Covalently bound polymer multilayers for efficient metal ion sorption

We report on the design, synthesis, and characterization of a polymeric multilayer assembly that can act as an efficient sorbent for selected metal ions. We use a covalent layer-by-layer deposition scheme for the construction of multilayer assemblies using the alternating copolymer of 4-hydroxyphenylmaleimide and ethyl vinyl ether 2-diisopropylphosphonate. The maleimide-vinyl ether (MVE) polymer layers are reacted at the maleimide hydroxyl functionality with adipoyl chloride to form an ester linkage, and the terminal acid chloride functionality is reactive toward subsequent deposition of another MVE polymer layer. The vinyl ether isopropylphosphonates remain protected during layer growth. Once layer growth is complete, the isopropylphosphonates are deprotected using bromotrimethylsilane to activate the multilayer for metal ion sorption. The uptake of Zr4+ by the multilayer is confirmed by ellipsometry, FTIR, X-ray photoelectron spectroscopy (XPS) and quartz crystal microbalance (QCM) gravimetry. The XPS data show similar to 13 atom % of the multilayer is Zr4+ after exposure to a 5 mM ethanolic solution of zirconyl chloride, and QCM data show that metal ion uptake is fast. The films also show changes in FTIR spectra and ellipsometric thickness, indicating substantial structural changes associated with metal ion uptake.