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46305-AC7
Surface Grafted Poly(ionic liquid)s

Scott M. Husson, Clemson University

Our goal is to develop methodologies to create poly(ionic liquid) (pIL) nanolayers. Program objectives are to prepare pIL nanolayers by surface-initiated atom transfer radical polymerization (ATRP), characterize the nanolayer structural properties and growth kinetics, and conduct fundamental thermodynamic property measurements to evaluate pIL nanolayer performance for selective sorption of CO₂. Results from our research will enable the design of membranes with ultrathin pIL separation layers to recover CO₂ emissions generated by combustion of fossil fuels, thusly lessening the effect of global climate change.

Proposed activities for Year 1 were these:

1. Synthesize IL monomers and develop the methodology to prepare pIL nanolayers on silicon substrates and silica beads.

2. Characterize the chemical and structural properties of the pIL nanolayers using FTIR, ellipsometry, AFM, DSC, and GPC. [proposed for Years 1-2]

Additional activities that we did in Year 1 were these:

3. Evaluate how the chemistry of pIL nanolayers impact their performance for selective sorption of CO₂ over N₂. [proposed for Year 2]

4. Measure how temperature influences the performance of pIL nanolayers in selective sorption of CO₂ over N₂. [proposed for Year 2]

5. Investigate selective transport of CO₂ through membranes modified by pIL nanolayers. [newly added activity]

Research progress

Our initial plan to examine the importance of pIL nanolayer chemistry was to prepare ammonium-based IL monomers, and to use these to grow pIL nanolayers. While the synthesis of the first monomer, 2-(methylacryloyloxy)ethyl-trimethylammonium tetrafluoroborate (1 in Figure 1), was successful, it required a solution-phase purification step to isolate the product monomer. After this first experiment, we redesigned our synthesis strategy to avoid the monomer purification step. Rather than synthesize each monomer separately and then prepare pIL nanolayers, we decided to use a single, commercially available monomer, 2-(methylacryloyloxy)ethyl-trimethylammonium chloride (METAC) (2 in Figure 1), to prepare the polymer nanolayers, and then carry out the ion exchange reaction on the grafted polymer. A simple rinse step replaces the monomer purification step.

ATRP in a methanol/water solvent was used to graft pMETAC from silicon wafers that had been coated with a 4-6 nm layer of initiator-functionalized poly(glycidyl methacrylate). FTIR measurements confirmed that polymerization was successful. pMETAC layer thickness evolution was monitored by ex-situ ellipsometry. Figure 2 shows the evolution of layer thickness with time for two catalyst formulations. As expected, increasing Cu(II) concentration decreased growth rates and improved control (as indicated by linear thickness evolution for open symbols).

Following nanolayer growth, ion exchange was done to replace Cl^- with BF₄^-, CH₃SO₃^-, and CF₃SO₃^-. To confirm that solid-phase ion exchange was successful, we used XPS to determine atomic compositions of the pIL nanolayers. Results verified that ion exchange reactions were quantitative. No residual Cl was seen in any of the pIL nanolayers, and experimental atomic ratios of F:N and S:N were similar to theoretical values.

Activities 3 and 4 were planned for Year 2; however, we were presented an opportunity to do gas adsorption measurements using a Wicke-Kallenbach cell and analysis methods developed by Prof. Seidel-Morgenstern at the Max Planck Institute for Dynamic Complex Technical Systems, Magdeburg, Germany. One student spent 3 months at the MPI measuring CO₂ and N₂ adsorption isotherms on pIL nanolayers grafted from cellulose membranes. We used membrane supports instead of the proposed silica beads since i) we envision pIL nanolayers to be used as CO₂-selective layers for membrane separations and ii) characterization of the pIL properties also can be done on cellulose.

Figure 3 shows the adsorption isotherms at 25 °C for CO₂ on pIL nanolayers comprising repeat units 3, 4, 5 (in Figure 1). There was no measureable N₂ adsorption at the highest pressure studied. Measurements were done at three temperatures. Data analysis is under way to determine thermodynamic properties for adsorption. A journal manuscript is being drafted to disseminate these results.

After demonstrating that the pIL nanolayers adsorb CO₂ selectively over N₂, we investigated if a pIL modified membrane would selectively transport CO₂ over N₂. A pIL modified membrane was loaded into a home-built diffusion cell. The downstream side of the cell was evacuated and the upstream side was pressurized with CO₂ or N₂. Downstream pressure was monitored over time. Figure 4a shows the results for an unmodified membrane. There is no selectivity for either gas; both permeate through the membrane at the same rate. Figure 4b shows the results for a membrane modified by a pIL nanolayer. CO₂ transport is much faster than N₂ for this membrane. Furthermore, the rate of pressure increase for CO₂ is similar to the unmodified membrane. These two results demonstrate that pIL nanolayers have excellent potential for membrane-based CO₂ separations.

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