Porous Polymer Membranes via Selectively Wetted Surfaces

Article pubs.acs.org/Langmuir

Porous Polymer Membranes via Selectively Wetted Surfaces Annemarie Magerl and Werner A. Goedel* Physical Chemistry, Chemnitz University of Technology, Strasse der Nationen 62, 09111 Chemnitz, Germany S Supporting Information *

ABSTRACT: Here, we show that porous polymeric membranes can be prepared using the principles of offset printing: an offset printing plate is structured into hydrophobic and hydrophilic regions with the help of photolithography and is selectively wetted with a solution of calcium chloride in water at the hydrophilic regions. Then, a polymer solution (poly(methyl methacrylate) in chloroform) is applied to this surface and forms a hydrophobic layer that is structured by the aqueous droplets. Deviating from standard offset printing, this layer is not transferred to another surface in its liquid state but is solidified and subsequently is separated from the printing plate. The thickness of the polymer film is chosen in such a way that the aqueous droplets on the surface protrude from the film. Thus, we obtain polymer membranes with pores in the size of the protruding aqueous droplets. These membranes are then characterized by the filtration of model dispersions.

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INTRODUCTION Porous membranes enjoy great popularity in a wide range of scientific and technological applications. They can be used, for example, as substrates for cell cultures in biology,1−3 in the production of emulsions,4 and are most prominent in filtration processes.5−7 A special kind of porous membranes are the so-called microsieves.8−10 They show a uniform pore size and a thickness smaller than the diameter of their pores and usually possess a high number of pores per area. Therefore, they offer a low flow resistance and a sharp size selectivity. The first microsieves were made via silicon-based photolithography.8 The pore sizes and shapes in these membranes can be varied through the application of various masks or the adoption of laser interference lithography.11−13 However, those lithography methods necessitate comparatively expensive machinery. In a simplified version, microsieves are directly made out of a photoresist without the need to transfer a pattern into an inorganic substrate.7,14 Microsieves can also be prepared via micromolding.15,16 Another approach is the usage of preformed sacrificial particles that are embedded in a solid layer (inorganic10a or polymer17,18) and later on are removed to generate pores. The resulting film contains pores in the size of the sacrificial particles and shows all the above-mentioned criteria of a microsieve. Recently, we used liquid sessile droplets instead of solid particles as sacrificial porogens.19 An inkjet materials printer was used to place uniform sessile droplets onto a solid substrate. This substrate was then covered with a polymer solution that surrounded the droplets, the solvent was evaporated, and a structured solid polymer film was obtained. The substrate was dissolved to yield a polymer microsieve with pores in the size of the sacrificial droplets. In this method, the size or the pattern of the liquid droplets could readily be chosen, and thus, it allowed an easy variation of the pore size and pattern. © 2012 American Chemical Society

However, inkjet printing is a serial process and thus is comparatively slow. This fact not only limited the rate of production but also posed the problem that the created droplets first had time to age, for example, by evaporation before the polymer solution was applied. Furthermore, this technique was not readily suitable to generate sacrificial sessile droplets of shapes other than spherical caps. It would be advantageous to have a process that (i) enables us to generate all the droplets at once (thus making evaporation effects similar for all the droplets) and that (ii) allows us to confine the droplets to predetermined places (thus enabling noncircular footprints). To achieve these goals, the usage of a substrate that has hydrophilic and hydrophobic regions seemed to be favorable. If a suitable liquid is applied to the surface of such a substrate, it might selectively wet only certain parts of the substrate. Even more important is that applying two immiscible liquids one after another to such a surface will yield a laterally structured layer that reflects the pattern of hydrophilicity on the substrate. This principle is used widely in offset printing,20 which employs aluminum printing plates that are partially covered with a polymer to achieve a variation in wettability. The regions free of polymer are hydrophilic and ink-repelling, whereas the polymercoated ones are hydrophobic and ink-accepting. Applying water and a hydrophobic ink yields a surface where only the printing areas actually carry ink, and the nonprinting areas carry water. This image made of ink can then be transferred to paper. Thus, the necessary equipment is readily available. One only needs to introduce one modification compared to offset printing: the structured layer shall not be transferred in its liquid state but Received: November 14, 2011 Revised: January 26, 2012 Published: February 27, 2012 5622

dx.doi.org/10.1021/la2044703 | Langmuir 2012, 28, 5622−5632

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squares were wetted with an aqueous solution of calcium chloride. To enlarge the resulting droplets on the substrate, the offset plate was placed within a moist atmosphere and was cooled. When the droplets reached a sufficient size through condensation, a solution of poly(methyl methacrylate) (PMMA) in chloroform (CHCl3) was applied to this selectively wetted surface, and the solvent was evaporated leaving behind a structured polymer film. Subsequently, the substrate was removed by etching with hydrochloric acid to yield a porous polymer membrane.31 Optimization of Preparation Process. Generation of a Wettability Pattern. A commercially available offset printing plate that consisted of aluminum with an anodized surface covered with a positive photoresist was used. As usual in offset printing, the surface of the plate was comparatively rough in order to enhance the wettability contrast between the hydrophilic alumina and the hydrophobic photoresist.32,33 In a first series of experiments, a lithographic mask made out of glass bearing a grid of chromium lines was utilized (see Figure 2a).34 The lines had a width of 20 μm enclosing square holes sized 40 × 40 μm. The exposure of the offset plate to UV-light through this mask and the subsequent development of the resist in sodium hydroxide solution yielded a structured surface. This structured surface showed a grid of smooth hydrophobic resist that enclosed rougher squares of hydrophilic alumina (Figure 2b and c, optical microscopy and scanning electron microscopy (SEM) image). Selective Wetting of the Pattern by Water. To selectively wet the hydrophilic squares, we first attempted to apply water with a foam roller mimicking the usual offset printing procedure. However, too many satellite droplets residing on the hydrophobic regions were obtained while the desired droplets on the hydrophilic regions were not sufficient in height and evaporated too fast. It was possible to remove the satellite droplets by wiping the wetted plate with a commercial squeegee. When such a squeegee is drawn over smooth wet glass, a thin film of water forms which evaporates instantly. If the squeegee was drawn over the wet substrate structured in hydrophilic and hydrophobic regions, film formation also occurred though only in the hydrophilic regions bare of photoresist. In these regions, the receding contact angle of water in air was below the detection limit, and complete wetting took place (Table 1), which is important because after the wiping step there still needed to be enough water for the droplets on the substrate left. On the other hand, the receding contact angle of water on the hydrophobic bridges was 69° ± 2.8°, and so the squeegee could only drag along a water front with a finite contact angle, and thus film formation did not occur on these regions. Thus, only the hydrophilic squares without photoresist were covered with water after wiping the substrate surface with a squeegee. Nevertheless, the wiping decreased the height of the desired droplets on the hydrophilic regions even further. Thus, in the next series of experiments, we tried to apply the water by placing the substrate in a moist atmosphere and by cooling it below the dew point (mimicking the process depicted in ref 24). However, as in the previous case, too many droplets residing on the hydrophobic regions were obtained. Often, even complete bridging by droplets extending over several hydrophilic squares could be observed. Thus, there was a need to facilitate evaporation on the hydrophobic regions while facilitating condensation on the hydrophilic regions or, in other words, to reduce the equilibrium partial pressure of water on the

first shall be solidified and then separated from the substrate to yield a free-standing porous membrane (see Figure 1).

Figure 1. Preparation of porous polymer membranes using the principles of offset printing.

To obtain substrates of structured wettability, one needs to locally influence their surface properties, for example, by surface reactions,21 deposition of self-assembled monolayers,22 chemical or physical vapor deposition,23,24 or transfer of layers,25 and do so in a laterally structured way, for example, by combining the abovementioned principles with microcontact printing,22,25 structural printing, photolithography, or deposition through a mask.23,24 The deposition of liquids onto these structured surfaces can be achieved by exposure to suitable vapors combined with cooling below the dew point,23,24,26,27 immersion into a suitable liquid and removal of any excess,28,29 or by spraying droplets onto the surface either using a commercial spray gun or an inkjet printer.30 Thus, in this paper, we report a method to produce porous polymer membranes utilizing the above-mentioned principles of offset printing. In this work, we also show the filtration of dispersions that comprise particles of various sizes with the new membranes.

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RESULTS AND DISCUSSION In our process (Figure 1), we structured a positive offset printing plate by photolithography creating hydrophilic squares. These 5623


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Figure 2. Structuring the offset printing plate: (a) optical microscopy image of the lithography mask, chromium lines on glass; (b, c) optical microscopy image and scanning electron microscopy image of the structured aluminum offset printing plate bearing a grid of hydrophobic photoresist enclosing rougher squares of hydrophilic alumina; (d) optical microscopy image of the offset printing plate, cooled below dewpoint in moist atmosphere. The hydrophilic regions are selectively wetted by an aqueous calcium chloride solution.

all satellite droplets, using a squeegee. The calcium chloride was used to decrease the equilibrium partial pressure of the water within the wetting liquid and so to increase the condensation at the hydrophilic regions. Subsequently, the substrate was placed in a moist atmosphere and was cooled down. Comparison of the contact angle of the calcium chloride solution on the substrate with the corresponding angle of water (Table 1) showed that the calcium chloride solution also wetted the hydrophilic regions completely on retraction and had a similar receding contact angle of 64° ± 2.8° on the hydrophobic photoresist. Thus, as in the previous section, the hydrophobic bridges were uncovered and the hydrophilic regions were covered with a thin aqueous film. To be sure that this film of aqueous calcium chloride solution was only present in the hydrophilic squares, the water was evaporated in vacuum and the samples were characterized with energy-dispersive X-ray spectroscopy (EDX) measurements. These measurements confirmed that the calcium chloride was only present in the hydrophilic squares (see Supporting Information for EDX element mappings). These aqueous films on the hydrophilic regions were too thin to serve as template droplets. Thus, they had to be converted into large enough droplets through condensation in a moist atmosphere. In detail, this growth step was executed in a closed cell with constant humidity. The humidity was controlled through a stream of argon carrying water from two washing bottles, the first filled with water and the second filled with a saturated aqueous solution of ammonium chloride. The substrate itself was placed onto a block of copper cooled with a cryostat to 6 °C inside the closed cell. That way, the temperature of the substrate and the relative humidity were constant. As a result, the gaseous water condensed onto the already existing aqueous films and let them grow into sessile droplets. Thus, sessile droplets were obtained on top of the hydrophilic regions while the number of satellite droplets or droplets bridging the hydrophobic regions was reduced by the previous wiping step.

Table 1. Contact Angles of Water, Calcium Chloride Solution, and Chloroform on the Substrate droplet phase

value on description of contact angle photoresist

value on alumina

93° ± 4.6° 66° ± 7.4° advancing (air/aqueous phase/substrate)a receding (air/aqueous phase/ 69° ± 2.8° wetting substrate)a calcium advancing (air/aqueous 98° ± 2.1° 102° ± 2.2° chloride phase/substrate)a solution receding (air/aqueous phase/ 64° ± 2.3° wetting substrate)a 105° ± 6.1° 97° ± 12.1° advancing (chloroform/ aqueous phase/substrate)b receding (chloroform/ 97° ± 5.8° wetting (70° ± 15.0°)d aqueous phase/substrate)b chloroform advancing (air/chloroform/ wetting wetting substrate)c receding (air/chloroform/ wetting wetting substrate)c

water

a

Angle between the air/aqueous phase interface and the aqueous phase/alumina interface at the position where both meet the threephase contact line. bAngle between the chloroform/aqueous phase interface and the aqueous phase/alumina interface at the position where both meet the three-phase contact line. The contact angle was measured upside down to compensate for the buoyancy of the aqueous calcium chloride solution in chloroform. cAngle between the air/chloroform interface and the chloroform interface at the position where both meet the three-phase contact line. In all cases, the term advancing is the equivalent of moving the three-phase contact line in such a way that the area of the interface (droplet phase/alumina) is enlarged. d The contact angle wildly varied from sample to sample between complete wetting and 70° ± 15.0°.

hydrophilic regions compared to the water on the hydrophobic ones. This was achieved as follows: instead of pure water, a hygroscopic aqueous solution of calcium chloride was applied with a foam roller and was wiped off almost completely, including 5624


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Figure 3. SEM images of the bottom side of prepared polymer films: (a, b) dense structured polymer film with dents left by the template droplets but with no pores penetrating through (droplet growth time too short, 3 min); (c, d) porous membrane with pores left by the template droplets (droplet growth time optimal, 6 min), polymer concentration 0.008 g/mL PMMA/CHCl3; (e, f) polymer film with wide dents and big pores left by the template droplets (droplet growth time too long, 30 min).

and form pores but would only leave shallow dents in the film (Figure 3a and b). While growing in volume, the contact area of the droplets with the substrate did not grow because the three-phase contact line was pinned to the borderline between the hydrophilic squares and the hydrophobic bridges. Thus, the contact angle of the droplets with respect to the substrate at this time also became larger. When the contact angle of the pinned contact line had reached the value of the contact angle of the aqueous solution on the hydrophobic photoresist (Table 1), the droplets also grew over the bridges as was already described in the literature.35,36 On further volume enlargement, the contact angle remained constant, while the contact area increased. Consequently, some droplets coalesced creating larger droplets

In the experiments depicted here, parameters like the block temperature, the relative humidity in the growth cell, and later on the polymer concentration and polymer amount per unit area were kept constant, while the growth timethe dwell time of the substrate in the growth cellwas chosen as the parameter to systematically influence the droplet height. For this reason, an optimal growth time had to be found. Because of the roughness of the plate, the water droplets were barely visible by optical microscopy (Figure 2d), and thus, it turned out to be more feasible to optimize the growing time by the inspection of the resulting polymer films. It could be shown that too short a growth time (tgrowth = 3 min) yielded insufficient droplet growth. The droplets would remain small and rather flat so that they could not penetrate the polymer film 5625


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Figure 4. Close-up SEM images of a porous polymer membrane prepared with a lithography mask bearing squares of 40 μm width: (a) top side, (b) bottom side.

perimeters seen from the bottom of the membrane which was facing the alumina were often jagged (Figure 4b). In some places, the three-phase contact line receded on application of the polymer solution from the hydrophobic−hydrophilic border of the substrate, and in others, it remained pinned to this border. The theory behind the varying positions of the three-phase contact line on rough surfaces is not fully understood yet, but the paper of Drelich, Miller, and Good37 provides some possible reasons for the observed behavior. They investigated contact angles of droplets of various volumes on rough surfaces. Their conclusion was that if the roughness is in the same order of magnitude as the radius of the droplet then the contact angles cannot be reliably measured. The same problem occurred in the shown experiments. Especially, the receding contact angle of the calcium chloride solution on the resist free substrate under chloroform could not be measured clearly. Many measurements yielded wildly scattered values between an angle of 70° and complete wetting as depicted in Table 1. Thus, it is not strange that some parts of the three-phase contact line were pinned and others were receded. The obtained membrane showed an average pore size of 38 μm ± 3.1 μm. While the pore size distribution was comparatively narrow, one cannot deny that it has a width considerably larger than could be expected from the lithography mask alone. This width predominantly arose from the roughness of the commercial printing plate because the template droplets were partially pinned on surface inhomogeneities and were not able to perfectly mimic the given pattern. The lithography mask used bore a whole patterned area of 7.2 cm2. Thus, the largest patterned polymer film was also 7.2 cm2 in size. The obtained membranes showed a mean thickness of 4.6 μm ± 0.78 μm measured by atomic force microscopy (AFM) (see Supporting Information for pictures) which is in accordance with the theoretical membrane thickness calculated from the applied volume and concentration of the PMMA solution per substrate area given by 4.4 μm. A membrane thickness could also be estimated using information from the SEM pictures and contact angle values. This estimation is based on a droplet that has just grown to the point of leaving the pinning introduced by the transition from the hydrophilic to the hydrophobic substrate at the borderlines of the structured pattern. For simplicity, it is assumed that the three-phase contact line sits in the hydrophobic region and is a circle with a radius r1 chosen to be half of the diagonal of one square, see Figure 5a. It is also assumed that the sessile drop as

that caused pores to be bigger than intended or that caused wide flat dents when the growth time was too long (tgrowth = 30 min) (Figure 3e and f). Using an optimized condensation time (tgrowth = 6 min), the desired membranes with pores reflecting the resist pattern (Figure 3c and d) were obtained finally. A summary of growth times and their influence on the membrane structure can be seen in Figure 8 and is discussed below. Application of the Polymer. The application of the polymer solution, taken for granted in the previous section, needed to be optimized as well. The solution of PMMA in chloroform had to be applied to the structured surface in such a way that it dewetted the template droplets and subsequently solidified through the evaporation of the solvent. Because the polymer film must not be higher than the droplets themselves, an optimal combination of droplet height and polymer amount had to be found. The height of the polymer film could be manipulated through the PMMA concentration in the chloroform, and the volume of the solution could be applied to a certain area of the substrate. For convenience, the volume of the polymer solution was kept constant at a volume (V = 300 μL) that could easily be spread out manually but that was still small enough that it would not flow off the substrate (substrate area 7.2 cm2) by itself. The PMMA concentration in chloroform was varied systematically until an optimum value was obtained (0.008 g/mL PMMA/ CHCl3). The polymer solution was applied by the use of a pipet, and it spread on its own accord to cover the whole substrate 1.5 cm × 4.8 cm in size (see Table 1 for advancing contact angles of chloroform). It took about 3 min for the chloroform to evaporate and for the polymer film to solidify. Finally, the aluminum substrate was removed by immersion into hydrochloric acid. The porous membrane resulting from this optimization is shown in Figure 3c and d. Using the optimized procedure, PMMA membranes were obtained of sizes up to 7.2 cm2. Their size was only limited by the size of the mask used for the photolithography. Those membranes could be handled carefully with tweezers or bare hands likewise without being broken. The membranes showed the desired pore pattern. Although the substrate showed hydrophilic squares and the template droplets had a quadratic base area in the beginning, the pores themselves showed a rounded perimeter. The pore top perimeters seen from the membrane side which was facing the air appeared to be smooth (Figure 4a), while the pore 5626


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from the simple geometry of a spherical cap penetrating a flat surface. Self-supporting porous membranes were successfully produced via a templating method using liquid droplets as porogenes. The pores in the membrane were of the same size and were arranged over wide areas in a predetermined pattern. The size of the membranes themselves was only limited by the size of the lithography mask used (here 7.2 cm2). In the example shown here, the lower limit (40 μm square length) of the size of the hydrophilic regions (the pore size) was given by the roughness of the substrate. Lithography masks with smaller holes could have been used. However, the photoresist on the substrate is of varying thickness because of the roughness of the underlying alumina substrate. The smaller the patterns become the more precisely tuned the exposure and the developing parameters need to be. A varying photoresist thickness makes it impossible to tune these parameters accordingly when the patterns become too small. Thus, to achieve smaller pores with a new lithography mask bearing smaller holes, one would have to choose other substrates with less roughness. With adequate substrates, one might achieve membranes with pores smaller than 10 μm. The selectively wetted substrates were very stable; even two weeks after wetting a substrate, porous membranes could be achieved by applying the polymer solution. Only longer dwell times in the moist atmosphere had to be realized. The process of membrane making in general was rather fast. The time needed for preparing one membrane was about 10 min. Variation of the Pore Geometry. Compared to the previously reported inkjet printing technique,19 a crucial advantage of the procedure depicted here may be the possibility to influence the pore size and shape via the lithography process. This possibility was explored in the next series of experiments. To have easy access to illumination patterns, transmission electron microscopy (TEM) grids were then used as masks. These are readily available in various patterns but unfortunately are limited in size and are easily distorted. Notwithstanding their drawbacks, the TEM grids were good enough to check if the principal idea of the templating method also works for different geometric requirements. To investigate the influence of a size change, TEM grids (200 mesh) showing square holes with a width of 100 μm were chosen. The selective wetting of the now bigger hydrophilic squares (100 μm) and the further processing worked as described before; even the same PMMA concentration in chloroform was used (c = 0.008 g/mL PMMA/CHCl3). The only difference compared to the smaller pattern was that the growth time needed to be extended to 25 min. The pores in the resulting membrane were 67 μm ± 8.5 μm in diameter (Figure 6). Concerning the perimeter of the pores, the same artifact as before was observed: the upper rims were smooth whereas the ones on the bottom were jagged because of template droplets being pinned to the roughness of the substrate (Figure 6b and d). Some bottom shapes of the droplets were almost quadratic this time. To learn if not only the size but also the shape of the pores could be tailored, TEM grids (100 × 400 mesh) showing rectangular openings (200 μm by 40 μm, see Supporting Information) were chosen as lithography mask to achieve rectangular hydrophilic regions to grow the droplets on. As before, it was possible to keep all parameters the same except the growth time which had to be extended to 30 min. As can be seen in Figure 7, one obtains slitlike pores in the resulting polymer

Figure 5. (a) Scheme of the position of the model droplet, (b) droplet has the shape of an ideal spherical cap, (c) real shape of the droplet and the polymer film. Polymer height H, contact angle φ, radius of the droplet base r1, radius of the pore r2.

a whole assumes a spherical cap geometry as was already done in ref 19. Thus, from theoretical assumptions (see Figure 5b), the radius of the droplet base r1, the contact angle φ, the radius of the open pore r2 in the polymer film, and the thickness H can be correlated (see Supporting Information for derivation). ⎛ ⎜ H = r1⎜tan(φ − 90◦) + ⎜ ⎝

⎞ ⎛ r2 ⎞2 ⎟ −⎜ ⎟ ⎟ ⎝ r1 ⎠ ⎟ [cos(φ − 90◦)]2 ⎠ 1

(1)

Using the values of r1 = 28 μm, half the length of the diagonal of one square; r1 = 19 μm, the pore radius obtained from electron microscopy; and φ = 97°, the receding contact angle on the hydrophobic region, we obtain an estimated membrane thickness of 21 μm. It can be seen from the AFM investigation that the height of the real membrane is significantly smaller than the estimate in this calculation. The reasons are first that the chosen model droplet is deviating in shape and size of its footprint from the real one acting as porogen. The real droplet is much smaller in height and shows no perfect spherical shape because of the interaction with the polymer (see Figure 5c). Second, the pores are formed in a dynamic process which might already be halted at a time of incomplete evaporation because of high viscosities, gel formation, and residual stress within a concentrated polymer solution.38 Furthermore, evaporation of polymer solutions usually leads to the formation of a skin which might not completely retract once the droplet penetrates the surface. Thus, the real pore radius becomes smaller than estimated 5627


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Figure 6. SEM images of a porous polymer membrane prepared with TEM grids (200 mesh) as mask bearing squares of 100 μm width: (a, b) top side and (c, d) bottom side.

Figure 7. SEM images of a porous polymer membrane prepared with TEM grids (100 × 400 mesh) as masks bearing rectangular openings of 200 μm length and 40 μm width: (a, b) top side and (c, d) bottom side. 5628


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the growth time. The larger the size of one single hydrophilic region, the longer the condensation time has to be to obtain droplets high enough to penetrate the polymer solution and to form permeable pores in the resulting polymer film. A summary of the parameters and results of the various patterns used can be found in Table 2. Filtration Tests. The permeance for clean water of a porous polymer membrane depicted in Figure 3c was 2.3 × 106 l h−1 bar−1 m−2. To determine whether the porous membranes in principle could serve as filtration media, a model suspension of micrometer-sized glass particles in water was filtered using the membranes described in the first section. The batch used here had holes of 30 μm ± 2.7 μm in diameter. The model suspension was applied to the top of the membrane and permeated it driven solely by gravity. The filtration was purely dead-end, without backflushing or cross-flow, but the applied amount was chosen small enough to prevent the formation of a filter cake. The particle size distributions of the original suspension, the permeate, and the retained particles were analyzed via scanning electron microscopy and are shown in Figure 9. The model suspension before filtration contained particles in sizes from 5 to 100 μm (Figure 9a). The mean pore radius of the membrane used for filtration was 30 μm ± 2.7 μm, and so a size cutoff of the permeate was expected at about 30 μm, which was the case as can be seen in Figure 9c. The largest particle measured in the permeate was 30 μm. In comparison to that, the residue on the membrane after filtration (Figure 9d) was significantly depleted of particles smaller than 30 μm but was not completely void of them. The latter fact is plausible considering that no emphasis was put on complete washing of the residue. The membranes do not show any damage after the filtration. (For pictures of the model suspension and the membrane after the filtration, see the Supporting Information.)

membrane showing the desired arrangement and shape and especially an almost uniform width. As expected, the upper rim of the droplets showed rounded down corners because of interactions between the polymer solvent and the aqueous solution (see Figure 7). The pores themselves were 29 μm ± 7.3 μm wide (length: 135 μm ± 23.7 μm). Depending on the used pattern size (size of one single hydrophilic area), the condensation time had to be adjusted (see Figure 8). The polymer solution used for membrane

Figure 8. Correlation of the observed structure to the area of one single hydrophilic region and the growth time.

making always had the same concentration and was used in the same amount per area substrate. Thus, the possible height of the polymer film was also always the same. Thus, independent of the used pattern, the template droplets had to have the same minimal necessary height to be able to break through the polymer film. If in a theoretical approach the droplets are assumed to be spherical caps, their height only depends on their base area and on their volume. The height is supposed to be constant; thus, the volume has to change with changing base radius. When the base radius of the hydrophilic region becomes larger, the volume of the droplet also has to become larger to achieve the minimal height. The volume in this example is a function of the growth time. Thus, the optimized growth time is expected to be a function of the base area of the used pattern. This connection can also be discovered in the empirical diagram in Figure 8. As expected, the lowest framed tics depicting the lowest possible growth time for the pores to be open show a linear trend between the base area of the hydrophilic region and

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CONCLUSION Concluding, it can be said that porous polymer membranes could be made by applying a templating method using sessile droplets resting on a patterned substrate as porogens. The key point here was to tailor the size and the geometry of the resulting pores by using offset printing plates that were illuminated by UV-light through various lithography masks to achieve various wetting patterns and sizes. The created droplets in the wetting patterns had to be enlarged before they could act as porogens as to be able to break through the polymer solution. The enlargement of the droplets was achieved through the exposition of the selectively wetted substrates to a moist atmosphere. A crucial parameter for the formation of sufficiently high template droplets was the dwell time of the substrates in this moist environment which determined whether the droplets could form commensurate pores in the resulting

Table 2. Data on Masks Used for Lithographic Patterning of the Photoresist and the Resulting Membranes property size of complete patterned substrate area geometry size of single hole area of single hole optimized growth time final pore size a

lithography mask 7.2 cm

2

grid with square holesa squares (40 μm by 40 μm) 1600 μm2 6 min rounded pores 38 μm ± 3.1 μm

TEM grid 200 mesh −4

7.1 × 10 cm (TEM grid diameter 0.03 cm) grid with square holesa squares (100 μm by 100 μm) 10 000 μm2 25 min rounded pores 67 μm ± 8.5 μm 2

TEM grid 400 mesh 7.1 × 10

−4

2

cm (TEM grid diameter 0.03 cm)

grid with rectangular holesa rectangles (40 μm by 200 μm) 8000 μm2 30 min slitlike pores of a width of 29 μm ± 7.3 μm (length: 135 μm ± 23.7 μm)

See Supporting Information for schematic drawings and optical microscopy images of the used mask patterns. 5629


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Figure 9. Membrane characterization and filtration: (a) pore size distribution of the membrane, (b) particle size distribution of the particle ensemble before filtration, (c) particle size distribution in the permeate after filtration, and (d) particle size distribution of the retained particles after filtration. illuminated with a low-pressure mercury arc lamp (Umex, Dresden, Germany, primary wavelength: 254 nm, intensity: 4.8 mW/cm2; secondary wavelength: 366 nm, intensity: 1 mW/cm2). Exposure took place for 150 s from a distance of 4.5 cm. For development, the exposed offset printing plate was put into an aqueous solution of NaOH (0.25 mol/L, 10 g/L) for 15 s and afterward was rinsed with deionized water and was dried in an argon flow. The structured offset printing plate was covered with a solution of CaCl2 in water (3.36 mol/L) with a foam roller (Lackierrolle, Perfecta Royal, 5.5 cm wide, shape and porosity not crucial). The excessive liquid was removed using a squeegee (Rival, Gerhard Haas KG, Stockach, Germany).39 Then, the selectively wetted offset plate was put onto a copper block cooled to a temperature of 6 °C which was surrounded by moist argon of a relative humidity of 60% (T = 25 °C, argon flow rate 2 Nl/h) in

polymer membrane. Depending on the used pattern size, the condensation time had to be adjusted. For patterns with larger sizes of one single hydrophilic area, a longer condensation time was needed.

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EXPERIMENTAL SECTION

Membrane Preparation. In the experiments depicted in this paper, commercial offset printing plates composed of an anodized aluminum sheet coated with a positive photoresist (sensitive to light of wavelengths between 350 and 420 nm) were used (Main Plate, positive analog plate, just4print Europe GmbH, Hainburg, Germany). The printing plate was pressed onto a lithography mask (made out of glass bearing a grid of chromium lines with a width of 20 μm enclosing square holes sized 40 × 40 μm, see Figure 2a) and was 5630


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a self-made growth cell. After 6 min (25 min for 100 μm square pattern, 30 min for rectangular pattern 200 μm by 40 μm), the substrate was taken off the copper block, was put into an atmosphere of relative humidity of 20−30%, and was covered with a solution (42 μL solution per 1 cm2 of substrate) of poly(methyl methacrylate) (PMMA molar mass Mn = 64 000 g/mol, Mw/Mn = 1.85) in technical grade CHCl3 (c = 0.008 g/mL PMMA/CHCl3). After the solidification of the polymer, the aluminum substrate was removed by etching for 3 min with diluted hydrochloric acid (7 mol/L). Residual photoresist was eliminated from the polymer membrane by renewed treatment with an aqueous solution of NaOH (0.25 mol/L, 10 g/L). Characterization. Contact angles were measured by the sessile drop technique using the G2 contact angle measurement system (Krüss). Optical micrographs were obtained with an Axioskop 40 Pol microscope from Zeiss. The membrane thickness was measured by atomic force microscopy (AFM) using a nanowizard II (JPK Instruments AG, Berlin, Germany). The membranes and particles were characterized by scanning electron microscopy using a Nova NanoSEM (Fei, Hillsboro, Oregon, United States). The Nova NanoSEM was also used for the energydispersive X-ray spectroscopy (EDX). The diameter of a pore generated by square templates was the longest of all distances from rim to rim measurable perpendicular to the orientation of the original hydrophobic lines. In the case of the rectangular pores, the width (length) was the maximum rim to rim distance that was measurable perpendicular (parallel) to the long axis. Filtration Experiments. A model dispersion with a broad size distribution was prepared combining two batches of Spheriglass solid glass microspheres (Potters Industries LLC, Valley Forge, Pennsylvania, United States). The used batches were Spheriglass 2530 (particle sizes between 40 and 100 μm) and Spheriglass 5000 (particle sizes between 5 and 45 μm). The model suspension consisted of 0.5 wt % particles in deionized water. The particle size distribution of the whole model suspension can be seen in Figure 9a. For filtration, one of the porous polymer membranes was put onto a grid made out of woven, polymer-coated glass fibers of 0.4 mm thickness and square meshes of 1.2 mm width (Windhager Handelsges. m. b. H., Thalgau, Austria), and the model suspension was applied to the top of the membrane by pipetting. The permeate was collected on a silicon wafer, was dried, and was analyzed by SEM. Furthermore, the membrane after filtration, which was decorated by the retained particles, was imaged by SEM as well. The diameters of the permeated particles as well as the retained particles were obtained from these images using the ImageJ software.

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characterizing the PMMA. Also, we thank Sabine Kaufmann (Department for Physical Chemistry, Chemnitz University of Technology) for the EDX measurements and Lutz Reinhardt (Department for Physical Chemistry, Chemnitz University of Technology) for the AFM measurements. We also thank Potters Industries LLC and Velox GmbH for providing free samples of the Spheriglass solid glass microspheres.

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NOTE ADDED AFTER ISSUE PUBLICATION The original supporting information file was incomplete but has now been replaced. This complete file was published on May 31, 2012. The associated addition/correction (doi: 10.1021/la301818j) also contains a link to the complete supporting information file.

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ASSOCIATED CONTENT

S Supporting Information *

Physical data on aqueous calcium chloride solution; photograph of the squeegee; schematic drawings and images of the masks; EDX measurement of a structured substrate wetted with CaCl2 solution; theoretical derivation of the polymer height; AFM measurement to obtain the membrane thickness; model suspension and membrane after filtration. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

(1) Kaji, H.; Ishibashi, T.; Nagamine, K.; Kanzaki, M.; Nishizawa, M. Biomaterials 2010, 31, 6981−6986. (2) Nishikawa, T.; Nishida, J.; Ookura, R.; Nishimura, S.-I.; Wada, S.; Karino, T.; Shimomura, M. Mater. Sci. Eng., C 1999, 10, 141−146. (3) Nishikawa, T.; Ookura, R.; Nishida, J.; Arai, K.; Hayashi, J.; Kurono, N.; Sawadaishi, T.; Hara, M.; Shimomura, M. Langmuir 2002, 18, 5734−5740. (4) Wagdare, N. A.; Marcelis, A. T. M.; Boom, R. M.; van Rijn, C. J. J. Colloid Interface Sci. 2011, 355, 453−457. (5) Kuiper, S.; van Rijn, C.; Nijdam, W.; Raspe, O.; van Wolferen, H.; Krijnen, G.; Elwenspoek, M. J. Membr. Sci. 2002, 196, 159−170. (6) Gutierrez-Rivera, L. E.; Katekawa, M. E.; Silva, M. A.; Cescato, L. Braz. J. Chem. Eng. 2010, 27, 677−685. (7) Warkiani, M. E.; Lou, C.-P.; Gong, H.-Q. J. Micromech. Microeng. 2011, 21, 035002. (8) van Rijn, C.; van der Wekken, M.; Nijdam, W.; Elwenspoek, M. J. Microelectromech. Syst. 1995, 6, 83−87. (9) Kuiper, S.; van Rijn, C. J. M.; Nijdam, W.; Elwenspoek, M. C. J. Membr. Sci. 1998, 150, 1−8. (10) van Rijn, C. J. M. Nano and Micro Engineered Membrane Technology; Elsevier: Amsterdam, The Netherlands, 2004; (a) p 289. (11) van Rijn, C. J. M.; Veldhuis, G. J.; Kuiper, S. Nanotechnology 1998, 9, 343−345. (12) van Rijn, C. J. M.; Nijdam, W.; Kulper, S.; Veldhuis, G. J.; van Wolferen, H.; Elwenspoek, M. J. Micromech. Microeng. 1999, 9, 170− 172. (13) Gutierrez-Rivera, L. E.; de Carvalho, E. J.; Silva, M. A.; Cescato, L. J. Micromech. Microeng. 2005, 15, 1932−1937. (14) Prenen, A. M.; van der Werf, J. C. A. H.; Bastiaansen, C. W. M.; Broer, D. J. Adv. Mater. 2009, 21, 1751−1755. (15) Vogelaar, L.; Barsema, J.; van Rijn, C.; Nijdam, W.; Wessling, M. Adv. Mater. 2003, 15, 1385−1389. (16) Gironès, M.; Akbarsyah, I.; Nijdam, W.; van Rijn, C.; Jansen, H.; Lammertink, R.; Wessling, M. J. Membr. Sci. 2006, 283, 411−424. (17) Xu, H.; Goedel, W. A. Langmuir 2003, 19, 4950−4952. (18) Yan, F.; Goedel, W. A. Chem. Mater. 2004, 16, 1622−1626. (19) Jahn, S. F.; Engisch, L.; Baumann, R. R.; Ebert, S.; Goedel, W. A. Langmuir 2009, 25, 606−610. (20) Kipphan, H. Handbook of Print Media; Springer-Verlag: Berlin, Germany, 2001; pp 206−357. (21) Choi, S.-J.; Suh, K. Y.; Lee, H. H. J. Am. Chem. Soc. 2008, 130, 6312−6313. (22) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002− 2004. (23) Gau, H.; Herminghaus, S. Phys. Rev. Lett. 2000, 84, 4156−4159. (24) Herminghaus, S.; Fery, A.; Schlagowski, S.; Jacobs, K.; Seemann, R.; Gau, H.; Monch, W.; Pompe, T. J. Phys.: Condens. Matter 2000, 12, A57−A74. (25) Zheng, Z.; Azzaroni, O.; Zhou, F.; Huck, W. T. S. J. Am. Chem. Soc. 2006, 128, 7730−7731. (26) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498−1511.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank M. Hietschold, G. Baumann, T. Jagemann, and S. Schulze, (Department for Solid Surfaces Analysis, Chemnitz University of Technology) for support in scanning electron microscopy and S. Spange and F. Riedel, (Department for Polymer Chemistry, Chemnitz University of Technology) for 5631


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(27) Schäfle, C.; Leiderer, P.; Bechinger, C. Europhys. Lett. 2003, 63, 394−400. (28) Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 2790− 2793. (29) Mastrangeli, M.; Ruythooren, W.; Van Hoof, C.; Celis, J. P. J. Micromech. Microeng. 2009, 19, 045015. (30) Leopoldes, J.; Dupuis, A.; Bucknall, D. G.; Yeomans, J. M. Langmuir 2003, 19, 9818−9822. (31) Magerl, A.; Ebert, S.; Goedel, W.A. DE 10 2008 045 540 B4, German Patent, Mar 4, 2010. (32) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988−994. (33) As a general rule, contact angles above (below) 90° increase (decrease) if the surface roughness is increased. (34) For a schematic drawing and optical microscopy images of all masks used, see the Supporting Information. (35) Lenz, P.; Lipowsky, R. Phys. Rev. Lett. 1998, 80, 1920−1923. (36) Lenz, P. Adv. Mater. 1999, 11, 1531−1534. (37) Drelich, J.; Miller, J. D.; Good, R. J. J. Colloid Interface Sci. 1996, 179, 37−50. (38) Gorand, Y.; Pauchard, L.; Calligari, G.; Hulin, J. P.; Allain, C. Langmuir 2004, 20, 5138−5140. (39) For a picture of the profile of the squeegee, see the Supporting Information.

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