Coordinative Layer-by-Layer Assembly of Thin Films Based on Metal Ion Complexes of Ligand-Substituted Polystyrene Copolymers and Their Use as Separation Membranes

Abstract

Polystyrene copolymers containing either 4′-vinyl-2,2′:6′,2″-terpyridine (P1) or 2,6-bis(1-methyl-1H-benzo[d]imidazol-2-yl)pyridin-4-yloxy)-methacrylate (P2) as comonomers are prepared and studied on metal ion coordination in solution and coordinative sequential assembly of films and membranes. Permeation and separation of ions and neutral aromatic molecules across the membranes are investigated. UV–vis titration of P1 with zinc acetate in chloroform/methanol (25:1) indicates formation of 2:1 ligand:metal bis-complexes, while titration of P2 led to 1:1 monocomplexes. It is demonstrated that coordinative interactions between the ligand groups and metal ions such as Zn(II) or Cu(II) can be used for layer-by-layer assembly of ultrathin films on solid quartz supports and porous supporting membranes. Formation of coordination polymer assemblies is characterized using UV-spectroscopy, quartz crystal microbalance measurements, scanning electron microscopy, and thickness measurements. Coordinative films prepared on porous polyacrylonitrile/polyethylene terephthalate supporting membranes exhibit a selective transport of ions and small molecules. Ion transport in aqueous and alcoholic solution is controlled by a sieving effect and by electrostatic rejection at the ligand–metal ion complexes in the membrane. The transport of organic aromatic molecules is only controlled by the sieving effect. Separation factors α(NaCl/BaCl2) up to 4.2 and α(naphthalene/perylene) up to 4.8 are found.