Investigation of ion transport and selectivity achieved with crown ether fixed site polymeric membranes: Grafting and photopolymerization membrane production methods.

摘要

The objective of this work has been to conduct an investigation into the factors which affect the ability of fixed site carrier membranes to transport ionic solutes in an effort to develop membranes that have the ability to selectively separate metal ions present in aqueous streams. In this work, the reactive sites within the membrane were generated by a crown ether-containing monomer, vinylbenzo-18-crown-6 (VB 18C6--which has preferential binding with the potassium ion) co-polymerized with di(ethylene glycol) ethyl ether acrylate and a crosslinker, hexamethylene diacrylate. Ion transport studies have been performed with aqueous solutions of salts of four alkali metal cations: potassium, sodium, rubidium, and cesium. The first class of system variables (transport solution) investigated for their effect on ion transport and selectivity includes the concentration of solutes, the number of solutes present (the level of competition), the salt pair identity and the pH of the feed and receiving solutions. The second class of variables investigated, membrane composition variables, includes the crosslinking density, the reactive site concentration, the inclusion of ionizable monomers, and the membrane preparation method.;Ion transport studies with the lightly crosslinked membranes produced normalized potassium flux values of 58,000-72,000*10;The flux of ions across the membrane was found to be dependent on the concentration and number of solutes in the feed solution of the transport cell. If KNO;Membranes for these studies were prepared by two methods: a crosslinking photopolymerization within the pores of a support and grafting the polymers directly onto the porous support material. The grafting procedure in this study is unique because we employ a two-step process that enables more efficient control of the resulting grafted membrane structure. In the first step, a photoactive species becomes grafted to the support. In the second step, the support with attached initiating species serves as a macroinitiator for polymerization reactions. By controlling the reaction conditions, the number of grafting sites, the chain length of the grafted polymer, and the depth of graft attachment are independently controlled.;Several studies have been performed in an effort to characterize the polymers formed in this research. Swelling studies on membranes with a variety of reactive site concentrations were performed with thermogravimetric analyses. Solid and liquid state nuclear magnetic resonance spectra were acquired to probe the organization of the polymer as it forms. Finally, the kinetics of the polymerization were investigated to elucidate the effect of solvent concentration, crosslinker concentration, photoinitiator concentration, and solution composition on the polymerization behavior.