(279a) Charged Nanopore Membranes 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. 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). Using cupric acetate in membrane casting solution was found to change the average size and the size distribution of the micelles. 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. 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. Using this method, we achieved membranes with pure water permeabilities up to ~8  L/m².h.bar. The resultant membranes exhibited rejection properties expected from negatively charged membranes with ~2 nm pore size, as indicated by the relative rejections of monovalent and divalent salts. The membrane showed charge-based selectivity between organic dye molecules, with a negatively charged dye being retained by >85% whereas a positively charged dye with a similar molecular size was retained by <20%. 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.