Article pubs.acs.org/Macromolecules
Copolymerization of 1‑Ethyl-3-vinylimidazolium Bis(trifluoromethylsulfonyl)imide via Initiated Chemical Vapor Deposition Laura C. Bradley and Malancha Gupta* Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, California 90089, United States
ABSTRACT: We studied the copolymerization of an ionic liquid (1-ethyl-3-vinylimidazolium bis(trifluoromethylsulfonyl)imide ([EVIm][TFSI])) with ethylene glycol diacrylate (EGDA) via initiated chemical vapor deposition to form polymerized ionic liquid (PIL) copolymer films. The copolymerization was carried out by placing droplets of [EVIm][TFSI] in the reactor and introducing EGDA and tert-butyl peroxide initiator in the vapor phase. The heterogeneous films that formed at the surface of the liquid droplets were composed of a homopolymer PEGDA top layer that formed by the polymerization of adsorbed EGDA at the liquidvapor interface and a poly([EVIm][TFSI]-co-EGDA) copolymer bottom layer that formed by the copolymerization of [EVIm][TFSI] with absorbed EGDA within the liquid. The copolymer layer contained a gradient composition with decreasing concentration of [EVIm][TFSI]. We showed that the composition of the copolymer films can be controlled by tuning the reaction time and pressure. In addition, we demonstrated that the films can be formed on solid supports which could allow these materials to be used as separation membranes and catalyst supports.
■
INTRODUCTION The polymerization of ionic liquid (IL) monomers is an emerging field that has led to the creation of a new class of materials know as polymerized ILs (PILs) which are being developed as ion conducting membranes,1,2 separation membranes,3,4 and catalyst supports.5,6 The development of PILs is motivated by the need to combine the chemical properties of ILs with the mechanical properties of polymers.7 PILs are commonly made by solution-phase, free-radical polymerization of imidazolium cations with vinyl moieties,8,9 and the PIL product can be isolated through precipitation. Other methods used to fabricate PILs include atom transfer polymerization,10 reversible addition−fragmentation transfer polymerization,11 and ring-opening metathesis polymerization.12 The immobilization of PILs onto polymer supports can be used to reduce the total amount of IL used,13 facilitate simple and effective recycling,14 and improve mechanical properties.15 Several researchers have demonstrated the use of grafting methods to attach PILs to support materials. For example, Lozano et al. grafted 1-decyl-2-methylimidazolium cations onto a polymer matrix to synthesize a support for biocatalysts.13 Hu © 2014 American Chemical Society
et al. grafted poly(ethylene glycol) onto PILs to make membranes that were less brittle than those made of pure homopolymer PIL.15 Samadi et al. demonstrated the grafting of PIL onto macroporous cellulose for the separation of carbon dioxide from a gaseous mixture.16 In addition to grafting methods, copolymerization can also be used to immobilize PILs within polymer networks. For example, Xiong and co-workers studied the copolymerization of a phosphorus IL and ethylene glycol dimethacrylate to form nanoparticles for use as reusable catalysts.14,17 Nulwala and co-workers fabricated a PIL block copolymer for use as a gas separation membrane with increased permeability.18 In addition, Winey and co-workers synthesized PIL diblock copolymers with high ionic conductivity for energy conversion and storage applications.19 In this paper, we study the copolymerization of 1-ethyl-3vinylimidazolium bis(trifluoromethylsulfonyl)imide ([EVIm][TFSI]) with ethylene glycol diacrylate (EGDA) via initiated chemical vapor deposition (iCVD) to form PIL copolymer Received: July 10, 2014 Revised: September 12, 2014 Published: September 29, 2014 6657
dx.doi.org/10.1021/ma501407q | Macromolecules 2014, 47, 6657−6663
Macromolecules
Article
FTIR peak area of the EGDA carbonyl stretching at 1732 cm−1 to the [EVIm][TFSI] imidazolium stretching vibrations between 3050 and 3200 cm−1. The peak ratio was compared to a calibration curve made from solutions with varying concentrations of EGDA and [EVIm][TFSI]. The reported values for the FTIR peak ratio and calculated [EVIm][TFSI] wt % are an average of 10 total samples from two depositions. Each FTIR sample was composed of three films on a silicon wafer and spectra were collected by performing 200 scans from 400 to 4000 cm−1. X-ray photoelectron spectroscopy (XPS) (Surface Science MProbe) was used to measure the atomic composition of the top (vapor) and bottom (liquid) sides of the PIL copolymer films using a monochromatic Al Kα X-ray source. Survey spectra were collected from 0 to 1000 eV at a step size of 1 eV and a total of five scans. The reported atomic compositions of the films are an average of four total samples from two depositions. In order to analyze the bottom side, the films were removed from the liquid droplets by placing a piece of copper tape (Ted Pella, Inc.) on a silicon wafer and pressing the tape onto the top of films. The removed films were then washed in an acetone bath for 2 min followed by an ethanol bath for 2 min. In order to analyze the top side, the films were removed from the liquid droplets by pressing a bare silicon wafer on the top of the films. When the silicon wafer was lifted off the droplets, the exposed bottom side of the films was gently washed with acetone, and a piece of copper tape was pressed onto the bottom side of the films, resulting in the top side of the films being exposed for analysis. These films were then washed for 2 min in acetone followed by 2 min in ethanol followed by an additional 24 h in an acetone bath to remove residual IL trapped between the film and the tape. The total film thickness was measured by imaging the cross-section of the films that had been prepared for XPS analysis using a scanning electron microscope (SEM) (Topcon, Aquila hybrid SEM). The crosssection of the films was prepared by dipping the films mounted on copper tape into liquid nitrogen for 30 s and then cutting the samples in half using a razor blade. The reported thicknesses are an average of measurements taken on three different films, with three locations imaged on each film, and five measurements taken 5 μm apart at each location. The equilibrium concentration of EGDA absorbed in [EVIm][TFSI] was measured using a quartz crystal microbalance. The mass of EGDA adsorbed at the liquid surface was estimated by measuring the mass uptake of EGDA onto a bare crystal. The concentration of EGDA absorbed in the bulk liquid was calculated by measuring the mass uptake of EGDA onto a crystal that contained a thin layer of [EVIm][TFSI] and subtracting the previously measured surface adsorption. The measurements were conducted under the deposition conditions; however, the initiator flow was replaced with nitrogen to measure only the uptake of EGDA into the liquid because nitrogen does not significantly absorb. For each measurement, the system was allowed to equilibrate for 20 min, and the reported values are an average of five trials.
films. In the iCVD process, vapor-phase monomer and initiator precursors are introduced into a vacuum chamber where a heated filament array decomposes the initiator into radicals. The monomer and initiator radicals diffuse to the surface of a cooled substrate where polymerization occurs through a freeradical mechanism.20,21 The advantages of the iCVD technique over other polymerization methods are that substrates with complex geometries can be coated,22,23 the functionality of the film can be tuned by varying the monomer,24−28 polymers can be deposited at low substrate temperatures,20,29 and the polymer thickness and growth rate can be controlled in situ.30,31 The iCVD process is typically used to deposit coatings onto solid substrates;32−35 however, we were recently the first group to deposit polymers onto liquid substrates such as ILs and silicone oils.36,37 These liquids have extremely low vapor pressures and are therefore stable in our vacuum system. We found that polymerization can occur at both the liquidvapor interface and within the liquid for cases in which the monomer is soluble,38,39 and we demonstrated that we can make nanoparticles,40 free-standing films,37 encapsulated liquid droplets,41 layered films,38 and polymerIL gels.39 Our previous studies involved nonreactive liquid substrates. In this work, we use a reactive IL as the substrate which copolymerizes with vapor-phase precursors to form PIL copolymer films at the liquid surface. We show that process conditions such as reaction time and pressure can be controlled to tune the composition and thickness of the copolymer films. We also demonstrate that we can form the films on wire mesh supports which is useful for using these films as separation membranes,42−44 solid-state electrolytes,45−47 and catalyst supports.5,48,49
■
EXPERIMENTAL SECTION
1-Ethyl-3-vinylimidazolium bis(trifluoromethylsulfonyl)imide ([EVIm][TFSI]) (98%, IoLiTec), ethylene glycol diacrylate (EGDA) (97%, Monomer-Polymer), and tert-butyl peroxide (TBPO) (97%, Sigma-Aldrich) were all used as received without further purification. All polymerization reactions were performed in a custom-built reactor chamber (GVD Corp, 250 mm diameter, 48 mm height). Droplets (12 μL) of [EVIm][TFSI] were placed on a glass slide, which was then placed on the reactor stage maintained at 30 °C by a recirculating chiller. To test the homopolymerization of [EVIm][TFSI], TBPO initiator was flown at room temperature into the chamber through a mass flow controller at 1.5 standard cubic centimeters per minute (sccm) with the filament heated to 230 °C for 60 min at a reactor pressure of 50 mTorr. After polymerization, the droplets were washed with ethanol to precipitate the polymer, which was then purified by alternating washes in acetone and ethanol. Nuclear magnetic resonance (NMR) spectroscopy was used to confirm the polymerization of [EVIm][TFSI] using deuterated acetone as the solvent and performing 60 scans from 0 to 10 ppm with a 10 s delay time. The copolymerization of [EVIm][TFSI] was carried out by flowing TBPO and EGDA into the reactor chamber at 1.5 and 1.0 sccm, respectively. The EGDA flow rate was achieved by heating the monomer jar to 35 and 40 °C for depositions at 30 and 50 mTorr, respectively, and the line temperature was kept 15 °C above the jar temperature. The deposition rate was measured on a reference silicon wafer using an in situ 633 nm helium−neon laser interferometer (Industrial Fiber Optics). After deposition, the polymer films were removed from the liquid surface by detaching the films at the edges of the droplets using tweezers and washed in an acetone bath for 2 min followed by an ethanol bath for 2 min. Fourier transform infrared spectroscopy (FTIR) (Thermo Nicolet iS10) was used to analyze the bulk composition of the PIL copolymer films. The concentration of [EVIm][TFSI] in the films was measured by taking the ratio of the
■
RESULTS AND DISCUSSION We first studied homopolymerization of [EVIm][TFSI] by placing droplets of the IL on a glass slide and flowing TBPO initiator into the iCVD chamber with the filament heated for 60 min. Rinsing the IL droplets with ethanol led to the precipitation of polymer which confirmed that radicals delivered through the vapor phase can initiate the polymerization of [EVIm][TFSI]. The NMR spectrum of the purified poly([EVIm][TFSI]) contains the characteristic signal for hydrogen on the polymer backbone at 4.2 ppm and does not contain the vinyl signals for the [EVIm][TFSI] monomer at 5.5, 6.0, or 7.4 ppm, confirming a free-radical polymerization mechanism (Figure 1).50 FTIR spectroscopy also verified the polymerization of [EVIm][TFSI] by the disappearance of the signal for the vinyl bond51 at 1660 cm−1. Our previous works have exclusively studied nonreactive liquid substrates, and this 6658
dx.doi.org/10.1021/ma501407q | Macromolecules 2014, 47, 6657−6663
Macromolecules
Article
Table 1. Atomic Compositions Measured by XPS of the Top and Bottom Sides of the PIL Copolymer Films from 40 min Reactions Compared to Reference PEGDA and Poly([EVIm][TFSI]) atomic composition sample
%C
%O
%F
%N
%S
PEGDA reference poly([EVIm] [TFSI]) reference top (vapor) side bottom (liquid) side
70 ± 1 31 ± 4
30 ± 1 18 ± 1
0±0 28 ± 4
0±0 13 ± 1
0±0 10 ± 1
83 ± 1 57 ± 3
17 ± 1 20 ± 1
0±0 13 ± 1
0±0 6±1
0±0 4±1
determine the structure of the films, the atomic compositions of the top (vapor) side and bottom (liquid) side of the films were measured using XPS which probes approximately 5 nm of the sample surface (Table 1). The top side of the films was composed of only carbon and oxygen indicating a top layer of homopolymer PEGDA that was formed by the polymerization of adsorbed EGDA at the liquidvapor interface.38 The high atomic concentration of carbon on the top side (83 ± 1%) compared to the PEGDA reference (70 ± 1%) is due to carbon contamination from the solvent washes which was confirmed by a measured increase in the carbon concentration (75 ± 1%) when the PEGDA reference was exposed to the same washing procedure as the top side of the films. The bottom side of the films contained fluorine, nitrogen, and sulfur which indicates the presence of [EVIm][TFSI], but the concentrations were less than the reference poly([EVIm][TFSI]) which indicates the presence of both monomers on the bottom side of the films. This was further confirmed by the carbon and oxygen concentrations which were between poly([EVIm][TFSI]) and PEGDA. This copolymer bottom layer was formed by the copolymerization of [EVIm][TFSI] with EGDA within the liquid, as we have previously shown that soluble precursors can absorb and also diffuse through polymer films at the liquid surface and then polymerize within the liquid.38 We estimated the concentration of [EVIm][TFSI] on the bottom side of the films to be 38 ± 4 wt % from the nitrogen and carbon concentrations. To estimate the thickness of the copolymer layer, we measured the total thickness of the films from crosssectional SEM images to be 1.6 ± 0.1 μm, and we assumed that the thickness of the top PEGDA layer was similar to the thickness of polymer formed on the reference silicon wafer (0.7 ± 0.1 μm) because we have previously shown that the molecular weight of polymer formed at the surface of liquid substrates and on reference silicon are similar, indicating comparable rates of polymerization.39 We estimated the thickness of the copolymer layer to be 0.9 ± 0.1 μm, which makes up a significant portion of the films formed by 40 min reactions. FTIR analysis of the PIL copolymer films also confirmed the incorporation of both monomers by the presence of the carbonyl peak at 1732 cm−1 characteristic of PEGDA52 and the imidazolium C−H symmetric stretching vibrations51 between 3050 and 3200 cm−1 and CN stretching53 at 1560 cm−1 characteristic of [EVIm][TFSI] (Figure 3a). We have previously shown that nonreactive IL integrated into polymer films can be removed by soaking the films in a solvent bath overnight.38 In contrast, we found that [EVIm][TFSI] could not be removed from the films even after soaking in an acetone bath for one week, further confirming that [EVIm][TFSI] is
Figure 1. Comparison of the NMR spectra of [EVIm][TFSI] monomer and poly([EVIm][TFSI]) formed by the introduction of TBPO initiator.
is the first demonstration that ILs can be polymerized in the iCVD process. We then studied the copolymerization of [EVIm][TFSI] by introducing EGDA and TBPO in the vapor phase (Figure 2). Since EGDA and TBPO are soluble in [EVIm][TFSI], we expect polymerization to occur at both the liquidvapor interface and within the IL as we have shown in other soluble systems.38,39 Reactions for 40 min at 50 mTorr resulted in the formation of PIL copolymer films at the liquid surface which completely encapsulated the IL droplets. The films were removed from the liquid surface and thoroughly washed. To
Figure 2. Schematic of the iCVD process for the copolymerization of [EVIm][TFSI] with EGDA delivered through the vapor phase. 6659
dx.doi.org/10.1021/ma501407q | Macromolecules 2014, 47, 6657−6663
Macromolecules
Article
Figure 4. (a) Atomic concentrations of fluorine, nitrogen, and sulfur measured by XPS and (b) the corresponding concentration of [EVIm][TFSI] on the bottom side of the films as a function of reaction time at a pressure of 50 mTorr.
to increase linearly between 5 and 90 min at a rate of 23 nm/ min (Figure 5). By subtracting the growth rate of the top PEGDA layer (18 nm/min), the growth rate of the copolymer layer was calculated to be 5 nm/min. The constant growth rate of the copolymer layer confirms that polymerization continues to occur within the liquid over the entire range of reaction times. It is important to note that the copolymer layer formed in the first 5 min had a high growth rate of 182 nm/min, which is due to a high absorption of precursors into the bulk liquid, while the subsequent slow growth rate for reaction times longer than 5 min (5 nm/min) is due to the lower flux of precursors through the film already formed at the liquid surface.38 We combined the XPS and SEM results to illustrate both the composition and thickness of the PIL copolymer films as a function of time as shown in Figure 5a. The copolymer layer contained a gradient composition with a decreasing concentration of IL from ∼80 to ∼35 wt %. The decrease in the concentration of [EVIm][TFSI] on the bottom side of the films from 5 to 20 min is likely due to the accumulation of absorbed EGDA monomer in the liquid. We confirmed the presence of unreacted EGDA in the liquid after 20 min reactions using NMR to detect the characteristic vinyl signals of the EGDA monomer at 5.9, 6.2, and 6.4 ppm which are unique from the vinyl signals of [EVIm][TFSI]. For reactions longer than 20 min, the constant composition of the bottom side and the constant growth rate of the copolymer layer suggest that polymerization reached a steady-state condition. To further validate the structures in Figure 5a, we measured the bulk composition of the films using FTIR and found good agreement with our proposed structures (Figure 5c). The concentration of [EVIm][TFSI] within the bulk films was measured to decrease continuously from 75 ± 3 to 26 ± 9 wt %
Figure 3. FTIR spectra of (a) the PIL copolymer films from 40 min reactions compared to reference PEGDA and poly([EVIm][TFSI]) and (b) the carbonyl peak of PIL copolymer films from varying reaction times compared to reference PEGDA.
copolymerized with EGDA. The copolymerization was also confirmed by the broadening of the carbonyl peak toward lower wavenumbers in the PIL copolymer films for a range of reaction times compared to the PEGDA reference (Figure 3b) which is likely due to the presence of hydrogen bonding in the copolymer layer.54−56 The weak signal for the vinyl bond in the spectra of the PIL copolymer films was determined to be unreacted [EVIm][TFSI] because trace amounts of the IL monomer were detected in the solvent bath using NMR spectroscopy. Furthermore, there is no vinyl signal in the FTIR spectrum of the PEGDA reference, indicating that all the vinyl bonds are reacted, and similarly we expect that all the vinyl bonds of EGDA are reacted in the PIL copolymer films. To study the growth of the films as a function of time, we varied the reaction time between 5 and 90 min at a constant reactor pressure of 50 mTorr and deposition rate of 18 ± 2 nm/min as measured on a reference silicon wafer using in situ interferometry. The composition of the bottom side of the films measured by XPS survey scans showed that the concentrations of fluorine, nitrogen, and sulfur decreased between 5 and 20 min, reflecting a decrease in the concentration of [EVIm][TFSI] from 79 ± 12 to 60 ± 13 to 37 ± 7 wt % at 5, 10, and 20 min, respectively, whereas the composition did not significantly change for reaction times between 20 and 90 min (Figure 4). The total thickness of the films was measured 6660
dx.doi.org/10.1021/ma501407q | Macromolecules 2014, 47, 6657−6663
Macromolecules
Article
decrease in the deposition rate measured on a reference silicon wafer from 18 ± 2 to 6 ± 1 nm/min. Decreasing the pressure also leads to a decrease in the equilibrium concentration of EGDA absorbed in [EVIm][TFSI] from 23 ± 2 to 6 ± 2 wt % as measured by a quartz crystal microbalance. We compared films from 40 min reactions and found that decreasing the reactor pressure from 50 to 30 mTorr resulted in an increase in the concentration of [EVIm][TFSI] on the bottom side from 38 ± 4 to 58 ± 12 wt %. The total film thicknesses were found to be 1.0 ± 0.1 and 1.6 ± 0.1 for 30 and 50 mTorr, respectively. The thicknesses of the PEGDA top layers were estimated to be 0.2 ± 0.1 and 0.7 ± 0.1 μm for 30 and 50 mTorr, respectively, and the thicknesses of the copolymer layers were calculated to be 0.8 ± 0.1 and 0.9 ± 0.1 μm, respectively. The thickness of the PEGDA layer decreases with decreasing pressure due to the lower deposition rate at the liquid surface; however, the thickness of the copolymer layer was similar in both cases likely because there is less resistance to the diffusion of precursors into the liquid at the lower pressure due to the thinner film at the liquid surface. Additionally, the concentration of [EVIm][TFSI] in the bulk films as measured by FTIR increased from 53 ± 7 to 86 ± 1 wt % with decreasing pressure due to both the thinner PEGDA layer and the higher concentration of [EVIm][TFSI] on the bottom side of the films. While the PIL copolymer films are strong enough to be removed from the liquid surface using tweezers, the films do not hold their shape and fold over on themselves. In order to control the shape of the films for different applications, we formed the films on supports by placing a piece of wire mesh over the top of the liquid. After the reaction, the wire mesh was removed from the liquid surface and thoroughly washed. The PIL copolymer films formed over the entire area of the mesh and filled the spaces between the wire grid (Figure 6a,b). The polymer films on the mesh supports were robust enough to be folded to a bend 1.5 mm wide (Figure 6c,d).
Figure 5. (a) Total film thickness as a function of reaction time measured by SEM cross-sectional images. Schematics depict the thickness and composition of the PEGDA and copolymer layers as determined by SEM and XPS analysis. (b) SEM cross-sectional images of PIL copolymer films made by 5 and 90 min reactions. (c) Concentration of [EVIm][TFSI] in the bulk films measured by FTIR compared to the estimated concentration calculated from the structrues shown in part a.
■
CONCLUSIONS
We have demonstrated the ability to copolymerize [EVIm][TFSI] with vapor-phase precursors in the iCVD system to fabricate PIL copolymer films. The films were heterogeneous composed of a homopolymer PEGDA top layer and a copolymer poly([EVIm][TFSI]-co-EGDA) bottom layer. The PEGDA layer was formed by the polymerization of adsorbed EGDA at the liquidvapor interface whereas the copolymer layer was formed by the copolymerization of [EVIm][TFSI] with absorbed EGDA within the liquid. We studied the growth of the PIL copolymer films by varying the reaction time and found that the copolymer layer has a high growth rate (182 nm/min) and a high concentration of [EVIm][TFSI] on the
between 5 and 90 min. The steady decrease in the concentration is due to the higher growth rate of the PEGDA layer (18 nm/min) compared to the copolymer layer (5 nm/min). In order to increase the concentration of IL on the bottom side of the films, we studied the effect of decreasing the reactor pressure. In the iCVD process, decreasing the reactor pressure from 50 to 30 mTorr decreases the concentration of EGDA adsorbed at the liquidvapor interface24,30 as measured by a
Figure 6. (a, b) PIL copolymer films formed on wire mesh supports. (c) Supported films can withstand being folded to a bend with 1.5 mm diameter. (d) Cross-sectional image of the bend in part c. 6661
dx.doi.org/10.1021/ma501407q | Macromolecules 2014, 47, 6657−6663
Macromolecules
Article
(17) Xiong, Y.; Wang, Y.; Wang, H.; Wang, R. Polym. Chem. 2011, 2, 2306−2315. (18) Adzima, B. J.; Venna, S. R.; Klara, S. S.; He, H.; Zhong, M.; Luebke, D. R.; Mauter, M. S.; Matyjaszewski, K.; Nulwala, H. B. J. Mater. Chem. A 2014, 2, 7967−7972. (19) Choi, J.-H.; Ye, Y.; Elabd, Y. A.; Winey, K. I. Macromolecules 2013, 46, 5290−5300. (20) Sreenivasan, R.; Gleason, K. K. Chem. Vap. Deposition 2009, 15, 77−90. (21) Alf, M. E.; Asatekin, A.; Barr, M. C.; Baxamusa, S. H.; Chelawat, H.; Ozaydin-Ince, G.; Petruczok, C. D.; Sreenivasan, R.; Tenhaeff, W. E.; Trujillo, N. J.; Vaddiraju, S.; Xu, J.; Gleason, K. K. Adv. Mater. 2010, 22, 1993−2027. (22) Ma, M.; Mao, Y.; Gupta, M.; Gleason, K. K.; Rutledge, G. C. Macromolecules 2005, 38, 9742−9748. (23) Bose, R. K.; Heming, A. M.; Lau, K. K. S. Macromol. Rapid Commun. 2012, 33, 1375−1380. (24) Gupta, M.; Gleason, K. K. Langmuir 2006, 22, 10047−10052. (25) Chan, K.; Gleason, K. K. Langmuir 2005, 21, 8930−8939. (26) Haller, P. D.; Flowers, C. A.; Gupta, M. Soft Matter 2011, 7, 2428−2432. (27) Kwong, P.; Gupta, M. Anal. Chem. 2012, 84, 10129−10135. (28) Alf, M. E.; Godfrin, P. D.; Hatton, T. A.; Gleason, K. K. Macromol. Rapid Commun. 2010, 31, 2166−2172. (29) Yagüe, J. L.; Coclite, A. M.; Petruczok, C.; Gleason, K. K. Macromol. Chem. Phys. 2012, 214, 302−312. (30) Lau, K. K. S.; Gleason, K. K. Macromolecules 2006, 39, 3688− 3694. (31) Ye, Y.; Mao, Y.; Wang, F.; Lu, H.; Qu, L.; Dai, L. J. Mater. Chem. 2011, 21, 837−842. (32) Baxamusa, S. H.; Gleason, K. K. Chem. Vap. Deposition 2008, 14, 313−318. (33) Gupta, M.; Kapur, V.; Pinkerton, N. M.; Gleason, K. K. Chem. Mater. 2008, 20, 1646−1651. (34) Kwong, P.; Flowers, C. A.; Gupta, M. Langmuir 2011, 27, 10634−10641. (35) Riche, C. T.; Marin, B. C.; Malmstadt, N.; Gupta, M. Lab Chip 2011, 11, 3049−3052. (36) Haller, P. D.; Frank-Finney, R. J.; Gupta, M. Macromolecules 2011, 44, 2653−2659. (37) Frank-Finney, R. J.; Haller, P. D.; Gupta, M. Macromolecules 2012, 45, 165−170. (38) Bradley, L. C.; Gupta, M. Langmuir 2013, 49, 10448−10454. (39) Frank-Finney, R. J.; Bradley, L. C.; Gupta, M. Macromolecules 2013, 46, 6852−6857. (40) Haller, P. D.; Bradley, L. C.; Gupta, M. Langmuir 2013, 29, 11640−11645. (41) Bradley, L. C.; Gupta, M. Langmuir 2012, 28, 10276−10280. (42) Gu, Y.; Lodge, T. P. Macromolecules 2011, 44, 1732−1736. (43) Chi, W. S.; Hong, S. U.; Jung, B.; Kang, S. W.; Kang, Y. S.; Kim, J. H. J. Membr. Sci. 2013, 443, 54−61. (44) Bara, J. E.; Hatakeyama, E. S.; Gabriel, C. J.; Zeng, X.; Lessmann, S.; Gin, D. L.; Noble, R. D. J. Membr. Sci. 2008, 316, 186− 191. (45) Chen, H.; Choi, J.-H.; Cruz, D. S.; Winey, K. I.; Elabd, Y. A. Macromolecules 2009, 42, 4809−4816. (46) Washiro, S.; Yoshizawa, M.; Nakajima, H.; Ohno, H. Polymer 2004, 45, 1577−1582. (47) Park, J. T.; Prosser, J. H.; Ahn, S. H.; Kim, S. J.; Kim, J. H.; Lee, D. Adv. Funct. Mater. 2013, 23, 2193−2200. (48) Tan, R.; Yin, D.; Yu, N.; Zhao, H.; Yin, D. J. Catal. 2009, 263, 284−291. (49) Wu, B.; Hu, D.; Kuang, Y.; Liu, B.; Zhang, X.; Chen, J. Angew. Chem., Int. Ed. 2009, 48, 4751−4754. (50) Nakamura, K.; Saiwaki, T.; Fukao, K.; Inoue, T. Macromolecules 2011, 44, 7719−7726. (51) Lee, J. H.; Lee, J. S.; Lee, J.-W.; Hong, S. M.; Koo, C. M. Eur. Polym. J. 2013, 49, 1017−1022.
bottom side (∼80 wt %) for short reactions of 5 min. With increasing reaction time between 5 and 20 min, the concentration of [EVIm][TFSI] decreases on the bottom side from ∼80 to ∼35 wt %, before reaching a steady-state condition for reaction times longer than 20 min. For all reaction times longer than 5 min, the copolymer layer grows at a constant rate of 5 nm/min. The concentration of [EVIm][TFSI] on the bottom side of the films could be increased by decreasing the reactor pressure. The ability for both the reaction time and pressure to be easily tuned in the iCVD process enables the relative thicknesses of the PEGDA and copolymer layers to be controlled and the reaction conditions to be optimized to obtain high concentrations of [EVIm][TFSI]. Furthermore, the PIL copolymer films can be formed on wire mesh supports to be shaped for different applications in separations and catalysis.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (M.G.). Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research. L.C.B. is supported by a National Science Foundation Graduate Research Fellowship under Grant DGE0937362. We thank the Molecular Materials Research Center of the Beckman Institute at the California Institute of Technology for use of their XPS.
■
REFERENCES
(1) Nakajima, H.; Ohno, H. Polymer 2005, 46, 11499−11504. (2) Ye, Y.; Elabd, Y. A. Polymer 2011, 52, 1309−1317. (3) Bara, J. E.; Lessmann, S.; Gabriel, C. J.; Hatakeyama, E. S.; Noble, R. D.; Gin, D. L. Ind. Eng. Chem. Res. 2007, 46, 5397−5404. (4) Li, P.; Paul, D. R.; Chung, T.-S. Green Chem. 2012, 14, 1052− 1063. (5) Muldoon, M. J.; Gordon, C. M. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 3865−3869. (6) Zhao, Q.; Zhang, P.; Antonietti, M.; Yuan, J. J. Am. Chem. Soc. 2012, 134, 11852−11855. (7) Mecerreyes, D. Prog. Polym. Sci. 2011, 36, 1629−1648. (8) López, M. S.-P.; Mecerreyes, D.; López-Cabarcos, E.; López-Ruiz, B. Biosens. Bioelectron. 2006, 21, 2320−2328. (9) Green, O.; Grubjesic, S.; Lee, S.; Firestone, M. A. Polym. Rev. 2009, 49, 339−360. (10) Tsarevsky, N. V.; Matyjaszewski, K. Chem. Rev. 2007, 107, 2270−2299. (11) Vijayakrishna, K.; Jewrajka, S. K.; Ruiz, A.; Marcilla, R.; Pomposo, J. A.; Mecerreyes, D.; Taton, D.; Gnanou, Y. Macromolecules 2008, 41, 6299−6308. (12) Vygodskii, Y. S.; Shaplov, A. S.; Lozinskaya, E. I.; Lyssenko, K. A.; Golovanov, D. G.; Malyshkina, I. A.; Gavrilova, N. D.; Buchmeiser, M. R. Macromol. Chem. Phys. 2008, 209, 40−51. (13) Lozano, P.; García-Verdugo, E.; Bernal, J. M.; Izquierdo, D. F.; Burguete, M. I.; Sánchez-Gómez, G.; Luis, S. V. ChemSusChem 2012, 5, 790−798. (14) Xiong, Y.; Wang, H.; Wang, R.; Yan, Y.; Zheng, B.; Wang, Y. Chem. Commun. 2010, 46, 3399−3401. (15) Hu, X.; Tang, J.; Blasig, A.; Shen, Y.; Radosz, M. J. Membr. Sci. 2006, 281, 130−138. (16) Samadi, A.; Kemmerlin, R. K.; Husson, S. M. Energy Fuels 2010, 24, 5797−5804. 6662
dx.doi.org/10.1021/ma501407q | Macromolecules 2014, 47, 6657−6663
Macromolecules
Article
(52) Ye, Y.; Mao, Y.; Wang, H.; Ren, Z. J. Mater. Chem. 2012, 22, 2449−2455. (53) Chowdhury, A.; Thynell, S. T. Thermochim. Acta 2006, 443, 159−172. (54) Coates, J. In Encyclopedia of Analytical Chemistry; John Wiley & Sons Ltd.: Chichester, 2000; pp 10815−10837. (55) Aggarwal, A.; Lancaster, N. L.; Sethi, A. R.; Welton, T. Green Chem. 2002, 4, 517−520. (56) Reinhardt, D.; Ilgen, F.; Kralisch, D.; König, B.; Kreisel, G. Green Chem. 2008, 10, 1170−1181.
6663
dx.doi.org/10.1021/ma501407q | Macromolecules 2014, 47, 6657−6663
