Thermo- and pH-Responsive Poly(A-ProOMe)-graft-poly(AAc) Membrane for Selective Separation of Metal Ions

Abstract

We investigated the permeation behavior of Li, Co, and Ni ions through thermo-and pH-responsive gel membranes, which were prepared by -ray grafting of pH-responsive poly(acrylic acid) (poly(AAc)) onto a thermo-responsive polymer gel of acryloyl-L-proline methyl ester (A-ProOMe). Using 15% AAc grafted membranes, the permeation constants of Li ions are 2.8 and 3.5 times higher than those of Co and Ni ions in a pH 6.0 buffer solution at 30 C. These ratios are two and three times higher than those of 5 and 43% grafted gel membranes, respectively. By comparison with the permeation of metal ions through the non-thermo-responsive porous polyethylene membranes grafted with AAc chains or random copolymer gel membranes (poly(A-ProOMe-co-AAc)), it is clear that the structure in which adjacent carboxyl groups of poly(AAc) are surrounded by thermo-responsive A-ProOMe matrix causes selective permeability of Li ion over Co and Ni ions. The distributions of Co and Ni ions in the microscopic structures clearly show that the structure consisting of flexible chains of the carboxyl groups grafted onto the thermo-response gel membrane plays a decisive role in the superior selective permeation of a Li ion.KEY WORDS: Gel / Graft / Polymerization / Radiation / Metal / Permeation / In recent years, there have been a plenty of attempts for developing environmentally responsive gels, which undergo discontinuous and reversible volume phase transitions in response to external stimuli such as temperature, pH, electric fields, and solvent composition, because of their potential applications in the fields of drug delivery, separation systems, chemomechanical actuators, and the like. 1-6The gel membrane can be prepared by a radiation induced polymerization method, characterized by simultaneously occurring processes of polymerization and self-crosslinking in aqueous solution.7-9 Since these gels can be formed from monomers only without initiator, crosslinker, or accelerator, the radiation method for gel preparation can be applied, making possible the formation of a super-clean gel. In addition to gel preparation, graft copolymers consisting of both random and block structures can be easily synthesized through simultaneous or stepwise polymerization by radiation (Figure 1).There were several reports about temperature-and pHresponsive polymers and only temperature responsive hydrogels.10,11 However, there have not been reported about both thermo-and pH-sensitive hydrogels to control permeation behavior of metal ions. Thus, we had reported the successful preparation of gel membranes with both thermo-and pHsensitivities using the radiation technique. The gel membranes consisted of poly(acrylic acid) (poly-(AAc) graft chains with pK a = 4.7, 11 onto a poly(acryloyl-L-proline methyl ester (A-ProOMe)) gel membrane, which undergoes volume phase transition at around 14 C (poly(AProOMe)-graft-poly(AAc)).12,13 The gel membrane composition of which 15% is AAc graft chain exhibited volume change in response to both temperature and pH-changes. It may be assum...

From pH- to Light-Response: Postpolymerization Modification of Polymer Brushes Grafted onto Microporous Polymeric Membranes

“…Alternatively, protonation below their pK a values induces a collapse of the noncharged polymer and an increase in pore size. 25 pH-Responsive membranes have been used for chemical separation processes such as for oil/water mixtures, 26 proteins, 27 ions 28 as well as in biotechnological applications such as biosensors, 4 drug-delivery systems, 29 or nonfouling surfaces. 30 However, research regarding light-responsive polymeric membranes has been exceedingly rare.…”