Preparation of Composite Membranes with Bicontinuous Structure

Article pubs.acs.org/Langmuir

Preparation of Composite Membranes with Bicontinuous Structure Marcel Mühlmann, Annemarie Magerl, and Werner A. Goedel* Physical Chemistry, Chemnitz University of Technology, Straße der Nationen 62, 09111 Chemnitz, Germany S Supporting Information *

ABSTRACT: Composite membranes with a hierarchical structure comprising thin regions with a bicontinuous structure and thick regions providing mechanical strength have been prepared by casting inorganic zeolite particles and mixtures that yield organic polymers onto substrates that were decorated with sessile droplets of aqueous solutions. Analysis by scanning electron microscopy (SEM) showed a membrane structure with well-ordered imprints caused by the sessile template droplets. These imprints were open at the bottom and covered on the top with a thin sheet composed of particles and polymer. The particles protruded out of the polymer sheet at the top and bottom of the membrane in the thin regions. A significant number of the particles protruded out of both interfaces at the same time. Thus, these parts of the membrane can be considered to be bicontinuous. The imprints are surrounded by thick regions. These regions act as a supporting structure. Thus, the membranes are stable enough to be handled without special precautions and might be applicable to membrane separation processes.

1. INTRODUCTION Membrane separation processes have been shown to be of considerable advantage in terms of cost, energy efficiency, and performance.1 One popular application is the separation of gas mixtures with the help of dense polymer membranes. Typical membrane materials are, for example, polysulfones,2 polyimides,3,4 and polyetherimides.5 With such membranes, gas mixtures of carbon dioxide and methane2,4 or volatile organic compounds and nitrogen5 could be separated. The major drawback of polymer membranes for gas separation is their permeability/selectivity trade-off relationship described by Robeson.6 Higher permeabilities generally lead to lower values for the separation factor of the gas pair to be separated. Thus, the general goal is the preparation of membranes with a favorable permeability/selectivity trade-off. One class of materials promising high selectivity and higher permeability consists of materials with a high free internal volume with rigid passages of well-defined diameter in a size range suitable to let desired molecules pass and to retain undesired molecules. Zeolites are one prominent example of such materials. Zeolite A, for example, has a suitable pore size that may even be tuned in the range of 3, 4, or 5 Å by an exchange of cations (K+, Na+, and Ca2+) accommodated in the skeletal structure.7,8 It thus might be suitable to discriminate oxygen from nitrogen or to remove the comparatively small water molecules from complex mixtures.9−14 There have been many efforts to manufacture zeolite membranes,15,16 but there are some general problems. In order to yield a sufficiently high permeance the thickness of such membranes shall be limited. Furthermore, membranes made just of pure zeolite are brittle and have a tendency to crack. Thus, often it is of more advantage to use composites and combine the mechanical sturdiness of a polymer with the high selectivity of a zeolite. © 2012 American Chemical Society

Furthermore, there are reports that these composites show a synergistic effect: the selectivity/permeability trade-off of composites may be more favorable than the corresponding values of each of the two components on its own.17,18 In principle, there are three ways to combine these two materials into a membrane, as shown in Figure 1.

Figure 1. Structures of composite membranes: (a) asymmetric structure giving rise to serial transport; (b) discontinuous phase in a continuous phase (mixed-matrix membrane); and (c) bicontinuous structure giving rise to parallel transport (black: nonselective supporting layer or surrounding matrix; gray: selective component).

(a) So called asymmetric membranes consist of two consecutive layers: Usually a selective, thin, but fragile top layer is supported by a highly permeable, mechanically sturdy second layer (Figure 1a). The transport through such a layer can be Received: January 24, 2012 Revised: March 19, 2012 Published: April 25, 2012 8197

dx.doi.org/10.1021/la300355h | Langmuir 2012, 28, 8197−8204

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described by a serial model.19,20 Theoretically, this would provide the best properties for a separation process. However, the selective layer needs to be defect-free to avoid bypasses that lower the selectivity. Such an asymmetric membrane can be obtained by the growth of zeolite crystals on an inelastic, porous support.21,22 However, such processes often give rise to zeolite layers with some degree of leaking. (b) In so-called mixed-matrix membranes, a discontinuous phase is embedded in a continuous phase or matrix (Figure 1b). For example, zeolites are incorporated as a filler, fully surrounded by a continuous matrix that is often an organic polymer.23 Polyimide zeolite mixed-matrix membranes have been prepared with amine-terminated silane coupling agents tethered to the zeolite surfaces to enhance the attachment between the zeolite and the polymer matrix through the formation of hydrogen bonds.24,25 Other mixed-matrix membranes were made on a polysulfone support and consisted of zeolite nanoparticles embedded in a polyamide matrix to be used for reverse osmosis.26,27 As matrix materials, more complicated systems can also be used, for example, ionic liquids in combination with ionic organic polymers based on polymerized room-temperature ionic liquids.28,29 These procedures may give rise to leak-free membranes; however, too high a permeability of the matrix leads to the bypassing of the filler; too low a permeability renders the transport through the matrix rate-determining. In both cases, the selectivity of the membrane is dominated by the continuous phase. The maximum influence of the filler on the selectivity is achieved if the permeabilities of the filler and the continuous phase are matched, but even in this case, the filler still is bypassed substantially and the selectivity of the membrane becomes significantly lower than the selectivity given by the filler.19,20 (c) A further possible configuration for composite membranes is a bicontinuous structure, giving rise to parallel transport. In this case, the selectivity of the membrane is dominated by the most permeable component. Thus, embedding highly selective but brittle particles within a mechanically sturdy and impermeable sheet of polymer may give rise to a membrane that takes full advantage of the selectivity of the embedded particles and the mechanical stability of the embedding polymer.17,30 By choosing a low membrane thickness, high permeabilities may be obtained. Such membranes have not been intensively investigated and discussed in the literature yet. One reason might be the challenging manufacturing process. Following this line of thought, we recently embedded zeolite A particles with a size of about 2 μm into a polymer layer in such a way that they protrude from this layer on both sides of the membrane at the same time.31 This was realized by float casting using the principle of particle-assisted wetting. In float casting, a liquid is applied to the surface of a second liquid that is not miscible with the first one, spreads out to form a thin layer of uniform thickness, solidifies, and is lifted off as a thin uniform sheet or membrane. The most prominent example is the Pilkington process used for the preparation of float glass, but there are also examples for the preparation of polymeric membranes on a water surface.32−34 However, there is hardly any organic liquid that wets a water surface under equilibrium conditions.35 As known for some time, particles have a strong tendency to adsorb to liquid/liquid or liquid/gas interfaces and may be used to stabilize emulsions.36−39 The driving force for this adsorption is the gain in energy that occurs upon the replacement of part of the interface between the two fluid phases by the particles. Adsorption energies easily exceed several kBT, and the adsorption of particles in many cases may

be regarded as irreversible. As described a few years ago, this reduction in total interfacial energy can not only be utilized to prevent the coalescence of emulsions but may also be utilized to achieve the spreading of a liquid on top of a second one.40−42 Following this line of thought, a mixture consisting of hydrophobized zeolite A particles, a multifunctional acrylic monomer, an initiator, and a volatile solvent was spread onto a water surface. After the evaporation of the solvent, the monomer was cured by photopolymerization, giving rise to zeolite polymer composite membranes with a parallel transport structure similar to that in Figure 1c.31 However, they were inherently thin and thus mechanically fragile. To increase the stability of these zeolite polymer composite membranes, supporting structures needed to be implemented. In another series of experiments, we created sieve-like membranes by applying sessile water droplets to a solid surface and partially covering them with a solution that finally was converted into a solid polymer layer.43,44 These membranes were mechanically sturdy and withstood manual handling without further support.

Figure 2. Process used for the preparation of microstructured composite membranes bearing thin regions of bicontinuous structure. 8198


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2. EXPERIMENTAL SECTION

Thus, the aim of this contribution is to combine both strategies (float casting of zeolite polymer composite membranes and structuring via sessile water droplets) to prepare a hierarchically structured composite membrane that comprises thin regions that allow for parallel transport and thick regions that provide mechanical stability. The principle of the manufacturing method is outlined in Figure 2. First, structured substrates with hydrophilic and hydrophobic areas are prepared, and the hydrophilic areas are selectively wetted with an aqueous solution to yield an ordered template structure composed of micrometer-sized sessile droplets. Then, hydrophobized zeolite A particles and a mixture of a nonvolatile monomer, a photoinitiator, and a volatile solvent are applied.45 After the evaporation of the solvent, the nonvolatile monomer is photopolymerized to yield a solid polymer and the resulting composite layer is separated from the support. As depicted in Figure 2, the goal is a membrane comprising thick parts in the regions in between the templating sessile droplets and thin parts with the desired bicontinuous structure in regions that covered the templating droplets.

Materials. Zeolites 4A and 5A were provided by SILKEM d.o.o., Kidričevo, Slowenija, as white powders. The average particle size was 1.8 ± 0.3 μm (Figure 3a).31 The acrylic monomer used, HEMATMDI (a mixture of 7,7,9- and 7,9,9-trimethyl-4,13-dioxo-3,14-dioxa5,12-diaza-hexadecan-1,16-diol-dimethacrylate (95%)) formerly known as PLEX 6661-O, was provided by Röhm Evonik GmbH. The coating agent 1H,1H,2H,2H-perfluorooctyltriethoxysilane (97%) and the photoinitiator benzoinisobutylether were purchased from Aldrich. All chemicals and solvents (technical grade) were used as received, with two exceptions. Chloroform was saturated with water,

Figure 3. (a) SEM image of zeolite A particles as used for membrane preparation after washing and coating. (b) Image (by optical microscopy) of a developed printing plate with the used pattern, consisting of the hydrophobic photoresist (dark grid) and the hydrophilic alumina surface (light squares).

Figure 5. SEM image of the bottom side of a polymer film with imprints, resulting from the sessile droplets that rested on the hydrophilic regions: (a) too short a condensation period (15 min), where the imprint formed by converged and protruding droplets is highlighted with a white, dashed frame.

Figure 4. SEM picture and EDX element mapping of a structured substrate after the application of an aqueous calcium chloride solution and drying in vacuum. 8199


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Table 1. Data on the Preparation Parameters and Results of the Discussed Membranesa figure

5a

matrix material solvent for matrix material concentration of matrix material (mg/mL) zeolite concentration (mg/mL) zeolite application

PMMA CHCl3 40

first condensation period (min) second condensation period (min) most prominent observation

1.5 droplets too small

5b PMMA CHCl3 40

5c

6

2 and 7

PMMA CHCl3 40

HEMA-TMDI CHCl3 20

HEMA-TMDI THF with 2% H2O 20

6

20

10 dispersed in monomer solution 6

optimal droplet size

droplets too large

15 in a separate step dispersed in CHCl3 6 10 optimal membrane structure

polymer skin on zeolites

a

All substrates were exposed and developed as described in the Experimental Section. All substrates were wetted with a saturated, aqueous solution of calcium chloride and put onto a copper block of T = 6 °C in moist nitrogen flow of 6−8 L/h and 60% relative humidity.

and toluene was dried by storing over zeolite 3A (activated by heating to 350 °C for 6 h) before use. Preparation of Hydrophobically Coated Zeolites. The zeolite particles were washed with demineralized water and ethanol. After drying at 350 °C for 6 h in air, the particles were hydrophobized by dispersing them overnight in a 10 mM solution of 1H,1H,2H,2Hperfluorooctyltriethoxysilane in toluene (about 10 mL/g zeolite). Subsequently, the zeolite was washed with toluene, demineralized water, and ethanol and thereafter dried in air at 120 °C for 6 h. Substrate Preparation. As a substrate, analog positive offset printing plates (MainPlate M-AP, just4print Europe GmbH, Hainburg, Germany) were used. These are anodized aluminum plates coated with a layer of positive photoresist. The spectral sensitivity of the photoresist layer, specified by the manufacturer, lies in the range of 350−420 nm. The plates were cut into pieces of 2 cm × 5.5 cm. The resist layer was covered with a lithography mask, made out of a glass plate and covered with a grid of 20-μm-wide lines of chromium that comprised uncovered squares of 40 × 40 μm2. Illumination was carried out with a UV hand lamp (NU - 6 KL, Benda Konrad Laborgeräte, Wiesloch, Germany) with a principal wavelength of 366 nm. The lamp was placed 2 cm from the lithography mask. Light exposure was performed for 115 s with an intensity of about 530 μW/cm2, measured though the mask and averaged over illuminated and shadowed areas. The development of the photoresist was done by immersing the exposed printing plate in a 0.25 M aqueous NaOH solution for 6 to 7 s. Immediately after this treatment, the substrate was rinsed with demineralized water. In the next step, microstructures were generated on the exposed and developed substrate. Therefore, a film of saturated aqueous CaCl2 solution (6.67 mol/L) was applied to the substrate by using a foam paint roller (shape and porosity not crucial). Subsequently, the thickness of the liquid film was reduced by wiping with a squeegee. (For a photograph of the squeegee, see the Supporting Information.) Then the substrate was brought into a moistening chamber in which a stream of 6−8 L/h of nitrogen of 60% relative humidity was channelled onto the wetted aluminum plate, which rested on a cooled copper block (6 °C, cryostat) for various time spans (optimum results obtained at 6 min). The moistened gas stream was realized by feeding nitrogen through two gas-washing bottles at room temperature: the first was filled with demineralized water, and the second was filled with a saturated NH4Cl solution. Membrane Preparation. The plate, decorated with sessile droplets using the procedure described above, was taken out of the moistening chamber and covered with 0.5 mL (≙ 0.0455 mL/cm2) of a dispersion of the hydrophobically coated zeolite A particles (predominantly zeolite 5A, in control experiments zeolite 4A) in chloroform. The concentration of zeolite in this dispersion was varied systematically. Membranes with optimized structure were obtained using a concentration of 10−15 mg of zeolite/mL of solvent. After the evaporation of the CHCl3, the substrate, now covered with sessile droplets and particles, was again brought into the moistening chamber described above for another 10 min. After this, the substrate was taken

out and covered with a mixture of the acrylic monomer, the photoinitiator (5−10% by mass with respect to the monomer) and a solvent. As the solvent, either chloroform or THF with 2 wt % water was used. The monomer concentration was varied systematically. Optimum thicknesses could be achieved with a HEMA-TMDI concentration of 20 mg/mL solvent. After this mixture was spread onto the substrate, it was covered with a Petri dish (turned upside down) for 15 min. Then the solvent was evaporated by removing the cover. In the next step, the substrate bearing the sessile droplets, particles, and monomer was exposed to UV light using a low-pressure mercury lamp from Umex Dresden (principal wavelength 254 nm, radiation intensity 4.8 mW/cm2) under an N2 atmosphere for 35−40 min with a distance of approximately 10 cm between the lamp and the substrate. After the membrane was cured this way, the aluminum substrate was completely removed by etching with a 0.5 M aqueous NaOH solution at 50 °C for 3 h. Characterization. The characterization of the membrane structure was realized by scanning electron microscopy with a NanoNovaSEM from Fei (Phillips). Before imaging, the nonconductive samples were coated with about 20 nm of platinum with a SCD 050 sputter coater from BAL-TEC. The sputtering was done at 40 mA for 120 s.

3. RESULTS AND DISCUSSION The manufacturing of the microstructured composite membranes according to Figure 2 can be separated into two parts. First, sessile water droplets have to be generated on top of microstructured substrates, and second, they are covered with zeolite particles and a nonvolatile monomer, which is subsequently solidified. Substrate Preparation. To realize a well-ordered microstructure consisting of sessile droplets on a solid substrate surface, we took advantage of the difference in wettability of hydrophilic and hydrophobic surfaces by water and organic liquids. If water is applied to a surface that comprises hydrophilic and hydrophobic regions in close proximity to each other, the water forms sessile droplets on top of the hydrophilic regions and the hydrophobic regions stay dry.46,47 Commercial offset printing plates were exposed to UV light through a lithography mask, and the subsequent development of the plates yielded a substrate showing a grid of the hydrophobic photoresist comprising squares of the hydrophilic alumina devoid of photoresist (Figure 3b). The next step, the application of the sessile droplets, might be done by applying liquid water directly to the structured surface or by condensing water out of a moist atmosphere. As observed earlier, neither of these two methods alone worked satisfactorily, but a combination was successful.43 The structured substrates were wetted with a saturated calcium chloride solution using a paint roller. Then the thickness of the liquid film was reduced, 8200


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and “satellite” droplets that had been applied to hydrophobic regions by the roller were removed by wiping with a squeegee. Thus, the liquid completely retracted from the hydrophobic areas and formed thin layers on top of the hydrophilic squares. To be sure that this film of aqueous calcium chloride solution was present only in the hydrophilic squares, the water was evaporated in vacuum and the samples were characterized with EDX measurements. These measurements confirmed that calcium chloride was present only in the hydrophilic squares (Figure 4). The aqueous films created this way had a small thickness. Hence, the amount of water on the hydrophilic regions needed to be increased. Therefore, water was condensed on them by placing the wetted substrates in a chamber in which they were cooled and purged with a gas stream of high humidity. Because of the presence of the calcium chloride, the partial pressure of the water vapor in equilibrium with the aqueous droplets resting on the hydrophilic regions was reduced in comparison to that of pure water. Consequently, it was low enough to prevent the nucleation of additional droplets (of pure water) on top of the hydrophobic regions. The increase in volume of the aqueous phases due to this condensation in principle might have been followed by optical microscopy; however, the growth was hard to quantify this way. Thus, a polymer solution was spread over the wetted substrate, and the solvent was evaporated until the polymer formed a solid layer bearing imprints that reflected the original shape of the sessile droplets (Figure 5). After this polymer layer was removed from the substrate, the imprints resulting from the droplets were analyzed, with an emphasis on their shape, size, and ordering. This allowed us to optimize the substrate temperature, the gas flow, the humidity of the purging gas, and the duration of the condensation. (See Table 1 for a summary of the preparation parameters.) With condensation periods that were too short, the imprints stayed flat and possibly did not cover the whole area of the hydrophilic squares (Figure 5a). If the condensation periods were too long, then there was the possibility that the growing droplets converged46 and the resulting larger droplets protruded through the covering polymer film (Figure 5c). When the time period for growing was chosen correctly, the resulting molds had a uniform size, with an adequate depth and a square base (Figure 5b). Membrane Preparation. In a first series of experiments, we tried to prepare composite membranes by covering the sessile droplets with a polymer solution that also comprised zeolite particles. However, this simple approach unfortunately always yielded composite membranes in which the particles were covered by a thin “skin” layer of the polymer. Obviously, this skin had already formed before the volume of the polymer solution was sufficiently reduced to enable the particles to protrude from the upper interface. Thus, we had to separate the evaporation of the solvent from the solidification of the polymer. Hence, instead of applying a solution of a polymer we applied a nonvolatile, liquid, acrylic monomer and subsequently solidified this monomer via photopolymerization. If we applied a solution of the monomer, which in addition comprised the zeolite particles, let the solvent evaporate, and solidified the monomer, then we obtained composite membranes. However, in the case of these membranes, only a few particles were protruding from the lower interface of the polymer film inside the imprints. Furthermore, the position of the particles protruding from the top surface seemed to be uncorrelated to the position of the former droplets (Figure 6).

Figure 6. SEM images of microstructured composite membranes prepared by casting monomer and zeolite particles in one step: (a) top view; (b, c) bottom view. Particles are visible (e.g., the one highlighted by the white, dashed circle) but seem blurred, and their polygonal shape is much less discernible than in the membranes shown in figure 7.

Both observations indicate that the particles adhere to the top interface but fail to make contact with the templating sessile droplets. Obviously, the particles did not have enough time or had too small a driving force to find the interface between the monomer and the sessile droplets. To increase this driving force, we decided to force the adsorption of the particles to the droplets by spreading dispersions of the particles in a completely volatile liquid void of the monomer first and to spread the monomer only after this forced adsorption took place. Thus, in the final series of experiments, first a dispersion of the particles in a completely volatile liquid was spread on top of 8201


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Figure 7. SEM images of optimized microstructured composite membranes prepared by applying particles and monomer solution sequentially (membranes prepared with tetrahydrofuran as a spreading solvent; for images of corresponding membranes prepared by using chloroform, see the Supporting Information): (a−c) top view; (d−f) bottom view. In images c and f, the polygonal shape of the protruding particles is clearly discernible.

the substrates decorated with sessile droplets and subsequently the monomer was applied, as depicted schematically in Figure 2. After the evaporation of the dispersing liquid, another condensation step was carried out for the further enlargement of the sessile droplets. Thereafter, a curable mixture consisting of monomer, photoinitiator, and solvent was poured over the substrate bearing the sessile droplets that were covered with particles. Photopolymerizing the monomer and removing the aluminum substrate yielded a composite membrane. To achieve the desired structure, the number of particles and the amount of monomer to be applied were optimized empirically. These parameters determine the thickness of the membrane and the coverage of the membrane with embedded zeolite particles. It has to be considered that the total membrane thickness is determined by the amounts of zeolite and monomer. Thus, the optimum monomer concentration is different for various concentrations of zeolite particles. Too few particles gave rise to membranes predominately without composite structures; too many particles yielded multilayers of particles instead of the desired monolayers. Too small a quantity of monomer yielded frail membranes with open porosity; too large a quantity of monomer gave rise to complete coverage of the zeolite particles, resting on the sessile droplets (Supporting Information). Furthermore, the solvent used to apply the monomer seems to influence the position of the particles in the thin parts of the membrane: if chloroform is used, then the particles in those thin parts of the membrane touch both interfaces, as desired; however, the particles protrude predominantly from the polymer on the lower side. Thus, the desired parallel transport structure is not fully achieved. If tetrahydrofuran is used instead of chloroform, then the particles protrude as desired from both interfaces of the thin parts in an equal manner. Obviously, the solvent influences the contact angle between the particles and the aqueous phase and thus the structure of the resulting membranes. Membranes prepared with optimized parameters showed the desired structure with particles protruding from the polymer in an appropriate way on both sides (Figure 7): The top sides of these membranes show a smooth polymer surface penetrated

by a significant number of particles that are embedded in the polymer on their sides and seem to be associated with the locations of the sessile droplets. (The latter fact is most clearly visible in the overview images Figure 7a,d.) This association should be caused by the chosen order of manufacturing steps: after the particle dispersion was applied, the organic liquid was evaporated and thus the particles were forced to be in contact with the template sessile droplets. Because of this, the interfaces between the particles and the aqueous phase were formed and the zeolite particles were pinned on the droplets. The particles stayed in their positions during the casting and curing of the monomer solution and formed the observed pattern. On the bottom sides of the membranes, regularly shaped and ordered imprints, resulting from the sessile droplets, can be observed. Inside these imprints, particles are embedded in a similar way as on the top sides of the membranes. A curious side effect occurred when we used zeolite 4A particles instead of zeolite 5A. In the case of zeolite 4A, the sessile droplets either did not form or disappeared during the casting and curing of the membranes. Membranes manufactured with zeolite 5A particles show the expected structure, as stated before. The reason for this behavior is currently not clear, but we assume that the difference in behavior is caused by an ion exchange between the calcium chloride solution, used as the template liquid, and the zeolite 4A particles, which are known for their ion-exchange properties.48 If the Na+ ions of zeolite 4A are exchanged with Ca2+ ions from the droplet solution and the calcium chloride solution in the extreme case is turned into a sodium chloride solution, then the sessile droplets lose their hygroscopic properties and thus may evaporate more easily. A simple estimation49 shows that the exchange capacity of zeolite 4A actually might suffice to generate this effect. Zeolite 5A, however, already has Ca2+ as its counterions and thus will not remove Ca2+ from the sessile droplets. From the SEM images shown in Figure 7, we conclude that particles protrude from the top and the bottom sides of the membrane. However, on the basis of these images we cannot discern that instead of the desired monolayer of particles we might have double layers or multilayers. Figure 8 shows the side 8202


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number of angularly shaped imprints. In the thin membrane areas, between one-third and two-thirds (depending on the individual sample) of the imprints are penetrating holes, resulting from particles that before etching penetrated both sides of the membrane at the same time. This observation can be made for both above-mentioned solvents that were used to manufacture the membranes. Thus, the desired bicontinuous structure has been achieved. As hoped for, the microstructured composite membrane is mechanically much more stable than a corresponding nonmicrostructured composite membrane with a thickness comparable to the particle width that may be prepared by float casting on a flat water surface. The latter membrane has a thickness of about 1 to 2 μm and thus will not withstand being lifted off the water surface; it needs to be transferred to a supporting structure immediately after preparation. By contrast, the structured membrane depicted in Figure 7 and Figure 8 has a thickness of about 6−8 μm in the thicker areas of the supporting structure and 1−2 μm in the thinner areas, caused by the sessile template droplets. The membrane is flexible and can be separated from the support and handled manually without further precautions being necessary.

Figure 8. SEM image of the same membrane as in Figure 7, cut and imaged from the side. One can see that the thin part of the membrane has a thickness comparable to the particle size.

of a membrane exposed by fracture, where different thicknesses of the membrane sections can be seen. In the thin zone over the imprint, resulting from a sessile droplet, individual zeolites and small angularly shaped imprints of zeolites can be observed, penetrating both the top and the bottom sides of the polymer film at the same time (Figure 8, arrows). This indicates that the thin parts of the membranes actually have the desired thickness, which is comparable to the particle size. At the positions of the small angularly shaped imprints, previously zeolite particles were embedded, fulfilling the conditions of a bicontinuous structure. The particles were mechanically removed by preparing the membrane for SEM imaging. To evaluate over larger areas how large a fraction of the particles actually penetrates both interfaces, the particles were dissolved in hydrochloric acid and the resulting membranes (consisting of the polymer only) were investigated with SEM (Figure 9). The dissolved particles left behind a significant

4. CONCLUSIONS It was possible to prepare a structured composite membrane by using sessile droplets as sacrificial templates. Thicker regions of the membrane, thus prepared, provide for mechanical stability, whereas thinner regions show the desired bicontinuous structure of particles embedded laterally but protruding simultaneously from the top and bottom of these thin regions. This structure in principle might allow a parallel transport mechanism; however, under the current status we are still far away from application in an industrial environment. The lateral dimensions are limited to the size of our mask of 1.5 × 4.8 cm2; the complete cycle of preparation needs a total of 1.5 h, and we cannot guarantee sufficiently large areas free of defects. We thus envision that these kinds of membranes might be used for advanced membrane separation processes if corresponding scale-up and optimization procedures will be conducted.

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ASSOCIATED CONTENT

S Supporting Information *

Photographs of the used squeegee. SEM images of membranes with different thicknesses. SEM images of membranes prepared with chloroform as a solvent for the monomer. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +49 371 531-31713. Fax: +49 371 531-21249. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank SILKEM d.o.o. (Kidričevo, Slovenia) for contributing the zeolite particles and Röhm Evonik GmbH for providing the monomer. Furthermore, we thank M. Hietschold, S. Schulze, and G. Baumann from TU Chemnitz (Chair of Solid Surfaces Analysis) for help in obtaining the SEM images.

Figure 9. SEM images of the polymer sheet that was part of the composite membranes (after the dissolution of the zeolite particles with HCl): (a, b) top view; (c, d) bottom view. Most of the imprints left behind by the particles penetrate the membrane in the thin regions. 8203


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(34) Hinrichsen, G.; Hoffmann, A.; Schleeh, T.; Macht, C. Adv. Polymer. Technol. 2003, 22, 120−125. (35) Therefore, refs 32−34 relied in their investigations on positive initial spreading coefficients and the solidification of the resulting nonequilibrium layer by rapid vitrification. (36) Ramsden, W. Proc. R. Soc. 1903, 72, 156−164. (37) Pickering, S. U. J. Chem. Soc. 1907, 91, 2001−2021. (38) Clint, J. H.; Taylor, S. E. Colloids Surf. 1992, 65, 61−67. (39) Binks, B. P.; Lumsdon, S. O. Phys. Chem. Chem. Phys. 1999, 1, 3007−3016. (40) Xu, H.; Goedel, W. A. Langmuir 2003, 19, 4950−4952. (41) Goedel, W. A. Europhys. Lett. 2003, 62, 607−613. (42) Ding, A.; Goedel, W. A. J. Am. Chem. Soc. 2006, 128, 4930− 4931. (43) Magerl, A; Goedel, W. A. Langmuir 2012, 28, 5622−5632. (44) Jahn, S. F.; Engisch, L.; Baumann, R. R.; Ebert, S.; Goedel, W. A. Langmuir 2009, 25, 606−610. (45) The channel width of the zeolite particles is too narrow to allow intrusion of the silane and monomer. In addition, once in contact with water their channels will preferentially be filled with water and this “natural” water content protects additionally the inner channels from uptake of polymer or monomer. (46) Gau, H.; Herminghaus, S. Phys. Rev. Lett. 2000, 84, 4156−4159. (47) Herminghaus, S.; Fery, A.; Schlagowski, S.; Jacobs, K.; Seemann, R.; Gau, H.; Monch, W.; Pompe, T. J. Phys.: Condens. Matter 2000, 12, A57−A74. (48) Sherry, H. S.; Walton, H. F. J. Phys. Chem. 1967, 71, 1457− 1465. (49) We assume that wiping with the squeegee gives rise to a film of thickness d = 1 μm, which covers an area fraction of f = 4/9 of the substrate (≙ hydrophilic areas). Given the saturation concentration of c = 6.67 mol/L, we thus deposit c·d·f = 3 × 10−3 mol/m2 Ca2+ ions. In the subsequent step, we apply at least approximately Γ = 4.55 g/m2 of zeolite particles and assume that a fraction of f = 4/9 finally makes contact with the water droplets. Given the ion exchange capacity of EC = 160 mg CaO/g zeolite = 2.85 × 10−3 mol Ca2+/g zeolite (provided by the manufacturer), we conclude that the zeolite gives rise to an ion exchange capacity of Γ·EC·f = 5.8 × 10−3 mol/m2, which is comparable to the surface concentration of Ca2+ ions (3 × 10−3 mol/m2) given above.

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