Article pubs.acs.org/Macromolecules
Ultrathin Free-Standing Polymer Films Deposited onto Patterned Ionic Liquids and Silicone Oil Robert J. Frank-Finney, Patrick D. Haller, and Malancha Gupta* Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, 925 Bloom Walk, Los Angeles, California 90089, United States ABSTRACT: We studied the vapor deposition of polymers onto the surfaces of silicone oil and imidazolium-based ionic liquids (ILs). We found that the deposition of poly(2hydroxyethyl methacrylate) (PHEMA) and poly(N-isopropylacrylamide) (PNIPAAm) resulted in polymer particles on silicone oil whereas continuous polymer skins formed on 1-butyl-3methylimidazolium hexafluorophosphate ([bmim][PF6]), 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), and 1ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]). The silicone oil and ILs were patterned onto a common substrate by exploiting their different wetting properties. Ultrathin free-standing PHEMA and PNIPAAm films of different shapes were produced by confining the shape of the IL within a wax barrier, surrounding it with silicone oil, and then depositing the polymer. The silicone oil prevented the polymer film from connecting to the underlying substrate and maintained the shape of the polymer film during deposition. Our process allows for multidimensional control over the resulting free-standing film: the area of the shape can be controlled by patterning the IL, and the thickness of the film can be controlled by adjusting the duration of polymer deposition. The films are highly pure and do not contain any residual monomer or solvent entrapment which extends their potential applications to include in vivo biomedical research.
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applications in optics,12 sensing,13,14 and separations.15 The fabrication of free-standing polymer films typically requires multiple steps such as spin-coating polymers onto sacrificial layers and then removal of the sacrificial layers using several steps of washing with various solvents.16−20 The fabrication process that we present in this paper is environmentally friendly because no organic solvents are used in any of the steps. The free-standing polymer films produced by our method are highly pure and do not contain any residual monomer or solvent entrapment which will allow biomedical researchers to use these films for in vivo applications such as tissue engineering, surgical applications, and drug delivery.
INTRODUCTION The initiated chemical vapor deposition (iCVD) technique is a one-step, solventless free radical polymerization process that can be used to deposit a wide range of polymer films such as poly(2-hydroxyethyl methacrylate) (PHEMA),1 poly(4-vinylpyridine) (P4VP),2 and poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA).3 The iCVD technique is typically used to deposit polymer coatings onto solid substrates such as silicon wafers,4 membranes,5 wires,6 carbon nanotubes,7 and fibers.8 We recently demonstrated the ability to deposit polymer coatings onto ionic liquids (ILs).9 ILs are salts that are liquids at ambient temperatures, and they have recently attracted significant interest as environmentally friendly alternatives to traditional volatile organic solvents because they are nonvolatile, nonflammable, and can be easily recycled.10,11 Our previous work examined the deposition of PHEMA and PPFDA in the presence of 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) droplets. We found that polymerization occurred at the vapor−IL interface and/or within the bulk IL depending on the solubility of the monomer within the IL and the reaction conditions such as the duration of deposition and stage temperature. In this paper, we use iCVD to deposit polymers onto silicone oil for the first time. We observe different polymer morphologies on the silicone oil as compared to the ILs, and we exploit this difference to fabricate ultrathin free-standing polymer films of different shapes by combining the silicone oil and ILs onto a common substrate. The generality of our fabrication method is demonstrated for multiple polymers and a range of imidazolium-based ILs. Our ability to produce freestanding polymer films is useful for a wide variety of © 2011 American Chemical Society
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RESULTS AND DISCUSSION We studied the deposition of PHEMA onto 5 μL droplets of silicone oil, [bmim][PF6], 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), and 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) placed on a silicon wafer. In the iCVD process, monomer and initiator molecules are flowed continuously into a vacuum chamber where the initiator molecules are broken into free radicals by a heated filament array. Polymerization occurs on the surface of the substrate via a free-radical mechanism.21 In the case of the imidazoliumbased ionic liquids, HEMA monomer molecules can absorb into the ILs and polymerization can occur at both the vapor−IL interface and within the bulk IL. In the case of silicone oil, Received: October 9, 2011 Revised: December 10, 2011 Published: December 19, 2011 165
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HEMA molecules do not appreciably absorb into the silicone oil, and therefore polymerization should only occur at the vapor−silicone oil interface. Figure 1 shows the images of the
Figure 2. FTIR spectra of (A) a PHEMA film deposited onto a wafer, (B) a PHEMA skin formed on [bmim][PF6], (C) a PHEMA skin formed on [bmim][BF4], (D) a PHEMA skin formed on [emim][BF4], and (E) PHEMA particles deposited onto silicone oil.
Figure 1. Images of 15 min of deposition of PHEMA onto (A) [bmim][PF6], (B) [bmim][BF4], (C) [emim][BF4], and (D) silicone oil. (E−H) The droplets were subjected to a continuous stream of air to show that a continuous skin of PHEMA formed on the ILs, but only particles of PHEMA formed on the silicone oil.
droplets taken after 15 min of deposition. A continuous polymer skin that completely encapsulates the droplet formed on all three ILs, while unconnected polymer particles formed on the silicone oil. An air stream was applied to the droplets to demonstrate the continuous nature of the polymer skins on the ILs and the granular nature of the polymer on the silicone oil. Fourier transform infrared spectroscopy (FTIR) was used to study the chemical structure of the PHEMA deposited on silicone oil and the ILs. Figure 2 shows that all of the spectra have the expected PHEMA peaks including the broad O−H stretching peak from 3600 to 3100 cm−1, C−H stretching peaks from 3050 to 2800 cm−1, the CO peak from 1750 to 1685 cm−1, C−H bending from 1520 to 1350 cm−1, and C−O stretching from 1310 to 1210 cm−1.9 The spectra are nearly identical, demonstrating that the polymer is highly pure and that varying the liquid substrate does not affect the composition of the polymer. The different polymer morphology on the silicone oil versus the ILs can be exploited to fabricate free-standing films. The skins that are formed on the IL droplet are connected to the underlying silicon wafer and therefore cannot be removed without tearing. For example, Figure 3A shows that PHEMA completely encapsulates [bmim][PF6] droplets that are placed
Figure 3. Images of 60 min of deposition of PHEMA onto a [bmim][PF6] droplet that was placed on (A) a silicon wafer and (C) a silicon wafer covered with a layer of silicone oil. (B, D) The substrate was tilted at a 15° angle after the deposition.
directly onto silicon wafers. These encapsulated droplets do not move when the substrate is tilted at a 15° angle (Figure 3B). In order to make free-standing polymer films, we combined the ILs and silicone oil onto a common substrate. Silicone oil was first dispensed onto silicon wafers and allowed to spread over the wafer surface. The silicone oil completely wets the surface of the silicon wafer, forming a thin layer (∼30 μm) onto which IL droplets can then be placed. Figure 3C shows that a continuous PHEMA film forms on the IL droplet, whereas only polymer particles form on the surrounding silicone oil. Figure 3D shows that the droplet slides when the substrate is tilted at a 15° angle. This verifies that the silicone oil acts as a lubricating layer to prevent the polymer that forms on the IL from connecting to the underlying wafer. The contact angles for [bmim][PF6], [emim][BF4], and [bmim][BF4] are 38°, 28°, 166
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Figure 4. Contact angle goniometer images of (A) IL droplets on silicon wafers, (B) IL droplets on a layer of silicone oil, and (C) the same droplets after 60 min of deposition of PHEMA.
using wax. The IL is then dispensed into the outline. The wax barrier contains the IL within the shape because the IL does not wet the wax. In the case of [bmim][BF4], the wax outline was drawn onto a bare silicon wafer. In the cases of [bmim][PF6] and [emim][BF4], the wax outline was drawn onto a silicon wafer that was precoated with PHEMA in order to increase the spreading of the IL into the corners of the shape. Silicone oil was then added in multiple locations around the outside of the wax barrier and allowed to spread over the barrier and encompass the IL. The silicone oil serves two purposes in this fabrication process: it maintains the shape of the original IL droplet during deposition, and it prevents the polymer film from connecting to the underlying substrate. We would like to note that there is no lubricating layer of silicone oil underneath the IL in this fabrication method since the IL is dispensed before the silicone oil. Therefore, the IL will not slide when the substrate is tilted. After deposition of polymer, the free-standing polymer film can be removed from the IL either by inserting a razor underneath the film and lifting it off or by submerging the entire substrate in silicone oil which allows the film to float off the IL. Figure 6 shows the generality of our fabrication method for two different ILs and two different polymers. A triangular PHEMA film was formed on [bmim][BF4] after 30 min of deposition. The FTIR spectrum of the PHEMA film showed no difference from the spectrum of PHEMA deposited on a silicon wafer, indicating the high purity of the film. The free-standing film had an average thickness of 510 ± 64 nm at the edge of the triangle and 663 ± 35 nm at the center. The increased thickness at the center of the film is caused by the integration of polymer chains that form within the bulk IL since polymerization takes place simultaneously at both the vapor−IL interface and within the IL at the conditions used for our study.9 Similar to PHEMA, the deposition of poly(N-isopropylacrylamide) (PNIPAAm) also results in polymer particles on silicone oil and polymer skins on each of the three ILs. Therefore, we can use our fabrication method to form shaped PNIPAAm films. A square PNIPAAm film was formed on [bmim][PF6] after 135 min of deposition. The film had an average thickness of 445 ± 30 nm at the edge of the square and 469 ± 41 nm at the center. The PNIPAAm film had the expected FTIR peaks: asymmetric −CH3 stretching at 2969 cm−1, asymmetric −CH2− stretching at 2931 cm−1, symmetric −CH3 stretching at 2880 cm−1, secondary amide CO stretching at 1652 cm−1, −CH3 and −CH2− deformation at 1458 cm−1, and −CH3 deformation at 1387 and 1366 cm−1.22 Compared to the PNIPAAm deposited
and 19°, respectively, on a silicon wafer (Figure 4A). When the IL is placed on top of the silicone oil, a thin layer of oil remains between the IL and the underlying silicon wafer, and the silicone oil forms a concave meniscus on the side of the IL droplet. This meniscus makes it impossible to measure a contact angle for the IL on the silicone oil; however, Figure 4B shows that the trend in wettability is the same on silicone oil as on a silicon wafer, i.e., that [bmim][BF4] wets the most and [bmim][PF6] wets the least. Without the use of silicone oil, the IL droplet spreads on the silicon wafer during PHEMA deposition due to both monomer absorption into the IL and the increased attraction between the IL and the PHEMA film formed on the silicon substrate surrounding the droplet. In contrast, the use of silicone oil prevents the IL droplets from spreading during PHEMA deposition. Comparison of parts B and C of Figure 4 shows no noticeable change in the curvature or diameter of the IL droplets after polymer deposition. We believe that the meniscus acts as a barrier to prevent the spreading of the IL droplet during deposition. The shape of the free-standing polymer film can be controlled by patterning the IL and silicone oil onto the substrate. Figure 5 shows a schematic of this fabrication method. First, an outline of a shape is drawn onto the substrate
Figure 5. Fabrication method for making shaped polymer films. 167
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Figure 6. Images and corresponding FTIR spectra of free-standing shaped films of (A, B) PHEMA formed on [bmim][BF4] and (C, D) PNIPAAm formed on [bmim][PF6]. The films were removed from the template and placed in a bath of silicone oil for imaging.
onto a wafer, the shaped free-standing PNIPAAm film had a shift in the location of the secondary amide N−H stretching from 1540 to 1575 cm−1. This is likely due to the mobility of the PNIPAAm chains in the free-standing film that allows for hydrogen bonding between the CO and N−H groups.23 In addition to PHEMA and PNIPAAm, we found that the deposition of several other polymers including poly(o-nitrobenzyl methacrylate) (PoNBMA) and poly(pentafluorophenyl methacrylate) (PPFM) also yields particles on silicone oil and skins on the ILs. The formation of particles on silicone oil has also been examined in the deposition of silver,24 copper,25 gold,26 and C4F827 precursors. Ye et al. proposed that silver clusters that formed on silicone oil do not merge into a film because an adsorbed layer of oil molecules surrounds the particles and thereby prevents coalescence.28 Similarly, we believe that a polymer skin does not form on the silicone oil in cases where the silicone oil wets the polymer. For example, the silicone oil wets PHEMA, PNIPAAm, PoNBMA, and PPFM (the contact angles of silicone oil on these polymers are 6°, 39°, 7°, and 24°, respectively), and the depositions of these polymers all result in the formation of particles. After long deposition times (e.g., ∼2 h in the case of PHEMA), the concentration of polymer particles becomes great enough to completely cover the oil surface such that additional polymer can no longer interact with the underlying silicone oil, and further deposition results in a continuous film that grows on the layer of particles. In contrast to the above polymers, we have found that the deposition of PPFDA results in continuous polymer films on droplets of both silicone oil and all three imidazolium-based ILs (Figure 7A−D). We believe that the formation of a continuous PPFDA skin on silicone oil is due to the poor wetting between PPFDA and silicone oil, as silicone oil has a high contact angle on PPFDA (70°). When PPFDA was deposited onto IL droplets placed on a layer of silicone oil, a continuous polymer skin formed that encapsulated the twoliquid system and connected to the underlying silicon wafer (Figure 7E). Therefore, the IL droplet did not move when the substrate was tilted at a 15° angle (Figure 7F), which is in contrast to the deposition of PHEMA which resulted in a
Figure 7. Images of 30 min of deposition of PPFDA onto droplets of (A) [bmim][PF6], (B) [bmim][BF4], (C) [emim][BF4], and (D) silicone oil placed on a silicon wafer. (E) Images of PPFDA deposited onto a droplet of [bmim][PF6] placed on a silicon wafer covered with a layer of silicone oil and (F) tilted at a 15° angle.
continuous film on only the IL surface (Figure 3C,D). Therefore, while free-standing films could be formed using PHEMA, PNIPAAm, PoNBMA, and PPFM, they could not be formed using PPFDA.
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CONCLUSION We have demonstrated that the deposition of both PHEMA and PNIPAAm on silicone oil results in the formation of polymer particles, whereas deposition onto imidazolium-based ILs results in polymer skins which completely encapsulate the ILs. We exploited this difference in polymer morphology to fabricate ultrathin free-standing polymer films of different shapes by combining the ILs and silicone oil onto a common substrate. Our study reveals some very interesting surface tension effects: when the IL is placed on top of the silicone oil, a thin layer of oil remains between the IL and the underlying silicon wafer and the silicone oil forms a concave meniscus on the side of the IL droplet. This meniscus helps maintain the shape of the IL and thereby the shape of the resulting polymer. FTIR analysis shows that the free-standing polymer films are highly pure (free of residual monomer, IL, and silicone oil) which will enable their use for biomedical applications. The free-standing PNIPAAm films have many potential uses due to their temperature-responsive hydrophilicity.29,30 Our fabrication process is environmentally friendly because no organic solvents are used in any of the steps and ionic liquids are nonvolatile, nonflammable, and can be easily recycled. We demonstrated the generality of our fabrication 168
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iS10) was used to study the chemical composition of the PHEMA and PNIPAAm films. Films were removed to a clean wafer by placing the wafer surface on top of the polymer film and lifting the film off. The films were rinsed with methanol and hexane before analysis. The FTIR of the PHEMA particles on silicone oil was measured by first separating the PHEMA from the silicone oil through extraction with methanol and then drop-casting the resulting solution onto a clean wafer.
method across a range of imidazolium-based ILs ([bmim][PF6], [bmim][BF4], and [emim][BF4]). Our ability to produce free-standing polymer films of controlled shape, size, and thickness is useful for a wide variety of applications in biosensing, biomimicry, and separations. In addition to PHEMA and PNIPAAm, we have found that the depositions of several other polymers including PoNBMA and PPFM also yield particles on silicone oil and skins on ILs. This allows us to extend our fabrication method to make light-responsive31 and click-active polymer films.32 Furthermore, films with multiple functionalities (e.g., mechanical strength, temperature-responsive swelling, photoresponsive solubility) can be made by sequentially stacking polymers.
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AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected]. ACKNOWLEDGMENTS This work was supported by the National Science Foundation under Grant EEC-0310723, the Mork Family Graduate Fellowship (R.F.F.), and the James H. Zumberge Faculty Research and Innovation Fund.
EXPERIMENTAL SECTION
1-Butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) (97%, Aldrich), 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) (97%, Aldrich), 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) (97%, Aldrich), poly(dimethylsiloxane) (Xiameter PMX-200 350 cSt, Aldrich), 2-hydroxyethyl methacrylate (HEMA) (98%, Aldrich), N-isopropylacrylamide (NIPAAm) (97%, Aldrich), 1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) (97%, Aldrich), and tert-butyl peroxide (TBPO) (98%, Aldrich) were used without further purification. All depositions were carried out in a custom designed reaction chamber (GVD Corp, 250 mm diameter, 48 mm height). For the deposition of PHEMA, the HEMA monomer was heated to a temperature of 55 °C, the stage temperature was maintained at 35 °C using a recirculating chiller, and the reactor pressure was kept constant at 110 mTorr. For the deposition of PNIPAAm, the NIPAAm monomer was heated to a temperature of 60 °C, the stage temperature was maintained at 55 °C using a recirculating chiller, and the reactor pressure was kept constant at 100 mTorr. For the deposition of PPFDA, the PFDA monomer was heated to a temperature of 50 °C, the stage temperature was maintained at 35 °C using a recirculating chiller, and the reactor pressure was kept constant at 140 mTorr. For all depositions, a nichrome filament array (80% Ni, 20% CR, Omega Engineering) was placed 32 mm above the substrate and was resistively heated to 240 °C. The TBPO initiator was maintained at room temperature and flowed into the reactor at a rate of 0.92 sccm using a mass flow controller (Model 1479A, MKS). The morphology of the polymer on the poly(dimethylsiloxane) silicone oil and the ILs was tested by first dispensing 5 μL of liquid directly onto a silicon wafer. Contact angles were then measured using a goniometer (ramé-hart Model 290-F1). After deposition, images of the polymer on the droplets were taken using a microscope and a Nikon D3000 camera. The continuity of the skins and particles was tested by subjecting the droplets to a continuous stream of air. In order to make shaped films, outlines of shapes were first drawn onto the substrate with a wax crayon using a ruler, and then IL was dispensed into the interior of the wax outline using a micropipet. For [bmim][ BF4], the shape was drawn onto an unmodified silicon wafer. For [bmim][PF6] and [emim][BF4], the shape was drawn onto a silicon wafer which had first been coated with a thin layer of PHEMA to increase the wetting of the IL into the corners of the shapes. Silicone oil was then dispensed (5 μL) at each edge of the shape and allowed to slowly spread over the wax and encircle the IL. After deposition of polymer, the polymer film was removed from the IL either by inserting a razor underneath the film and lifting it off or by submerging the entire substrate in silicone oil which allowed the film to float off the IL. The thickness of the polymer films was determined from JEOL-6610 low-vacuum scanning electron microscopy (SEM) images. For SEM sample preparation, the films were transferred onto a clean silicon wafer and blown flush against the wafer. The wafer underneath the skin was then cracked and mounted in a substrate holder such that the cross section could be visualized. A thin gold coating was sputtered onto the surface of the sample before imaging. Fourier transform infrared spectroscopy (FTIR) (Thermo Nicolet
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