Encapsulation of Ionic Liquids within Polymer Shells via Vapor

We demonstrate the use of vapor phase deposition to completely encapsulate ionic liquid (IL) droplets within robust polymer shells. The IL droplets we...
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Encapsulation of Ionic Liquids within Polymer Shells via Vapor Phase 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 S Supporting Information *

ABSTRACT: We demonstrate the use of vapor phase deposition to completely encapsulate ionic liquid (IL) droplets within robust polymer shells. The IL droplets were first rolled into liquid marbles using poly(tetrafluoroethylene) (PTFE) particles because the marble structure facilitates polymerization onto the entire surface area of the IL. Polymer shells composed of 1H,1H,2H,2H-perfluorodecyl acrylate cross-linked with ethylene glycol diacrylate (P(PFDA-co-EGDA)) were found to be stronger than the respective homopolymers. Fourier transform infrared spectroscopy showed that the PTFE particles become incorporated into the polymer shells. The integration of the particles increased the rigidity of the polymer shells and enabled the pure IL to be recovered or replaced with other fluids. Our encapsulation technique can be used to form polymer shells onto dozens of droplets at once and can be extended to encapsulate any low vapor pressure liquid that is stable under vacuum conditions.

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INTRODUCTION Ionic liquids (ILs) have recently attracted significant interest as environmentally friendly alternatives to traditional organic solvents because they are nonvolatile and nonflammable and can be easily recycled.1,2 A significant amount of research has been focused on using ILs in chemical synthesis,3 cellulose dissolution,4 and thermal energy storage.5 ILs have also been shown to absorb harmful gases such as carbon dioxide6 and sulfur dioxide.7 Immobilization and encapsulation of ILs is important to implementing ILs at the industrial scale to bypass issues caused by their high viscosity and offers a way to increase the surface area to volume ratio for specific applications, such as gas absorption.8,9 In this paper, we demonstrate the ability to simultaneously encapsulate dozens of millimeter-sized IL droplets in robust polymer shells using initiated chemical vapor deposition (iCVD). The iCVD process is a one-step, solventless polymerization technique in which monomer and initiator vapors are flown into a vacuum chamber, and a heated filament array decomposes the initiator into radicals.10,11 The initiating radicals and monomer molecules diffuse to a cooled stage and polymerization occurs on the surface of the substrate via a free radical mechanism. ILs have been immobilized onto the surfaces of solid supports such as porous silica particles,12,13 sol−gel materials,14−17 polymer membranes,18,19 and particles,20 and they have been encapsulated in polymer materials using suspension spraying,9 emulsion polymerization,21 and microfluidic processes.22 The iCVD technique is unique from conventional encapsulation methods because it enables ILs to be encapsulated within a wide range of polymers including insoluble fluoropolymers and cross-linked polymers. Also, iCVD does not require the use of solvents, and therefore the IL is not lost to a solvent phase during encapsulation. © 2012 American Chemical Society

The iCVD technique is typically used to deposit polymer coatings onto solid substrates;23,24 however, we have recently shown that low vapor pressure liquids such as ILs and silicone oil can also be introduced into the iCVD process.25,26 In our first study, we examined surface versus bulk polymerization of hydroxyethyl methacrylate and 1H,1H,2H,2H-perfluorodecyl acrylate at different temperatures. In our next study, we compared polymerization on ILs versus silicone oil surfaces. We found that continuous films formed on the IL whereas only polymer particles formed on the silicone oil. In our current study, we demonstrate for the first time that we can completely encapsulate IL droplets within polymer shells. The IL droplets are first rolled into liquid marbles using micrometer-sized particles. The IL marbles are then coated on a bed of particles which enables the encapsulated IL droplets to remain intact when they are removed from the substrate.

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EXPERIMENTAL SECTION

1-Ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) (97%, Aldrich), 1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) (97% Aldrich), ethylene glycol diacrylate (EGDA) (97%, Monomer-Polymer and Dajac Laboratories), tert-butyl peroxide (TBPO) (98%, Aldrich), and poly(tetrafluoroethylene) (PTFE) particles (Aldrich, 35 μm) were used without further purification. Marbles were made by dispensing 2 mm diameter droplets of IL into a Petri dish (5 cm diameter) containing 0.5 g of PTFE particles. The Petri dish was tilted until the IL droplet was completely covered with PTFE particles. The marbles were then transferred to another Petri dish (5 cm diameter) containing 1.5 g of PTFE uniformly spread across the bottom. The dish was then placed into the iCVD chamber. All polymer depositions were carried out in a custom-designed reaction chamber (GVD Corp., 250 mm Received: March 19, 2012 Revised: June 11, 2012 Published: June 27, 2012 10276

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diameter, 48 mm height). A nichrome filament array (80% Ni, 20% CR, Omega Engineering) was placed 32 mm above the substrate and was resistively heated to 250 °C. The TBPO initiator was maintained at room temperature and flowed into the reactor at a rate of 1.35 sccm using a mass flow controller (Model 1479A, MKS) for all polymer depositions. The stage temperature was maintained at 30 °C using a recirculating chiller. For all depositions, the PFDA and EGDA monomers were heated to 50 and 35 °C, respectively. The reactor pressure was kept constant at 80 mTorr for the depositions of PEGDA and P(PFDA-co-EGDA) and 60 mTorr for the deposition of PPFDA. Fourier transform infrared spectroscopy (FTIR) (Thermo Nicolet iS10) was used to analyze the polymer coatings and the bulk IL. X-ray photoelectron spectroscopy (XPS) experiments were carried out using a Surface Science Instruments M-Probe spectrometer with a monochromatic Al Kα X-ray source. High-resolution spectra were taken between 280 and 296 eV binding energies with a step size of 0.065 eV. Scanning electron microscopy (SEM) (JEOL-6610) was used to visualize the polymer shells.

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RESULTS AND DISCUSSION Figure 1a shows a schematic of our fabrication process. To ensure that the entire surface area of the IL was coated during deposition, we first rolled 2 mm diameter droplets of IL into liquid marbles using 35 μm diameter poly(tetrafluoroethylene) (PTFE) particles. Aussillous and Quéré were the first to fabricate liquid marbles in 2001 by rolling water droplets over hydrophobic silane-treated lycopodium grains.27 Gao and McCarthy used this rolling technique to fabricate IL marbles using hydrophobic oligomeric and polymeric tetrafluoroethylene particles.28 For our study, 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) was chosen as the model IL because its low viscosity (37.7 cP29) relative to other alkylimidazolium-based ILs enabled small marbles to be made reproducibly and its yellow color allowed for visualization of the IL. We placed our IL marbles in Petri dishes filled with 1.5 g of loose PTFE particles and inserted the Petri dishes into the iCVD reactor. A thinner layer of PTFE would increase the deposition onto the IL; however, the bed of loose PTFE tends to shift during the pump down of the reactor. We use excess PTFE to ensure that the entire surface of the Petri dish remains covered. We then continuously flowed gaseous monomer and initiator molecules into the iCVD chamber. The heated filament array inside the chamber cleaved the initiator molecules into free radicals. These free radicals and the monomer molecules diffused to the surface of the marbles and polymerized via a free radical mechanism. The PTFE particles on the marble surface created an air gap between the IL and the surface underneath the marble which enabled initiator and monomer molecules to diffuse and polymerize on the underside of the IL. The iCVD technique can uniformly coat surfaces with features on the order of micrometers;30 therefore, using 35 μm diameter PTFE particles ensured that polymerization occurred on the entire surface of the IL and formed a continuous shell. The IL marbles were coated with three different polymer shells: poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA), poly(ethylene glycol diacrylate) (PEGDA), and PFDA cross-linked with EGDA (P(PFDA-co-EGDA)). These compositions were chosen because PFDA polymerizes only at the surface of the IL since it does not absorb into the bulk IL and EGDA enhances the mechanical strength of the polymer shell through crosslinking. It was necessary to coat the IL marbles on a bed of loose PTFE to prevent bridging between the polymer deposited onto the marble and the Petri dish. Figure 1b compares IL

Figure 1. (a) Schematic of our process for encapsulating ionic liquids in polymer shells via iCVD. (b) IL marbles sitting on the surface of a water bath that were coated on a bed of loose PTFE (left) and a bare Petri dish (right). The polymer shell to the left retains the yellow IL, whereas the marble to the right had a hole torn in the polymer coating and the IL leaked into the water bath.

marbles sitting on the surface of a water bath that were coated on a bed of loose PTFE and a bare Petri dish. When a coated marble was lifted off the bare Petri dish, a hole was torn in the polymer shell where it was connected to the Petri dish, and therefore the yellow IL leaked out when placed on the water bath. In contrast, the IL marble coated on a bed of loose PTFE particles retained the yellow IL inside the polymer shell when placed on water demonstrating that the polymer shell was continuous. When the marble was removed from the bed of PTFE after deposition, the polymer was torn between the loose PTFE particles, preserving the continuous polymer shell. We tried to dye the IL to measure the release rate of IL from the polymer shells, but we were unable to find a dye that was fully miscible with the IL. Instead, we placed ten coated IL marbles on a piece of cellulose paper to measure the release rate. If the IL leaked, it would show up on the cellulose paper which readily wicks IL. After 22 days, there was no IL on the cellulose paper, indicating no penetration of the IL through the polymer shells. Figure S-1 in the Supporting Information shows that our method can be scaled down to smaller droplets by dispensing smaller volumes of IL. The mechanical strength of the three polymer shells was compared by dropping uncoated and coated marbles (∼2 to 3 μm thick coatings on a reference wafer) onto a water bath from a height of 0.5 in. (Figure S-2 in the Supporting Information). This drop method enabled us to determine if the polymer shells remained continuous after impact by observing the retention of the yellow IL inside the polymer shells. The uncoated marbles 10277

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Figure 2. (a) FTIR spectra of a P(PFDA-co-EGDA) coating deposited onto an [emim][BF4] droplet placed directly onto a silicon wafer and covered with a layer of PTFE particles, a reference P(PFDA-co-EGDA) film deposited onto a bare silicon wafer, the bulk [emim][BF4] after polymer deposition, and reference [emim][BF4]. (b) XPS spectra of the top and bottom surfaces of the P(PFDA-co-EGDA) film.

top 5 nm of the surface (Figure 2b). The top of the sample is defined as the polymer−air interface, and the bottom of the sample is the polymer−IL interface. Neither spectrum contained an overwhelming CF2 signal at 292 eV which is characteristic of PTFE.34 This indicates that the PTFE particles are buried within the polymer coating. The spectrum of the top surface matched well with the reference spectrum of P(PFDAco-EGDA). The intensity of the signal at 292 eV is similar to that in the reference polymer film, indicating that the CF2 groups in the top of the polymer film are from the P(PFDA-coEGDA) polymer and not from the PTFE particles. The spectrum of the bottom surface contained the three identifying peaks for PEGDA35 but no identifying peaks for PPFDA. This is likely due to the fact that only EGDA can absorb into the IL and polymerize within the bulk. The resulting PEGDA becomes integrated into the bottom portion of the film. The small signal at 292 eV indicates that some PTFE particles are not completely coated on the bottom side. To study the effect of the PTFE particles on the rigidity of the polymer coating, we compared P(PFDA-co-EGDA) coatings deposited onto IL droplets placed on a silicon wafer without PTFE and with PTFE at the IL surface (Figure 3). The smooth film deposited onto the IL droplet without PTFE rolled up on itself when it was lifted off the IL while the rough film on the IL with PTFE held its shape after it was removed from the IL, demonstrating that the integration of PTFE increased the rigidity of the polymer shell. The PTFE particles are incorporated into the polymer film as shown by the SEM images in Figure 4. The reference polymer was ∼3 μm, and the thickness of the polymer shell is ∼4 μm measured from the cross section in Figure 4c. The increase in the thickness is most likely due to the presence of IL. The mechanical strength of the P(PFDA-co-EGDA) polymer shells was tested by examining their ability to hold their shape under the weight of other coated marbles. When uncoated and coated IL marbles were stacked in pyramids, the uncoated marbles conformed to one another because they are malleable while the robust polymer shells of the coated marbles

and the PPFDA and PEGDA polymer shells broke on impact with the water bath, whereas the P(PFDA-co-EGDA) polymer shell remained intact. We compared the rigidity of the three polymer compositions by depositing coatings onto [emim][BF4] droplets that were placed directly onto a silicon wafer and covered with a layer of PTFE particles since this produced large (∼5 mm diameter) and relatively flat coatings that could be easily examined. We found that the P(PFDA-coEGDA) polymer coatings were robust enough to be easily lifted off the IL droplets with tweezers in one continuous piece, whereas the PPFDA and PEGDA coatings ripped when grasped with tweezers. These observations and the drop method results suggest that the P(PFDA-co-EGDA) shells have increased mechanical strength relative to either the PPFDA or PEGDA shells. When the P(PFDA-co-EGDA) coating was lifted off the IL droplet, there was no visible PTFE at the surface or within the bulk IL which suggested that the PTFE particles became incorporated into the polymer coating. Fourier transform infrared (FTIR) spectroscopy was used to verify the integration of the PTFE particles into the polymer coating (Figure 2a). The spectrum of the polymer coating shows the characteristic CF2 wagging vibrations for PTFE at 504 and 556 cm−1 which are distinct from the reference P(PFDA-co-EGDA) spectrum in the 450−600 cm−1 region verifying the integration of the PTFE into the polymer coating.31 The polymer coating also contains IL identified by the presence of the imidazolium ring C−H symmetric stretching vibrations between 3050 and 3250 cm−1.32 The bulk [emim][BF4] was tested to confirm the absence of PTFE. The spectrum does not have the characteristic PTFE signals and is nearly identical to the reference [emim][BF4] spectrum displaying the representative C−H out of plane bending vibration at 521 cm−1 and the C−H symmetric stretching vibrations of the imidazolium ring.33 FTIR cannot distinguish the location of the PTFE particles within the film; therefore, we used X-ray photoelectron spectroscopy (XPS) to study the chemical composition of the top and bottom surfaces of the film since XPS only probes the 10278

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Figure 5. A P(PFDA-co-EGDA) polymer shell (a) encapsulating IL, (b) after the IL is removed with a syringe, and (c) injected with dyed red water.

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CONCLUSION This work demonstrated the encapsulation of IL droplets within robust polymer shells via iCVD. The P(PFDA-coEGDA) polymer shells were stronger than the respective homopolymer shells. The incorporation of the PTFE particles into the polymer shell eliminated PTFE contamination of the interior fluid and allowed the coated IL to be treated as a twophase product consisting of an exterior polymer shell and interior IL that can be easily replaced if necessary. Encapsulating ILs in solid polymer shells may reduce the effect of their high viscosity which is a challenge in their implementation into industrial processes. For example, the immobilized IL droplets can be stacked and used in gas separation processes to allow the desired species to diffuse into the IL phase while the remaining species can easily flow between the spherical droplets. It has been shown that small gas molecules such as carbon dioxide and oxygen can diffuse through dense polymer films.36−38 Future studies will focus on optimizing the thickness and chemical functionality of the shell for maximizing gas absorption. Our encapsulation process can be used to encapsulate low vapor pressure liquids that are stable under iCVD conditions such as ILs, silicone oil, and glycerol. Our technique can be easily extended to fabricate stimuli-responsive polymer shells by using precursors such as N-isopropylacrylamide39 or o-nitrobenzyl methacrylate.40

Figure 3. Schematic and corresponding images of P(PFDA-co-EGDA) deposited onto IL droplets on a silicon wafer (a, c) without PTFE and (b, d) with PTFE at the IL surface. (e) The polymer coating lifted off the droplet shown in (c) rolled up on itself, and (f) the polymer coating lifted off the droplet shown in (d) held its shape, demonstrating that the integration of PTFE increases the rigidity of the polymer shell.

maintained their shape (Figure S-3). We tried using microtensile testing (DEBEN, 5 kN), but our films are extremely thin and therefore the force required for pulling the film is below the threshold of the instrument. We expect the compression strength would be very low as well. We were not able to use nanoindentation because the technique requires smooth films and our coatings are rough due to the incorporation of PTFE particles. However, the polymer shells are robust enough for our intended applications because they can be dropped and stacked as we show in Figures S-2 and S-3. The polymer shells are also strong enough for the IL to be replaced with dyed red water (Figure 5). A small hole was made in the polymer shell, the IL was removed using a syringe, and dyed red water was injected into the polymer shell through the same hole. Figure 5b illustrates that the polymer shell did not retain its shape when the IL was removed; however, the polymer shell regained its shape when it was filled with the dyed red water (Figure 5c). This showed that the polymer shells can also contain high vapor pressure liquids that cannot be introduced into the iCVD vacuum chamber by easily exchanging the interior fluid using a syringe.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S-1−S-3. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

Figure 4. SEM images of the (a) top side, (b) bottom side, and (c) cross section of the P(PFDA-co-EGDA) polymer film with PTFE incorporation. 10279

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ACKNOWLEDGMENTS L.C.B. is supported by a fellowship from the Chevron Corporation (USC-CVX UPP). We thank the Molecular Materials Research Center of the Beckman Institute of the California Institute of Technology for use of their XPS.

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