger, M. C.; Antonietti, M. Angew. Chem. 1998, 110, 633-636. (6) Go¨ltner, C. G.; Antonietti, M. Adv. Mater. 1997, 9, 431-436. (7) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, 1st ed.; Academic Press Inc.: New York, 1990.
adapt to even the smallest structures. Compatibilization via PEO tails, first demonstrated by Pinnavaia et al.,8-10 is one approach shown to work well. Here, the formation of hydrogen bridges between the silicic acid framework and the ether oxygens of the PEO chain stabilizes the interface. The gelation of the SiO2 network is preferentially done at about pH 2, close to the isoelectric point of silicic acid. The assembled structures of amphiphilic block copolymers turned out to be very useful templates, which allow a wider variation of chemistry and casting conditions. Here, the range of accessible pore sizes is extended to pore diameters between 5 and 100 nm, and a variety of pore connectivities have been made by nanocasting of the corresponding self-assembly structures in water. Because of the size and the higher stability of polymer assemblies, it was possible to check directly the quality and precision of the nanocasting process. This was shown by fixing a lyotropic phase of a poly(1,2-butadiene)-poly(ethylene oxide) (PB-PEO) block copolymer by γ-radiation and comparing this structure with the silica nanocast of the parent PB-PEO liquid crystal phase.11 The solid silica imprint had the inverted structure of the organic template, but the template structure was not affected by the casting process itself. Meanwhile, a large variety of amphiphilic block copolymers has been used. Besides nonionic structures with PEO chains to mediate silica compatibility,5,12-14 also cationic and anionic polyelectrolyte blocks were employed.15 The first reported systems were polystyrene-PEO block copolymers of low molecular weight. A polymer with Mw (8) Bagshaw, S. A.; Prouzet, E.; Pinnavaia, T. J. Science 1995, 269, 1242-1244. (9) Tanev, P. T.; Pinnavaia, T. J. Science 1995, 267, 865-867. (10) Prouzet, E.; Pinnavaia, T. J. Angew. Chem. 1997, 109, 533-536. (11) Hentze, H. P.; Kra¨mer, E.; Berton, B.; Fo¨rster, S.; Antonietti, M. Macromolecules 1999, 32, 5803-5809. (12) Go¨ltner, C. G.; Berton, B.; Kra¨mer, E.; Antonietti, M. Adv. Mater. 1999, 11, 395-398. (13) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024-6036. (14) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548-552. (15) Kra¨mer, E.; Fo¨rster, S.; Go¨ltner, C.; Antonietti, M. Langmuir 1998, 14, 2027-2031.
10.1021/la0340807 CCC: $25.00 © 2003 American Chemical Society Published on Web 04/05/2003
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Scheme 1. Synthesis of Poly(ethylene-co-butylene)-block-poly(ethylene oxide) (KLE)
Thomas et al. Table 1. Molecular Properties of the Template Block Copolymersa sample
x
y
fEO
wEO
PDI
KLE-1 KLE-2 KLE-3 KLE-4
99 104 104 99
72 56 52 109
0.42 0.35 0.33 0.52
0.46 0.39 0.37 0.57
1.03 1.02 1.03 1.06
a x, y: number of ethylene/butylene and ethylene oxide repeating units, respectively. fEO, wEO: mole and weight fraction, respectively, of PEO in the copolymer. PDI: polydispersity index.
Table 2. Experimental Conditions for the Preparation of Silica Samples 1-4a
) 1000 g mol-1 in each of its block segments (named SE1010) gives a pore size of 4.8 nm with some distorted hexagonal structure with many branches.5 In the case of the PEO-PB block copolymers, also unconventional pore morphologies, such as a branched lamellar phases and a bicontinuous spongelike structure, were generated.16-18 Throughout these examinations, it became obvious that, due to the strict relation between the template structure and the resulting pore system, nanocasting can obviously be used to learn about the structure of the templates itself. Since those are sometimes fragile equilibrium structures or aggregates in water, which are not easy to depict by electron microscopy or X-ray experiments under natural conditions (insufficient contrast, beam damage, and preparation artifacts), one can preferably examine the “hardcopy” of the pore structure. The pores show high contrast, reveal even fine structural details, and (for example for silica materials) are stable against most external conditions. Recent examples of this approach include characterization of the self-assembly of cyclodextrin moieties19 and the superstructures of cyclodextrin based rotaxanes.20 The scope of the present work is to analyze in more detail the micelle formation and the mutual packing motives of the spherical micelles of a simple KLE-type amphiphilic block copolymer, made by polymerization of ethylene oxide (EO) using Kraton Liquid as macroinitiator (see Scheme 1). The length of the PEO chain is varied in a certain range to fine-tune the assembly patterns, and the resulting structures are characterized by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) of their solid silica replicas in dependence of template concentration. 2. Materials and Methods Materials. Kraton Liquid is a ω-hydroxypoly(ethylene-cobutylene) supplied by Exxon. The molecular weight was 3700 or 3900 g/mol, as provided by the supplier for two different lots. The mole fraction of butylene units was determined by 1H NMR to be 0.33. Size exclusion chromatography (SEC) in THF, calibrated with polystyrene standards, revealed an apparent polydispersity index (PDI) of 1.03. EO and tetrahydrofuran (THF) were received from Fluka and BASF AG, respectively, and were purified as described elsewhere.21 All other chemicals were obtained from Aldrich and were used without further purification. Synthesis of Poly(ethylene-co-butylene)-block-poly(ethylene oxide). KLE-type block copolymers were prepared (16) Go¨ltner, C. G.; Berton, B.; Kra¨mer, E.; Antonietti, M. Adv. Mater. 1999, 11, 395-398. (17) Go¨ltner, C. G.; Berton, B.; Kra¨mer, E.; Antonietti, M. Chem. Commun. 1998, 2287-2288. (18) Fo¨rster, S.; Berton, B.; Hentze, H. P.; Kra¨mer, E.; Antonietti, M.; Lindner, P. Macromolecules 2001, 34, 4610-4623. (19) Polarz, S.; Smarsly, B.; Bronstein, L.; Antonietti, M. Angew. Chem. 2001, 113, 4549-4553. (20) Han, B.-H.; Antonietti, M. Chem. Mater. 2002, 14, 3477-3485. (21) Schlaad, H.; Kukula, H.; Rudloff, J.; Below, I. Macromolecules 2001, 34, 4302-4304.
sample
template
TEOS/HCl/template (g)
wKLE
φKLE
1 2a 2b 2c 2d 2e 2f 2g 2h 3 4
KLE-1 KLE-2 KLE-2 KLE-2 KLE-2 KLE-2 KLE-2 KLE-2 KLE-2 KLE-3 KLE-4
1:0.5:0.2 1:0.5:0.8 1:0.5:0.7 1:0.5:0.6 1:0.5:0.5 1:0.5:0.4 1:0.5:0.3 1:0.5:0.2 1:0.5:0.1 1:0.5:0.2 1:0.5:0.2
0.21 0.51 0.48 0.44 0.39 0.34 0.28 0.21 0.12 0.21 0.21
0.38 0.70 0.68 0.64 0.59 0.54 0.47 0.38 0.24 0.38 0.38
a w KLE, φKLE: weight and volume fraction, respectively, of the KLE template.
according to a procedure described elsewhere.21 Briefly, to a ∼20 wt % THF solution of Kraton Liquid was added an equimolar amout of the phosphazene base t-BuP4 (1 M solution in hexane) using a syringe. The solution was cooled to -30 °C, and EO was cryodistilled into the reactor. The reaction mixture was stirred at -30 °C for 1 h and then slowly heated to 50 °C and stirred for 2 days under a dry argon atmosphere. After the polymerization was quenched with acetic acid, the reaction solution was concentrated and the crude product was precipitated into cold acetone (-30 °C). The polymer was redissolved in distilled water, washed with a strongly acidic cation exchanger to remove traces of protonated t-BuP4, filtered, and freeze-dried. The chemical composition of polymer samples was determined by 1H NMR (δ/ppm ) 0.83 (CH3), 1.0-1.5 (CH, CH2), 3.65 (CH2O)). Note that in any case the composition of the polymer was as targeted, indicating that EO had been quantitatively consumed. Size exclusion chromatography revealed a monomodal molecular weight distribution; no Kraton Liquid residuals were detected. Table 1 summarizes the different block copolymers employed as templates. Silica Nanocasting. The porous silica materials were prepared as follows. The appropriate amount of the block copolymer (e.g. 0.1 g) was dissolved in 2.0 g of ethanol under gentle heating, and then 1.0 g of tetraethoxysilane (TEOS) and 0.5 g of aqueous hydrochloric acid (pH 2) were added. Homogenization occurred in some minutes while the TEOS is hydrolyzed. The ethanol was removed under vacuum prior to solidification, diminishing the influence of the ethanol on the phase structure. The resulting viscous solutions or gels were aged at 60 °C in a drying oven for 3 days. The organic template was removed via calcination in air at 550 °C for 5 h. Table 2 summarizes the relative compositions of the different silica samples described in the present contribution. Measurements. Transmission electron microscopy (TEM) images were taken with a Zeiss EM 912Ω at an acceleration voltage of 120 kV. Samples were ground in a ball mill and taken up in acetone. One droplet of the suspension was applied to a 400 mesh carbon-coated copper grid and left to dry in air. For a closer inspection, some specimens were prepared by embedding the ground silica powders in an epoxy resin and ultramicrotoming. Nitrogen adsorption data were obtained with a Micromeritics Tristar. SAXS curves were recorded employing a Kratky camera and also a rotating anode with pinhole collimation. A Nonius rotating anode (P ) 4 kW, Cu KR) and an image-plate detector system were used. Placing the image plates at a distance of 40 cm from the sample, a scattering vector range from s ) 0.05 to
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Figure 1. SAXS data for the silica nanocasts 2a-2h, prepared using different concentrations of the KLE-2 template (cf. Table 2).
Figure 2. TEM micrographs of calcined silica nanocasts obtained at different concentrations of the template KLE-2: left, 2b (wKLE ) 0.48); middle, 2f (wKLE ) 0.28); right, 2g (wKLE ) 0.21). Scale bar ) 100 nm. 1.6 nm-1 was available. The samples were irradiated for 18 h to reduce the noise level and to obtain a sufficiently high scattering intensity. 2D diffraction patterns were transformed into a 1D profile by radial averaging of the scattering intensity.
3. Results All silica hybrids are homogeneous, transparent materials, indicating the absence of demixing throughout the silica solidification process. Polarization microscopy reveals in all cases the absence of birefringence, indicating the presence of either a cubic or a disordered micellar structure. We essentially performed two series of experiments: (i) a concentration series with the optimal template KLE-2 with template contents between 12 and 51 wt % (f silicas 2a-h) and (ii) a series varying the length of the PEO tail at a fixed optimal template concentration of 21 wt %. Figure 1 shows the SAXS curves of the silica samples 2a-h, which were prepared within the first series. It is obvious that the samples prepared at low template concentrations show nice oscillations of intensity. These oscillations are getting less pronounced with increasing template content. This is, however, not necessarily related to the order of the system or an increased polydispersity of the micelles, as the form factor of spherical micelles and the lattice factor of the mutual packing of the system do overlap. A quantitative description of data would then depend on the assumed model and a whole set of assumptions;this is the general weakness of any scattering on nonoriented polydomains.
The TEM micrographs of the corresponding silica replicas, shown in Figure 2, further highlight the advantages of the nanocasting technique. It is seen that, for the highest template concentration (2b), the constituting units are “about-spherical” micelles, which however do deform into a tightly packed foamlike structure. The system rather tries to preserve a certain minimal distance between the hydrophobic cores (the visible big pores in the replica) than to keep the spherical shape. Hence, surface energy contributions are of lower importance than the entropic contributions to the free energy, brought in by the tethered solvating chains. It is obvious that the key aspects of the underlying thermodynamics are neglected if such a system is treated in a hard-sphere approximation. From statistical evaluations, the average diameter of the pores is determined to be 21.0 nm, and the averaged wall thickness is estimated as 2.8 nm. Here, we want to repeat that the observed structure is most probably not an artifact of nanocasting, as the overall shrinking of the sample throughout is low and the change of the scattering curves negligible. There is a good chance that the crumpled character is indeed due to the preparation, but neither the overall size nor the mutual arrangement is expected to have seriously changed. The missing long-range order for very high volume fractions of template (ΦKLE ≈ 0.70, not considering swelling) has to be seen in analogy to the so-called “glassy colloidal phases” of hard colloidal spheres. Above a certain packing concentration, such spheres are dynamically
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immobilized and form a kinetically stable “glassy phase”.22 As our data correspond to those observations, we will describe the structure, which is formed by the rather soft KLE micelles at high concentration, also as a “glassy phase”. For decreasing template concentration, the micelles are getting more and more spherical, and better ordered structures are formed; see Figure 2 (2f: pore size 13.9 nm, wall thickness 3.3 nm). Here, we have to point to the fact that the range of order represents a snapshot of a dynamic system, fixed by our applied condensation conditions. A slower condensation reaction would presumably increase the order of species with intermediate concentration. In our series, the highest order was found for the 21 wt % template sample (2g), where quite narrowly distributed, spherical micelles are formed. Here the pore diameter is 13.1 ( 0.5 nm, and the minimal wall thickness is 3.9 nm, as estimated from the repeat period. For a template content of 12 wt % (2h, data not shown), the structural order between the micelles is decreasing again, which is due to the increasing distance between the micelles and the coupled softer effective interparticle potentials of the system. In this range, we cannot exclude an influence of silica addition on the structural order, as the interaction potentials are obviously getting softer and more sensitive. It is interesting to note that the size of micelles increases with increasing block copolymer concentration. On the basis of standard theories of block copolymer aggregation for low concentrations,23 this can be explained by the following: for higher concentrations with their higher osmotic background, the steric repulsion between different solvating chains of the same micelle is getting weaker, and more densely packed structures within the micelle interface can be made. In other words, the system moves from good solvent conditions in the diluted state to the θ-conditions of the limiting melt, and the aggregate size increases. This effect of a slight increase of the micelle size with concentration is usually not considered in the data evaluation of such systems. It is expected that also the minimal wall thickness of the silicas should depend on polymer concentration. Indeed, values from 2.2 nm ((0.3 nm) for the highest concentration (2a, picture not shown) up to 6.0 nm ((0.3 nm) for the lowest concentration (2h, picture not shown) were found. Electron microscopy has the potential disadvantage to be affected with sampling problems, especially when mutual and long-range order is involved. Figure 3 shows different pictures of the same recipe reproduced twice. Independent of the grain and the batch, the same size of micelles and the same type of structure are found. The bare observations of distinct sites of the sample indicate a face centered cubic (FCC) type arrangement, but defects of the hexagonal closed packing (HCP) type also seem to be present. This is in good agreement with the behavior of similar self-organizing systems. A coexistence of FCC and partial HCP ordering was found for the mesoporous silicas SBA-2 (with high-resolution TEM)24 and SBA-12 (electron crystallography).25 In lyotropic block copolymer phases, usually FCC structures are obtained for “hard sphere” systems, whereas BCC prevails for “soft” (22) Pusey, P. N.; Vanmegen, W. Nature 1986, 320, 340-342. (23) Fo¨rster, S.; Zisenis, M.; Wenz, E.; Antonietti, M. J. Chem. Phys. 1996, 104, 9956-9970. (24) Zhou, W.; Hunter, H. M. A.; Wright, P. A.; Ge, Q.; Thomas, J. M. J. Phys. Chem. B 1998, 102, 6933-6936. (25) Sakamoto, Y.; Dı´az, I.; Terasaki, O.; Zhao, D.; Pe´rez-Pariente, J.; Kim, J. M.; Stucky, G. D. J. Phys. Chem. B 2002, 106, 3118-3123.
Thomas et al.
Figure 3. Sampling of TEM micrographs of sample 2g obtained for different grains and for different batches. Scale bar ) 100 nm (bottom right: 50 nm).
spherical block copolymer micelles with long-range repulsive interactions.26 Meanwhile, there are elaborate theories describing the occurrence of those phases for colloidal systems with “soft and ultrasoft” interactions, such as block copolymer micelles, dendritic structures, and star polymers.27 To the best of our knowledge, deformation of the constituting units and formation of a constant separation layer, as observed above for the high concentration systems, is however not considered or supported by those theories. Because of the monodispersity of the micelles and the high structural order, it is possible to use structure models to calculate properties of the micellar aggregates. For sample 2g, the average distance between micelles of 17.7 nm together with the applied polymer concentration gives 2.5 × 1017 micelles per cm3. With the hybrid density, a micelle molecular mass of ∼840 000 g/mol and an aggregation number of 133 are obtained. In addition, it is possible to calculate a micelle core weight based on the pore size, and a value of about 700 000 g/mol is revealed. As the micelle is composed of 39 wt % PEO, this indicates that a majority of the PEO chains (∼60%) condenses onto the hydrophobic core. The remaining part is compatible with the silica, mediates the solubilization, and forms micropores. This result supports the recently discussed “three-phase model” of nanocasting.28,29 It is, however, an open question if the separation into two types of PEO is induced by the nanocasting process or an inherent structural feature of amphiphilic micelles, as recently demonstrated by Fo¨rster et al.30 In a second set of experiments, we varied the relative composition of the block copolymers by changing the length of the PEO chain at fixed hydrophobic moiety. For the (26) McConnell, G. A.; Gast, A. P. Phys. Rev. E 1996, 54, 5447-5455. (27) Watzlawek, M.; Likos, C. N.; Lo¨wen, H. Phys. Rev. Lett. 1999, 82, 5289-5292. (28) Go¨ltner, C. G.; Smarsly, B.; Berton, B.; Antonietti, M. Chem. Mater. 2001, 13, 1617-1624. (29) Smarsly, B.; Polarz, S.; Antonietti, M. J. Phys. Chem. B 2001, 105, 10473-10483. (30) Fo¨rster, S.; Hermsdorf, N.; Bo¨ttcher, C.; Lindner, P. Macromolecules 2002, 35, 4096-4105.
Replication of Lyotropic Block Copolymer Mesophases
Figure 4. SAXS curves of the silica nanocasts 1, 2g, 3, and 4, prepared by different KLE templates at a fixed weight fraction of 0.21 polymer (ΦKLE ) 0.38; cf. Table 2).
comparison of the corresponding lyotropic phases, we also kept the volume fraction of template constant at 0.38. All the presented polymers, KLE-1, KLE-2, and KLE3, form highly ordered cubic phases. Polymers with longer hydrophilic blocks such as KLE-4 as templates provided only weakly ordered phases; see the SAXS curve in Figure 4. As taken from the statistical evaluation of electron micrographs, the pore size is changing from 11.5 to 13.1 to 15.8 nm ((0.5 nm) for 1 (f KLE-1), 2g (f KLE-2), and 3 (f KLE-3), respectively (pictures not shown). The increase of the pore size and the corresponding micelle size with decreasing molar mass of the hydrophilic block is in agreement with a general scaling law of the aggregation number on the block sizes of amphiphilic block copolymers.23 In the case of 1, the scattering diagram seems to be suited for a quantitative data evaluation. In general, the SAXS powder diffraction pattern of any alignment of monodisperse spherical objects is given by
I(s) ∝ P(s) Z(s) where P(s) is the form factor of a sphere and Z(s) the lattice factor of the respective lattice type, which is the sum of the structure factors Fhkl multiplied by the multiplicities of the lattice planes (hkl). Figure 5 shows a simulation of the SAXS curve of 1 where Lorenzian profiles with a certain width were used to simulate the FCC lattice factor. It is seen that the simulation assuming FCC structure describes the experimental curve correctly with respect to the peak positions and the relative peak intensities, revealing the pore diameter 13.8 nm ((0.6 nm) and a FCC unit cell size of 24 nm as fit parameters. It is interesting to note that TEM provides about the same cell size but slightly smaller pores, which fits better to the known stoichiometry of the system. The reason for this deviation is unknown, but it is presumably due to the fact that simulation always depends on many structural assumptions, as mentioned above. In this context it is also worth mentioning that variation of polydispersity did not improve the data description; that is, the fit is optimal within the assumptions of the chosen model. 4. Conclusion In this contribution, the self-assembly of amphiphilic block copolymers, made by simple grafting of EO from an
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Figure 5. Numerical simulation (solid line) of the experimental SAXS diagram (squares) of 1, assuming an FCC structure of spherical voids possessing a Gaussian dispersity of 0.6 nm.
easily available industrial hydrophobic block, was analyzed. In the described composition range, the polymers exclusively formed spherical micelles with very low polydispersity, as shown by silica nanocasting. No phase transitions to other assembly morphologies such as hexagonal mesophases were observed. This makes the present systems good model systems for the examination of concentration effects on the packing of micelles. Depending on polymer concentration, different structural packing motives were identified. At high volume fractions, a “glassy structure” with deformed micellar cores and constant layer thickness of the hydrophilic phase was observed. With decreasing concentration, the micelles started to form an FCC-type arrangement with possible HCP stacking defects. The long-range order was increasing with decreasing concentration, which might be due to a kinetic effect, characterizing the fixed preparation conditions chosen. At volume fractions as low as 0.20, the micelles just weakly interacted with each other and lost their ability for structural packing. A quantitative evaluation on the basis of unit cell size, pore size, and stoichiometry revealed an aggregation number of the block copolymers of about 133 (KLE-2 at 21 wt %), which agrees well with theories of micelle formation in dilute solution.23 As expected, the aggregate size was found to increase with decreasing length of the hydrophilic block and to increase with increasing concentration; the latter effect is usually neglected. From a materials point of view, nanocasting of those block copolymer mesophases enabled the generation of very fine and homogeneous porous amorphous silica with an interesting intermediate pore size in the range 11-20 nm and a high structural order. The samples are obtained as very homogeneous monoliths or thin films. To generate silicas with long-range cubic order, volume fractions of the template of about 0.40 are most appropriate, as they combine high template mobility with sufficiently strong intermicellar forces to drive colloidal crystallization. Acknowledgment. We thank Sebastian Polarz for fruitful discussions, Ines Below for help with the polymer synthesis, and Rona Pitschke for technical help throughout electron microscopy. Erich C. is especially acknowledged for keeping the creative atmosphere alive. The Max Planck Society is acknowledged for financial support. LA0340807
