(747d) Charged Nanochannels By Random Copolymer Micelle Assembly

Membranes that can separate solutes by factors other than size (e.g. charge, hydrophobicity, chemical functionality) would enable the use of this energy-efficient, green technology in new applications. However, achieving membrane selectivity based on parameters other than size has been a challenge, especially when the molecules to be separated are small. Biological pores such as ion channels achieve exceptional selectivity by confining flow into very small pores lined with functional groups. Therefore, incorporating nanostructures that confine permeation into membrane selective layers can significantly enhance selectivity. In this study, we created membrane selective layers with nanopores lined with carboxyl groups using the deposition of micelles formed in methanol onto porous membranes. To prepare these membranes, we prepared an amphiphilic random copolymer that combines highly hydrophobic, fluorinated repeat units of 2,2,2-trifluroethyl methacrylate (TFEMA) with repeat units of methacrylic acid (MAA), PTFEMA-r-PMAA, by free radical polymerization, because it a simple, robust and scalable method. The copolymer contained 55 wt% TFEMA, and was insoluble in water. We found that this copolymer forms micelles in methanol with a multimodal size distribution, as shown by dynamic light scattering (DLS). When these micelles are coated onto the surface of a porous support membrane whose pores are smaller than the micelles, a selective layer of micelles packed together is formed in between which permeation occur. If the membrane pore size is too large, micelles go inside the membrane, clogging inner pores and resulting in a membrane with very low flux. Also, if the solvent evaporation time is too long, the micelle merge and create a dense selective layer with a very low permeability. By proper selection of conditions (i.e. copolymer concentration, solvent evaporation time, base membrane MWCO, etc.), we achieved membranes with pure water permeabilities up to ~6 L/m².h.bar and high selectivity (separation coefficient up to 24) between small molecules with similar size (~1nm) but opposite charges. For example, these membranes can retain a negatively charged dye by > 85% whereas a positively charged dye with a similar size was retained by < 20%. Membrane rejection can be modeled by Donnan exclusion theory. This is the first demonstration of a new approach to forming membranes from easily synthesized copolymers by micelle assembly. We believe this approach can be adapted to address various pore sizes and pore surface chemistries, leading to novel membranes with promising selectivity.